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

blob: 13461b55da5151d6270fed331c8b119eeb8fbebd [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
Andy McFadden	278c12b	2009-09-21 16:28:39 -0700	[diff] [blame]	162	<code>DeleteGlobalRef</code>. This pattern is frequently used for
				163	cached copies of class objects obtained from <code>FindClass</code>.
				164	</p><p>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	165	</p><p>
				166	All JNI methods accept both local and global references as arguments.
Andy McFadden	5aca603	2009-05-06 16:48:33 -0700	[diff] [blame]	167	It's possible for references to the same object to have different values;
				168	for example, the return values from consecutive calls to
				169	<code>NewGlobalRef</code> on the same object may be different.
				170	<strong>To see if two references refer to the same object,
				171	you must use the <code>IsSameObject</code> function.</strong> Never compare
				172	references with "==" in native code.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	173	</p><p>
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame]	174	One consequence of this is that you
				175	<strong>must not assume object references are constant</strong>
				176	in native code. The 32-bit value representing an object may be different
				177	from one invocation of a method to the next, and it's possible that two
				178	different objects could have the same 32-bit value at different times. Do
				179	not use jobjects as keys.
				180	</p><p>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	181	Programmers are required to "not excessively allocate" local references. In practical terms this means
				182	that if you're creating large numbers of local references, perhaps while running through an array of
				183	Objects, you should free them manually with
				184	<code>DeleteLocalRef</code> instead of letting JNI do it for you. The
				185	VM is only required to reserve slots for
				186	16 local references, so if you need more than that you should either delete as you go or use
				187	<code>EnsureLocalCapacity</code> to reserve more.
				188	</p><p>
				189	Note: method and field IDs are just 32-bit identifiers, not object
				190	references, and should not be passed to <code>NewGlobalRef</code>. The raw data
				191	pointers returned by functions like <code>GetStringUTFChars</code>
				192	and <code>GetByteArrayElements</code> are also not objects.
				193	</p><p>
				194	One unusual case deserves separate mention. If you attach a native
				195	thread to the VM with AttachCurrentThread, the code you are running will
				196	never "return" to the VM until the thread detaches from the VM. Any local
				197	references you create will have to be deleted manually unless the thread
				198	is about to exit or detach.
				199	</p><p>
				200	</p><p>
				201	</p><p>
				202	</p><h2><a name="UTF_8_and_UTF_16_strings"> </a> UTF-8 and UTF-16 Strings </h2>
				203	<p>
				204	The Java programming language uses UTF-16. For convenience, JNI provides methods that work with "modified UTF-8" encoding
				205	as well. (Some VMs use the modified UTF-8 internally to store strings; ours do not.) The
				206	modified encoding only supports the 8- and 16-bit forms, and stores ASCII NUL values in a 16-bit encoding.
				207	The nice thing about it is that you can count on having C-style zero-terminated strings,
				208	suitable for use with standard libc string functions. The down side is that you cannot pass
				209	arbitrary UTF-8 data into the VM and expect it to work correctly.
				210	</p><p>
				211	It's usually best to operate with UTF-16 strings. With our current VMs, the
				212	<code>GetStringChars</code> method
				213	does not require a copy, whereas <code>GetStringUTFChars</code> requires a malloc and a UTF conversion. Note that
				214	<strong>UTF-16 strings are not zero-terminated</strong>, and \u0000 is allowed,
				215	so you need to hang on to the string length as well as
				216	the string pointer.
				217
				218	</p><p>
Andy McFadden	5aca603	2009-05-06 16:48:33 -0700	[diff] [blame]	219	<strong>Don't forget to Release the strings you Get</strong>. The
				220	string functions return <code>jchar</code> or <code>jbyte</code>, which
				221	are C-style pointers to primitive data rather than local references. They
				222	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]	223	released when the native method returns.
				224	</p><p>
				225	</p><p>
				226
				227
				228	</p><h2><a name="Arrays"> </a> Primitive Arrays </h2>
				229	<p>
				230	JNI provides functions for accessing the contents of array objects.
				231	While arrays of objects must be accessed one entry at a time, arrays of
				232	primitives can be read and written directly as if they were declared in C.
				233	</p><p>
				234	To make the interface as efficient as possible without constraining
				235	the VM implementation,
				236	the <code>Get<PrimitiveType>ArrayElements</code> family of calls
				237	allows the VM to either return a pointer to the actual elements, or
				238	allocate some memory and make a copy. Either way, the raw pointer returned
				239	is guaranteed to be valid until the corresponding <code>Release</code> call
				240	is issued (which implies that, if the data wasn't copied, the array object
				241	will be pinned down and can't be relocated as part of compacting the heap).
