original

[gb-231r1-is01/Gingerbread_2.3.3_r1_IS01.git] / frameworks / base / docs / html / guide / topics / fundamentals.jd

page.title=Application Fundamentals
@jd:body

<div id="qv-wrapper">
<div id="qv">

<h2>In this document</h2>
<ol>
<li><a href="#appcomp">Application Components</a>
  <ol>
    <li><a href="#actcomp">Activating components: intents</a></li>
    <li><a href="#endcomp">Shutting down components</a></li>
    <li><a href="#manfile">The manifest file</a></li>
    <li><a href="#ifilters">Intent filters</a></li>
  </ol></li>
<li><a href="#acttask">Activities and Tasks</a>
  <ol>
    <li><a href="#afftask">Affinities and new tasks</a></li>
    <li><a href="#lmodes">Launch modes</a></li>
    <li><a href="#clearstack">Clearing the stack</a></li>
    <li><a href="#starttask">Starting tasks</a></li>
  </ol></li>
<li><a href="#procthread">Processes and Threads</a>
  <ol>
    <li><a href="#procs">Processes</a></li>
    <li><a href="#threads">Threads</a></li>
    <li><a href="#rpc">Remote procedure calls</a></li>
    <li><a href="#tsafe">Thread-safe methods</a></li>
  </ol></li>
<li><a href="#lcycles">Component Lifecycles</a>
  <ol>
    <li><a href="#actlife">Activity lifecycle</a></li>
    <li><a href="#servlife">Service lifecycle</a></li>
    <li><a href="#broadlife">Broadcast receiver lifecycle</a></li>
    <li><a href="#proclife">Processes and lifecycles</a></li>
  </ol></li>
</ol>

<h2>Key classes</h2>
<ol>
<li>{@link android.app.Activity}</li>
<li>{@link android.app.Service}</li>
<li>{@link android.content.BroadcastReceiver}</li>
<li>{@link android.content.ContentProvider}</li>
<li>{@link android.content.Intent}</li>
</ol>

</div>
</div>

<p>
Android applications are written in the Java programming language. 
The compiled Java code &mdash; along with any data and resource 
files required by the application &mdash; is bundled by the 
<a href="{@docRoot}guide/developing/tools/aapt.html"><code>aapt</code> 
tool</a> into an <i>Android package</i>, an archive file 
marked by an {@code .apk} suffix.  This file is the vehicle 
for distributing the application and installing it on mobile devices; 
it's the file users download to their devices.  All the code in a 
single {@code .apk} file is considered to be one <i>application</i>.
</p>

<p>
In many ways, each Android application lives in its own world:
</p>

<ul>
<li>By default, every application runs in its own Linux process.
Android starts the process when any of the application's code needs to be 
executed, and shuts down the process when it's no longer needed and system 
resources are required by other applications.</li>

<li>Each process has its own virtual machine (VM), so application code 
runs in isolation from the code of all other applications.</li>

<li>By default, each application is assigned a unique Linux user ID.  
Permissions are set so that the application's files are visible only to
that user and only to the application itself &mdash; although there are ways
to export them to other applications as well.</li>
</ul>

<p>
It's possible to arrange for two applications to share the same user ID, 
in which case they will be able to see each other's files.  To conserve 
system resources, applications with the same ID can also arrange to run 
in the same Linux process, sharing the same VM.
</p>


<h2 id="appcomp">Application Components</h2>

<p>
A central feature of Android is that one application can make use of elements 
of other applications (provided those applications permit it).  For example, 
if your application needs to display a scrolling list of images and another 
application has developed a suitable scroller and made it available to others, 
you can call upon that scroller to do the work, rather than develop your own.  
Your application doesn't incorporate the code of the other application or 
link to it.  Rather, it simply starts up that piece of the other application 
when the need arises.
</p>

<p>
For this to work, the system must be able to start an application process 
when any part of it is needed, and instantiate the Java objects for that part.  
Therefore, unlike applications on most other systems, Android applications don't 
have a single entry point for everything in the application (no {@code main()} 
function, for example).  Rather, they have essential <i>components</i> that 
the system can instantiate and run as needed.  There are four types of components:
</p>

<dl>

<dt><b>Activities</b></dt>
<dd>An <i>activity</i> presents a visual user interface for one focused endeavor 
the user can undertake.  For example, an activity might present a list of 
menu items users can choose from or it might display photographs along
with their captions.  A text messaging application might have one activity 
that shows a list of contacts to send messages to, a second activity to write 
the message to the chosen contact, and other activities to review old messages 
or change settings.  Though they work together to form a cohesive user interface, 
each activity is independent of the others.  
Each one is implemented as a subclass of the {@link android.app.Activity} base class.  

<p>
An application might consist of just one activity or, like the text messaging
application just mentioned, it may contain several.  
What the activities are, and how many there are depends, of course, on the 
application and its design.  Typically, one of the activities is marked
as the first one that should be presented to the user when the application is 
launched.  Moving from one activity to another is accomplished by having the 
current activity start the next one.  
</p>

<p>
Each activity is given a default window to draw in.  Typically, the window 
fills the screen, but it might be smaller than the screen and float on top 
of other windows.  An activity can also make use of additional windows &mdash; 
for example, a pop-up dialog that calls for a user response in the midst of 
the activity, or a window that presents users with vital information when they 
select a particular item on-screen.
</p>

<p>
The visual content of the window is provided by a hierarchy of views &mdash; 
objects derived from the base {@link android.view.View} class.  Each view 
controls a particular rectangular space within the window.  Parent views 
contain and organize the layout of their children.  Leaf views (those at the 
bottom of the hierarchy) draw in the rectangles they control and respond to 
user actions directed at that space.  Thus, views are where the activity's 
interaction with the user takes place.  For example, a view might display 
a small image and initiate an action when the user taps that image.  Android 
has a number of ready-made views that you can use &mdash; including buttons, 
text fields, scroll bars, menu items, check boxes, and more.
</p>

<p>
A view hierarchy is placed within an activity's window by the 
<code>{@link android.app.Activity#setContentView Activity.setContentView()}</code> 
method.  The <i>content view</i> is the View object at the root of the hierarchy.  
(See the separate <a href="{@docRoot}guide/topics/ui/index.html">User Interface</a> 
document for more information on views and the hierarchy.)
</p>

<p><dt><b>Services</b></dt>
<dd>A <i>service</i> doesn't have a visual user interface, but rather runs in 
the background for an indefinite period of time.  For example, a service might 
play background music as the user attends to other matters, or it might fetch 
data over the network or calculate something and provide the result to activities 
that need it.  Each service extends the {@link android.app.Service} base class.

<p>
A prime example is a media player playing songs from a play list.  The player 
application would probably have one or more activities that allow the user to 
choose songs and start playing them.  However, the music playback itself would 
not be handled by an activity because users will expect the music to keep 
playing even after they leave the player and begin something different.  
To keep the music going, the media player activity could start a service to run 
in the background.  The system would then keep the music playback service running 
even after the activity that started it leaves the screen.
</p>

<p> 
It's possible to connect to (bind to) an ongoing service (and start the service 
if it's not already running).  While connected, you can communicate with the 
service through an interface that the service exposes.  For the music service, 
this interface might allow users to pause, rewind, stop, and restart the playback.
</p>

<p>
Like activities and the other components, services run in the main thread of 
the application process.  So that they won't block other components or the 
user interface, they often spawn another thread for time-consuming tasks 
(like music playback).  See <a href="#procthread">Processes and Threads</a>, later.
</p></dd>

<dt><b>Broadcast receivers</b></dt>
<dd>A <i>broadcast receiver</i> is a component that does nothing but 
receive and react to broadcast announcements.  Many broadcasts originate in 
system code &mdash; for example, announcements that the timezone has changed, 
that the battery is low, that a picture has been taken, or that the user 
changed a language preference.  Applications can also initiate broadcasts
&mdash; for example, to let other applications know that some data has been
downloaded to the device and is available for them to use.

