3 <title>Android JNI Tips</title>
4 <link rel=stylesheet href="android.css">
8 <h1><a name="JNI_Tips"></a>Android JNI Tips</h1>
12 <li> <a href="#What_s_JNI_">What's JNI?</a>
14 <li> <a href="#JavaVM_and_JNIEnv">JavaVM and JNIEnv</a>
17 <li> <a href="#jclass_jmethodID_and_jfieldID">jclass, jmethodID, and jfieldID</a>
19 <li> <a href="#local_vs_global_references">Local vs. Global References</a>
21 <li> <a href="#UTF_8_and_UTF_16_strings">UTF-8 and UTF-16 Strings</a>
23 <li> <a href="#Arrays">Primitive Arrays</a>
25 <li> <a href="#RegionCalls">Region Calls</a>
27 <li> <a href="#Exceptions">Exceptions</a>
30 <li> <a href="#Extended_checking">Extended Checking</a>
32 <li> <a href="#Native_Libraries">Native Libraries</a>
34 <li> <a href="#64bit">64-bit Considerations</a>
37 <li> <a href="#Unsupported">Unsupported Features</a>
42 </p><h2><a name="What_s_JNI_"> </a> What's JNI? </h2>
45 JNI is the Java Native Interface. It defines a way for code written in the
46 Java programming language to interact with native
47 code, e.g. functions written in C/C++. It's VM-neutral, has support for loading code from
48 dynamic shared libraries, and while cumbersome at times is reasonably efficient.
50 You really should read through the
51 <a href="http://java.sun.com/javase/6/docs/technotes/guides/jni/spec/jniTOC.html">JNI spec for J2SE 1.6</a>
52 to get a sense for how JNI works and what features are available. Some
53 aspects of the interface aren't immediately obvious on
54 first reading, so you may find the next few sections handy.
55 The more detailed <i>JNI Programmer's Guide and Specification</i> can be found
56 <a href="http://java.sun.com/docs/books/jni/html/jniTOC.html">here</a>.
59 </p><h2><a name="JavaVM_and_JNIEnv"> </a> JavaVM and JNIEnv </h2>
61 JNI defines two key data structures, "JavaVM" and "JNIEnv". Both of these are essentially
62 pointers to pointers to function tables. (In the C++ version, it's a class whose sole member
63 is a pointer to a function table.) The JavaVM provides the "invocation interface" functions,
64 which allow you to create and destroy the VM. In theory you can have multiple VMs per process,
65 but Android's VMs only allow one.
67 The JNIEnv provides most of the JNI functions. Your native functions all receive a JNIEnv as
71 On some VMs, the JNIEnv is used for thread-local storage. For this reason, <strong>you cannot share a JNIEnv between threads</strong>.
72 If a piece of code has no other way to get its JNIEnv, you should share
73 the JavaVM, and use JavaVM->GetEnv to discover the thread's JNIEnv.
75 The C and C++ declarations of JNIEnv and JavaVM are different. "jni.h" provides different typedefs
76 depending on whether it's included into ".c" or ".cpp". For this reason it's a bad idea to
77 include JNIEnv arguments in header files included by both languages. (Put another way: if your
78 header file requires "#ifdef __cplusplus", you may have to do some extra work if anything in
79 that header refers to JNIEnv.)
82 </p><h2><a name="jclass_jmethodID_and_jfieldID"> jclass, jmethodID, and jfieldID </a></h2>
84 If you want to access an object's field from native code, you would do the following:
87 <li> Get the class object reference for the class with <code>FindClass</code>
89 <li> Get the field ID for the field with <code>GetFieldID</code>
91 <li> Get the contents of the field with something appropriate, e.g.
92 <code>GetIntField</code>
96 Similarly, to call a method, you'd first get a class object reference and then a method ID. The IDs are often just
97 pointers to internal VM data structures. Looking them up may require several string
98 comparisons, but once you have them the actual call to get the field or invoke the method
101 If performance is important, it's useful to look the values up once and cache the results
102 in your native code. Because we are limiting ourselves to one VM per process, it's reasonable
103 to store this data in a static local structure.
