dalvik/docs/jni-tips.html

<html>
  <head>
    <title>Android JNI Tips</title>
    <link rel=stylesheet href="android.css">
  </head>

  <body>
    <h1><a name="JNI_Tips"></a>Android JNI Tips</h1>
<p>
</p><p>
</p><ul>
<li> <a href="#What_s_JNI_">What's JNI?</a>
</li>
<li> <a href="#JavaVM_and_JNIEnv">JavaVM and JNIEnv</a>

</li>
<li> <a href="#jclass_jmethodID_and_jfieldID">jclass, jmethodID, and jfieldID</a>
</li>
<li> <a href="#local_vs_global_references">Local vs. Global References</a>
</li>
<li> <a href="#UTF_8_and_UTF_16_strings">UTF-8 and UTF-16 Strings</a>
</li>
<li> <a href="#Arrays">Primitive Arrays</a>
</li>
<li> <a href="#RegionCalls">Region Calls</a>
</li>
<li> <a href="#Exceptions">Exceptions</a>
</li>

<li> <a href="#Extended_checking">Extended Checking</a>
</li>
<li> <a href="#Native_Libraries">Native Libraries</a>
</li>
<li> <a href="#64bit">64-bit Considerations</a>
</li>

<li> <a href="#Unsupported">Unsupported Features</a>
</ul>
<p>
<noautolink>
</noautolink></p><p>
</p><h2><a name="What_s_JNI_"> </a> What's JNI? </h2>
<p>

JNI is the Java Native Interface.  It defines a way for code written in the
Java programming language to interact with native
code, e.g. functions written in C/C++.  It's VM-neutral, has support for loading code from
dynamic shared libraries, and while cumbersome at times is reasonably efficient.
</p><p>
You really should read through the
<a href="http://java.sun.com/javase/6/docs/technotes/guides/jni/spec/jniTOC.html">JNI spec for J2SE 1.6</a>
to get a sense for how JNI works and what features are available.  Some
aspects of the interface aren't immediately obvious on
first reading, so you may find the next few sections handy.
The more detailed <i>JNI Programmer's Guide and Specification</i> can be found
<a href="http://java.sun.com/docs/books/jni/html/jniTOC.html">here</a>.
</p><p>
</p><p>
</p><h2><a name="JavaVM_and_JNIEnv"> </a> JavaVM and JNIEnv </h2>
<p>
JNI defines two key data structures, "JavaVM" and "JNIEnv".  Both of these are essentially
pointers to pointers to function tables.  (In the C++ version, it's a class whose sole member
is a pointer to a function table.)  The JavaVM provides the "invocation interface" functions,
which allow you to create and destroy the VM.  In theory you can have multiple VMs per process,
but Android's VMs only allow one.
</p><p>
The JNIEnv provides most of the JNI functions.  Your native functions all receive a JNIEnv as
the first argument.
</p><p>

On some VMs, the JNIEnv is used for thread-local storage.  For this reason, <strong>you cannot share a JNIEnv between threads</strong>.
If a piece of code has no other way to get its JNIEnv, you should share
the JavaVM, and use JavaVM-&gt;GetEnv to discover the thread's JNIEnv.
</p><p>
The C and C++ declarations of JNIEnv and JavaVM are different.  "jni.h" provides different typedefs
depending on whether it's included into ".c" or ".cpp".  For this reason it's a bad idea to
include JNIEnv arguments in header files included by both languages.  (Put another way: if your
header file requires "#ifdef __cplusplus", you may have to do some extra work if anything in
that header refers to JNIEnv.)
</p><p>
</p><p>
</p><h2><a name="jclass_jmethodID_and_jfieldID"> jclass, jmethodID, and jfieldID </a></h2>
<p>
If you want to access an object's field from native code, you would do the following:
</p><p>
</p><ul>
<li> Get the class object reference for the class with <code>FindClass</code>
</li>
<li> Get the field ID for the field with <code>GetFieldID</code>
</li>
<li> Get the contents of the field with something appropriate, e.g.
<code>GetIntField</code>
</li>
</ul>
<p>
Similarly, to call a method, you'd first get a class object reference and then a method ID.  The IDs are often just
pointers to internal VM data structures.  Looking them up may require several string
comparisons, but once you have them the actual call to get the field or invoke the method
is very quick.
</p><p>
If performance is important, it's useful to look the values up once and cache the results
in your native code.  Because we are limiting ourselves to one VM per process, it's reasonable
to store this data in a static local structure.
</p><p>
The class references, field IDs, and method IDs are guaranteed valid until the class is unloaded.  Classes
are only unloaded if all classes associated with a ClassLoader can be garbage collected,
which is rare but will not be impossible in our system.  Note however that
the <code>jclass</code>
is a class reference and <strong>must be protected</strong> with a call
to <code>NewGlobalRef</code> (see the next section).
</p><p>
If you would like to cache the IDs when a class is loaded, and automatically re-cache them
if the class is ever unloaded and reloaded, the correct way to initialize
the IDs is to add a piece of code that looks like this to the appropriate class:
</p><p>

