JNI(1)

JNI是Android系统中Java与Native交互的桥梁

MediaRecorder举例

  • java：MediaRecorder.java
  • JNI：动态库libmedia_jni.so
  • Native：动态库libmedia.so （实际功能）

java层MediaRecorder

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//frameworks/base/media/java/android/media/MediaRecorder.java
public class MediaRecorder{
    static {
        //加载mediajni动态库
        System.loadLibrary("media_jni");
        native_init();
    }
    //两个native方法，交给jni处理
    private static native final void native_init();
    public native void start() throws IllegalStateException;  
}

JNI层的MediaRecorder

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//frameworks/base/media/jni/android_media_MediaRecorder.cpp
static void android_media_MediaRecorder_native_init(JNIEnv *env){
    jclass clazz;
    clazz = env->FindClass("android/media/MediaRecorder");
    if (clazz == NULL) {return;}
    ......
    fields.post_event = env->GetStaticMethodID(clazz, "postEventFromNative","(Ljava/lang/Object;IIILjava/lang/Object;)V");
    if (fields.post_event == NULL) {
        return;
    }
}

static void android_media_MediaRecorder_start(JNIEnv *env, jobject thiz){
    ALOGV("start");
    sp<MediaRecorder> mr = getMediaRecorder(env, thiz);
    process_media_recorder_call(env, mr->start(), "java/lang/RuntimeException", "start failed.");
}

native_init如何找到对应的android_media_MediaRecorder_native_init ？

JNI注册

静态注册

静态注册就是根据方法名，将Java方法和JNI方法建立关联，但是它有一些缺点：

  • JNI层的方法名称过长。
  • 声明Native方法的类需要用javah生成头文件。
  • 初次调用JIN方法时需要建立关联，影响效率。
动态注册

JNI中的JNINativeMethod，用来关联Java中的Native方法和JNI中的方法，它在jni.h中被定义：

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typedef struct {
    const char* name;//Java方法的名字
    const char* signature;//Java方法的签名信息
    void*       fnPtr;//JNI中对应的方法指针
} JNINativeMethod;

系统的MediaRecorder采用的就是动态注册，定义数组，然后注册

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//frameworks/base/media/jni/android_media_MediaRecorder.cpp
static const JNINativeMethod gMethods[] = {
    ......
    {"start",            "()V",      (void *)android_media_MediaRecorder_start},
    {"stop",             "()V",      (void *)android_media_MediaRecorder_stop},
    {"pause",            "()V",      (void *)android_media_MediaRecorder_pause},
    {"resume",           "()V",      (void *)android_media_MediaRecorder_resume},
    {"native_reset",     "()V",      (void *)android_media_MediaRecorder_native_reset},
    {"release",          "()V",      (void *)android_media_MediaRecorder_release},
    {"native_init",      "()V",      (void *)android_media_MediaRecorder_native_init},
   ......
};

//JNI_OnLoad in android_media_MediaPlayer.cpp
int register_android_media_MediaRecorder(JNIEnv *env){
    return AndroidRuntime::registerNativeMethods(env,
                "android/media/MediaRecorder", gMethods, NELEM(gMethods));
}

//frameworks/base/core/jni/AndroidRuntime.cpp
/*static*/ int AndroidRuntime::registerNativeMethods(JNIEnv* env,
    const char* className, const JNINativeMethod* gMethods, int numMethods){
    return jniRegisterNativeMethods(env, className, gMethods, numMethods);
}

//libnativehelper/JNIHelp.cpp
extern "C" int jniRegisterNativeMethods(JNIEnv* env, const char* className,
    const JNINativeMethod* gMethods, int numMethods){
   ......
    if (env->RegisterNatives(c.get(), gMethods, numMethods) < 0) {
        char* msg;
        (void)asprintf(&msg, "RegisterNatives failed for '%s'; aborting...", className);
        env->FatalError(msg);
    }
    return 0;
}

最终调用JNIEnv的RegisterNatives
从注释//JNI_OnLoad in android_media_MediaPlayer.cpp可以看到register_android_media_MediaRecorder的调用点

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//frameworks/base/media/jni/android_media_MediaPlayer.cpp
jint JNI_OnLoad(JavaVM* vm, void* /* reserved */){
    JNIEnv* env = NULL;
    jint result = -1;
    if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
        ALOGE("ERROR: GetEnv failed\n");
        goto *bail;
    }
    assert(env != NULL);
    ......
    if (register_android_media_MediaPlayer(env) < 0) {
        ALOGE("ERROR: MediaPlayer native registration failed\n");
        goto *bail;
    }
    if (register_android_media_MediaRecorder(env) < 0) {//1
        ALOGE("ERROR: MediaRecorder native registration failed\n");
        goto *bail;
    }
   ......
   result = JNI_VERSION_1_4;
bail:
    return result;
}

参考：

http://liuwangshu.cn/tags/Android%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3JNI/