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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>
Andy McFadden	40f3271	2009-09-22 13:55:42 -0700	[diff] [blame]	17	<li> <a href="#jclass_jmethodID_and_jfieldID">jclass, jmethodID, and jfieldID</a>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	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>
Andy McFadden	40f3271	2009-09-22 13:55:42 -0700	[diff] [blame]	82	</p><h2><a name="jclass_jmethodID_and_jfieldID"> jclass, jmethodID, and jfieldID </a></h2>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	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,
Andy McFadden	40f3271	2009-09-22 13:55:42 -0700	[diff] [blame]	107	which is rare but will not be impossible in our system. Note however that
				108	the <code>jclass</code>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	109	is a class reference and <strong>must be protected</strong> with a call
				110	to <code>NewGlobalRef</code> (see the next section).
				111	</p><p>
				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:
				115	</p><p>
				116
				117	</p><pre> /*
				118	* We use a class initializer to allow the native code to cache some
				119	* field offsets.
				120	*/
				121
				122	/*
				123	* A native function that looks up and caches interesting
				124	* class/field/method IDs for this class. Returns false on failure.
				125	*/
				126	native private static boolean nativeClassInit();
Carl Shapiro	de75089	2010-06-08 16:37:12 -0700	[diff] [blame]	127
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	128	/*
				129	* Invoke the native initializer when the class is loaded.
				130	*/
				131	static {
				132	if (!nativeClassInit())
				133	throw new RuntimeException("native init failed");
				134	}
				135	</pre>
				136	<p>
				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.)
				141	</p><p>
				142	</p><p>
				143	</p><p>
				144	</p><h2><a name="local_vs_global_references"> Local vs. Global References </a></h2>
				145	<p>
				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>
Andy McFadden	40f3271	2009-09-22 13:55:42 -0700	[diff] [blame]	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
				152	checks are enabled.)
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	153	</p><p>
				154
Andy McFadden	40f3271	2009-09-22 13:55:42 -0700	[diff] [blame]	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>.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	160
Andy McFadden	40f3271	2009-09-22 13:55:42 -0700	[diff] [blame]	161	</p><p>
				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);
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	166	</pre>
				167
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	168	</p><p>
				169	All JNI methods accept both local and global references as arguments.
Andy McFadden	5aca603	2009-05-06 16:48:33 -0700	[diff] [blame]	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.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	176	</p><p>
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame]	177	One consequence of this is that you
Andy McFadden	40f3271	2009-09-22 13:55:42 -0700	[diff] [blame]	178	<strong>must not assume object references are constant or unique</strong>
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame]	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
Andy McFadden	40f3271	2009-09-22 13:55:42 -0700	[diff] [blame]	181	different objects could have the same 32-bit value on consecutive calls. Do
				182	not use <code>jobject</code> values as keys.
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame]	183	</p><p>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	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.
				191	</p><p>
				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.
				196	</p><p>
				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
Andy McFadden	40f3271	2009-09-22 13:55:42 -0700	[diff] [blame]	200	references you create will have to be deleted manually unless you're going
				201	to detach the thread soon.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	202	</p><p>
				203	</p><p>
				204	</p><p>
				205	</p><h2><a name="UTF_8_and_UTF_16_strings"> </a> UTF-8 and UTF-16 Strings </h2>
				206	<p>
				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.
				213	</p><p>
				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
				219	the string pointer.
				220
				221	</p><p>
Andy McFadden	5aca603	2009-05-06 16:48:33 -0700	[diff] [blame]	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
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	226	released when the native method returns.
				227	</p><p>
Andy McFadden	7e7a332	2010-01-28 12:27:06 -0800	[diff] [blame]	228	<strong>Data passed to NewStringUTF must be in "modified" UTF-8 format</strong>. A
				229	common mistake is reading character data from a file or network stream
				230	and handing it to <code>NewStringUTF</code> without filtering it.