</p><pre>    /*
     * We use a class initializer to allow the native code to cache some
     * field offsets.
     */

    /*
     * A native function that looks up and caches interesting
     * class/field/method IDs for this class.  Returns false on failure.
     */
    native private static boolean nativeClassInit();

    /*
     * Invoke the native initializer when the class is loaded.
     */
    static {
        if (!nativeClassInit())
            throw new RuntimeException("native init failed");
    }
</pre>
<p>
Create a nativeClassInit method in your C/C++ code that performs the ID lookups.  The code
will be executed once, when the class is initialized.  If the class is ever unloaded and
then reloaded, it will be executed again.  (See the implementation of java.io.FileDescriptor
for an example in our source tree.)
</p><p>
</p><p>
</p><p>
</p><h2><a name="local_vs_global_references"> Local vs. Global References </a></h2>
<p>
Every object that JNI returns is a "local reference".  This means that it's valid for the
duration of the current native method in the current thread.
<strong>Even if the object itself continues to live on after the native method returns, the reference is not valid.</strong>
This applies to all sub-classes of <code>jobject</code>, including
<code>jclass</code>, <code>jstring</code>, and <code>jarray</code>.
(Dalvik VM will warn you about most reference mis-uses when extended JNI
checks are enabled.)
</p><p>

If you want to hold on to a reference for a longer period, you must use
a "global" reference.  The <code>NewGlobalRef</code> function takes the
local reference as an argument and returns a global one.
The global reference is guaranteed to be valid until you call
<code>DeleteGlobalRef</code>.

</p><p>
This pattern is commonly used when caching copies of class objects obtained
from <code>FindClass</code>, e.g.:
<p><pre>jclass* localClass = env-&gt;FindClass("MyClass");
jclass* globalClass = (jclass*) env-&gt;NewGlobalRef(localClass);
</pre>

</p><p>
All JNI methods accept both local and global references as arguments.
It's possible for references to the same object to have different values;
for example, the return values from consecutive calls to
<code>NewGlobalRef</code> on the same object may be different.
<strong>To see if two references refer to the same object,
you must use the <code>IsSameObject</code> function.</strong>  Never compare
references with "==" in native code.
</p><p>
One consequence of this is that you
<strong>must not assume object references are constant or unique</strong>
in native code.  The 32-bit value representing an object may be different
from one invocation of a method to the next, and it's possible that two
different objects could have the same 32-bit value on consecutive calls.  Do
not use <code>jobject</code> values as keys.
</p><p>
Programmers are required to "not excessively allocate" local references.  In practical terms this means
that if you're creating large numbers of local references, perhaps while running through an array of
Objects, you should free them manually with
<code>DeleteLocalRef</code> instead of letting JNI do it for you.  The
VM is only required to reserve slots for
16 local references, so if you need more than that you should either delete as you go or use
<code>EnsureLocalCapacity</code> to reserve more.
</p><p>
Note: method and field IDs are just 32-bit identifiers, not object
references, and should not be passed to <code>NewGlobalRef</code>.  The raw data
pointers returned by functions like <code>GetStringUTFChars</code>
and <code>GetByteArrayElements</code> are also not objects.
</p><p>
One unusual case deserves separate mention.  If you attach a native
thread to the VM with AttachCurrentThread, the code you are running will
never "return" to the VM until the thread detaches from the VM.  Any local
references you create will have to be deleted manually unless you're going
to detach the thread soon.
</p><p>
</p><p>
</p><p>
</p><h2><a name="UTF_8_and_UTF_16_strings"> </a> UTF-8 and UTF-16 Strings </h2>
<p>
The Java programming language uses UTF-16.  For convenience, JNI provides methods that work with "modified UTF-8" encoding
as well.  (Some VMs use the modified UTF-8 internally to store strings; ours do not.)  The
modified encoding only supports the 8- and 16-bit forms, and stores ASCII NUL values in a 16-bit encoding.
The nice thing about it is that you can count on having C-style zero-terminated strings,
suitable for use with standard libc string functions.  The down side is that you cannot pass
arbitrary UTF-8 data into the VM and expect it to work correctly.
</p><p>
It's usually best to operate with UTF-16 strings.  With our current VMs, the
<code>GetStringChars</code> method
does not require a copy, whereas <code>GetStringUTFChars</code> requires a malloc and a UTF conversion.  Note that
<strong>UTF-16 strings are not zero-terminated</strong>, and \u0000 is allowed,
so you need to hang on to the string length as well as
the string pointer.

