Android平台上的JNI技术介绍

package com.example.hellojni;
import android.app.Activity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.os.Bundle;
import android.view.View.OnClickListener;

public class HelloJni extends Activity
{
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        Button btn = (Button)findViewById(R.id.btn);
        final TextView txtv = (TextView)findViewById(R.id.txtv);
        btn.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                txtv.setText(stringFromJNI());//调用native函数

            }
        });
    }

    /* A native method that is implemented by the
     * "hello-jni" native library, which is packaged
     * with this application.
     * 声明含有native关键词的函数，就可以在类中使用了。
     */
    public native String  stringFromJNI();

    /* 
     * 该函数并没有在共享库中实现，但是仍然可以声明。
     * 没有实现的native函数也可以在类中声明，native方法仅在首次调用时才开始搜索。
     * 若没有找到该方法，会抛出java.lang.UnsatisfiedLinkError异常
     */
    public native String  unimplementedStringFromJNI();

    /* this is used to load the "hello-jni" library on application
     * startup. The library has already been unpacked into
     * /data/data/com.example.HelloJni/lib/libhello-jni.so at
     * installation time by the package manager.
     * 使用静态方式再创建类时就载入共享库，该共享库（后面会介绍）在程序安装后
     * 位于/data/data/com.example.HelloJni/lib/libhello-jni.so
     */
    static {
        System.loadLibrary("hello-jni");
    }
}

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_example_hellojni_HelloJni */

#ifndef _Included_com_example_hellojni_HelloJni
#define _Included_com_example_hellojni_HelloJni
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_example_hellojni_HelloJni
 * Method:    stringFromJNI
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_example_hellojni_HelloJni_stringFromJNI
  (JNIEnv *, jobject);

/*
 * Class:     com_example_hellojni_HelloJni
 * Method:    unimplementedStringFromJNI
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_example_hellojni_HelloJni_unimplementedStringFromJNI
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

#include <string.h>
#include <jni.h>
#include <stdio.h>

/* This is a trivial JNI example where we use a native method
 * to return a new VM String. See the corresponding Java source
 * file located at:
 *
 *   apps/samples/hello-jni/project/src/com/example/HelloJni/HelloJni.java
 */
jstring
Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env,
                                                  jobject thiz )
{
    char msg[100];
    sprintf(msg,"Hello from JNI.");
    return (*env)->NewStringUTF(env, msg);
}

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE    := hello-jni
LOCAL_SRC_FILES := hello-jni2.c

include $(BUILD_SHARED_LIBRARY)

Gdbserver      : [arm-linux-androideabi-4.4.3] libs/armeabi/gdbserver
Gdbsetup       : libs/armeabi/gdb.setup
Compile thumb  : hello-jni <= hello-jni.c
SharedLibrary  : libhello-jni.so
Install        : libhello-jni.so => libs/armeabi/libhello-jni.so

#include <string.h>
#include <jni.h>
#include <stdio.h>

jstring getHelloString();

static JNINativeMethod gMethods[] = {
    {
    "stringFromJNI",
    "()Ljava/lang/String;",
    (void *)getHelloString
    }
};

static int nMethods = 1;
static JNIEnv *env = NULL;

jstring getHelloString()
{
    char msg[100];
    sprintf(msg,"Hello from JNI.");
    return (*env)->NewStringUTF(env, msg);
}

jint JNI_OnLoad(JavaVM *vm,void *reserved){

    jint result = -1;
    jclass clz = NULL;
    if ((*vm)->GetEnv(vm,(void**) &env, JNI_VERSION_1_4) != JNI_OK){
        return -1;
    }
    clz = (*env)->FindClass(env,"com/example/hellojni/HelloJni");
    if((*env)->RegisterNatives(env,clz,gMethods,nMethods) < 0) {
        return -1;
    }
    return JNI_VERSION_1_4;//根据JNI规范，JNI_OnLoad必须返回版本号常量否则出错。
}

`RegisterNatives` 用来注册一组native函数，其中使用到了 `JNINativeMethod` 结构，具体定义如下3：

    typedef struct {

        char *name; //Java代码中声明的native函数的名称

        char *signature; //对应Java代码层native函数的签名，下面会介绍

        void *fnPtr; //共享库中函数指针

    } JNINativeMethod;