Blame - docs/jni-tips.html - platform/dalvik

blob: 30207bf3752bee30f114ff0afecc55d89e403b14 [file] [log] [blame]

The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	1	<html>
				2	<head>
				3	<title>Android JNI Tips</title>
				4	<link rel=stylesheet href="android.css">
				5	</head>
				6
				7	<body>
				8	<h1><a name="JNI_Tips"></a>Android JNI Tips</h1>
				9	<p>
				10	</p><p>
				11	</p><ul>
				12	<li> <a href="#What_s_JNI_">What's JNI?</a>
				13	</li>
				14	<li> <a href="#JavaVM_and_JNIEnv">JavaVM and JNIEnv</a>
				15
				16	</li>
				17	<li> <a href="#jclassID_jmethodID_and_jfieldID">jclassID, jmethodID, and jfieldID</a>
				18	</li>
				19	<li> <a href="#local_vs_global_references">Local vs. Global References</a>
				20	</li>
				21	<li> <a href="#UTF_8_and_UTF_16_strings">UTF-8 and UTF-16 Strings</a>
				22	</li>
				23	<li> <a href="#Arrays">Primitive Arrays</a>
				24	</li>
				25	<li> <a href="#RegionCalls">Region Calls</a>
				26	</li>
				27	<li> <a href="#Exceptions">Exceptions</a>
				28	</li>
				29
				30	<li> <a href="#Extended_checking">Extended Checking</a>
				31	</li>
				32	<li> <a href="#Native_Libraries">Native Libraries</a>
				33	</li>
				34	<li> <a href="#64bit">64-bit Considerations</a>
				35	</li>
				36
				37	<li> <a href="#Unsupported">Unsupported Features</a>
				38	</ul>
				39	<p>
				40	<noautolink>
				41	</noautolink></p><p>
				42	</p><h2><a name="What_s_JNI_"> </a> What's JNI? </h2>
				43	<p>
				44
				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.
				49	</p><p>
				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>.
				57	</p><p>
				58	</p><p>
				59	</p><h2><a name="JavaVM_and_JNIEnv"> </a> JavaVM and JNIEnv </h2>
				60	<p>
				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.
				66	</p><p>
				67	The JNIEnv provides most of the JNI functions. Your native functions all receive a JNIEnv as
				68	the first argument.
				69	</p><p>
				70
				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.
				74	</p><p>
				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.)
				80	</p><p>
				81	</p><p>
				82	</p><h2><a name="jclassID_jmethodID_and_jfieldID"> jclassID, jmethodID, and jfieldID </a></h2>
				83	<p>
				84	If you want to access an object's field from native code, you would do the following:
				85	</p><p>
				86	</p><ul>
				87	<li> Get the class object reference for the class with <code>FindClass</code>
				88	</li>
				89	<li> Get the field ID for the field with <code>GetFieldID</code>
				90	</li>
				91	<li> Get the contents of the field with something appropriate, e.g.
				92	<code>GetIntField</code>
				93	</li>
				94	</ul>
				95	<p>
				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
				99	is very quick.
				100	</p><p>
				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.
				104	</p><p>
				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. The jclassID
				108	is a class reference and <strong>must be protected</strong> with a call
				109	to <code>NewGlobalRef</code> (see the next section).
				110	</p><p>
				111	If you would like to cache the IDs when a class is loaded, and automatically re-cache them
				112	if the class is ever unloaded and reloaded, the correct way to initialize
				113	the IDs is to add a piece of code that looks like this to the appropriate class:
				114	</p><p>
				115
				116	</p><pre> /*
				117	* We use a class initializer to allow the native code to cache some
				118	* field offsets.
				119	*/
				120
				121	/*
				122	* A native function that looks up and caches interesting
				123	* class/field/method IDs for this class. Returns false on failure.
				124	*/
				125	native private static boolean nativeClassInit();
				126
				127	/*
				128	* Invoke the native initializer when the class is loaded.
				129	*/
				130	static {
				131	if (!nativeClassInit())
				132	throw new RuntimeException("native init failed");
				133	}
				134	</pre>
				135	<p>
				136	Create a nativeClassInit method in your C/C++ code that performs the ID lookups. The code
				137	will be executed once, when the class is initialized. If the class is ever unloaded and
				138	then reloaded, it will be executed again. (See the implementation of java.io.FileDescriptor
				139	for an example in our source tree.)
				140	</p><p>
				141	</p><p>
				142	</p><p>
				143	</p><h2><a name="local_vs_global_references"> Local vs. Global References </a></h2>
				144	<p>
				145	Every object that JNI returns is a "local reference". This means that it's valid for the
				146	duration of the current native method in the current thread.