				231	Unless you know the data is 7-bit ASCII, you need to strip out high-ASCII
				232	characters or convert them to proper "modified" UTF-8 form. If you don't,
				233	the UTF-16 conversion will likely not be what you expect. The extended
				234	JNI checks will scan strings and warn you about invalid data, but they
				235	won't catch everything.
				236	</p><p>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	237	</p><p>
				238
				239
				240	</p><h2><a name="Arrays"> </a> Primitive Arrays </h2>
				241	<p>
				242	JNI provides functions for accessing the contents of array objects.
				243	While arrays of objects must be accessed one entry at a time, arrays of
				244	primitives can be read and written directly as if they were declared in C.
				245	</p><p>
				246	To make the interface as efficient as possible without constraining
				247	the VM implementation,
				248	the <code>Get<PrimitiveType>ArrayElements</code> family of calls
				249	allows the VM to either return a pointer to the actual elements, or
				250	allocate some memory and make a copy. Either way, the raw pointer returned
				251	is guaranteed to be valid until the corresponding <code>Release</code> call
				252	is issued (which implies that, if the data wasn't copied, the array object
				253	will be pinned down and can't be relocated as part of compacting the heap).
				254	<strong>You must Release every array you Get.</strong> Also, if the Get
				255	call fails, you must ensure that your code doesn't try to Release a NULL
				256	pointer later.
				257	</p><p>
				258	You can determine whether or not the data was copied by passing in a
				259	non-NULL pointer for the <code>isCopy</code> argument. This is rarely
				260	useful.
				261	</p><p>
				262	The <code>Release</code> call takes a <code>mode</code> argument that can
				263	have one of three values. The actions performed by the VM depend upon
				264	whether it returned a pointer to the actual data or a copy of it:
				265	<ul>
				266	<li><code>0</code>
				267	<ul>
				268	<li>Actual: the array object is un-pinned.
				269	<li>Copy: data is copied back. The buffer with the copy is freed.
				270	</ul>
				271	<li><code>JNI_COMMIT</code>
				272	<ul>
				273	<li>Actual: does nothing.
				274	<li>Copy: data is copied back. The buffer with the copy
				275	<strong>is not freed</strong>.
				276	</ul>
				277	<li><code>JNI_ABORT</code>
				278	<ul>
				279	<li>Actual: the array object is un-pinned. Earlier
				280	writes are <strong>not</strong> aborted.
				281	<li>Copy: the buffer with the copy is freed; any changes to it are lost.
				282	</ul>
				283	</ul>
				284	</p><p>
				285	One reason for checking the <code>isCopy</code> flag is to know if
				286	you need to call <code>Release</code> with <code>JNI_COMMIT</code>
				287	after making changes to an array -- if you're alternating between making
				288	changes and executing code that uses the contents of the array, you may be
				289	able to
				290	skip the no-op commit. Another possible reason for checking the flag is for
				291	efficient handling of <code>JNI_ABORT</code>. For example, you might want
				292	to get an array, modify it in place, pass pieces to other functions, and
				293	then discard the changes. If you know that JNI is making a new copy for
				294	you, there's no need to create another "editable" copy. If JNI is passing
				295	you the original, then you do need to make your own copy.
				296	</p><p>
				297	Some have asserted that you can skip the <code>Release</code> call if
				298	<code>*isCopy</code> is false. This is not the case. If no copy buffer was
				299	allocated, then the original memory must be pinned down and can't be moved by
				300	the garbage collector.
				301	</p><p>
				302	Also note that the <code>JNI_COMMIT</code> flag does NOT release the array,
				303	and you will need to call <code>Release</code> again with a different flag
				304	eventually.
