java - Using GetDirectBufferAddress from JNI

Question

I am trying to understand how to use GetDirectBufferAddress from the JNI layer. To understand I've build a very simple example:

public class my_image_info {
  static {
      System.loadLibrary("my_jni");
  }
  private java.nio.ByteBuffer image_info_bb;
  native static void initc( java.nio.ByteBuffer bb );
  my_image_info()
    {
    image_info_bb = java.nio.ByteBuffer.allocateDirect( 5 * 4 );
    initc( image_info_bb );
    }
  public java.nio.ByteBuffer getBB() {
    return image_info_bb;
  }
  static public void main(String argv[]) {
    my_image_info fii = new my_image_info();
    java.nio.ByteBuffer bb = fii.getBB();
    System.out.println("1: " + bb.getInt(0));
    System.out.println("2: " + bb.getInt(4));
    System.out.println("3: " + bb.getInt(8));
    System.out.println("4: " + bb.getInt(12));
    System.out.println("5: " + bb.getInt(16));
}

And then from the native JNI layer:

JNIEXPORT void JNICALL Java_my_1image_1info_initc
  (JNIEnv *env, jclass cls, jobject jobj)
{
  int *iBuf = (*env)->GetDirectBufferAddress(env, jobj);
  iBuf[0] = -2;
  iBuf[1] = -1;
  iBuf[2] = 0;
  iBuf[3] = 1;
  iBuf[4] = 2;
}

If I run this example over here (debian/linux wheezy amd64) with openjdk :

$ java -version
java version "1.6.0_34"
OpenJDK Runtime Environment (IcedTea6 1.13.6) (6b34-1.13.6-1~deb7u1)
OpenJDK 64-Bit Server VM (build 23.25-b01, mixed mode)

Here is what I see:

1: -16777217
2: -1
3: 0
4: 16777216
5: 33554432

I understand the values for index 2 & 3. But all other values do not make any sense to me, I would have expected something like:

1: -2
2: -1
3: 0
4: 1
5: 2

What did I misunderstood from the ByteBuffer usage in JNI?

Answer 1

What I missed from the documentation is that by default java.nio.ByteBuffer is actually using BIG_ENDIAN byte order. Which explains the behavior I was seeing on my LITTLE_ENDIAN system. See ref here.

My code now reads as:

image_info_bb = java.nio.ByteBuffer.allocateDirect( 5 * 4 );
image_info_bb.order( java.nio.ByteOrder.LITTLE_ENDIAN );

It appears that by default it is always BIG_ENDIAN, and no effort has been made so far to provide an API for LITTLE_ENDIAN, as explained in the bug report here (JDK-5043362 : (bf) NewDirectByteBuffer always has order ByteOrder.BIG_ENDIAN).

---

Documentation has been updated recently to reflect that:

• JDK-8225152 : Release Note: JNI NewDirectByteBuffer Creates Direct Buffer That Is java.nio.ByteOrder.BIG_ENDIAN