<p>
An application can have any number of broadcast receivers to respond to any 
announcements it considers important.  All receivers extend the {@link 
android.content.BroadcastReceiver} base class.
</p>

<p>
Broadcast receivers do not display a user interface.  However, they may start
an activity in response to the information they receive, or they may use 
the {@link android.app.NotificationManager} to alert the user.  Notifications 
can get the user's attention in various ways &mdash; flashing 
the backlight, vibrating the device, playing a sound, and so on.  They 
typically place a persistent icon in the status bar, which users can open to 
get the message. 
</p></dd>

<dt><b>Content providers</b></dt>
<dd>A <i>content provider</i> makes a specific set of the application's data 
available to other applications. The data can be stored in the file system, 
in an SQLite database, or in any other manner that makes sense.  
The content provider extends the {@link android.content.ContentProvider} base 
class to implement a standard set of methods that enable other applications 
to retrieve and store data of the type it controls.  However, applications 
do not call these methods directly.  Rather they use a {@link 
android.content.ContentResolver} object and call its methods instead.  
A ContentResolver can talk to any content provider; it cooperates with the
provider to manage any interprocess communication that's involved. 

<p>
See the separate 
<a href="{@docRoot}guide/topics/providers/content-providers.html">Content 
Providers</a> document for more information on using content providers.
</p></dd>

</dl>

<p>
Whenever there's a request that should be handled by a particular component, 
Android makes sure that the application process of the component is running, 
starting it if necessary, and that an appropriate instance of the component 
is available, creating the instance if necessary.  
</p>


<h3 id="actcomp">Activating components: intents</h3> 

<p>
Content providers are activated when they're targeted by a request from a 
ContentResolver.  The other three components &mdash; activities, services, 
and broadcast receivers &mdash; are activated by asynchronous messages 
called <i>intents</i>.  An intent is an {@link android.content.Intent} 
object that holds the content of the message.  For activities and services,
it names the action being requested and specifies the URI of the data to 
act on, among other things. For example, it might convey a request for 
an activity to present an image to the user or let the user edit some
text.  For broadcast receivers, the Intent object names the action being 
announced.  For example, it might announce to interested parties that the 
camera button has been pressed.
</p>

<p>
There are separate methods for activating each type of component: 
</p>

<ul>

<li>An activity is launched (or given something new to do) by passing an 
Intent object to <code>{@link android.content.Context#startActivity 
Context.startActivity()}</code> or <code>{@link 
android.app.Activity#startActivityForResult 
Activity.startActivityForResult()}</code>.  The responding activity can 
look at the initial intent that caused it to be launched by calling its 
<code>{@link android.app.Activity#getIntent getIntent()}</code> method.  
Android calls the activity's <code>{@link 
android.app.Activity#onNewIntent onNewIntent()}</code> method to pass 
it any subsequent intents.

<p>
One activity often starts the next one.  If it expects a result back from 
the activity it's starting, it calls {@code startActivityForResult()} 
instead of {@code startActivity()}.  For example, if it starts an activity 
that lets the user pick a photo, it might expect to be returned the chosen 
photo.  The result is returned in an Intent object that's passed to the 
calling activity's <code>{@link android.app.Activity#onActivityResult 
onActivityResult()}</code> method.
</p>
</li>

<li><p>A service is started (or new instructions are given to an ongoing 
service) by passing an Intent object to <code>{@link 
android.content.Context#startService Context.startService()}</code>.  
Android calls the service's <code>{@link android.app.Service#onStart 
onStart()}</code> method and passes it the Intent object.</p>

<p>
Similarly, an intent can be passed to <code>{@link 
android.content.Context#bindService Context.bindService()}</code> to 
establish an ongoing connection between the calling component and a 
target service.  The service receives the Intent object in
an <code>{@link android.app.Service#onBind onBind()}</code> call.
(If the service is not already running, {@code bindService()} can
optionally start it.)  For example, an activity might establish a connection 
with the music playback service mentioned earlier so that it can provide 
the user with the means (a user interface) for controlling the playback.  
The activity would call {@code bindService()} to set up that connection, 
and then call methods defined by the service to affect the playback.
</p>

<p>
A later section, <a href="#rpc">Remote procedure calls</a>, has more details 
about binding to a service.
</p>
</li>

<li><p>An application can initiate a broadcast by passing an Intent object to 
methods like <code>{@link 
android.content.Context#sendBroadcast(Intent) Context.sendBroadcast()}</code>, 
<code>{@link android.content.Context#sendOrderedBroadcast(Intent, String) 
Context.sendOrderedBroadcast()}</code>, and <code>{@link 
android.content.Context#sendStickyBroadcast Context.sendStickyBroadcast()}</code>
in any of their variations.  Android delivers the intent to all interested 
broadcast receivers by calling their <code>{@link 
android.content.BroadcastReceiver#onReceive onReceive()}</code> methods.</p></li>

</ul>

<p>
For more on intent messages, see the separate article, 
<a href="{@docRoot}guide/topics/intents/intents-filters.html">Intents 
and Intent Filters</a>.
</p>


<h3 id="endcomp">Shutting down components</h3>

<p>
A content provider is active only while it's responding to a request from 
a ContentResolver.  And a broadcast receiver is active only while it's 
responding to a broadcast message.  So there's no need to explicitly shut 
down these components.
</p>

<p>
Activities, on the other hand, provide the user interface.  They're 
in a long-running conversation with the user and may remain active, 
even when idle, as long as the conversation continues.  Similarly, services 
may also remain running for a long time.  So Android has methods to shut 
down activities and services in an orderly way:
</p>

<ul>
<li>An activity can be shut down by calling its
<code>{@link android.app.Activity#finish finish()}</code> method.  One activity can
shut down another activity (one it started with {@code startActivityForResult()}) by 
calling <code>{@link android.app.Activity#finishActivity finishActivity()}</code>.</li>

<li>A service can be stopped by calling its
<code>{@link android.app.Service#stopSelf stopSelf()}</code> method, or by calling 
<code>{@link android.content.Context#stopService Context.stopService()}</code>.</li>
</ul>

<p>
Components might also be shut down by the system when they are no longer being
used or when Android must reclaim memory for more active components.  A later
section, <a href="#lcycles">Component Lifecycles</a>, discusses this
possibility and its ramifications in more detail.
</p>


<h3 id="manfile">The manifest file</h3>

<p>
Before Android can start an application component, it must learn that 
the component exists.  Therefore, applications declare their components 
in a manifest file that's bundled into the Android package, the {@code .apk} 
file that also holds the application's code, files, and resources.  
</p>

<p>
The manifest is a structured XML file and is always named AndroidManifest.xml 
for all applications.  It does a number of things in addition to declaring the 
application's components, such as naming any libraries the application needs 
to be linked against (besides the default Android library) and identifying 
any permissions the application expects to be granted.
</p>

<p>
But the principal task of the manifest is to inform Android about the application's 
components.  For example, an activity might be declared as follows:
</p>

<pre>&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;manifest . . . &gt;
    &lt;application . . . &gt;
        &lt;activity android:name="com.example.project.FreneticActivity"
                  android:icon="@drawable/small_pic.png"
                  android:label="@string/freneticLabel" 
                  . . .  &gt;
        &lt;/activity&gt;
        . . .
    &lt;/application&gt;
&lt;/manifest&gt;</pre>

<p>
The {@code name} attribute of the 
<code><a href="{@docRoot}guide/topics/manifest/activity-element.html">&lt;activity&gt;</a></code>
element names the {@link android.app.Activity} subclass that implements the 
activity.  The {@code icon} and {@code label} attributes point to 
resource files containing an icon and label that can be displayed 
to users to represent the activity.
</p>

<p>
The other components are declared in a similar way &mdash; 
<code><a href="{@docRoot}guide/topics/manifest/service-element.html">&lt;service&gt;</a></code>
elements for services,
<code><a href="{@docRoot}guide/topics/manifest/receiver-element.html">&lt;receiver&gt;</a></code>
elements for broadcast receivers, and 
<code><a href="{@docRoot}guide/topics/manifest/provider-element.html">&lt;provider&gt;</a></code>
elements for content providers.  Activities, services, and content providers 
that are not declared in the manifest are not visible to the system and are 
consequently never run.  However, broadcast receivers can either be 
declared in the manifest, or they can be created dynamically in code 
(as {@link android.content.BroadcastReceiver} objects) 
and registered with the system by calling 
<code>{@link android.content.Context#registerReceiver Context.registerReceiver()}</code>.
</p>

