使用JNI中的GetDirectBufferAddress

时间:2015-04-20 10:39:21

标签: java c java-native-interface

我试图了解如何使用JNI图层中的GetDirectBufferAddress。要理解我构建了一个非常简单的例子:

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));
}

然后从原生JNI层:

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;
}

如果我用openjdk在这里运行这个例子(debian / linux wheezy amd64):

$ 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)

以下是我看到的内容:

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

我理解索引2&的值。但是所有其他价值观对我来说都没有任何意义,我会期待类似的东西:

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

我从JNI中的ByteBuffer用法中误解了什么?

1 个答案:

答案 0 :(得分:8)

我从文档中遗漏的是默认情况下java.nio.ByteBuffer实际上使用BIG_ENDIAN字节顺序。这解释了我在LITTLE_ENDIAN系统上看到的行为。请参阅参考here

我的代码现在读作:

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

默认情况下,它始终为BIG_ENDIAN,并且目前尚无法为LITTLE_ENDIAN提供API,如错误报告here(JDK-中所述) 5043362:(bf)NewDirectByteBuffer总是有订单ByteOrder.BIG_ENDIAN)。