105 The class references, field IDs, and method IDs are guaranteed valid until the class is unloaded. Classes
106 are only unloaded if all classes associated with a ClassLoader can be garbage collected,
107 which is rare but will not be impossible in our system. Note however that
108 the <code>jclass</code>
109 is a class reference and <strong>must be protected</strong> with a call
110 to <code>NewGlobalRef</code> (see the next section).
112 If you would like to cache the IDs when a class is loaded, and automatically re-cache them
113 if the class is ever unloaded and reloaded, the correct way to initialize
114 the IDs is to add a piece of code that looks like this to the appropriate class:
118 * We use a class initializer to allow the native code to cache some
123 * A native function that looks up and caches interesting
124 * class/field/method IDs for this class. Returns false on failure.
126 native private static boolean nativeClassInit();
129 * Invoke the native initializer when the class is loaded.
132 if (!nativeClassInit())
133 throw new RuntimeException("native init failed");
137 Create a nativeClassInit method in your C/C++ code that performs the ID lookups. The code
138 will be executed once, when the class is initialized. If the class is ever unloaded and
139 then reloaded, it will be executed again. (See the implementation of java.io.FileDescriptor
140 for an example in our source tree.)
144 </p><h2><a name="local_vs_global_references"> Local vs. Global References </a></h2>
146 Every object that JNI returns is a "local reference". This means that it's valid for the
147 duration of the current native method in the current thread.
148 <strong>Even if the object itself continues to live on after the native method returns, the reference is not valid.</strong>
149 This applies to all sub-classes of <code>jobject</code>, including
150 <code>jclass</code>, <code>jstring</code>, and <code>jarray</code>.
151 (Dalvik VM will warn you about most reference mis-uses when extended JNI
155 If you want to hold on to a reference for a longer period, you must use
156 a "global" reference. The <code>NewGlobalRef</code> function takes the
157 local reference as an argument and returns a global one.
158 The global reference is guaranteed to be valid until you call
159 <code>DeleteGlobalRef</code>.
162 This pattern is commonly used when caching copies of class objects obtained
163 from <code>FindClass</code>, e.g.:
164 <p><pre>jclass* localClass = env->FindClass("MyClass");
165 jclass* globalClass = (jclass*) env->NewGlobalRef(localClass);
169 All JNI methods accept both local and global references as arguments.
170 It's possible for references to the same object to have different values;
171 for example, the return values from consecutive calls to
172 <code>NewGlobalRef</code> on the same object may be different.
173 <strong>To see if two references refer to the same object,
174 you must use the <code>IsSameObject</code> function.</strong> Never compare
175 references with "==" in native code.
177 One consequence of this is that you
178 <strong>must not assume object references are constant or unique</strong>
179 in native code. The 32-bit value representing an object may be different
180 from one invocation of a method to the next, and it's possible that two
181 different objects could have the same 32-bit value on consecutive calls. Do
182 not use <code>jobject</code> values as keys.
184 Programmers are required to "not excessively allocate" local references. In practical terms this means
185 that if you're creating large numbers of local references, perhaps while running through an array of
186 Objects, you should free them manually with
187 <code>DeleteLocalRef</code> instead of letting JNI do it for you. The
188 VM is only required to reserve slots for
189 16 local references, so if you need more than that you should either delete as you go or use
190 <code>EnsureLocalCapacity</code> to reserve more.
192 Note: method and field IDs are just 32-bit identifiers, not object
193 references, and should not be passed to <code>NewGlobalRef</code>. The raw data
194 pointers returned by functions like <code>GetStringUTFChars</code>
195 and <code>GetByteArrayElements</code> are also not objects.
197 One unusual case deserves separate mention. If you attach a native
198 thread to the VM with AttachCurrentThread, the code you are running will
199 never "return" to the VM until the thread detaches from the VM. Any local
200 references you create will have to be deleted manually unless you're going
201 to detach the thread soon.
205 </p><h2><a name="UTF_8_and_UTF_16_strings"> </a> UTF-8 and UTF-16 Strings </h2>
207 The Java programming language uses UTF-16. For convenience, JNI provides methods that work with "modified UTF-8" encoding
208 as well. (Some VMs use the modified UTF-8 internally to store strings; ours do not.) The
209 modified encoding only supports the 8- and 16-bit forms, and stores ASCII NUL values in a 16-bit encoding.