<p>
For more on how to structure a manifest file for your application, see 
<a href="{@docRoot}guide/topics/manifest/manifest-intro.html">The 
AndroidManifest.xml File</a>.
</p>


<h3 id="ifilters">Intent filters</h3>

<p>
An Intent object can explicitly name a target component.  If it does,
Android finds that component (based on the declarations in the manifest 
file) and activates it.  But if a target is not explicitly named, 
Android must locate the best component to respond to the intent.  
It does so by comparing the Intent object to the <i>intent filters</i> 
of potential targets.  A component's intent filters inform Android of 
the kinds of intents the component is able to handle.  Like other 
essential information about the component, they're declared in the 
manifest file.  Here's an extension of the previous example that adds 
two intent filters to the activity:
</p>

<pre>&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;manifest . . . &gt;
    &lt;application . . . &gt;
        &lt;activity android:name="com.example.project.FreneticActivity"
                  android:icon="@drawable/small_pic.png"
                  android:label="@string/freneticLabel" 
                  . . .  &gt;
            &lt;intent-filter . . . &gt;
                &lt;action android:name="android.intent.action.MAIN" /&gt;
                &lt;category android:name="android.intent.category.LAUNCHER" /&gt;
            &lt;/intent-filter&gt;
            &lt;intent-filter . . . &gt;
                &lt;action android:name="com.example.project.BOUNCE" /&gt;
                &lt;data android:mimeType="image/jpeg" /&gt;
                &lt;category android:name="android.intent.category.DEFAULT" /&gt;
            &lt;/intent-filter&gt;
        &lt;/activity&gt;
        . . .
    &lt;/application&gt;
&lt;/manifest&gt;</pre>

<p>
The first filter in the example &mdash; the combination of the action
"{@code android.intent.action.MAIN}" and the category
"{@code android.intent.category.LAUNCHER}" &mdash; is a common one.
It marks the activity as one that should be represented in the
application launcher, the screen listing applications users can launch 
on the device.  In other words, the activity is the entry point for 
the application, the initial one users would see when they choose 
the application in the launcher.
</p>

<p>
The second filter declares an action that the activity can perform on 
a particular type of data.
</p>

<p>
A component can have any number of intent filters, each one declaring a 
different set of capabilities.  If it doesn't have any filters, it can 
be activated only by intents that explicitly name the component as the 
target.
</p>

<p>
For a broadcast receiver that's created and registered in code, the
intent filter is instantiated directly as an {@link android.content.IntentFilter}
object.  All other filters are set up in the manifest.
</p>

<p>
For more on intent filters, see a separate document, 
<a href="{@docRoot}guide/topics/intents/intents-filters.html">Intents 
and Intent Filters</a>.
</p>


<h2 id="acttask">Activities and Tasks</h2>

<p>
As noted earlier,  one activity can start another, including one defined 
in a different application.  Suppose, for example, that you'd like 
to let users display a street map of some location.  There's already an 
activity that can do that, so all your activity needs to do is put together 
an Intent object with the required information and pass it to 
{@code startActivity()}.  The map viewer will display the map.  When the user 
hits the BACK key, your activity will reappear on screen.
</p>

<p>
To the user, it will seem as if the map viewer is part of the same application 
as your activity, even though it's defined in another application and runs in 
that application's process.  Android maintains this user experience by keeping 
both activities in the same <i>task</i>.  Simply put, a task is what the user 
experiences as an "application."  It's a group of related activities, arranged 
in a stack.  The root activity in the stack is the one that began the task 
&mdash; typically, it's an activity the user selected in the application launcher.  
The activity at the top of the stack is one that's currently running &mdash; 
the one that is the focus for user actions.  When one activity starts another, 
the new activity is pushed on the stack; it becomes the running activity.  
The previous activity remains in the stack.  When the user presses the BACK key, 
the current activity is popped from the stack, and the previous one resumes as 
the running activity.  
</p>

<p>
The stack contains objects, so if a task has more than one instance of the same 
Activity subclass open &mdash; multiple map viewers, for example &mdash; the 
stack has a separate entry for each instance.  Activities in the stack are never 
rearranged, only pushed and popped.
</p>

<p>
A task is a stack of activities, not a class or an element in the manifest file. 
So there's no way to set values for a task independently of its activities.  
Values for the task as a whole are set in the root activity.  For example, the 
next section will talk about the "affinity of a task"; that value is read from 
the affinity set for the task's root activity.
</p>

<p>
All the activities in a task move together as a unit.  The entire task (the entire 
activity stack) can be brought to the foreground or sent to the background.  
Suppose, for instance, that the current task has four activities in its stack 
&mdash; three under the current activity.  The user presses the HOME key, goes 
to the application launcher, and selects a new application (actually, a new <i>task</i>).  
The current task goes into the background and the root activity for the new task is displayed.  
Then, after a short period, the user goes back to the home screen and again selects 
the previous application (the previous task).  That task, with all four 
activities in the stack, comes forward.  When the user presses the BACK 
key, the screen does not display the activity the user just left (the root
activity of the previous task).  Rather, the activity on the top of the stack 
is removed and the previous activity in the same task is displayed. 
</p>

<p>
The behavior just described is the default behavior for activities and tasks.  
But there are ways to modify almost all aspects of it.  The association of 
activities with tasks, and the behavior of an activity within a task, is 
controlled by the interaction between flags set in the Intent object that
started the activity and attributes set in the activity's 
<code><a href="{@docRoot}guide/topics/manifest/activity-element.html">&lt;activity&gt;</a></code>
element in the manifest.  Both requester and respondent have a say in what happens.
</p>

<p>
In this regard, the principal Intent flags are:

<p style="margin-left: 2em">{@code FLAG_ACTIVITY_NEW_TASK}
<br/>{@code FLAG_ACTIVITY_CLEAR_TOP}
<br/>{@code FLAG_ACTIVITY_RESET_TASK_IF_NEEDED}
<br/>{@code FLAG_ACTIVITY_SINGLE_TOP}</p>

<p>
The principal {@code &lt;activity&gt;} attributes are:
  
<p style="margin-left: 2em">{@code taskAffinity}
<br/>{@code launchMode}
<br/>{@code allowTaskReparenting}
<br/>{@code clearTaskOnLaunch}
<br/>{@code alwaysRetainTaskState}
<br/>{@code finishOnTaskLaunch}</p>

<p>
The following sections describe what some of these flags and attributes do,
how they interact, and what considerations should govern their use.
</p>


<h3 id="afftask">Affinities and new tasks</h3>

<p>
By default, all the activities in an application have an <i>affinity</i> for each 
other &mdash; that is, there's a preference for them all to belong to the 
same task.  However, an individual affinity can be set for each activity 
with the {@code taskAffinity} attribute of the {@code &lt;activity&gt;} element. 
Activities defined in different applications can share an affinity, or activities 
defined in the same application can be assigned different affinities.  
The affinity comes into play in two circumstances:  When the Intent object 
that launches an activity contains the {@code FLAG_ACTIVITY_NEW_TASK} flag, 
and when an activity has its {@code allowTaskReparenting} attribute set 
to "{@code true}". 
</p>

<dl>
<dt>The <code>{@link android.content.Intent#FLAG_ACTIVITY_NEW_TASK}</code> flag</dt>
<dd>As described earlier, a new activity is, by default, launched into 
the task of the activity that called {@code startActivity()}.  It's pushed
 onto the same stack as the caller.  However, if the Intent object passed 
to {@code startActivity()} contains the {@code FLAG_ACTIVITY_NEW_TASK} 
flag, the system looks for a different task to house the new activity.  
Often, as the name of the flag implies, it's a new task.  However, it 
doesn't have to be.  If there's already an existing task with the same 
affinity as the new activity, the activity is launched into that task.  If 
not, it begins a new task.</dd>

<dt>The <code><a 
href="{@docRoot}guide/topics/manifest/activity-element.html#reparent">allowTaskReparenting</a></code>
attribute</dt>
<dd>If an activity has its {@code allowTaskReparenting} attribute set 
to "{@code true}", it can move from the task it starts in to the task 
it has an affinity for when that task comes to the fore.  For example, 
suppose that an activity that reports weather conditions in selected 
cities is defined as part of a travel application.  It has the same
affinity as other activities in the same application (the default 
affinity) and it allows reparenting.  One of your activities 
starts the weather reporter, so it initially belongs to the same task as 
your activity.  However, when the travel application next comes forward, 
the weather reporter will be reassigned to and displayed with that task.</dd>
</dl>

