Firstly, I want to say that the old answers doesn't work (for example, GetDirectBufferAddress function in below answers want one parameter now) at the moment as here:
JNI - native method with ByteBuffer parameter
and here,
how to write and read from bytebuffer passing from java to jni
It would be better if someone helps..
So, I can't send correctly my ByteBuffer, which's some elements filled, to C from Java using JNI and I can't return that ByteBuffer's elements again to C
My native function decleration:
public native int myfunc(ByteBuffer pkt);
Allocation for it
private ByteBuffer pkt = ByteBuffer.allocate(1000);
I am calling it in this way:
System.out.println(myfunc(pkt)); // Doesn't works, throws exception
pkt.position(0);
System.out.println(pkt.get()); // works, when I do comment line above .println
And my C codes as below:
JNIEXPORT jint JNICALL Java_xxx_myfunc(JNIEnv *, jobject, jobject); // header
JNIEXPORT jint JNICALL Java_xxx_myfunc(JNIEnv *env, jobject obj, jobject pkt) // function
{
jbyte *buff = (jbyte *) env->GetDirectBufferAddress(pkt);
// buff[0] = 0; I've also tried in this way
return buff[0];
//return 1; if I return 1, it returns correctly
}
when I run the java program it throws exception. How can I return my filled ByteBuffer values from C?
EDIT
A fatal error has been detected by the Java Runtime Environment:
EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x719c16b8, pid=1096, tid=1900
To use GetDirectBufferAddress()
you must guarantee that pkt is direct (you can check this with isDirect()). The easy way to obtain such object is ByteBuffer.allocateDirect()
. Direct ByteBuffer can be created from C++, too.
The difference between direct and indirect ByteBuffer is clearly explained in the Java oficial doc.
Update I see your update. No, ByteBuffer.allocate(1000)
does not produce a DirectByteBuffer.
As @Tom Blodget correctly noted, the JNI spec explicitly says that GetDirectBufferAddress()
can return NULL. This works both ways: the call itself should not crash if pkt is an indirect ByteBuffer, but you must check the result even if you are sure that pkt is a DirectByteBuffer.
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