210 The nice thing about it is that you can count on having C-style zero-terminated strings,
211 suitable for use with standard libc string functions. The down side is that you cannot pass
212 arbitrary UTF-8 data into the VM and expect it to work correctly.
214 It's usually best to operate with UTF-16 strings. With our current VMs, the
215 <code>GetStringChars</code> method
216 does not require a copy, whereas <code>GetStringUTFChars</code> requires a malloc and a UTF conversion. Note that
217 <strong>UTF-16 strings are not zero-terminated</strong>, and \u0000 is allowed,
218 so you need to hang on to the string length as well as
222 <strong>Don't forget to Release the strings you Get</strong>. The
223 string functions return <code>jchar*</code> or <code>jbyte*</code>, which
224 are C-style pointers to primitive data rather than local references. They
225 are guaranteed valid until Release is called, which means they are not
226 released when the native method returns.
231 </p><h2><a name="Arrays"> </a> Primitive Arrays </h2>
233 JNI provides functions for accessing the contents of array objects.
234 While arrays of objects must be accessed one entry at a time, arrays of
235 primitives can be read and written directly as if they were declared in C.
237 To make the interface as efficient as possible without constraining
238 the VM implementation,
239 the <code>Get<PrimitiveType>ArrayElements</code> family of calls
240 allows the VM to either return a pointer to the actual elements, or
241 allocate some memory and make a copy. Either way, the raw pointer returned
242 is guaranteed to be valid until the corresponding <code>Release</code> call
243 is issued (which implies that, if the data wasn't copied, the array object
244 will be pinned down and can't be relocated as part of compacting the heap).
245 <strong>You must Release every array you Get.</strong> Also, if the Get
246 call fails, you must ensure that your code doesn't try to Release a NULL
249 You can determine whether or not the data was copied by passing in a
250 non-NULL pointer for the <code>isCopy</code> argument. This is rarely
253 The <code>Release</code> call takes a <code>mode</code> argument that can
254 have one of three values. The actions performed by the VM depend upon
255 whether it returned a pointer to the actual data or a copy of it:
259 <li>Actual: the array object is un-pinned.
260 <li>Copy: data is copied back. The buffer with the copy is freed.
262 <li><code>JNI_COMMIT</code>
264 <li>Actual: does nothing.
265 <li>Copy: data is copied back. The buffer with the copy
266 <strong>is not freed</strong>.
268 <li><code>JNI_ABORT</code>
270 <li>Actual: the array object is un-pinned. Earlier
271 writes are <strong>not</strong> aborted.
272 <li>Copy: the buffer with the copy is freed; any changes to it are lost.
276 One reason for checking the <code>isCopy</code> flag is to know if
277 you need to call <code>Release</code> with <code>JNI_COMMIT</code>
278 after making changes to an array -- if you're alternating between making
279 changes and executing code that uses the contents of the array, you may be
281 skip the no-op commit. Another possible reason for checking the flag is for
282 efficient handling of <code>JNI_ABORT</code>. For example, you might want
283 to get an array, modify it in place, pass pieces to other functions, and
284 then discard the changes. If you know that JNI is making a new copy for
285 you, there's no need to create another "editable" copy. If JNI is passing
286 you the original, then you do need to make your own copy.
288 Some have asserted that you can skip the <code>Release</code> call if
289 <code>*isCopy</code> is false. This is not the case. If no copy buffer was
290 allocated, then the original memory must be pinned down and can't be moved by
291 the garbage collector.
293 Also note that the <code>JNI_COMMIT</code> flag does NOT release the array,
294 and you will need to call <code>Release</code> again with a different flag
300 </p><h2><a name="RegionCalls"> Region Calls </a></h2>
303 There is an alternative to calls like <code>Get<Type>ArrayElements</code>
304 and <code>GetStringChars</code> that may be very helpful when all you want
305 to do is copy data in or out. Consider the following:
307 jbyte* data = env->GetByteArrayElements(array, NULL);
309 memcpy(buffer, data, len);
310 env->ReleaseByteArrayElements(array, data, JNI_ABORT);
314 This grabs the array, copies the first <code>len</code> byte
315 elements out of it, and then releases the array. Depending upon the VM
316 policies the <code>Get</code> call will either pin or copy the array contents.