<p>
If an {@code .apk} file contains more than one "application"
from the user's point of view, you will probably want to assign different 
affinities to the activities associated with each of them.
</p>


<h3 id="lmodes">Launch modes</h3>

<p>
There are four different launch modes that can be assigned to an {@code
&lt;activity&gt;} element's 
<code><a href="{@docRoot}guide/topics/manifest/activity-element.html#lmode">launchMode</a></code> 
attribute:
</p>

<p style="margin-left: 2em">"{@code standard}" (the default mode)
<br>"{@code singleTop}"
<br>"{@code singleTask}"
<br>"{@code singleInstance}"</p>

<p>
The modes differ from each other on these four points:
</p>

<ul>

<li><b>Which task will hold the activity that responds to the intent</b>.  
For the "{@code standard}" and "{@code singleTop}" modes, it's the task that 
originated the intent (and called 
<code>{@link android.content.Context#startActivity startActivity()}</code>) 
&mdash; unless the Intent object contains the 
<code>{@link android.content.Intent#FLAG_ACTIVITY_NEW_TASK}</code> flag.  
In that case, a different task is chosen as described in the previous 
section, <a href="#afftask">Affinities and new tasks</a>.  

<p>
In contrast, the "{@code singleTask}" and "{@code singleInstance}" modes mark 
activities that are always at the root of a task.  They define a task; they're
never launched into another task.
</p>  

<li><p><b>Whether there can be multiple instances of the activity</b>.  
A "{@code standard}" or "{@code singleTop}" activity can be instantiated
many times.  They can belong to multiple tasks, and a given task can have 
multiple instances of the same activity.
</p> 

<p>
In contrast, "{@code singleTask}" and "{@code singleInstance}" activities 
are limited to just one instance.  Since these activities are at the root
of a task, this limitation means that there is never more than a single
instance of the task on the device at one time.
</p>    

<li><p><b>Whether the instance can have other activities in its task</b>.  
A "{@code singleInstance}" activity stands alone as the only activity in its 
task.  If it starts another activity, that activity will be launched into a 
different task regardless of its launch mode &mdash; as if {@code
FLAG_ACTIVITY_NEW_TASK} was in the intent.  In all other respects, the 
"{@code singleInstance}" mode is identical to "{@code singleTask}".</p>

<p>
The other three modes permit multiple activities to belong to the task.
A "{@code singleTask}" activity will always be the root activity of the task, 
but it can start other activities that will be assigned to its
task.  Instances of "{@code standard}" and "{@code singleTop}"
activities can appear anywhere in a stack.  
</p></li>

<li><b>Whether a new instance of the class will be launched 
to handle a new intent</b>.  For the default "{@code standard}" mode, a 
new instance is created to respond to every new intent.  Each instance 
handles just one intent.  For the "{@code singleTop}" mode, an existing 
instance of the class is re-used to handle a new intent if it resides 
at the top of the activity stack of the target task.  If it does not 
reside at the top, it is not re-used.  Instead, a new instance 
is created for the new intent and pushed on the stack.

<p>
For example, suppose a task's activity stack consists of root activity A with 
activities B, C, and D on top in that order, so the stack is A-B-C-D.  An intent 
arrives for an activity of type D.  If D has the default "{@code standard}" launch 
mode, a new instance of the class is launched and the stack becomes A-B-C-D-D.  
However, if D's launch mode is "{@code singleTop}", the existing instance is 
expected to handle the new intent (since it's at the top of the stack) and the 
stack remains A-B-C-D.  
</p>

<p>
If, on the other hand, the arriving intent is for an activity of type B, a new 
instance of B would be launched no matter whether B's mode is "{@code standard}" 
or "{@code singleTop}" (since B is not at the top of the stack), so the resulting 
stack would be A-B-C-D-B.
</p>

<p>
As noted above, there's never more than one instance of a "{@code singleTask}" 
or "{@code singleInstance}" activity, so that instance is expected to handle
all new intents.  A "{@code singleInstance}" activity is always at the top of 
the stack (since it is the only activity in the task), so it is always in 
position to handle the intent.  However, a "{@code singleTask}" activity may 
or may not have other activities above it in the stack.  If it does, it is not 
in position to handle the intent, and the intent is dropped.  (Even though the 
intent is dropped, its arrival would have caused the task to come to the 
foreground, where it would remain.)
</p>
</li>

</ul>

<p>
When an existing activity is asked to handle a new intent, the Intent
object is passed to the activity in an 
<code>{@link android.app.Activity#onNewIntent onNewIntent()}</code> call.  
(The intent object that originally started the activity can be retrieved by 
calling <code>{@link android.app.Activity#getIntent getIntent()}</code>.)
</p>

<p>
Note that when a new instance of an Activity is created to handle a new
intent, the user can always press the BACK key to return to the previous state
(to the previous activity).  But when an existing instance of an 
Activity handles a new intent, the user cannot press the BACK key to 
return to what that instance was doing before the new intent arrived.
</p>

<p>
For more on launch modes, see the description of the <code><a 
href="{@docRoot}guide/topics/manifest/activity-element.html#lmode">&lt;activity&gt;</a></code>
element.
</p>


<h3 id="clearstack">Clearing the stack</h3>

<p>
If the user leaves a task for a long time, the system clears the task of all
activities except the root activity.  When the user returns to the task again, 
it's as the user left it, except that only the initial activity is present. 
The idea is that, after
a time, users will likely have abandoned what they were doing before and are
returning to the task to begin something new.
</p>

<p>
That's the default.  There are some activity attributes that can be used to 
control this behavior and modify it:
</p>

<dl>
<dt>The <code><a 
href="{@docRoot}guide/topics/manifest/activity-element.html#always">alwaysRetainTaskState</a></code>
attribute</dt>
<dd>If this attribute is set to "{@code true}" in the root activity of a task, 
the default behavior just described does not happen.  
The task retains all activities in its stack even after a long period.</dd>

<dt>The <code><a 
href="{@docRoot}guide/topics/manifest/activity-element.html#clear">clearTaskOnLaunch</a></code>
attribute</dt>
<dd>If this attribute is set to "{@code true}" in the root activity of a task, 
the stack is cleared down to the root activity whenever the user leaves the task 
and returns to it.  In other words, it's the polar opposite of 
{@code alwaysRetainTaskState}.  The user always returns to the task in its
initial state, even after a momentary absence.</dd>

<dt>The <code><a 
href="{@docRoot}guide/topics/manifest/activity-element.html#finish">finishOnTaskLaunch</a></code>
attribute</dt>
<dd>This attribute is like {@code clearTaskOnLaunch}, but it operates on a 
single activity, not an entire task.  And it can cause any activity to go
away, including the root activity.  When it's set to "{@code true}", the 
activity remains part of the task only for the current session.  If the user 
leaves and then returns to the task, it no longer is present.</dd>
</dl>

<p>
There's another way to force activities to be removed from the stack.  
If an Intent object includes the <code>{@link 
android.content.Intent#FLAG_ACTIVITY_CLEAR_TOP FLAG_ACTIVITY_CLEAR_TOP}</code> 
flag, and the target task already has an instance of the type of activity that 
should handle the intent in its stack, all activities above that instance 
are cleared away so that it stands at the top of the stack and can respond 
to the intent. 
If the launch mode of the designated activity is "{@code standard}", it too 
will be removed from the stack, and a new instance will be launched to handle 
the incoming intent.  That's because a new instance is always created for 
a new intent when the launch mode is "{@code standard}".
</p>

<p>
{@code FLAG_ACTIVITY_CLEAR_TOP} is most often used in conjunction
with {@code FLAG_ACTIVITY_NEW_TASK}.  When used together, these flags are
a way of locating an existing activity in another task and putting it in
a position where it can respond to the intent.  
</p>


<h3 id="starttask">Starting tasks</h3>

<p>
An activity is set up as the entry point for a task by giving it 
an intent filter with "{@code android.intent.action.MAIN}" as the 
specified action and "{@code android.intent.category.LAUNCHER}" as 
the specified category.  (There's an example of this type of filter 
in the earlier <a href="#ifilters">Intent Filters</a> section.)  
A filter of this kind causes an icon and label for the activity to be 
displayed in the application launcher, giving users a way both to 
launch the task and to return to it at any time after it has been 
launched.
</p>