				242	<strong>You must Release every array you Get.</strong> Also, if the Get
				243	call fails, you must ensure that your code doesn't try to Release a NULL
				244	pointer later.
				245	</p><p>
				246	You can determine whether or not the data was copied by passing in a
				247	non-NULL pointer for the <code>isCopy</code> argument. This is rarely
				248	useful.
				249	</p><p>
				250	The <code>Release</code> call takes a <code>mode</code> argument that can
				251	have one of three values. The actions performed by the VM depend upon
				252	whether it returned a pointer to the actual data or a copy of it:
				253	<ul>
				254	<li><code>0</code>
				255	<ul>
				256	<li>Actual: the array object is un-pinned.
				257	<li>Copy: data is copied back. The buffer with the copy is freed.
				258	</ul>
				259	<li><code>JNI_COMMIT</code>
				260	<ul>
				261	<li>Actual: does nothing.
				262	<li>Copy: data is copied back. The buffer with the copy
				263	<strong>is not freed</strong>.
				264	</ul>
				265	<li><code>JNI_ABORT</code>
				266	<ul>
				267	<li>Actual: the array object is un-pinned. Earlier
				268	writes are <strong>not</strong> aborted.
				269	<li>Copy: the buffer with the copy is freed; any changes to it are lost.
				270	</ul>
				271	</ul>
				272	</p><p>
				273	One reason for checking the <code>isCopy</code> flag is to know if
				274	you need to call <code>Release</code> with <code>JNI_COMMIT</code>
				275	after making changes to an array -- if you're alternating between making
				276	changes and executing code that uses the contents of the array, you may be
				277	able to
				278	skip the no-op commit. Another possible reason for checking the flag is for
				279	efficient handling of <code>JNI_ABORT</code>. For example, you might want
				280	to get an array, modify it in place, pass pieces to other functions, and
				281	then discard the changes. If you know that JNI is making a new copy for
				282	you, there's no need to create another "editable" copy. If JNI is passing
				283	you the original, then you do need to make your own copy.
				284	</p><p>
				285	Some have asserted that you can skip the <code>Release</code> call if
				286	<code>*isCopy</code> is false. This is not the case. If no copy buffer was
				287	allocated, then the original memory must be pinned down and can't be moved by
				288	the garbage collector.
				289	</p><p>
				290	Also note that the <code>JNI_COMMIT</code> flag does NOT release the array,
				291	and you will need to call <code>Release</code> again with a different flag
				292	eventually.
				293	</p><p>
				294	</p><p>
				295
				296
				297	</p><h2><a name="RegionCalls"> Region Calls </a></h2>
				298
				299	<p>
				300	There is an alternative to calls like <code>Get<Type>ArrayElements</code>
				301	and <code>GetStringChars</code> that may be very helpful when all you want
				302	to do is copy data in or out. Consider the following:
				303	<pre>
				304	jbyte* data = env->GetByteArrayElements(array, NULL);
				305	if (data != NULL) {
				306	memcpy(buffer, data, len);
				307	env->ReleaseByteArrayElements(array, data, JNI_ABORT);
				308	}
				309	</pre>
				310	<p>
				311	This grabs the array, copies the first <code>len</code> byte
				312	elements out of it, and then releases the array. Depending upon the VM
				313	policies the <code>Get</code> call will either pin or copy the array contents.
				314	We copy the data (for perhaps a second time), then call Release; in this case
				315	we use <code>JNI_ABORT</code> so there's no chance of a third copy.
				316	</p><p>
				317	We can accomplish the same thing with this:
				318	<pre>
				319	env->GetByteArrayRegion(array, 0, len, buffer);
				320	</pre>
				321	</p><p>
				322	This accomplishes the same thing, with several advantages:
				323	<ul>
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame]	324	<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]	325	<li>Doesn't require pinning or extra data copies.
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame]	326	<li>Reduces the risk of programmer error -- no risk of forgetting
				327	to call <code>Release</code> after something fails.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	328	</ul>
				329	</p><p>
				330	Similarly, you can use the <code>Set<Type>ArrayRegion</code> call
				331	to copy data into an array, and <code>GetStringRegion</code> or
				332	<code>GetStringUTFRegion</code> to copy characters out of a
				333	<code>String</code>.
				334
				335
				336	</p><h2><a name="Exceptions"> Exceptions </a></h2>
				337	<p>
				338	<strong>You may not call most JNI functions while an exception is pending.</strong>
				339	Your code is expected to notice the exception (via the function's return value,
				340	<code>ExceptionCheck()</code>, or <code>ExceptionOccurred()</code>) and return,
				341	or clear the exception and handle it.
