从使用JNI GetDirectBufferAddress

问题描述

我想了解如何使用 GetDirectBufferAddress 从JNI层。要理解我已经建立一个非常简单的例子：

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));
}

然后再从本地JNI层：

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;
}

如果我的OpenJDK运行这个例子在这里（的Debian / Linux的喘息AMD64）：

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)

下面是我看到的：

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

我理解为指数2和值; 3.但是，所有其他值不作任何意义，我，我本来期望是这样的：

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

我是怎么从ByteBuffer的使用在误解JNI？

What did I misunderstood from the ByteBuffer usage in JNI ?

推荐答案

我从文档错过的是，默认情况下 java.nio.ByteBuffer中实际上是用 BIG_ENDIAN 字节顺序。这也解释了，我看到我的 LITTLE_ENDIAN 系统上的行为。见参考文献（JDK-5043362解释：（BF）NewDirectByteBuffer总是为了ByteOrder.BIG_ENDIAN）

It appears that by default it is always BIG_ENDIAN, an not 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).

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