<p>
This second ability is important:  Users must be able to leave a task
and then come back to it later.  For this reason, the two launch modes
that mark activities as always initiating a task, "{@code singleTask}" 
and "{@code singleInstance}", should be used only when the activity has 
a {@code MAIN} and {@code LAUNCHER} filter.  
Imagine, for example, what could happen if the filter is missing:
An intent launches a "{@code singleTask}" activity, initiating a new task, 
and the user spends some time working in that task.  The user then presses 
the HOME key.  The task is now ordered behind and obscured by the home 
screen.  And, because it is not represented in the application launcher, 
the user has no way to return to it.
</p>

<p>
A similar difficulty attends the {@code FLAG_ACTIVITY_NEW_TASK} flag.
If this flag causes an activity to
begin a new task and the user presses the HOME key to leave it, there
must be some way for the user to navigate back to it again.  Some 
entities (such as the notification manager) always start activities 
in an external task, never as part of their own, so they always put 
{@code FLAG_ACTIVITY_NEW_TASK} in the intents they pass to 
{@code startActivity()}.  If you have an activity that can be invoked 
by an external entity that might use this flag, take care that the user 
has a independent way to get back to the task that's started.
</p> 

<p>
For those cases where you don't want the user to be able to return
to an activity, set the {@code &lt;activity&gt;} element's {@code
finishOnTaskLaunch} to "{@code true}".  
See <a href="#clearstack">Clearing the stack</a>, earlier.
</p>


<h2 id="procthread">Processes and Threads</h2>

<p>
When the first of an application's components needs to be run, Android 
starts a Linux process for it with a single thread of execution.  By default, 
all components of the application run in that process and thread.
</p>

<p>
However, you can arrange for components to run in other processes, and you 
can spawn additional threads for any process.
</p>


<h3 id="procs">Processes</h3>

<p>
The process where a component runs is controlled by the manifest file.  
The component elements &mdash; {@code &lt;activity&gt;}, 
{@code &lt;service&gt;}, {@code &lt;receiver&gt;}, and {@code &lt;provider&gt;} 
&mdash; each have a {@code process} attribute that can specify a process
where that component should run.  These attributes can be set so that each 
component runs in its own process, or so that some components share a process 
while others do not.  They can also be set so that components of 
different applications run in the same process &mdash; provided that the 
applications share the same Linux user ID and are signed by the same authorities.
The {@code &lt;application&gt;} element also has a {@code process} attribute,
for setting a default value that applies to all components.
</p>

<p>
All components are instantiated in the main thread of the specified
process, and system calls to the component are dispatched from that
thread.  Separate threads are not created for each instance.  Consequently,
methods that respond to those calls &mdash; methods like 
<code>{@link android.view.View#onKeyDown View.onKeyDown()}</code> that report
user actions and the lifecycle notifications discussed later in the 
<a href="#lcycles">Component Lifecycles</a> section &mdash; always run in the
main thread of the process.  This means
that no component should perform long or blocking operations (such as networking 
operations or computation loops) when called by the system, since this will block
any other components also in the process.  You can spawn separate threads for 
long operations, as discussed under <a href="#threads">Threads</a>, next.
</p>

<p>
Android may decide to shut down a process at some point, when memory is 
low and required by other processes that are more immediately serving 
the user.  Application components running in the process are consequently 
destroyed.  A process is restarted for those components when there's again
work for them to do.
</p>  

<p>
When deciding which processes to terminate, Android weighs their relative
importance to the user.  For example, it more readily shuts down a process 
with activities that are no longer visible on screen than a process with
visible activities.
The decision whether to terminate a process, therefore, depends on the state 
of the components running in that process.  Those states are the subject of
a later section, <a href="#lcycles">Component Lifecycles</a>.
</p>


<h3 id="threads">Threads</h3>

<p>
Even though you may confine your application to a single process, there will
likely be times when you will need to spawn a thread to do some background 
work.  Since the user interface must always be quick to respond to user actions, 
the thread that hosts an activity should not also host time-consuming operations 
like network downloads.  Anything that may not be completed quickly should be
assigned to a different thread. 
</p>

<p>
Threads are created in code using standard Java {@link java.lang.Thread}
objects.  Android provides a number of convenience classes for managing 
threads &mdash; {@link android.os.Looper} for running a message loop within 
a thread, {@link android.os.Handler} for processing messages, and 
{@link android.os.HandlerThread} for setting up a thread with a message loop.
</p>


<h3 id="rpc">Remote procedure calls</h3>

<p>
Android has a lightweight mechanism for remote procedure calls (RPCs) 
&mdash; where a method is called locally, but executed remotely (in another
process), with any result returned back to the caller.
This entails decomposing the method call and all its attendant data to a 
level the operating system can understand, transmitting it from the local 
process and address space to the remote process and address space, and 
reassembling and reenacting the call there.  Return values have to be 
transmitted in the opposite direction.  Android provides all the code 
to do that work, so that you can concentrate on defining and implementing 
the RPC interface itself.
</p>

<p>
An RPC interface can include only methods. By default,
all methods are executed synchronously (the local method blocks until the
remote method finishes), even if there is no return value.
</p>

<p>
In brief, the mechanism works as follows:  You'd begin by declaring the
RPC interface you want to implement using a simple IDL (interface definition 
language).  From that declaration, the 
<code><a href="{@docRoot}guide/developing/tools/aidl.html">aidl</a></code> 
tool generates a Java interface definition that must be made available to 
both the local and the remote process.  It contains two inner class, as shown 
in the following diagram:
</p>

<p style="margin-left: 2em">
<img src="{@docRoot}images/binder_rpc.png" alt="RPC mechanism." />
</p>

<p>
The inner classes have all the code needed to administer remote procedure
calls for the interface you declared with the IDL. 
Both inner classes implement the {@link android.os.IBinder}
interface.  One of them is used locally and internally by the system;
the code you write can ignore it.
The other, called Stub, extends the {@link android.os.Binder}
class.  In addition to internal code for effectuating the IPC calls, it 
contains declarations for the methods in the RPC interface you declared.
You would subclass Stub to implement those methods, as indicated in the 
diagram.  
</p>

<p>
Typically, the remote process would be managed by a service (because a 
service can inform the system about the process and its connections to 
other processes).  It would have both the interface file generated by 
the {@code aidl} tool and the Stub subclass implementing the 
RPC methods.  Clients of the service would have only the interface file
generated by the {@code aidl} tool.
</p>

<p>
Here's how a connection between a service and its clients is set up:
</p>

<ul>
<li>Clients of the service (on the local side) would implement 
<code>{@link android.content.ServiceConnection#onServiceConnected
onServiceConnected()}</code> and 
<code>{@link android.content.ServiceConnection#onServiceDisconnected
onServiceDisconnected()}</code> methods so they can be notified 
when a successful connection to the remote service is established, and 
when it goes away.  They would then call
<code>{@link android.content.Context#bindService bindService()}</code>
to set up the connection.
</li>  

<li> 
The service's <code>{@link android.app.Service#onBind onBind()}</code> 
method would be implemented to either accept or reject the connection, 
depending on the intent it receives (the intent passed to
{@code bindService()}).  If the connection is accepted, it returns 
an instance of the Stub subclass.
</li>

<li>If the service accepts the connection, Android calls the 
client's {@code onServiceConnected()} method and passes it an IBinder 
object, a proxy for the Stub subclass managed by the service.  Through
the proxy, the client can make calls on the remote service.  
</li>
</ul>

<p>
This brief description omits some details of the RPC mechanism.  For more 
information, see 
<a href="{@docRoot}guide/developing/tools/aidl.html">Designing a Remote 
Interface Using AIDL</a> and the {@link android.os.IBinder IBinder} class 
description.
</p>  


<h3 id="tsafe">Thread-safe methods</h3>

<p>
In a few contexts, the methods you implement may be called from more 
than one thread, and therefore must be written to be thread-safe.
</p>