				147	<strong>Even if the object itself continues to live on after the native method returns, the reference is not valid.</strong>
				148	This applies to all sub-classes of jobject, including jclass and jarray.
				149	(Dalvik VM will warn you about this when -Xcheck:jni is enabled.)
				150	</p><p>
				151
				152	If you want to hold on to a reference for a longer period, you must use a "global" reference.
				153	The <code>NewGlobalRef</code> function takes the local reference as
				154	an argument and returns a global one:
				155
				156	<p><pre>jobject* localRef = [...];
				157	jobject* globalRef;
				158	globalRef = env->NewGlobalRef(localRef);
				159	</pre>
				160
				161	The global reference is guaranteed to be valid until you call
				162	<code>DeleteGlobalRef</code>.
				163	</p><p>
				164	All JNI methods accept both local and global references as arguments.
Andy McFadden	5aca603	2009-05-06 16:48:33 -0700	[diff] [blame]	165	It's possible for references to the same object to have different values;
				166	for example, the return values from consecutive calls to
				167	<code>NewGlobalRef</code> on the same object may be different.
				168	<strong>To see if two references refer to the same object,
				169	you must use the <code>IsSameObject</code> function.</strong> Never compare
				170	references with "==" in native code.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	171	</p><p>
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame^]	172	One consequence of this is that you
				173	<strong>must not assume object references are constant</strong>
				174	in native code. The 32-bit value representing an object may be different
				175	from one invocation of a method to the next, and it's possible that two
				176	different objects could have the same 32-bit value at different times. Do
				177	not use jobjects as keys.
				178	</p><p>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	179	Programmers are required to "not excessively allocate" local references. In practical terms this means
				180	that if you're creating large numbers of local references, perhaps while running through an array of
				181	Objects, you should free them manually with
				182	<code>DeleteLocalRef</code> instead of letting JNI do it for you. The
				183	VM is only required to reserve slots for
				184	16 local references, so if you need more than that you should either delete as you go or use
				185	<code>EnsureLocalCapacity</code> to reserve more.
				186	</p><p>
				187	Note: method and field IDs are just 32-bit identifiers, not object
				188	references, and should not be passed to <code>NewGlobalRef</code>. The raw data
				189	pointers returned by functions like <code>GetStringUTFChars</code>
				190	and <code>GetByteArrayElements</code> are also not objects.
				191	</p><p>
				192	One unusual case deserves separate mention. If you attach a native
				193	thread to the VM with AttachCurrentThread, the code you are running will
				194	never "return" to the VM until the thread detaches from the VM. Any local
				195	references you create will have to be deleted manually unless the thread
				196	is about to exit or detach.
				197	</p><p>
				198	</p><p>
				199	</p><p>
				200	</p><h2><a name="UTF_8_and_UTF_16_strings"> </a> UTF-8 and UTF-16 Strings </h2>
				201	<p>
				202	The Java programming language uses UTF-16. For convenience, JNI provides methods that work with "modified UTF-8" encoding
				203	as well. (Some VMs use the modified UTF-8 internally to store strings; ours do not.) The
				204	modified encoding only supports the 8- and 16-bit forms, and stores ASCII NUL values in a 16-bit encoding.
				205	The nice thing about it is that you can count on having C-style zero-terminated strings,
				206	suitable for use with standard libc string functions. The down side is that you cannot pass
				207	arbitrary UTF-8 data into the VM and expect it to work correctly.
				208	</p><p>
				209	It's usually best to operate with UTF-16 strings. With our current VMs, the
				210	<code>GetStringChars</code> method
				211	does not require a copy, whereas <code>GetStringUTFChars</code> requires a malloc and a UTF conversion. Note that
				212	<strong>UTF-16 strings are not zero-terminated</strong>, and \u0000 is allowed,
				213	so you need to hang on to the string length as well as
				214	the string pointer.
				215
				216	</p><p>
Andy McFadden	5aca603	2009-05-06 16:48:33 -0700	[diff] [blame]	217	<strong>Don't forget to Release the strings you Get</strong>. The
				218	string functions return <code>jchar</code> or <code>jbyte</code>, which
				219	are C-style pointers to primitive data rather than local references. They
				220	are guaranteed valid until Release is called, which means they are not
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	221	released when the native method returns.
				222	</p><p>
				223	</p><p>
				224
				225
				226	</p><h2><a name="Arrays"> </a> Primitive Arrays </h2>
				227	<p>
				228	JNI provides functions for accessing the contents of array objects.
				229	While arrays of objects must be accessed one entry at a time, arrays of
				230	primitives can be read and written directly as if they were declared in C.