				305	</p><p>
				306	</p><p>
				307
				308
				309	</p><h2><a name="RegionCalls"> Region Calls </a></h2>
				310
				311	<p>
				312	There is an alternative to calls like <code>Get<Type>ArrayElements</code>
				313	and <code>GetStringChars</code> that may be very helpful when all you want
				314	to do is copy data in or out. Consider the following:
				315	<pre>
				316	jbyte* data = env->GetByteArrayElements(array, NULL);
				317	if (data != NULL) {
				318	memcpy(buffer, data, len);
				319	env->ReleaseByteArrayElements(array, data, JNI_ABORT);
				320	}
				321	</pre>
				322	<p>
				323	This grabs the array, copies the first <code>len</code> byte
				324	elements out of it, and then releases the array. Depending upon the VM
				325	policies the <code>Get</code> call will either pin or copy the array contents.
				326	We copy the data (for perhaps a second time), then call Release; in this case
				327	we use <code>JNI_ABORT</code> so there's no chance of a third copy.
				328	</p><p>
				329	We can accomplish the same thing with this:
				330	<pre>
				331	env->GetByteArrayRegion(array, 0, len, buffer);
				332	</pre>
				333	</p><p>
				334	This accomplishes the same thing, with several advantages:
				335	<ul>
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame]	336	<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]	337	<li>Doesn't require pinning or extra data copies.
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame]	338	<li>Reduces the risk of programmer error -- no risk of forgetting
				339	to call <code>Release</code> after something fails.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	340	</ul>
				341	</p><p>
				342	Similarly, you can use the <code>Set<Type>ArrayRegion</code> call
				343	to copy data into an array, and <code>GetStringRegion</code> or
				344	<code>GetStringUTFRegion</code> to copy characters out of a
				345	<code>String</code>.
				346
				347
				348	</p><h2><a name="Exceptions"> Exceptions </a></h2>
				349	<p>
				350	<strong>You may not call most JNI functions while an exception is pending.</strong>
				351	Your code is expected to notice the exception (via the function's return value,
				352	<code>ExceptionCheck()</code>, or <code>ExceptionOccurred()</code>) and return,
				353	or clear the exception and handle it.
				354	</p><p>
				355	The only JNI functions that you are allowed to call while an exception is
				356	pending are:
				357	<font size="-1"><ul>
				358	<li>DeleteGlobalRef
				359	<li>DeleteLocalRef
				360	<li>DeleteWeakGlobalRef
				361	<li>ExceptionCheck
				362	<li>ExceptionClear
				363	<li>ExceptionDescribe
				364	<li>ExceptionOccurred
				365	<li>MonitorExit
				366	<li>PopLocalFrame
				367	<li>PushLocalFrame
Elliott Hughes	b7906ed	2009-09-09 15:16:59 -0700	[diff] [blame]	368	<li>Release<PrimitiveType>ArrayElements
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	369	<li>ReleasePrimitiveArrayCritical
				370	<li>ReleaseStringChars
				371	<li>ReleaseStringCritical
				372	<li>ReleaseStringUTFChars
				373	</ul></font>
				374	</p><p>
				375	Note that exceptions thrown by interpreted code do not "leap over" native code,
				376	and C++ exceptions thrown by native code are not handled by Dalvik.
				377	The JNI <code>Throw</code> and <code>ThrowNew</code> instructions just
				378	set an exception pointer in the current thread. Upon returning to the VM from
				379	native code, the exception will be noted and handled appropriately.
				380	</p><p>
				381	Native code can "catch" an exception by calling <code>ExceptionCheck</code> or
				382	<code>ExceptionOccurred</code>, and clear it with
				383	<code>ExceptionClear</code>. As usual,
				384	discarding exceptions without handling them can lead to problems.
				385	</p><p>
				386	There are no built-in functions for manipulating the Throwable object
				387	itself, so if you want to (say) get the exception string you will need to
				388	find the Throwable class, look up the method ID for
				389	<code>getMessage "()Ljava/lang/String;"</code>, invoke it, and if the result
				390	is non-NULL use <code>GetStringUTFChars</code> to get something you can
				391	hand to printf or a LOG macro.