				342	</p><p>
				343	The only JNI functions that you are allowed to call while an exception is
				344	pending are:
				345	<font size="-1"><ul>
				346	<li>DeleteGlobalRef
				347	<li>DeleteLocalRef
				348	<li>DeleteWeakGlobalRef
				349	<li>ExceptionCheck
				350	<li>ExceptionClear
				351	<li>ExceptionDescribe
				352	<li>ExceptionOccurred
				353	<li>MonitorExit
				354	<li>PopLocalFrame
				355	<li>PushLocalFrame
				356	<li>Release<PrimitiveType>ArrayElements
				357	<li>ReleasePrimitiveArrayCritical
				358	<li>ReleaseStringChars
				359	<li>ReleaseStringCritical
				360	<li>ReleaseStringUTFChars
				361	</ul></font>
				362	</p><p>
				363	Note that exceptions thrown by interpreted code do not "leap over" native code,
				364	and C++ exceptions thrown by native code are not handled by Dalvik.
				365	The JNI <code>Throw</code> and <code>ThrowNew</code> instructions just
				366	set an exception pointer in the current thread. Upon returning to the VM from
				367	native code, the exception will be noted and handled appropriately.
				368	</p><p>
				369	Native code can "catch" an exception by calling <code>ExceptionCheck</code> or
				370	<code>ExceptionOccurred</code>, and clear it with
				371	<code>ExceptionClear</code>. As usual,
				372	discarding exceptions without handling them can lead to problems.
				373	</p><p>
				374	There are no built-in functions for manipulating the Throwable object
				375	itself, so if you want to (say) get the exception string you will need to
				376	find the Throwable class, look up the method ID for
				377	<code>getMessage "()Ljava/lang/String;"</code>, invoke it, and if the result
				378	is non-NULL use <code>GetStringUTFChars</code> to get something you can
				379	hand to printf or a LOG macro.
				380
				381	</p><p>
				382	</p><p>
				383	</p><h2><a name="Extended_checking"> Extended Checking </a></h2>
				384	<p>
				385	JNI does very little error checking. Calling <code>SetFieldInt</code>
				386	on an Object field will succeed, even if the field is marked
				387	<code>private</code> and <code>final</code>. The
				388	goal is to minimize the overhead on the assumption that, if you've written it in native code,
				389	you probably did it for performance reasons.
				390	</p><p>
				391	Some VMs support extended checking with the "<code>-Xcheck:jni</code>" flag. If the flag is set, the VM
				392	puts a different table of functions into the JavaVM and JNIEnv pointers. These functions do
				393	an extended series of checks before calling the standard implementation.
				394
				395	</p><p>
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame]	396	Some things that may be checked:
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	397	</p><p>
				398	</p>
				399	<ul>
				400	<li> Check for null pointers where not allowed.
				401	<li>
				402	<li> Verify argument type correctness (jclass is a class object,
				403	jfieldID points to field data, jstring is a java.lang.String).
				404	</li>
				405	<li> Field type correctness, e.g. don't store a HashMap in a String field.
				406	</li>
				407	<li> Check to see if an exception is pending on calls where pending exceptions are not legal.
				408	</li>
				409	<li> Check for calls to inappropriate functions between Critical get/release calls.
				410	</li>
				411	<li> Check that JNIEnv structs aren't being shared between threads.
				412
				413	</li>
				414	<li> Make sure local references aren't used outside their allowed lifespan.
				415	</li>
				416	<li> UTF-8 strings contain valid "modified UTF-8" data.
				417	</li>
				418	</ul>
				419	<p>Accessibility of methods and fields (i.e. public vs. private) is not
				420	checked.
				421	<p>
				422	The Dalvik VM supports the <code>-Xcheck:jni</code> flag. For a
				423	description of how to enable it for Android apps, see
				424	<a href="embedded-vm-control.html">Controlling the Embedded VM</a>.
				425	It's currently enabled by default in the Android emulator and on
				426	"engineering" device builds.
				427
				428	</p><p>
				429	JNI checks can be modified with the <code>-Xjniopts</code> command-line
				430	flag. Currently supported values include:
				431	</p>
				432	<blockquote><dl>
				433	<dt>forcecopy
				434	<dd>When set, any function that can return a copy of the original data
				435	(array of primitive values, UTF-16 chars) will always do so. The buffers
				436	are over-allocated and surrounded with a guard pattern to help identify
				437	code writing outside the buffer, and the contents are erased before the
				438	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]	439	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]	440	<dt>warnonly
				441	<dd>By default, JNI "warnings" cause the VM to abort. With this flag
				442	it continues on.