				231	</p><p>
				232	To make the interface as efficient as possible without constraining
				233	the VM implementation,
				234	the <code>Get<PrimitiveType>ArrayElements</code> family of calls
				235	allows the VM to either return a pointer to the actual elements, or
				236	allocate some memory and make a copy. Either way, the raw pointer returned
				237	is guaranteed to be valid until the corresponding <code>Release</code> call
				238	is issued (which implies that, if the data wasn't copied, the array object
				239	will be pinned down and can't be relocated as part of compacting the heap).
				240	<strong>You must Release every array you Get.</strong> Also, if the Get
				241	call fails, you must ensure that your code doesn't try to Release a NULL
				242	pointer later.
				243	</p><p>
				244	You can determine whether or not the data was copied by passing in a
				245	non-NULL pointer for the <code>isCopy</code> argument. This is rarely
				246	useful.
				247	</p><p>
				248	The <code>Release</code> call takes a <code>mode</code> argument that can
				249	have one of three values. The actions performed by the VM depend upon
				250	whether it returned a pointer to the actual data or a copy of it:
				251	<ul>
				252	<li><code>0</code>
				253	<ul>
				254	<li>Actual: the array object is un-pinned.
				255	<li>Copy: data is copied back. The buffer with the copy is freed.
				256	</ul>
				257	<li><code>JNI_COMMIT</code>
				258	<ul>
				259	<li>Actual: does nothing.
				260	<li>Copy: data is copied back. The buffer with the copy
				261	<strong>is not freed</strong>.
				262	</ul>
				263	<li><code>JNI_ABORT</code>
				264	<ul>
				265	<li>Actual: the array object is un-pinned. Earlier
				266	writes are <strong>not</strong> aborted.
				267	<li>Copy: the buffer with the copy is freed; any changes to it are lost.
				268	</ul>
				269	</ul>
				270	</p><p>
				271	One reason for checking the <code>isCopy</code> flag is to know if
				272	you need to call <code>Release</code> with <code>JNI_COMMIT</code>
				273	after making changes to an array -- if you're alternating between making
				274	changes and executing code that uses the contents of the array, you may be
				275	able to
				276	skip the no-op commit. Another possible reason for checking the flag is for
				277	efficient handling of <code>JNI_ABORT</code>. For example, you might want
				278	to get an array, modify it in place, pass pieces to other functions, and
				279	then discard the changes. If you know that JNI is making a new copy for
				280	you, there's no need to create another "editable" copy. If JNI is passing
				281	you the original, then you do need to make your own copy.
				282	</p><p>
				283	Some have asserted that you can skip the <code>Release</code> call if
				284	<code>*isCopy</code> is false. This is not the case. If no copy buffer was
				285	allocated, then the original memory must be pinned down and can't be moved by
				286	the garbage collector.
				287	</p><p>
				288	Also note that the <code>JNI_COMMIT</code> flag does NOT release the array,
				289	and you will need to call <code>Release</code> again with a different flag
				290	eventually.
				291	</p><p>
				292	</p><p>
				293
				294
				295	</p><h2><a name="RegionCalls"> Region Calls </a></h2>
				296
				297	<p>
				298	There is an alternative to calls like <code>Get<Type>ArrayElements</code>
				299	and <code>GetStringChars</code> that may be very helpful when all you want
				300	to do is copy data in or out. Consider the following:
				301	<pre>
				302	jbyte* data = env->GetByteArrayElements(array, NULL);
				303	if (data != NULL) {
				304	memcpy(buffer, data, len);
				305	env->ReleaseByteArrayElements(array, data, JNI_ABORT);
				306	}
				307	</pre>
				308	<p>
				309	This grabs the array, copies the first <code>len</code> byte
				310	elements out of it, and then releases the array. Depending upon the VM
				311	policies the <code>Get</code> call will either pin or copy the array contents.
				312	We copy the data (for perhaps a second time), then call Release; in this case
				313	we use <code>JNI_ABORT</code> so there's no chance of a third copy.
				314	</p><p>
				315	We can accomplish the same thing with this:
				316	<pre>
				317	env->GetByteArrayRegion(array, 0, len, buffer);
				318	</pre>
				319	</p><p>
				320	This accomplishes the same thing, with several advantages:
				321	<ul>
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame^]	322	<li>Requires one JNI call instead of 2, reducing overhead.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	323	<li>Doesn't require pinning or extra data copies.