				392
				393	</p><p>
				394	</p><p>
				395	</p><h2><a name="Extended_checking"> Extended Checking </a></h2>
				396	<p>
Andy McFadden	40f3271	2009-09-22 13:55:42 -0700	[diff] [blame]	397	JNI does very little error checking. Calling <code>SetIntField</code>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	398	on an Object field will succeed, even if the field is marked
				399	<code>private</code> and <code>final</code>. The
				400	goal is to minimize the overhead on the assumption that, if you've written it in native code,
				401	you probably did it for performance reasons.
				402	</p><p>
				403	Some VMs support extended checking with the "<code>-Xcheck:jni</code>" flag. If the flag is set, the VM
				404	puts a different table of functions into the JavaVM and JNIEnv pointers. These functions do
				405	an extended series of checks before calling the standard implementation.
				406
				407	</p><p>
Andy McFadden	c7659ec	2009-09-18 16:14:41 -0700	[diff] [blame]	408	Some things that may be checked:
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	409	</p><p>
				410	</p>
				411	<ul>
				412	<li> Check for null pointers where not allowed.
Andy McFadden	3ccaec4	2010-06-04 16:51:47 -0700	[diff] [blame]	413	</li>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	414	<li> Verify argument type correctness (jclass is a class object,
				415	jfieldID points to field data, jstring is a java.lang.String).
				416	</li>
				417	<li> Field type correctness, e.g. don't store a HashMap in a String field.
				418	</li>
				419	<li> Check to see if an exception is pending on calls where pending exceptions are not legal.
				420	</li>
				421	<li> Check for calls to inappropriate functions between Critical get/release calls.
				422	</li>
				423	<li> Check that JNIEnv structs aren't being shared between threads.
				424
				425	</li>
				426	<li> Make sure local references aren't used outside their allowed lifespan.
				427	</li>
Andy McFadden	7e7a332	2010-01-28 12:27:06 -0800	[diff] [blame]	428	<li> UTF-8 strings contain only valid "modified UTF-8" data.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	429	</li>
				430	</ul>
				431	<p>Accessibility of methods and fields (i.e. public vs. private) is not
				432	checked.
				433	<p>
				434	The Dalvik VM supports the <code>-Xcheck:jni</code> flag. For a
				435	description of how to enable it for Android apps, see
				436	<a href="embedded-vm-control.html">Controlling the Embedded VM</a>.
				437	It's currently enabled by default in the Android emulator and on
				438	"engineering" device builds.
				439
				440	</p><p>
				441	JNI checks can be modified with the <code>-Xjniopts</code> command-line
				442	flag. Currently supported values include:
				443	</p>
				444	<blockquote><dl>
				445	<dt>forcecopy
				446	<dd>When set, any function that can return a copy of the original data
				447	(array of primitive values, UTF-16 chars) will always do so. The buffers
				448	are over-allocated and surrounded with a guard pattern to help identify
				449	code writing outside the buffer, and the contents are erased before the
				450	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]	451	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]	452	<dt>warnonly
				453	<dd>By default, JNI "warnings" cause the VM to abort. With this flag
				454	it continues on.
				455	</dl></blockquote>
				456
				457
				458	</p><p>
				459	</p><h2><a name="Native_Libraries"> Native Libraries </a></h2>
				460	<p>
				461	You can load native code from shared libraries with the standard
				462	<code>System.loadLibrary()</code> call. The
				463	preferred way to get at your native code is:
				464	</p><p>
				465	</p><ul>
Andy McFadden	40f3271	2009-09-22 13:55:42 -0700	[diff] [blame]	466	<li> Call <code>System.loadLibrary()</code> from a static class
				467	initializer. (See the earlier example, where one is used to call
				468	<code>nativeClassInit()</code>.) The argument is the "undecorated"
				469	library name, e.g. to load "libfubar.so" you would pass in "fubar".
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	470
				471	</li>
				472	<li> Provide a native function: <code><strong>jint JNI_OnLoad(JavaVM* vm, void* reserved)</strong></code>
				473	</li>
				474	<li>In <code>JNI_OnLoad</code>, register all of your native methods. You
				475	should declare
				476	the methods "static" so the names don't take up space in the symbol table
				477	on the device.