<p>
This is primarily true for methods that can be called remotely &mdash;
as in the RPC mechanism discussed in the previous section.
When a call on a method implemented in an IBinder object originates
in the same process as the IBinder, the method is executed in the
caller's thread.  However, when the call originates in another process, 
the method is executed in a thread chosen from a pool of threads that 
Android maintains in the same process as the IBinder; it's not executed 
in the main thread of the process.  For example, whereas a service's 
{@code onBind()} method would be called from the main thread of the 
service's process, methods implemented in the object that {@code onBind()} 
returns (for example, a Stub subclass that implements RPC methods) would 
be called from threads in the pool. 
Since services can have more than one client, more than one pool thread
can engage the same IBinder method at the same time.  IBinder methods
must, therefore, be implemented to be thread-safe.
</p>  

<p>
Similarly, a content provider can receive data requests that originate in 
other processes.  Although the ContentResolver and ContentProvider classes 
hide the details of how the interprocess communication is managed, 
ContentProvider methods that respond to those requests &mdash; the methods
<code>{@link android.content.ContentProvider#query query()}</code>, 
<code>{@link android.content.ContentProvider#insert insert()}</code>, 
<code>{@link android.content.ContentProvider#delete delete()}</code>, 
<code>{@link android.content.ContentProvider#update update()}</code>, and
<code>{@link android.content.ContentProvider#getType getType()}</code>
&mdash; are called from a pool of threads in the content provider's
process, not the main thread of the process.  Since these methods
may be called from any number of threads at the same time, they too must
be implemented to be thread-safe.
</p> 


<h2 id="lcycles">Component Lifecycles</h2>

<p>
Application components have a lifecycle &mdash; a beginning when 
Android instantiates them to respond to intents through to an end when 
the instances are destroyed.  In between, they may sometimes be active 
or inactive,or, in the case of activities, visible to the user or
invisible.  This section discusses the lifecycles of activities,
services, and broadcast receivers &mdash; including the states that they 
can be in during their lifetimes, the methods that notify you of transitions
between states, and the effect of those states on the possibility that
the process hosting them might be terminated and the instances destroyed.
</p> 


<h3 id="actlife">Activity lifecycle</h3>

<p>An activity has essentially three states:</p>

<ul>
<li> It is <em>active</em> or <em>running</em> when it is in the foreground of the 
screen (at the top of the activity stack for the current task).  This is the
activity that is the focus for the user's actions.</li>

<li><p>It is <em>paused</em> if it has lost focus but is still visible to the user.
That is, another activity lies on top of it and that activity either is transparent
or doesn't cover the full screen, so some of the paused activity can show through. 
A paused activity is completely alive (it maintains all state and member information 
and remains attached to the window manager), but can be killed by the system in 
extreme low memory situations.</p></li>

<li><p>It is <em>stopped</em> if it is completely obscured by another activity.
It still retains all state and member information.  However, it is no longer 
visible to the user so its window is hidden and it will often be killed by the 
system when memory is needed elsewhere.</p></li>
</ul>

<p>
If an activity is paused or stopped, the system can drop it from memory either 
by asking it to finish (calling its {@link android.app.Activity#finish finish()}
method), or simply killing its process.  When it is displayed again 
to the user, it must be completely restarted and restored to its previous state.
</p>

<p>
As an activity transitions from state to state, it is notified of the change 
by calls to the following protected methods:
</p>

<p style="margin-left: 2em">{@code void onCreate(Bundle <i>savedInstanceState</i>)}
<br/>{@code void onStart()}
<br/>{@code void onRestart()}
<br/>{@code void onResume()}
<br/>{@code void onPause()}
<br/>{@code void onStop()}
<br/>{@code void onDestroy()}</p>

<p>
All of these methods are hooks that you can override to do appropriate work 
when the state changes.  All activities must implement 
<code>{@link android.app.Activity#onCreate onCreate()}</code> to do the 
initial setup when the object is first instantiated.  
Many will also implement <code>{@link android.app.Activity#onPause onPause()}</code> 
to commit data changes and otherwise prepare to stop interacting with the user.
</p>

<div class="sidebox-wrapper">
<div class="sidebox">
<h2>Calling into the superclass</h2>
<p>
An implementation of any activity lifecycle method should always first 
call the superclass version.  For example:
</p>

<pre>protected void onPause() {
    super.onPause();
    . . .
}</pre>
</div>
</div> 


<p>
Taken together, these seven methods define the entire lifecycle of an 
activity.  There are three nested loops that you can monitor by
implementing them: 
</p> 

<ul>
<li>The <b>entire lifetime</b> of an activity happens between the first call
to <code>{@link android.app.Activity#onCreate onCreate()}</code> through to a 
single final call to <code>{@link android.app.Activity#onDestroy}</code>.  
An activity does all its initial setup of "global" state in {@code onCreate()}, 
and releases all remaining resources in {@code onDestroy()}.  For example, 
if it has a thread running in the background to download data from the network, 
it may create that thread in {@code onCreate()} and then stop the thread in 
{@code onDestroy()}.</li>

<li><p>The <b>visible lifetime</b> of an activity happens between a call to
<code>{@link android.app.Activity#onStart onStart()}</code> until a 
corresponding call to <code>{@link android.app.Activity#onStop onStop()}</code>.  
During this time, the user can see the activity on-screen, though it may not 
be in the foreground and interacting with the user.  Between these two methods, 
you can maintain resources that are needed to show the activity to the user.  
For example, you can register a {@link android.content.BroadcastReceiver} in 
{@code onStart()} to monitor for changes that impact your UI, and unregister 
it in {@code onStop()} when the user can no longer see what you are displaying.  
The {@code onStart()} and {@code onStop()} methods can be called multiple times, 
as the activity alternates between being visible and hidden to the user.</p></li>

<li><p>The <b>foreground lifetime</b> of an activity happens between a call 
to <code>{@link android.app.Activity#onResume onResume()}</code> until a 
corresponding call to <code>{@link android.app.Activity#onPause onPause()}</code>.  
During this time, the activity is in front of all other activities on screen and 
is interacting with the user.  An activity can frequently transition between the 
resumed and paused states &mdash; for example, {@code onPause()} is called when 
the device goes to sleep or when a new activity is started, {@code onResume()} 
is called when an activity result or a new intent is delivered.  Therefore, the 
code in these two methods should be fairly lightweight.</p></li>
</ul>

<p>
The following diagram illustrates these loops and the paths an activity
may take between states.  The colored ovals are major states the activity 
can be in.  The square rectangles represent the callback methods you can implement 
to perform operations when the activity transitions between states.
<p>

<p style="margin-left: 2em"><img src="{@docRoot}images/activity_lifecycle.png"
alt="State diagram for an Android activity lifecycle." /></p>  
  
<p>
The following table describes each of these methods in more detail and 
locates it within the activity's overall lifecycle:
</p>

<table border="2" width="85%" frame="hsides" rules="rows">
<colgroup align="left" span="3"></colgroup>
<colgroup align="left"></colgroup>
<colgroup align="center"></colgroup>
<colgroup align="center"></colgroup>

<thead>
<tr><th colspan="3">Method</th> <th>Description</th> <th>Killable?</th> <th>Next</th></tr>
</thead>

<tbody>
<tr>
  <td colspan="3" align="left"><code>{@link android.app.Activity#onCreate onCreate()}</code></td>
  <td>Called when the activity is first created.
      This is where you should do all of your normal static set up &mdash;
      create views, bind data to lists, and so on.  This method is passed
      a Bundle object containing the activity's previous state, if that 
      state was captured (see <a href="#actstate">Saving Activity State</a>, 
      later).
      <p>Always followed by {@code onStart()}.</p></td>
  <td align="center">No</td>
      <td align="center">{@code onStart()}</td>
</tr>

<tr>
   <td rowspan="5" style="border-left: none; border-right: none;">&nbsp;&nbsp;&nbsp;&nbsp;</td>
   <td colspan="2" align="left"><code>{@link android.app.Activity#onRestart 
onRestart()}</code></td>
   <td>Called after the activity has been stopped, just prior to it being
       started again.
       <p>Always followed by {@code onStart()}</p></td>
   <td align="center">No</td>
   <td align="center">{@code onStart()}</td>
</tr>

<tr>
   <td colspan="2" align="left"><code>{@link android.app.Activity#onStart onStart()}</code></td>
   <td>Called just before the activity becomes visible to the user.
       <p>Followed by {@code onResume()} if the activity comes
       to the foreground, or {@code onStop()} if it becomes hidden.</p></td>
    <td align="center">No</td>
    <td align="center">{@code onResume()} <br/>or<br/> {@code onStop()}</td>
</tr>