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame^]	324	<li>Reduces the risk of programmer error -- no risk of forgetting
				325	to call <code>Release</code> after something fails.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	326	</ul>
				327	</p><p>
				328	Similarly, you can use the <code>Set<Type>ArrayRegion</code> call
				329	to copy data into an array, and <code>GetStringRegion</code> or
				330	<code>GetStringUTFRegion</code> to copy characters out of a
				331	<code>String</code>.
				332
				333
				334	</p><h2><a name="Exceptions"> Exceptions </a></h2>
				335	<p>
				336	<strong>You may not call most JNI functions while an exception is pending.</strong>
				337	Your code is expected to notice the exception (via the function's return value,
				338	<code>ExceptionCheck()</code>, or <code>ExceptionOccurred()</code>) and return,
				339	or clear the exception and handle it.
				340	</p><p>
				341	The only JNI functions that you are allowed to call while an exception is
				342	pending are:
				343	<font size="-1"><ul>
				344	<li>DeleteGlobalRef
				345	<li>DeleteLocalRef
				346	<li>DeleteWeakGlobalRef
				347	<li>ExceptionCheck
				348	<li>ExceptionClear
				349	<li>ExceptionDescribe
				350	<li>ExceptionOccurred
				351	<li>MonitorExit
				352	<li>PopLocalFrame
				353	<li>PushLocalFrame
				354	<li>Release<PrimitiveType>ArrayElements
				355	<li>ReleasePrimitiveArrayCritical
				356	<li>ReleaseStringChars
				357	<li>ReleaseStringCritical
				358	<li>ReleaseStringUTFChars
				359	</ul></font>
				360	</p><p>
				361	Note that exceptions thrown by interpreted code do not "leap over" native code,
				362	and C++ exceptions thrown by native code are not handled by Dalvik.
				363	The JNI <code>Throw</code> and <code>ThrowNew</code> instructions just
				364	set an exception pointer in the current thread. Upon returning to the VM from
				365	native code, the exception will be noted and handled appropriately.
				366	</p><p>
				367	Native code can "catch" an exception by calling <code>ExceptionCheck</code> or
				368	<code>ExceptionOccurred</code>, and clear it with
				369	<code>ExceptionClear</code>. As usual,
				370	discarding exceptions without handling them can lead to problems.
				371	</p><p>
				372	There are no built-in functions for manipulating the Throwable object
				373	itself, so if you want to (say) get the exception string you will need to
				374	find the Throwable class, look up the method ID for
				375	<code>getMessage "()Ljava/lang/String;"</code>, invoke it, and if the result
				376	is non-NULL use <code>GetStringUTFChars</code> to get something you can
				377	hand to printf or a LOG macro.
				378
				379	</p><p>
				380	</p><p>
				381	</p><h2><a name="Extended_checking"> Extended Checking </a></h2>
				382	<p>
				383	JNI does very little error checking. Calling <code>SetFieldInt</code>
				384	on an Object field will succeed, even if the field is marked
				385	<code>private</code> and <code>final</code>. The
				386	goal is to minimize the overhead on the assumption that, if you've written it in native code,
				387	you probably did it for performance reasons.
				388	</p><p>
				389	Some VMs support extended checking with the "<code>-Xcheck:jni</code>" flag. If the flag is set, the VM
				390	puts a different table of functions into the JavaVM and JNIEnv pointers. These functions do
				391	an extended series of checks before calling the standard implementation.
				392
				393	</p><p>
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame^]	394	Some things that may be checked:
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	395	</p><p>
				396	</p>
				397	<ul>
				398	<li> Check for null pointers where not allowed.
				399	<li>
				400	<li> Verify argument type correctness (jclass is a class object,
				401	jfieldID points to field data, jstring is a java.lang.String).
				402	</li>
				403	<li> Field type correctness, e.g. don't store a HashMap in a String field.
				404	</li>
				405	<li> Check to see if an exception is pending on calls where pending exceptions are not legal.
				406	</li>
				407	<li> Check for calls to inappropriate functions between Critical get/release calls.
				408	</li>
				409	<li> Check that JNIEnv structs aren't being shared between threads.
				410
				411	</li>
				412	<li> Make sure local references aren't used outside their allowed lifespan.
				413	</li>
				414	<li> UTF-8 strings contain valid "modified UTF-8" data.
				415	</li>
				416	</ul>
				417	<p>Accessibility of methods and fields (i.e. public vs. private) is not
				418	checked.
				419	<p>
				420	The Dalvik VM supports the <code>-Xcheck:jni</code> flag. For a
				421	description of how to enable it for Android apps, see
				422	<a href="embedded-vm-control.html">Controlling the Embedded VM</a>.
				423	It's currently enabled by default in the Android emulator and on
				424	"engineering" device builds.