317 We copy the data (for perhaps a second time), then call Release; in this case
318 we use <code>JNI_ABORT</code> so there's no chance of a third copy.
320 We can accomplish the same thing with this:
322 env->GetByteArrayRegion(array, 0, len, buffer);
325 This accomplishes the same thing, with several advantages:
327 <li>Requires one JNI call instead of 2, reducing overhead.
328 <li>Doesn't require pinning or extra data copies.
329 <li>Reduces the risk of programmer error -- no risk of forgetting
330 to call <code>Release</code> after something fails.
333 Similarly, you can use the <code>Set<Type>ArrayRegion</code> call
334 to copy data into an array, and <code>GetStringRegion</code> or
335 <code>GetStringUTFRegion</code> to copy characters out of a
339 </p><h2><a name="Exceptions"> Exceptions </a></h2>
341 <strong>You may not call most JNI functions while an exception is pending.</strong>
342 Your code is expected to notice the exception (via the function's return value,
343 <code>ExceptionCheck()</code>, or <code>ExceptionOccurred()</code>) and return,
344 or clear the exception and handle it.
346 The only JNI functions that you are allowed to call while an exception is
351 <li>DeleteWeakGlobalRef
354 <li>ExceptionDescribe
355 <li>ExceptionOccurred
359 <li>Release<PrimitiveType>ArrayElements
360 <li>ReleasePrimitiveArrayCritical
361 <li>ReleaseStringChars
362 <li>ReleaseStringCritical
363 <li>ReleaseStringUTFChars
366 Note that exceptions thrown by interpreted code do not "leap over" native code,
367 and C++ exceptions thrown by native code are not handled by Dalvik.
368 The JNI <code>Throw</code> and <code>ThrowNew</code> instructions just
369 set an exception pointer in the current thread. Upon returning to the VM from
370 native code, the exception will be noted and handled appropriately.
372 Native code can "catch" an exception by calling <code>ExceptionCheck</code> or
373 <code>ExceptionOccurred</code>, and clear it with
374 <code>ExceptionClear</code>. As usual,
375 discarding exceptions without handling them can lead to problems.
377 There are no built-in functions for manipulating the Throwable object
378 itself, so if you want to (say) get the exception string you will need to
379 find the Throwable class, look up the method ID for
380 <code>getMessage "()Ljava/lang/String;"</code>, invoke it, and if the result
381 is non-NULL use <code>GetStringUTFChars</code> to get something you can
382 hand to printf or a LOG macro.
386 </p><h2><a name="Extended_checking"> Extended Checking </a></h2>
388 JNI does very little error checking. Calling <code>SetIntField</code>
389 on an Object field will succeed, even if the field is marked
390 <code>private</code> and <code>final</code>. The
391 goal is to minimize the overhead on the assumption that, if you've written it in native code,
392 you probably did it for performance reasons.
394 Some VMs support extended checking with the "<code>-Xcheck:jni</code>" flag. If the flag is set, the VM
395 puts a different table of functions into the JavaVM and JNIEnv pointers. These functions do
396 an extended series of checks before calling the standard implementation.
399 Some things that may be checked:
403 <li> Check for null pointers where not allowed.
405 <li> Verify argument type correctness (jclass is a class object,
406 jfieldID points to field data, jstring is a java.lang.String).
408 <li> Field type correctness, e.g. don't store a HashMap in a String field.
410 <li> Check to see if an exception is pending on calls where pending exceptions are not legal.
412 <li> Check for calls to inappropriate functions between Critical get/release calls.
414 <li> Check that JNIEnv structs aren't being shared between threads.
417 <li> Make sure local references aren't used outside their allowed lifespan.
419 <li> UTF-8 strings contain valid "modified UTF-8" data.
422 <p>Accessibility of methods and fields (i.e. public vs. private) is not
425 The Dalvik VM supports the <code>-Xcheck:jni</code> flag. For a
426 description of how to enable it for Android apps, see
427 <a href="embedded-vm-control.html">Controlling the Embedded VM</a>.
428 It's currently enabled by default in the Android emulator and on
429 "engineering" device builds.
