JNI

This page is over JNI, The Java Native Interface of the java language. For other features, see the Java page.

References

• Sheng Liang, The Java Native Interace — Programmer's Guide and Specification, Addison Wesley.

Frequent Caveats

From :

Don't cache JNIEnv between threads

Each thread gets its own value of JNIEnv

Don't truncate jboolean arguments

jboolean type is actually a 8-bit type, so value like 256/512 would actually be treated like false

Don't mix thread models (green thread vs native thread) if models are different

Cache field / Method ID at the time class is created

Better performance, and avoid using the wrong field/method if a sub-class that overrides some field/method is passed.

Use a same JNIEnv reference in different threads

A JNIEnv pointer is only valid in the thread associated with it. Don't pass this pointer from one thread to another, or cache and use it in multiple threads.

Pass local references to different threads

Local referneces are valid only in the thread that created them. Instead convert local references to global reference whenever there is a possibility that multiple threads may use the same reference.

JNI and Threads

Monitors

Entry and Exit

This block in java

synchronized (obj) {
    ...                             // synchronized block
}

is equivalent to native code

if ((*env)->MonitorEnter(env, obj) != JNI_OK) {
    ... /* error handling */
}
...                                /* synchronized block */
if ((*env)->MonitorExit(env, obj) != JNI_OK) {
    ... /* error handling */
};

MonitorEnter and MonitorExit work on jclass, jstring, jarray types.

Wait and Notify

There are no particular JNI functions for these. Instead native code can simply call the corresponding methods in the Java API. Example for wait():

/* precomputed method IDs */
static jmethodID MID_Object_wait;    /* same for _notify and ņotifyAll */

void
JNU_MonitorWait(JNIEnv *env, jobject object, jlong timeout)
{
    (*env)->CallVoidMethod(env, object, MID_Object_wait, timeout);
}

Obtain the JNIEnv Pointer

Obtaining a reference to JNIEnv is necessary in order to issue JNI function calls.

For native methods called from the JVM, this is easy since the the JNIEnv reference for the current thread is always passed as a parameter to the call.

In other circumstance, one must first call AttachCurrentThread to associate the current thread to an existing JVM. If the current native thread is not yet associated, AttachCurrentThead will create a new java.long.instance in the JVM, and returns the corresponding JNIEnv reference. Attached thread can then issue JNI function calls. Use DetachCurrentThread if the current thread does not need to make native calls anymore (doing so will enable clean-up and freeing resources).

Since Java 2 SDK release 1.2, a new method GetEnv allows to test whether the current thread is attached to a given VM instance, and to return the JNIEnv reference if it is the case. Note that GetEnv and AttachCurrentThread are functionally equivalent if the current thread is already attached to the VM.

Registering Native Methods

A given JNI method declared in java must be associated to a native method implementation before it is possible to call that method. This is the purpose of the native method registration.

The standard method is to rely on the standard java 2-step mechanism:

• System.loadLibrary to locate and load a named native library.
• The VM locates the native method implementation in one of the loaded native libraries (e.g. Foo.g() requires locating and linking the native method Java_Foo_g

Another method is to do the association manually:

void JNICALL g_impl(JNIEnv *env, jobject self);

//...

JNINativeMethod nm;
nm.name = "g";
/* method descriptor assigned to signature field */
nm.signature = "()V";
nm.fmPtr = g_impl;
(*env)->RegisterNatives(env, cls, &nm, 1);

The advantage of the manual method:

• The standard method is slow (lazy).
• The manual method allow to update the native method implementation at runtime.
• If the VM is launched by a native application, RegisterNatives allows to register native methods of that application, whereas the standard method only search native implementation in native libraries, not in the application itself.