				425
				426	</p><p>
				427	JNI checks can be modified with the <code>-Xjniopts</code> command-line
				428	flag. Currently supported values include:
				429	</p>
				430	<blockquote><dl>
				431	<dt>forcecopy
				432	<dd>When set, any function that can return a copy of the original data
				433	(array of primitive values, UTF-16 chars) will always do so. The buffers
				434	are over-allocated and surrounded with a guard pattern to help identify
				435	code writing outside the buffer, and the contents are erased before the
				436	storage is freed to trip up code that uses the data after calling Release.
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame^]	437	This will have a noticeable performance impact on some applications.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	438	<dt>warnonly
				439	<dd>By default, JNI "warnings" cause the VM to abort. With this flag
				440	it continues on.
				441	</dl></blockquote>
				442
				443
				444	</p><p>
				445	</p><h2><a name="Native_Libraries"> Native Libraries </a></h2>
				446	<p>
				447	You can load native code from shared libraries with the standard
				448	<code>System.loadLibrary()</code> call. The
				449	preferred way to get at your native code is:
				450	</p><p>
				451	</p><ul>
				452	<li> Call <code>System.loadLibrary()</code> from a static class initializer. (See the earlier example, where one is used to call nativeClassInit().) The argument is the "undecorated" library name, e.g. to load "libfubar.so" you would pass in "fubar".
				453
				454	</li>
				455	<li> Provide a native function: <code><strong>jint JNI_OnLoad(JavaVM* vm, void* reserved)</strong></code>
				456	</li>
				457	<li>In <code>JNI_OnLoad</code>, register all of your native methods. You
				458	should declare
				459	the methods "static" so the names don't take up space in the symbol table
				460	on the device.
				461	</li>
				462	</ul>
				463	<p>
				464	The <code>JNI_OnLoad</code> function should look something like this if
				465	written in C:
				466	</p><blockquote><pre>jint JNI_OnLoad(JavaVM* vm, void* reserved)
				467	{
				468	JNIEnv* env;
				469	if ((vm)->GetEnv(vm, (void*) &env, JNI_VERSION_1_4) != JNI_OK)
				470	return -1;
				471
				472	/* get class with (env)->FindClass /
				473	/* register methods with (env)->RegisterNatives /
				474
				475	return JNI_VERSION_1_4;
				476	}
				477	</pre></blockquote>
				478	</p><p>
				479	You can also call <code>System.load()</code> with the full path name of the
				480	shared library. For Android apps, you can get the full path to the
				481	application's private data storage area from the context object.
				482	</p><p>
				483	Dalvik does support "discovery" of native methods that are named in a
				484	specific way (see <a href="http://java.sun.com/javase/6/docs/technotes/guides/jni/spec/design.html#wp615">
				485	the JNI spec</a> for details), but this is a less desirable
				486	approach. It requires more space in the shared object symbol table,
				487	loading is slower because it requires string searches through all of the
				488	loaded shared libraries, and if a method signature is wrong you won't know
				489	about it until the first time the method is actually used.
				490	</p><p>
				491
				492
				493	</p><h2><a name="64bit"> 64-bit Considerations </a></h2>
				494
				495	<p>
				496	Android is currently expected to run on 32-bit platforms. In theory it
				497	could be built for a 64-bit system, but that is not a goal at this time.
				498	For the most part this isn't something that you will need to worry about
				499	when interacting with native code,
				500	but it becomes significant if you plan to store pointers to native
				501	structures in integer fields in an object. To support architectures
				502	that use 64-bit pointers, <strong>you need to stash your native pointers in a
				503	<code>long</code> field rather than an <code>int</code></strong>.
				504
				505
				506	</p><h2><a name="Unsupported"> Unsupported Features </a></h2>
				507	<p>All JNI 1.6 features are supported, with the following exceptions:
				508	<ul>
				509	<li><code>DefineClass</code> is not implemented. Dalvik does not use
				510	Java bytecodes or class files, so passing in binary class data
				511	doesn't work. Translation facilities may be added in a future
				512	version of the VM.</li>
				513	<li><code>NewWeakGlobalRef</code> and <code>DeleteWeakGlobalRef</code>
				514	are not implemented. The
				515	VM supports weak references, but not JNI "weak global" references.
				516	These will be supported in a future release.</li>
				517	<li><code>GetObjectRefType</code> (new in 1.6) is implemented but not fully
				518	functional -- it can't always tell the difference between "local" and
				519	"global" references.</li>
				520	</ul>
				521
				522	</p>
				523
				524	<address>Copyright © 2008 The Android Open Source Project</address>
				525
				526	</body>
				527	</html>