</p><p>
<strong>Don't forget to Release the strings you Get</strong>.  The
string functions return <code>jchar*</code> or <code>jbyte*</code>, which
are C-style pointers to primitive data rather than local references.  They
are guaranteed valid until Release is called, which means they are not
released when the native method returns.
</p><p>
</p><p>


</p><h2><a name="Arrays"> </a> Primitive Arrays </h2>
<p>
JNI provides functions for accessing the contents of array objects.
While arrays of objects must be accessed one entry at a time, arrays of
primitives can be read and written directly as if they were declared in C.
</p><p>
To make the interface as efficient as possible without constraining
the VM implementation,
the <code>Get&lt;PrimitiveType&gt;ArrayElements</code> family of calls
allows the VM to either return a pointer to the actual elements, or
allocate some memory and make a copy.  Either way, the raw pointer returned
is guaranteed to be valid until the corresponding <code>Release</code> call
is issued (which implies that, if the data wasn't copied, the array object
will be pinned down and can't be relocated as part of compacting the heap).
<strong>You must Release every array you Get.</strong>  Also, if the Get
call fails, you must ensure that your code doesn't try to Release a NULL
pointer later.
</p><p>
You can determine whether or not the data was copied by passing in a
non-NULL pointer for the <code>isCopy</code> argument.  This is rarely
useful.
</p><p>
The <code>Release</code> call takes a <code>mode</code> argument that can
have one of three values.  The actions performed by the VM depend upon
whether it returned a pointer to the actual data or a copy of it:
<ul>
    <li><code>0</code>
    <ul>
        <li>Actual: the array object is un-pinned.
        <li>Copy: data is copied back.  The buffer with the copy is freed.
    </ul>
    <li><code>JNI_COMMIT</code>
    <ul>
        <li>Actual: does nothing.
        <li>Copy: data is copied back.  The buffer with the copy
        <strong>is not freed</strong>.
    </ul>
    <li><code>JNI_ABORT</code>
    <ul>
        <li>Actual: the array object is un-pinned.  Earlier
        writes are <strong>not</strong> aborted.
        <li>Copy: the buffer with the copy is freed; any changes to it are lost.
    </ul>
</ul>
</p><p>
One reason for checking the <code>isCopy</code> flag is to know if
you need to call <code>Release</code> with <code>JNI_COMMIT</code>
after making changes to an array -- if you're alternating between making
changes and executing code that uses the contents of the array, you may be
able to
skip the no-op commit.  Another possible reason for checking the flag is for
efficient handling of <code>JNI_ABORT</code>.  For example, you might want
to get an array, modify it in place, pass pieces to other functions, and
then discard the changes.  If you know that JNI is making a new copy for
you, there's no need to create another "editable" copy.  If JNI is passing
you the original, then you do need to make your own copy.
</p><p>
Some have asserted that you can skip the <code>Release</code> call if
<code>*isCopy</code> is false.  This is not the case.  If no copy buffer was
allocated, then the original memory must be pinned down and can't be moved by
the garbage collector.
</p><p>
Also note that the <code>JNI_COMMIT</code> flag does NOT release the array,
and you will need to call <code>Release</code> again with a different flag
eventually.
</p><p>
</p><p>


