通过MediaScanner理解JNI原理

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public class MediaScanner
{
    //加载对应的so库
    static {
        System.loadLibrary("media_jni");
        native_init();
    }

    //声明native方法
    private native void processDirectory(String path, String extensions, MediaScannerClient client);
    private native void processFile(String path, String mimeType, MediaScannerClient client);
    public native void setLocale(String locale);

    public native byte[] extractAlbumArt(FileDescriptor fd);

    private static native final void native_init();
    private native final void native_setup();
    private native final void native_finalize();
}

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static void
android_media_MediaScanner_processDirectory(
    JNIEnv *env, jobject thiz, jstring path, jstring extensions, jobject ent)
{
    MediaScanner *mp = (MediaScanner *)env->GetIntField(thiz, fields.context);

    if (path == NULL) {
        jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
        return;
    }
    if (extensions == NULL) {
        jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
        return;
    }

    const char *pathStr = env->GetStringUTFChars(path, NULL);
    if (pathStr == NULL) {  // Out of memory
        jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
        return;
    }
    const char *extensionsStr = env->GetStringUTFChars(extensions, NULL);
    if (extensionsStr == NULL) {  // Out of memory
        env->ReleaseStringUTFChars(path, pathStr);
        jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
        return;
    }

    MyMediaScannerClient myClient(env, client);
    mp->processDirectory(pathStr, extensionsStr, myClient, ExceptionCheck, env);
    env->ReleaseStringUTFChars(path, pathStr);
    env->ReleaseStringUTFChars(extensions, extensionsStr);
}

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typedef struct {
    const char* name;           //java中的native函数名
    const char* signature;      //java函数的签名信息，是参数和返回值类型的组合
    void*       fnPtr;          //JNI层对应的函数指针，注意他是void*类型
} JNINativeMethod;

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int register_android_media_MediaScanner(JNIEnv *env)
{
    return AndroidRuntime::registerNativeMethods(env,
                "android/media/MediaScanner", gMethods, NELEM(gMethods));
}

static JNINativeMethod gMethods[] = {
    {"processDirectory",  "(Ljava/lang/String;Ljava/lang/String;Landroid/media/MediaScannerClient;)V",
                                                        (void *)android_media_MediaScanner_processDirectory},
    {"processFile",       "(Ljava/lang/String;Ljava/lang/String;Landroid/media/MediaScannerClient;)V",
                                                        (void *)android_media_MediaScanner_processFile},
    {"setLocale",         "(Ljava/lang/String;)V",      (void *)android_media_MediaScanner_setLocale},
    {"extractAlbumArt",   "(Ljava/io/FileDescriptor;)[B",     (void *)android_media_MediaScanner_extractAlbumArt},
    {"native_init",        "()V",                      (void *)android_media_MediaScanner_native_init},
    {"native_setup",        "()V",                      (void *)android_media_MediaScanner_native_setup},
    {"native_finalize",     "()V",                      (void *)android_media_MediaScanner_native_finalize},
};

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jint JNI_OnLoad(JavaVM* vm, void* reserved)
{
    JNIEnv* env = NULL;
    jint result = -1;

    if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
        LOGE("ERROR: GetEnv failed\n");
        goto bail;
    }
    assert(env != NULL);

    if (register_android_media_MediaScanner(env) < 0) {
        LOGE("ERROR: MediaScanner native registration failed\n");
        goto bail;
    }

    //....省略
}

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private native void processFile(String path, String mimeType, MediaScannerClient client);

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static void
android_media_MediaScanner_processFile(
    JNIEnv *env, jobject thiz, jstring path, jstring mimeType, jobject client)
{
    //...
}

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/*
 * C++ object wrapper.
 *
 * This is usually overlaid on a C struct whose first element is a
 * JNINativeInterface*.  We rely somewhat on compiler behavior.
 */
struct _JNIEnv {
    /* do not rename this; it does not seem to be entirely opaque */
    const struct JNINativeInterface* functions;

    //...结构体内容太多，不方便贴出来
}

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struct _JNIEnv {
    /* do not rename this; it does not seem to be entirely opaque */
    const struct JNINativeInterface* functions;

#if defined(__cplusplus)

    //....

    jfieldID GetFieldID(jclass clazz, const char* name, const char* sig)
    { return functions->GetFieldID(this, clazz, name, sig); }

    jmethodID GetMethodID(jclass clazz, const char* name, const char* sig)
    { return functions->GetMethodID(this, clazz, name, sig); }

    //....
}

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MyMediaScannerClient(JNIEnv *env, jobject client)
    :   mEnv(env),
        mClient(env->NewGlobalRef(client)),
        mScanFileMethodID(0),
        mHandleStringTagMethodID(0),
        mSetMimeTypeMethodID(0)
{
    jclass mediaScannerClientInterface = env->FindClass("android/media/MediaScannerClient");
    if (mediaScannerClientInterface == NULL) {
        fprintf(stderr, "android/media/MediaScannerClient not found\n");
    }
    else {
        mScanFileMethodID = env->GetMethodID(mediaScannerClientInterface, "scanFile",
                                                 "(Ljava/lang/String;JJ)V");
        mHandleStringTagMethodID = env->GetMethodID(mediaScannerClientInterface, "handleStringTag",
                                                 "(Ljava/lang/String;Ljava/lang/String;)V");
        mSetMimeTypeMethodID = env->GetMethodID(mediaScannerClientInterface, "setMimeType",
                                                 "(Ljava/lang/String;)V");
        mAddNoMediaFolderMethodID = env->GetMethodID(mediaScannerClientInterface, "addNoMediaFolder",
                                                 "(Ljava/lang/String;)V");
    }
}

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// returns true if it succeeded, false if an exception occured in the Java code
virtual bool scanFile(const char* path, long long lastModified, long long fileSize)
{
    jstring pathStr;
    if ((pathStr = mEnv->NewStringUTF(path)) == NULL) return false;
    mEnv->CallVoidMethod(mClient, mScanFileMethodID, pathStr, lastModified, fileSize);
    mEnv->DeleteLocalRef(pathStr);
    return (!mEnv->ExceptionCheck());
}

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save_this = thiz;

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MyMediaScannerClient(JNIEnv *env, jobject client)
       :   mEnv(env),
           mClient(env->NewGlobalRef(client)),  //使用NewGlobalRef创建全局引用
           mScanFileMethodID(0),
           mHandleStringTagMethodID(0),
           mSetMimeTypeMethodID(0)
   {
      //....
   }

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virtual ~MyMediaScannerClient()
{
    mEnv->DeleteGlobalRef(mClient);  //释放全局引用
}

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// returns true if it succeeded, false if an exception occured in the Java code
virtual bool scanFile(const char* path, long long lastModified, long long fileSize)
{
    jstring pathStr;

    //使用NewStringUTF创建了一个本地引用的String对象
    if ((pathStr = mEnv->NewStringUTF(path)) == NULL) return false;

    //当执行完CallVoidMethod方法后pathStr对象会被回收。
    mEnv->CallVoidMethod(mClient, mScanFileMethodID, pathStr, lastModified, fileSize);

    //既然上面说了pathStr会回收，这个DeleteLocalRef函数是不是多余的呢？
    mEnv->DeleteLocalRef(pathStr);
    return (!mEnv->ExceptionCheck());
}