Android 4.2 JNI机制(MediaScanner)

一、分析的文件路径
./frameworks/base/media/java/android/media/MediaScanner.java
./frameworks/base/media/jni/android_media_MediaScanner.cpp
./frameworks/base/media/jni/android_media_MediaPlayer.cpp
./frameworks/base/media/jni/AndroidRuntime.cpp
./libnativehelper/JNIHelp.cpp
二、代码分析
1、java层
MediaScanner.java

public class MediaScanner
{
    static {
    /*加载对应的JNI库*/
        System.loadLibrary("media_jni");
        native_init();//调用native_init()函数
    }
      ........
    private static native final void native_init();//声明一个native函数，native为关键字
    private native final void native_setup();
      ........
}

注：native_init函数位于android.media这个包中，其全路径名称为android.media.MediaScanner.nantive_init,根据规则其对应的JNI层函数名称为：android_media_MediaScanner_native_init。
2、JNI层
android_media_MediaScanner.cpp

static const char* const kClassMediaScanner =
        "android/media/MediaScanner";
     ........

/*native_init函数的JNI层实现*/
static void android_media_MediaScanner_native_init(JNIEnv *env)
{
    ALOGV("native_init");
    jclass clazz = env->FindClass(kClassMediaScanner);
    if (clazz == NULL) {
        return;
    }

    fields.context = env->GetFieldID(clazz, "mNativeContext", "I");
    if (fields.context == NULL) {
        return;
    }
}

    (1)JNI函数的注册
    Java native函数与JNI函数是一一对应关系，这里采用JNINativeMetho的结构体来记录这种关系。

typedef struct {
    const char* name;
    const char* signature;
    void* fnPtr;
} JNINativeMethod;

/*定义一个JNINativeMethod数组*/
    static JNINativeMethod gMethods[] = {
        {
            "processDirectory",
            "(Ljava/lang/String;Landroid/media/MediaScannerClient;)V",
            (void *)android_media_MediaScanner_processDirectory
        },
        .......
        {
            "native_init",//java中native函数的函数名
            "()V",
            /*JNI对应的函数指针*/
            (void *)android_media_MediaScanner_native_init
        },
        {
            "native_setup",
            "()V",
            (void *)android_media_MediaScanner_native_setup
        },
         .......
    };
/*注册JNINativeMethod数组*/
int register_android_media_MediaScanner(JNIEnv *env)
{
    return AndroidRuntime::(env,
                kClassMediaScanner, gMethods, NELEM(gMethods));
}

接着调用AndroidRuntime中的registerNativeMethods:
AndroidRuntime.cpp

/*static*/ int AndroidRuntime::registerNativeMethods(JNIEnv* env,
    const char* className, const JNINativeMethod* gMethods, int numMethods)
{
    return jniRegisterNativeMethods(env, className, gMethods, numMethods);
}

JNIHelp.cpp

extern "C" int jniRegisterNativeMethods(C_JNIEnv* env, const char* className,
    const JNINativeMethod* gMethods, int numMethods)
{
    JNIEnv* e = reinterpret_cast<JNIEnv*>(env);

    ALOGV("Registering %s natives", className);

    scoped_local_ref<jclass> c(env, findClass(env, className));
    if (c.get() == NULL) {
        ALOGE("Native registration unable to find class '%s', aborting", className);
        abort();
    }

    if ((*env)->RegisterNatives(e, c.get(), gMethods, numMethods) < 0) {
        ALOGE("RegisterNatives failed for '%s', aborting", className);
        abort();
    }

    return 0;
}

当Java层通过System.loadLibrary加载完JNI动态库后，紧接着会查找一个JNI_OnLoad的函数。如果有就调用它，动态注册的工作在这里完成。
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_MediaScanner(env) < 0) {
        ALOGE("ERROR: MediaScanner native registration failed\n");
        goto bail;
    }
   .......
    /* success -- return valid version number */
    result = JNI_VERSION_1_4;

bail:
    return result;
}