<tr>
   <td rowspan="2" style="border-left: none;">&nbsp;&nbsp;&nbsp;&nbsp;</td>
   <td align="left"><code>{@link android.app.Activity#onResume onResume()}</code></td>
   <td>Called just before the activity starts
       interacting with the user.  At this point the activity is at
       the top of the activity stack, with user input going to it.
       <p>Always followed by {@code onPause()}.</p></td>
   <td align="center">No</td>
   <td align="center">{@code onPause()}</td>
</tr>

<tr>
   <td align="left"><code>{@link android.app.Activity#onPause onPause()}</code></td>
   <td>Called when the system is about to start resuming another
       activity.  This method is typically used to commit unsaved changes to
       persistent data, stop animations and other things that may be consuming
       CPU, and so on.  It should do whatever it does very quickly, because
       the next activity will not be resumed until it returns.
       <p>Followed either by {@code onResume()} if the activity
       returns back to the front, or by {@code onStop()} if it becomes
       invisible to the user.</td>
   <td align="center"><strong style="color:#800000">Yes</strong></td>
   <td align="center">{@code onResume()} <br/>or<br/> {@code onStop()}</td>
</tr>

<tr>
   <td colspan="2" align="left"><code>{@link android.app.Activity#onStop onStop()}</code></td>
   <td>Called when the activity is no longer visible to the user.  This
       may happen because it is being destroyed, or because another activity 
       (either an existing one or a new one) has been resumed and is covering it. 
       <p>Followed either by {@code onRestart()} if
       the activity is coming back to interact with the user, or by
       {@code onDestroy()} if this activity is going away.</p></td>
   <td align="center"><strong style="color:#800000">Yes</strong></td>
   <td align="center">{@code onRestart()} <br/>or<br/> {@code onDestroy()}</td>
</tr>

<tr>
   <td colspan="3" align="left"><code>{@link android.app.Activity#onDestroy 
onDestroy()}</code></td>
   <td>Called before the activity is destroyed.  This is the final call 
       that the activity will receive.  It could be called either because the
       activity is finishing (someone called <code>{@link android.app.Activity#finish 
       finish()}</code> on it), or because the system is temporarily destroying this
       instance of the activity to save space.  You can distinguish
       between these two scenarios with the <code>{@link
       android.app.Activity#isFinishing isFinishing()}</code> method.</td>
   <td align="center"><strong style="color:#800000">Yes</strong></td>
   <td align="center"><em>nothing</em></td>
</tr>
</tbody>
</table>

<p>
Note the <b>Killable</b> column in the table above.  It indicates
whether or not the system can kill the process hosting the activity 
<em>at any time after the method returns, without executing another
line of the activity's code</em>.  Three methods ({@code onPause()},
{@code onStop()}, and {@code onDestroy()}) are marked "Yes."  Because
{@code onPause()} is the first of the three, it's the only one that's
guaranteed to be called before the process is killed &mdash; 
{@code onStop()} and {@code onDestroy()} may not be.  Therefore, you 
should use {@code onPause()} to write any persistent data (such as user 
edits) to storage.
</p>

<p>
Methods that are marked "No" in the <b>Killable</b> column protect the
process hosting the activity from being killed from the moment they are 
called.  Thus an activity is in a killable state, for example, from the 
time {@code onPause()} returns to the time {@code onResume()} is called.
It will not again be killable until {@code onPause()} again returns.
</p>

<p>
As noted in a later section, <a href="#proclife">Processes and lifecycle</a>,
an activity that's not technically "killable" by this definition might
still be killed by the system &mdash; but that would happen only in
extreme and dire circumstances when there is no other recourse.
</p>


<h4 id="actstate">Saving activity state</h4>

<p>
When the system, rather than the user, shuts down an activity to conserve 
memory, the user may expect to return to the activity and find it in its 
previous state.
</p>

<p>
To capture that state before the activity is killed, you can implement
an <code>{@link android.app.Activity#onSaveInstanceState 
onSaveInstanceState()}</code> method for the activity.  Android calls this 
method before making the activity vulnerable to being destroyed &mdash;
that is, before {@code onPause()} is called.  It
passes the method a {@link android.os.Bundle} object where you can record 
the dynamic state of the activity as name-value pairs.  When the activity is 
again started, the Bundle is passed both to {@code onCreate()} and to a
method that's called after {@code onStart()}, <code>{@link 
android.app.Activity#onRestoreInstanceState onRestoreInstanceState()}</code>, 
so that either or both of them can recreate the captured state.
</p>

<p>
Unlike {@code onPause()} and the other methods discussed earlier,
{@code onSaveInstanceState()} and {@code onRestoreInstanceState()} are
not lifecycle methods.  They are not always called.  For example, Android
calls {@code onSaveInstanceState()} before the activity becomes 
vulnerable to being destroyed by the system, but does not bother
calling it when the instance is actually being destroyed by a user action
(such as pressing the BACK key).  In that case, the user won't expect to
return to the activity, so there's no reason to save its state.
</p>

<p>
Because {@code onSaveInstanceState()} is not always called, you should 
use it only to record the transient state of the activity, not to store 
persistent data.  Use {@code onPause()} for that purpose instead.
</p>


<h4 id="coordact">Coordinating activities</h4>

<p>
When one activity starts another, they both experience lifecycle
transitions.  One pauses and may stop, while the other starts up.
On occasion, you may need to coordinate these activities, one with
the other.
</p>

<p>
The order of lifecycle callbacks is well defined,
particularly when the two activities are in the same process:
</p>

<ol>
<li>The current activity's {@code onPause()} method is called.</li>

<li>Next, the starting activity's {@code onCreate()}, {@code onStart()},
and {@code onResume()} methods are called in sequence.</li>

<li>Then, if the starting activity is no longer visible
on screen, its {@code onStop()} method is called.</li>
</ol>


<h3 id="servlife">Service lifecycle</h3>

<p>
A service can be used in two ways:
</p>

<ul>
<li>It can be started and allowed to run until someone stops it or
it stops itself.  In this mode, it's started by calling 
<code>{@link android.content.Context#startService Context.startService()}</code>
and stopped by calling 
<code>{@link android.content.Context#stopService Context.stopService()}</code>.
It can stop itself by calling 
<code>{@link android.app.Service#stopSelf() Service.stopSelf()}</code> or
<code>{@link android.app.Service#stopSelfResult Service.stopSelfResult()}</code>.  
Only one {@code stopService()} call is needed to stop the service, no matter how 
many times {@code startService()} was called.</li>

<li><p>It can be operated programmatically using an interface that
it defines and exports.  Clients establish a connection to the Service 
object and use that connection to call into the service.  The connection is 
established by calling  
<code>{@link android.content.Context#bindService Context.bindService()}</code>,
and is closed by calling
<code>{@link android.content.Context#unbindService Context.unbindService()}</code>.
Multiple clients can bind to the same service.
If the service has not already been launched, {@code bindService()} can optionally
launch it.
</p></li>
</ul>

<p>
The two modes are not entirely separate.  You can bind to a service that 
was started with {@code startService()}.  For example, a background music
service could be started by calling {@code startService()} with an Intent
object that identifies the music to play.  Only later, possibly when the 
user wants to exercise some control over the player or get information 
about the current song, would an activity
establish a connection to the service by calling {@code bindService()}.  
In cases like this, {@code stopService()} 
will not actually stop the service until the last binding is closed.
</p>

<p>
Like an activity, a service has lifecycle methods that you can implement
to monitor changes in its state.  But they are fewer than the activity 
methods &mdash; only three &mdash; and they are public, not protected:
</p>

<p style="margin-left: 2em">{@code void onCreate()}
<br/>{@code void onStart(Intent <i>intent</i>)}
<br/>{@code void onDestroy()}</p>

<p>
By implementing these methods, you can monitor two nested loops of the
service's lifecycle:
</p>

<ul>
<li>The <b>entire lifetime</b> of a service happens between the time
<code>{@link android.app.Service#onCreate onCreate()}</code> is called and
the time <code>{@link android.app.Service#onDestroy}</code> returns.  
Like an activity, a service does its initial setup in {@code onCreate()}, 
and releases all remaining resources in {@code onDestroy()}.  For example, 
a music playback service could create the thread where the music will be played  
in {@code onCreate()}, and then stop the thread in {@code onDestroy()}.</li>