</p><h2><a name="RegionCalls"> Region Calls </a></h2>

<p>
There is an alternative to calls like <code>Get&lt;Type&gt;ArrayElements</code>
and <code>GetStringChars</code> that may be very helpful when all you want
to do is copy data in or out.  Consider the following:
<pre>
    jbyte* data = env->GetByteArrayElements(array, NULL);
    if (data != NULL) {
        memcpy(buffer, data, len);
        env->ReleaseByteArrayElements(array, data, JNI_ABORT);
    }
</pre>
<p>
This grabs the array, copies the first <code>len</code> byte
elements out of it, and then releases the array.  Depending upon the VM
policies the <code>Get</code> call will either pin or copy the array contents.
We copy the data (for perhaps a second time), then call Release; in this case
we use <code>JNI_ABORT</code> so there's no chance of a third copy.
</p><p>
We can accomplish the same thing with this:
<pre>
    env->GetByteArrayRegion(array, 0, len, buffer);
</pre>
</p><p>
This accomplishes the same thing, with several advantages:
<ul>
    <li>Requires one JNI call instead of 2, reducing overhead.
    <li>Doesn't require pinning or extra data copies.
    <li>Reduces the risk of programmer error -- no risk of forgetting
    to call <code>Release</code> after something fails.
</ul>
</p><p>
Similarly, you can use the <code>Set&lt;Type&gt;ArrayRegion</code> call
to copy data into an array, and <code>GetStringRegion</code> or
<code>GetStringUTFRegion</code> to copy characters out of a
<code>String</code>.


</p><h2><a name="Exceptions"> Exceptions </a></h2>
<p>
<strong>You may not call most JNI functions while an exception is pending.</strong>
Your code is expected to notice the exception (via the function's return value,
<code>ExceptionCheck()</code>, or <code>ExceptionOccurred()</code>) and return,
or clear the exception and handle it.
</p><p>
The only JNI functions that you are allowed to call while an exception is
pending are:
<font size="-1"><ul>
    <li>DeleteGlobalRef
    <li>DeleteLocalRef
    <li>DeleteWeakGlobalRef
    <li>ExceptionCheck
    <li>ExceptionClear
    <li>ExceptionDescribe
    <li>ExceptionOccurred
    <li>MonitorExit
    <li>PopLocalFrame
    <li>PushLocalFrame
    <li>Release<PrimitiveType>ArrayElements
    <li>ReleasePrimitiveArrayCritical
    <li>ReleaseStringChars
    <li>ReleaseStringCritical
    <li>ReleaseStringUTFChars
</ul></font>
</p><p>
Note that exceptions thrown by interpreted code do not "leap over" native code,
and C++ exceptions thrown by native code are not handled by Dalvik.
The JNI <code>Throw</code> and <code>ThrowNew</code> instructions just
set an exception pointer in the current thread.  Upon returning to the VM from
native code, the exception will be noted and handled appropriately.
</p><p>
Native code can "catch" an exception by calling <code>ExceptionCheck</code> or
<code>ExceptionOccurred</code>, and clear it with
<code>ExceptionClear</code>.  As usual,
discarding exceptions without handling them can lead to problems.
</p><p>
There are no built-in functions for manipulating the Throwable object
itself, so if you want to (say) get the exception string you will need to
find the Throwable class, look up the method ID for
<code>getMessage "()Ljava/lang/String;"</code>, invoke it, and if the result
is non-NULL use <code>GetStringUTFChars</code> to get something you can
hand to printf or a LOG macro.

</p><p>
</p><p>
</p><h2><a name="Extended_checking"> Extended Checking </a></h2>
<p>
JNI does very little error checking.  Calling <code>SetIntField</code>
on an Object field will succeed, even if the field is marked
<code>private</code> and <code>final</code>.  The
goal is to minimize the overhead on the assumption that, if you've written it in native code,
you probably did it for performance reasons.
</p><p>
Some VMs support extended checking with the "<code>-Xcheck:jni</code>" flag.  If the flag is set, the VM
puts a different table of functions into the JavaVM and JNIEnv pointers.  These functions do
an extended series of checks before calling the standard implementation.

</p><p>
Some things that may be checked:
</p><p>
</p>
<ul>
<li> Check for null pointers where not allowed.
<li>
<li> Verify argument type correctness (jclass is a class object,
jfieldID points to field data, jstring is a java.lang.String).
</li>
<li> Field type correctness, e.g. don't store a HashMap in a String field.
</li>
<li> Check to see if an exception is pending on calls where pending exceptions are not legal.
</li>
<li> Check for calls to inappropriate functions between Critical get/release calls.
</li>
<li> Check that JNIEnv structs aren't being shared between threads.

</li>
<li> Make sure local references aren't used outside their allowed lifespan.
</li>
<li> UTF-8 strings contain valid "modified UTF-8" data.
</li>
</ul>
<p>Accessibility of methods and fields (i.e. public vs. private) is not
checked.
<p>
The Dalvik VM supports the <code>-Xcheck:jni</code> flag.  For a
description of how to enable it for Android apps, see
<a href="embedded-vm-control.html">Controlling the Embedded VM</a>.
It's currently enabled by default in the Android emulator and on
"engineering" device builds.

