jni jnienv NewDirectByteBuffer example c c++ java

NewDirectByteBuffer

jobject NewDirectByteBuffer(JNIEnv* env, void* address, jlong capacity);

Allocates and returns a direct java.nio.ByteBuffer referring to the block of memory starting at the memory address address and extending capacity bytes. (NewDirectByteBuffer)

Native code that calls this function and returns the resulting byte-buffer object to Java-level code should ensure that the buffer refers to a valid region of memory that is accessible for reading and, if appropriate, writing. An attempt to access an invalid memory location from Java code will either return an arbitrary value, have no visible effect, or cause an unspecified exception to be thrown.

LINKAGE:

Index 229 in the JNIEnv interface function table.

PARAMETERS of NewDirectByteBuffer

env: the JNIEnv interface pointer

address: the starting address of the memory region (must not be NULL)

capacity: the size in bytes of the memory region (must be positive)

RETURNS of NewDirectByteBuffer

Returns a local reference to the newly-instantiated java.nio.ByteBuffer object. Returns NULL if an exception occurs, or if JNI access to direct buffers is not supported by this virtual machine.

EXCEPTIONS:

OutOfMemoryError: if allocation of the ByteBuffer object fails

Example of NewDirectByteBuffer

JNIEXPORT jobject JNICALL Java_com_foo_allocNativeBuffer(JNIEnv* env, jobject thiz, jlong size)
{
    void* buffer = malloc(size);
    jobject directBuffer = env->NewDirectByteBuffer(buffer, size);
    jobject globalRef = env->NewGlobalRef(directBuffer);

    return globalRef;
}

JNIEXPORT void JNICALL Java_comfoo_freeNativeBuffer(JNIEnv* env, jobject thiz, jobject globalRef)
{
    void *buffer = env->GetDirectBufferAddress(globalRef);

    env->DeleteGlobalRef(globalRef);
    free(buffer);
}

Then in Java...

mImageData = (ByteBuffer)allocNativeBuffer( mResX * mResY * mBPP );

and

freeNativeBuffer(mImageData);
mImageData = null;