432 JNI checks can be modified with the <code>-Xjniopts</code> command-line
433 flag. Currently supported values include:
437 <dd>When set, any function that can return a copy of the original data
438 (array of primitive values, UTF-16 chars) will always do so. The buffers
439 are over-allocated and surrounded with a guard pattern to help identify
440 code writing outside the buffer, and the contents are erased before the
441 storage is freed to trip up code that uses the data after calling Release.
442 This will have a noticeable performance impact on some applications.
444 <dd>By default, JNI "warnings" cause the VM to abort. With this flag
450 </p><h2><a name="Native_Libraries"> Native Libraries </a></h2>
452 You can load native code from shared libraries with the standard
453 <code>System.loadLibrary()</code> call. The
454 preferred way to get at your native code is:
457 <li> Call <code>System.loadLibrary()</code> from a static class
458 initializer. (See the earlier example, where one is used to call
459 <code>nativeClassInit()</code>.) The argument is the "undecorated"
460 library name, e.g. to load "libfubar.so" you would pass in "fubar".
463 <li> Provide a native function: <code><strong>jint JNI_OnLoad(JavaVM* vm, void* reserved)</strong></code>
465 <li>In <code>JNI_OnLoad</code>, register all of your native methods. You
467 the methods "static" so the names don't take up space in the symbol table
472 The <code>JNI_OnLoad</code> function should look something like this if
474 </p><blockquote><pre>jint JNI_OnLoad(JavaVM* vm, void* reserved)
477 if ((*vm)->GetEnv(vm, (void**) &env, JNI_VERSION_1_4) != JNI_OK)
480 /* get class with (*env)->FindClass */
481 /* register methods with (*env)->RegisterNatives */
483 return JNI_VERSION_1_4;
487 You can also call <code>System.load()</code> with the full path name of the
488 shared library. For Android apps, you may find it useful to get the full
489 path to the application's private data storage area from the context object.
491 This is the recommended approach, but not the only approach. The VM does
492 not require explicit registration, nor that you provide a
493 <code>JNI_OnLoad</code> function.
494 You can instead use "discovery" of native methods that are named in a
495 specific way (see <a href="http://java.sun.com/javase/6/docs/technotes/guides/jni/spec/design.html#wp615">
496 the JNI spec</a> for details), though this is less desirable.
497 It requires more space in the shared object symbol table,
498 loading is slower because it requires string searches through all of the
499 loaded shared libraries, and if a method signature is wrong you won't know
500 about it until the first time the method is actually used.
502 One other note about <code>JNI_OnLoad</code>: any <code>FindClass</code>
503 calls you make from there will happen in the context of the class loader
504 that was used to load the shared library. Normally <code>FindClass</code>
505 uses the loader associated with the method at the top of the interpreted
506 stack, or if there isn't one (because the thread was just attached to
507 the VM) it uses the "system" class loader.
511 </p><h2><a name="64bit"> 64-bit Considerations </a></h2>
514 Android is currently expected to run on 32-bit platforms. In theory it
515 could be built for a 64-bit system, but that is not a goal at this time.
516 For the most part this isn't something that you will need to worry about
517 when interacting with native code,
518 but it becomes significant if you plan to store pointers to native
519 structures in integer fields in an object. To support architectures
520 that use 64-bit pointers, <strong>you need to stash your native pointers in a
521 <code>long</code> field rather than an <code>int</code></strong>.
524 </p><h2><a name="Unsupported"> Unsupported Features </a></h2>
525 <p>All JNI 1.6 features are supported, with the following exceptions:
527 <li><code>DefineClass</code> is not implemented. Dalvik does not use
528 Java bytecodes or class files, so passing in binary class data
529 doesn't work. Translation facilities may be added in a future
530 version of the VM.</li>
531 <li>"Weak global" references are implemented, but may only be passed
532 to <code>NewLocalRef</code>, <code>NewGlobalRef</code>, and
533 <code>DeleteWeakGlobalRef</code>. (The spec strongly encourages
534 programmers to create hard references to weak globals before doing
535 anything with them, so this should not be at all limiting.)</li>
536 <li><code>GetObjectRefType</code> (new in 1.6) is implemented but not fully
537 functional -- it can't always tell the difference between "local" and
538 "global" references.</li>
543 <address>Copyright © 2008 The Android Open Source Project</address>