</p><p>
JNI checks can be modified with the <code>-Xjniopts</code> command-line
flag.  Currently supported values include:
</p>
<blockquote><dl>
<dt>forcecopy
<dd>When set, any function that can return a copy of the original data
(array of primitive values, UTF-16 chars) will always do so.  The buffers
are over-allocated and surrounded with a guard pattern to help identify
code writing outside the buffer, and the contents are erased before the
storage is freed to trip up code that uses the data after calling Release.
This will have a noticeable performance impact on some applications.
<dt>warnonly
<dd>By default, JNI "warnings" cause the VM to abort.  With this flag
it continues on.
</dl></blockquote>


</p><p>
</p><h2><a name="Native_Libraries"> Native Libraries </a></h2>
<p>
You can load native code from shared libraries with the standard
<code>System.loadLibrary()</code> call.  The
preferred way to get at your native code is:
</p><p>
</p><ul>
<li> Call <code>System.loadLibrary()</code> from a static class
initializer.  (See the earlier example, where one is used to call
<code>nativeClassInit()</code>.)  The argument is the "undecorated"
library name, e.g. to load "libfubar.so" you would pass in "fubar".

</li>
<li> Provide a native function: <code><strong>jint JNI_OnLoad(JavaVM* vm, void* reserved)</strong></code>
</li>
<li>In <code>JNI_OnLoad</code>, register all of your native methods.  You
should declare
the methods "static" so the names don't take up space in the symbol table
on the device.
</li>
</ul>
<p>
The <code>JNI_OnLoad</code> function should look something like this if
written in C:
</p><blockquote><pre>jint JNI_OnLoad(JavaVM* vm, void* reserved)
{
    JNIEnv* env;
    if ((*vm)->GetEnv(vm, (void**) &env, JNI_VERSION_1_4) != JNI_OK)
        return -1;

    /* get class with (*env)->FindClass */
    /* register methods with (*env)->RegisterNatives */

    return JNI_VERSION_1_4;
}
</pre></blockquote>
</p><p>
You can also call <code>System.load()</code> with the full path name of the
shared library.  For Android apps, you may find it useful to get the full
path to the application's private data storage area from the context object.
</p><p>
This is the recommended approach, but not the only approach.  The VM does
not require explicit registration, nor that you provide a
<code>JNI_OnLoad</code> function.
You can instead use "discovery" of native methods that are named in a
specific way (see <a href="http://java.sun.com/javase/6/docs/technotes/guides/jni/spec/design.html#wp615">
    the JNI spec</a> for details), though this is less desirable.
It requires more space in the shared object symbol table,
loading is slower because it requires string searches through all of the
loaded shared libraries, and if a method signature is wrong you won't know
about it until the first time the method is actually used.
</p><p>
One other note about <code>JNI_OnLoad</code>: any <code>FindClass</code>
calls you make from there will happen in the context of the class loader
that was used to load the shared library.  Normally <code>FindClass</code>
uses the loader associated with the method at the top of the interpreted
stack, or if there isn't one (because the thread was just attached to
the VM) it uses the "system" class loader.
</p><p>


</p><h2><a name="64bit"> 64-bit Considerations </a></h2>

<p>
Android is currently expected to run on 32-bit platforms.  In theory it
could be built for a 64-bit system, but that is not a goal at this time.
For the most part this isn't something that you will need to worry about
when interacting with native code,
but it becomes significant if you plan to store pointers to native
structures in integer fields in an object.  To support architectures
that use 64-bit pointers, <strong>you need to stash your native pointers in a
<code>long</code> field rather than an <code>int</code></strong>.


</p><h2><a name="Unsupported"> Unsupported Features </a></h2>
<p>All JNI 1.6 features are supported, with the following exceptions:
<ul>
    <li><code>DefineClass</code> is not implemented.  Dalvik does not use
    Java bytecodes or class files, so passing in binary class data
    doesn't work.  Translation facilities may be added in a future
    version of the VM.</li>
    <li><code>NewWeakGlobalRef</code> and <code>DeleteWeakGlobalRef</code>
    are not implemented.  The
    VM supports weak references, but not JNI "weak global" references.
    These will be supported in a future release.</li>
    <li><code>GetObjectRefType</code> (new in 1.6) is implemented but not fully
    functional -- it can't always tell the difference between "local" and
    "global" references.</li>
</ul>

</p>

<address>Copyright &copy; 2008 The Android Open Source Project</address>

  </body>
</html>