<li><p>The <b>active lifetime</b> of a service begins with a call to 
<code>{@link android.app.Service#onStart onStart()}</code>.  This method 
is handed the Intent object that was passed to {@code startService()}.
The music service would open the Intent to discover which music to 
play, and begin the playback.</p>

<p>
There's no equivalent callback for when the service stops &mdash; no
{@code onStop()} method.
</p></li>
</ul>

<p>
The {@code onCreate()} and {@code onDestroy()} methods are called for all
services, whether they're started by 
<code>{@link android.content.Context#startService Context.startService()}</code>
or 
<code>{@link android.content.Context#bindService Context.bindService()}</code>.
However, {@code onStart()} is called only for services started by {@code
startService()}.
</p>

<p>
If a service permits others to
bind to it, there are additional callback methods for it to implement:
</p>

<p style="margin-left: 2em">{@code IBinder onBind(Intent <i>intent</i>)}
<br/>{@code boolean onUnbind(Intent <i>intent</i>)}
<br/>{@code void onRebind(Intent <i>intent</i>)}</p>

<p>
The <code>{@link android.app.Service#onBind onBind()}</code> callback is passed 
the Intent object that was passed to {@code bindService} and 
<code>{@link android.app.Service#onUnbind onUnbind()}</code> is handed
the intent that was passed to {@code unbindService()}.   
If the service permits the binding, {@code onBind()} 
returns the communications channel that clients use to interact with the service. 
The {@code onUnbind()} method can ask for 
<code>{@link android.app.Service#onRebind onRebind()}</code>
to be called if a new client connects to the service.
</p>

<p>
The following diagram illustrates the callback methods for a service.  
Although, it separates services that are created via {@code startService}
from those created by {@code bindService()}, keep in mind that any service,
no matter how it's started, can potentially allow clients to bind to it,
so any service may receive {@code onBind()} and {@code onUnbind()} calls.
</p>

<p style="margin-left: 2em"><img src="{@docRoot}images/service_lifecycle.png"
alt="State diagram for Service callbacks." /></p>


<h3 id="broadlife">Broadcast receiver lifecycle</h3>

<p>
A broadcast receiver has single callback method:
</p>

<p style="margin-left: 2em">{@code void onReceive(Context <i>curContext</i>, Intent <i>broadcastMsg</i>)}</p>

<p>
When a broadcast message arrives for the receiver, Android calls its 
<code>{@link android.content.BroadcastReceiver#onReceive onReceive()}</code> 
method and passes it the Intent object containing the message.  The broadcast 
receiver is considered to be active only while it is executing this method.  
When {@code onReceive()} returns, it is inactive.
</p>

<p>
A process with an active broadcast receiver is protected from being killed. 
But a process with only inactive components can be killed by the system at 
any time, when the memory it consumes is needed by other processes.
</p>

<p>
This presents a problem when the response to a broadcast message is time 
consuming and, therefore, something that should be done in a separate thread, 
away from the main thread where other components of the user interface run.
If {@code onReceive()} spawns the thread and then returns, the entire process,
including the new thread, is judged to be inactive (unless other application 
components are active in the process), putting it in jeopardy of being killed.  
The solution to this problem is for {@code onReceive()} to start a service 
and let the service do the job, so the
system knows that there is still active work being done in the process.
</p>

<p>
The next section has more on the vulnerability of processes to being killed.
</p>


<h3 id="proclife">Processes and lifecycles</h3>

<p>The Android system tries to maintain an application process for as
long as possible, but eventually it will need to remove old processes when
memory runs low.  To determine which processes to keep and which to kill, 
Android places each process into an "importance hierarchy" based on the 
components running in it and the state of those components.  Processes 
with the lowest importance are eliminated first, then those with the next
lowest, and so on.  There are five levels in the hierarchy.  The following 
list presents them in order of importance:
</p>

<ol>

<li>A <b>foreground process</b> is one that is required for
what the user is currently doing.  A process is considered to be 
in the foreground if any of the following conditions hold:

<ul>
<li>It is running an activity that the user is interacting with 
(the Activity object's <code>{@link android.app.Activity#onResume 
onResume()}</code> method has been called).</li>

<li><p>It hosts a service that's bound 
to the activity that the user is interacting with.</p></li>

<li><p>It has a {@link android.app.Service} object that's executing
one of its lifecycle callbacks (<code>{@link android.app.Service#onCreate 
onCreate()}</code>, <code>{@link android.app.Service#onStart onStart()}</code>, 
or <code>{@link android.app.Service#onDestroy onDestroy()}</code>).</p></li>

<li><p>It has a {@link android.content.BroadcastReceiver} object that's 
executing its <code>{@link android.content.BroadcastReceiver#onReceive 
onReceive()}</code> method.</p></li>
</ul>

<p>
Only a few foreground processes will exist at any given time.  They 
are killed only as a last resort &mdash; if memory is so low that 
they cannot all continue to run.  Generally, at that point, the device has
reached a memory paging state, so killing some foreground processes is 
required to keep the user interface responsive.
</p></li>

<li><p>A <b>visible process</b> is one that doesn't have any foreground
components, but still can affect what the user sees on screen.  
A process is considered to be visible if either of the following conditions 
holds:</p>

<ul>
<li>It hosts an activity that is not in the foreground, but is still visible 
to the user (its <code>{@link android.app.Activity#onPause onPause()}</code> 
method has been called).  This may occur, for example, if the foreground 
activity is a dialog that allows the previous activity to be seen behind it.</li>

<li><p>It hosts a service that's bound to a visible activity.</p></li>
</ul>

<p>
A visible process is considered extremely important and will not be killed 
unless doing so is required to keep all foreground processes running.
</p></li>

<li><p>A <b>service process</b> is one that is running a service that 
has been started with the 
<code>{@link android.content.Context#startService startService()}</code>
method and that does not fall into either of the two higher categories.  
Although service processes are not directly tied to anything the 
user sees, they are generally doing things that the user cares about (such 
as playing an mp3 in the background or downloading  data on the network), 
so the system keeps them running unless there's not enough 
memory to retain them along with all foreground and visible processes.  
</p></li>

<li><p>A <b>background process</b> is one holding an activity
that's not currently visible to the user  (the Activity object's
<code>{@link android.app.Activity#onStop onStop()}</code> method has been called).  
These processes have no direct impact on the user experience, and can be killed 
at any time to reclaim memory for a foreground, visible, or service process.  
Usually there are many background processes running, so they are kept in an 
LRU (least recently used) list to ensure that the process with the activity that 
was most recently seen by the user is the last to be killed.
If an activity implements its lifecycle methods correctly, and captures its current 
state, killing its process will not have a deleterious effect on the user experience. 
</p></li>

<li><p>An <b>empty process</b> is one that doesn't hold any active application
components.  The only reason to keep such a process around is as a cache to
improve startup time the next time a component needs to run in it.  The system 
often kills these processes in order to balance overall system resources between 
process caches and the underlying kernel caches.</p></li>

</ol>

<p>
Android ranks a process at the highest level it can, based upon the
importance of the components currently active in the process.  For example, 
if a process hosts a service and a visible activity, the process will be 
ranked as a visible process, not a service process.
</p>

<p>
In addition, a process's ranking may be increased because other processes are
dependent on it.  A process that is serving another process can never be 
ranked lower than the process it is serving.  For example, if a content 
provider in process A is serving a client in process B, or if a service in 
process A is bound to a component in process B, process A will always be 
considered at least as important as process B.
</p> 

<p>
Because a process running a service is ranked higher than one with background
activities, an activity that initiates a long-running operation might do
well to start a service for that operation, rather than simply spawn a thread
&mdash; particularly if the operation will likely outlast the activity.  
Examples of this are playing music in the background 
and uploading a picture taken by the camera to a web site.  Using a service
guarantees that the operation will have at least "service process" priority,
regardless of what happens to the activity.  As noted in the 
<a href="#broadlife">Broadcast receiver lifecycle</a> section earlier, this
is the same reason that broadcast receivers should employ services rather
than simply put time-consuming operations in a thread.
</p>