				478	</li>
				479	</ul>
				480	<p>
				481	The <code>JNI_OnLoad</code> function should look something like this if
				482	written in C:
				483	</p><blockquote><pre>jint JNI_OnLoad(JavaVM* vm, void* reserved)
				484	{
				485	JNIEnv* env;
				486	if ((vm)->GetEnv(vm, (void*) &env, JNI_VERSION_1_4) != JNI_OK)
				487	return -1;
				488
				489	/* get class with (env)->FindClass /
				490	/* register methods with (env)->RegisterNatives /
				491
				492	return JNI_VERSION_1_4;
				493	}
				494	</pre></blockquote>
				495	</p><p>
				496	You can also call <code>System.load()</code> with the full path name of the
Andy McFadden	40f3271	2009-09-22 13:55:42 -0700	[diff] [blame]	497	shared library. For Android apps, you may find it useful to get the full
				498	path to the application's private data storage area from the context object.
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	499	</p><p>
Andy McFadden	278c12b	2009-09-21 16:28:39 -0700	[diff] [blame]	500	This is the recommended approach, but not the only approach. The VM does
				501	not require explicit registration, nor that you provide a
				502	<code>JNI_OnLoad</code> function.
				503	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]	504	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]	505	the JNI spec</a> for details), though this is less desirable.
				506	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]	507	loading is slower because it requires string searches through all of the
				508	loaded shared libraries, and if a method signature is wrong you won't know
				509	about it until the first time the method is actually used.
				510	</p><p>
Andy McFadden	278c12b	2009-09-21 16:28:39 -0700	[diff] [blame]	511	One other note about <code>JNI_OnLoad</code>: any <code>FindClass</code>
				512	calls you make from there will happen in the context of the class loader
				513	that was used to load the shared library. Normally <code>FindClass</code>
				514	uses the loader associated with the method at the top of the interpreted
				515	stack, or if there isn't one (because the thread was just attached to
				516	the VM) it uses the "system" class loader.
				517	</p><p>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	518
				519
				520	</p><h2><a name="64bit"> 64-bit Considerations </a></h2>
				521
				522	<p>
				523	Android is currently expected to run on 32-bit platforms. In theory it
				524	could be built for a 64-bit system, but that is not a goal at this time.
				525	For the most part this isn't something that you will need to worry about
				526	when interacting with native code,
				527	but it becomes significant if you plan to store pointers to native
				528	structures in integer fields in an object. To support architectures
				529	that use 64-bit pointers, <strong>you need to stash your native pointers in a
				530	<code>long</code> field rather than an <code>int</code></strong>.
				531
				532
				533	</p><h2><a name="Unsupported"> Unsupported Features </a></h2>
				534	<p>All JNI 1.6 features are supported, with the following exceptions:
				535	<ul>
				536	<li><code>DefineClass</code> is not implemented. Dalvik does not use
				537	Java bytecodes or class files, so passing in binary class data
				538	doesn't work. Translation facilities may be added in a future
				539	version of the VM.</li>
Andy McFadden	b18992f	2009-09-25 10:42:15 -0700	[diff] [blame]	540	<li>"Weak global" references are implemented, but may only be passed
				541	to <code>NewLocalRef</code>, <code>NewGlobalRef</code>, and
				542	<code>DeleteWeakGlobalRef</code>. (The spec strongly encourages
				543	programmers to create hard references to weak globals before doing
				544	anything with them, so this should not be at all limiting.)</li>
The Android Open Source Project	f6c3871	2009-03-03 19:28:47 -0800	[diff] [blame]	545	<li><code>GetObjectRefType</code> (new in 1.6) is implemented but not fully
				546	functional -- it can't always tell the difference between "local" and
				547	"global" references.</li>
				548	</ul>
				549
				550	</p>
				551
				552	<address>Copyright © 2008 The Android Open Source Project</address>
				553
				554	</body>
				555	</html>