				443	</dl></blockquote>
				444
				445
				446	</p><p>
				447	</p><h2><a name="Native_Libraries"> Native Libraries </a></h2>
				448	<p>
				449	You can load native code from shared libraries with the standard
				450	<code>System.loadLibrary()</code> call. The
				451	preferred way to get at your native code is:
				452	</p><p>
				453	</p><ul>
				454	<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".
				455
				456	</li>
				457	<li> Provide a native function: <code><strong>jint JNI_OnLoad(JavaVM* vm, void* reserved)</strong></code>
				458	</li>
				459	<li>In <code>JNI_OnLoad</code>, register all of your native methods. You
				460	should declare
				461	the methods "static" so the names don't take up space in the symbol table
				462	on the device.
				463	</li>
				464	</ul>
				465	<p>
				466	The <code>JNI_OnLoad</code> function should look something like this if
				467	written in C:
				468	</p><blockquote><pre>jint JNI_OnLoad(JavaVM* vm, void* reserved)
				469	{
				470	JNIEnv* env;
				471	if ((vm)->GetEnv(vm, (void*) &env, JNI_VERSION_1_4) != JNI_OK)
				472	return -1;
				473
				474	/* get class with (env)->FindClass /
				475	/* register methods with (env)->RegisterNatives /
				476
				477	return JNI_VERSION_1_4;
				478	}
				479	</pre></blockquote>
				480	</p><p>
				481	You can also call <code>System.load()</code> with the full path name of the
				482	shared library. For Android apps, you can get the full path to the
				483	application's private data storage area from the context object.
				484	</p><p>
Andy McFadden	278c12b	2009-09-21 16:28:39 -0700	[diff] [blame]	485	This is the recommended approach, but not the only approach. The VM does
				486	not require explicit registration, nor that you provide a
				487	<code>JNI_OnLoad</code> function.
				488	You can instead use "discovery" of native methods that are named in a
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	489	specific way (see <a href="http://java.sun.com/javase/6/docs/technotes/guides/jni/spec/design.html#wp615">
Andy McFadden	278c12b	2009-09-21 16:28:39 -0700	[diff] [blame]	490	the JNI spec</a> for details), though this is less desirable.
				491	It requires more space in the shared object symbol table,
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	492	loading is slower because it requires string searches through all of the
				493	loaded shared libraries, and if a method signature is wrong you won't know
				494	about it until the first time the method is actually used.
				495	</p><p>
Andy McFadden	278c12b	2009-09-21 16:28:39 -0700	[diff] [blame]	496	One other note about <code>JNI_OnLoad</code>: any <code>FindClass</code>
				497	calls you make from there will happen in the context of the class loader
				498	that was used to load the shared library. Normally <code>FindClass</code>
				499	uses the loader associated with the method at the top of the interpreted
				500	stack, or if there isn't one (because the thread was just attached to
				501	the VM) it uses the "system" class loader.
				502	</p><p>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	503
				504
				505	</p><h2><a name="64bit"> 64-bit Considerations </a></h2>
				506
				507	<p>
				508	Android is currently expected to run on 32-bit platforms. In theory it
				509	could be built for a 64-bit system, but that is not a goal at this time.
				510	For the most part this isn't something that you will need to worry about
				511	when interacting with native code,
				512	but it becomes significant if you plan to store pointers to native
				513	structures in integer fields in an object. To support architectures
				514	that use 64-bit pointers, <strong>you need to stash your native pointers in a
				515	<code>long</code> field rather than an <code>int</code></strong>.
				516
				517
				518	</p><h2><a name="Unsupported"> Unsupported Features </a></h2>
				519	<p>All JNI 1.6 features are supported, with the following exceptions:
				520	<ul>
				521	<li><code>DefineClass</code> is not implemented. Dalvik does not use
				522	Java bytecodes or class files, so passing in binary class data
				523	doesn't work. Translation facilities may be added in a future
				524	version of the VM.</li>
				525	<li><code>NewWeakGlobalRef</code> and <code>DeleteWeakGlobalRef</code>
				526	are not implemented. The
				527	VM supports weak references, but not JNI "weak global" references.
				528	These will be supported in a future release.</li>
				529	<li><code>GetObjectRefType</code> (new in 1.6) is implemented but not fully
				530	functional -- it can't always tell the difference between "local" and
				531	"global" references.</li>
				532	</ul>
				533
				534	</p>
				535
				536	<address>Copyright © 2008 The Android Open Source Project</address>
				537
				538	</body>
				539	</html>