JAVA通过Jni层与C++代码相互调用

Native.java

package com.ticktick.jnibuffer;

import java.nio.ByteBuffer;

import android.util.Log;

public class Native {
	
	public static final int TEST_BUFFER_SIZE = 128;

	private byte[] mTestBuffer1;
	private byte[] mTestBuffer2;
	private ByteBuffer mDirectBuffer;
	
	private native void nativeSetBuffer1(byte[] buffer,int len);
	private native void nativeSetBuffer2(byte[] buffer,int len);
	private native void nativeSetDirectBuffer(Object buffer, int len);
	private native byte[] nativeGetByteArray();
	
	public Native() {
		mTestBuffer1 = new byte[TEST_BUFFER_SIZE];
		mTestBuffer2 = new byte[TEST_BUFFER_SIZE];
		mDirectBuffer = ByteBuffer.allocateDirect(TEST_BUFFER_SIZE);
	}
	
	public void test() {
        
		nativeSetBuffer1(mTestBuffer1,TEST_BUFFER_SIZE);
        printBuffer("nativeInitNormalBuffer",mTestBuffer1);
                
        nativeSetBuffer2(mTestBuffer2,TEST_BUFFER_SIZE);
        printBuffer("nativeInitNormalBuffer2",mTestBuffer2);
        
        nativeSetDirectBuffer(mDirectBuffer,TEST_BUFFER_SIZE);
        printBuffer("nativeInitDirectBuffer",mDirectBuffer.array());
        
        byte[] buffer = nativeGetByteArray();        
        printBuffer("nativeGetByteArray",buffer);
	}
	
	private void printBuffer( String tag, byte[] buffer ) {
		StringBuffer sBuffer = new StringBuffer();
		for( int i=0; i<buffer.length; i++ ) {
			sBuffer.append(buffer[i]);
			sBuffer.append(" ");	
		}
		Log.d("Native", tag + sBuffer.toString());
	}
	
    static {
        System.loadLibrary("native");
    }
}

Jni基本的命令:

//编译JAVA类
javac Native.java
//生成对应的jni头文件
javah -jni Native
//查看各函数的标签
javap -s -p Native

Native.c

#include <stdio.h>
#include <stdlib.h>
#include <jni.h>
#include <android/log.h>

#define LOG(...) __android_log_print(ANDROID_LOG_DEBUG,"Native",__VA_ARGS__)

#define TEST_BUFFER_SIZE 128

/*
 * Class:     com_ticktick_jnibuffer_Native
 * Method:    nativeSetBuffer1
 * Signature: ([BI)V
 */
JNIEXPORT void JNICALL Java_com_ticktick_jnibuffer_Native_nativeSetBuffer1(JNIEnv *env, jobject thiz, jbyteArray buffer, jint len)
{
	unsigned char array[TEST_BUFFER_SIZE];

	//直接将Java端的数组拷贝到本地的数据中，建议使用这种方式，更加安全
	(*env)->GetByteArrayRegion(env, buffer, 0, len, array);

	//可以通过array来访问这段数组的值了,注意，修改的只是本地的值，Java端不会同时改变数组的值
	int i=0;
	for( i=0; i<TEST_BUFFER_SIZE; i++ ) {
		array[i] = i;
	}
}

/*
 * Class:     com_ticktick_jnibuffer_Native
 * Method:    nativeSetBuffer2
 * Signature: ([BI)V
 */
JNIEXPORT void JNICALL Java_com_ticktick_jnibuffer_Native_nativeSetBuffer2(JNIEnv *env, jobject thiz, jbyteArray buffer, jint len)
{
	//将本地指针指向含有Java端数组的内存地址，依赖Jvm的具体实现，可能是锁住Java端的那段数组不被回收（增加引用计数），
	//也可能所Jvm在堆上对该数组的一份拷贝
	//速度和效率比GetByteArrayRegion方法要高很多
	unsigned char * pBuffer = (*env)->GetByteArrayElements(env,buffer,NULL);
	if( pBuffer == NULL ) {
		LOG("GetByteArrayElements Failed!");
		return;
	}

	//可以通过pBuffer指针来访问这段数组的值了,注意，修改的是堆上的值，Java端可能会同步改变，依赖于Jvm的具体实现,不建议通过本方法改变Java端的数组值
	int i=0;
	for( i=0; i<TEST_BUFFER_SIZE; i++ ) {
		pBuffer[i] = i;
	}

	//最后不要忘记释放指针（减小引用计数）
	(*env)->ReleaseByteArrayElements(env,buffer,pBuffer,0);
}

/*
 * Class:     com_ticktick_jnibuffer_Native
 * Method:    nativeSetDirectBuffer
 * Signature: (Ljava/lang/Object;I)V
 */
JNIEXPORT void JNICALL Java_com_ticktick_jnibuffer_Native_nativeSetDirectBuffer(JNIEnv *env, jobject thiz, jobject buffer, jint len)
{
	//无需拷贝，直接获取与Java端共享的直接内存地址(效率比较高，但object的构造析构开销大，建议长期使用的大型buffer采用这种方式)
	unsigned char * pBuffer = (unsigned char *)(*env)->GetDirectBufferAddress(env,buffer);
	if( pBuffer == NULL ) {
		LOG("GetDirectBufferAddress Failed!");
		return;
	}

	//可以通过pBuffer指针来访问这段数组的值了,注意，修改数组的值后，Java端同时变化
	int i=0;
	for( i=0; i<TEST_BUFFER_SIZE; i++ ) {
		pBuffer[i] = i;
	}
}

/*
 * Class:     com_ticktick_jnibuffer_Native
 * Method:    nativeGetByteArray
 * Signature: ()[B
 */
JNIEXPORT jbyteArray JNICALL Java_com_ticktick_jnibuffer_Native_nativeGetByteArray(JNIEnv *env, jobject thiz)
{
	//传递JNI层的数组数据到Java端，有两种方法，一种是本例所示的通过返回值来传递
	//另一种是通过回调Java端的函数来传递(多用于jni线程中回调java层)
	unsigned char buffer[TEST_BUFFER_SIZE];

	int i=0;
	for( i=0; i<TEST_BUFFER_SIZE; i++ ) {
		buffer[i] = i;
	}

	//分配ByteArray
	jbyteArray array = (*env)->NewByteArray(env,TEST_BUFFER_SIZE);

	//将传递数据拷贝到java端
	(*env)->SetByteArrayRegion(env, array, 0, TEST_BUFFER_SIZE, buffer);

	return array;
}

JAVA通过Jni层与C++代码相互调用

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