尊敬原創(chuàng)：http://blog.csdn.net/yuanzeyao/article/details/42418977
JNI技巧對(duì)于多java開(kāi)辟的同伙信賴并不陌生，即(java native interface)，本地調(diào)用接口，重要功能有以下兩點(diǎn)：
1、java層調(diào)用C/C++層代碼
2、C/C++層調(diào)用java層代碼
可能有些人會(huì)認(rèn)為jni技巧破壞了Java說(shuō)話的跨平臺(tái)性，有這種設(shè)法主意可能是因?yàn)槟銓?duì)java懂得得還不敷深，如不雅你看看jdk源碼，你會(huì)發(fā)明在jdk瑯綾擎大年夜量應(yīng)用了jni技巧，并且java虛擬機(jī)就是用本地說(shuō)話寫(xiě)的，所以導(dǎo)致jvm并不克不及跨平臺(tái)性，所以說(shuō)java的跨平臺(tái)性并不是100%的跨平臺(tái)的。相反你應(yīng)當(dāng)看到應(yīng)用Jni的優(yōu)勢(shì)：
1、因?yàn)镃/C++說(shuō)話本來(lái)機(jī)比java說(shuō)話出生早，所以很多庫(kù)代碼都是應(yīng)用C/C++寫(xiě)的，有了Jni我們就可以直接應(yīng)用了，不消反復(fù)造輪子。
2、弗成否定，C/C++履行效力比java 高，對(duì)于一些對(duì)效力有請(qǐng)求的功能，必須應(yīng)用C/C++.
因?yàn)楸P(pán)似揭捉究Android 中java層和native層是若何連接起來(lái)的，所以想研究一下Android中的jni技巧（在瀏覽之前，最好懂得jni中的根本常識(shí)，如jni中數(shù)據(jù)類(lèi)型，簽名格局，不然看起來(lái)可能有些吃力），因?yàn)楣ぷ骱蚆ediaPlayer有關(guān)，這里就應(yīng)用MediaPlayer為例吧。
下面給出一張圖，經(jīng)由過(guò)程此圖，我們扼要解釋一下jni是若何連接Java層和本地層的。
[img]http://img.blog.csdn.net/20150105104601352?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQveXVhbnpleWFv/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/Center
當(dāng)我們的app要播放視頻的時(shí)刻，我們應(yīng)用的是java層的MediaPlayer類(lèi)，我們進(jìn)入到MediaPlayer.java看看(提示：我這里應(yīng)用的是源碼4.1)
重要留意的有兩點(diǎn)：
1、靜態(tài)代碼塊：

 static {
        System.loadLibrary("media_jni");
        native_init();
    }

2、native_init的簽名：

private static native final void native_init();

看到靜態(tài)代碼塊后，我們可以知道MediaPlayer對(duì)應(yīng)的jni層代碼在Media_jni.so庫(kù)中
本地層對(duì)應(yīng)的so庫(kù)是libmedia.so，所以MediaPlayer.java經(jīng)由過(guò)程Media_jni.so和MediaPlayer.cpp(libmedia.so)進(jìn)行交互
下面我們就深刻到細(xì)節(jié)吧。不過(guò)在深刻細(xì)節(jié)前，我先要告訴你一個(gè)規(guī)矩，在Android中，平日java層類(lèi)和jni層類(lèi)的名字有如下關(guān)系，拿MediaPlayer為例，java層叫android.media.MediaPlayer.java，那么jni層叫做android_media_MediaPlayer.cpp
因?yàn)閚ative_init是一個(gè)本處所法，那么我們就到android_media_MediaPlayer.cpp找到native_init的對(duì)應(yīng)辦法吧

static void
android_media_MediaPlayer_native_init(JNIEnv *env)
{
    jclass clazz;

    clazz = env->FindClass("android/media/MediaPlayer");
    if (clazz == NULL) {
        return;
    }

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

    fields.post_event = env->GetStaticMethodID(clazz, "postEventFromNative",
                                               "(Ljava/lang/Object;IIILjava/lang/Object;)V");
    if (fields.post_event == NULL) {
        return;
    }

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

對(duì)應(yīng)膳綾擎的代碼，如不雅你對(duì)java中的反射懂得得很透辟的話，其實(shí)很好懂得，起首找到j(luò)ava層的MediaPlayer的Class對(duì)象，jclass是java層Class在native層的代碼，然后分別保存mNaviceContext字段，postEventFromNative辦法，mNativeSurfaceTexture字段。
其拭魅這里我最想解釋的是別的一個(gè)問(wèn)題，就是MediaPlayer中的native_init辦法時(shí)若何跟android_media_MediaPlayer.cpp中的android_media_MediaPlayer_native_init對(duì)應(yīng)起來(lái)的，因?yàn)槲覀冎廊绮谎艖?yīng)用javah主動(dòng)生成的頭文件，那么在jni層的名字應(yīng)當(dāng)是java_android_media_MediaPlayer_native_linit。其拭魅這里涉及到一個(gè)動(dòng)態(tài)注冊(cè)的過(guò)程。
其實(shí)袈溱java層代用System.loadLibrary成功后，就會(huì)調(diào)用jni文件中的JNI_onLoad辦法，android_media_MediaPlayer.cpp中的JNI_onLoad辦法如下(朝長(zhǎng)進(jìn)步部分)

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;
    }

  

    /* success -- return valid version number */
    result = JNI_VERSION_1_4;

bail:
    return result;
}

這里有一個(gè)辦法叫做register_android_media_MediaPlayer，我們進(jìn)入此辦法，看看注冊(cè)了什么

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

這里就是調(diào)用了AndroidRuntime供給的registerNativeMethods辦法，這里涉及到一個(gè)gMethods的變量，它其實(shí)是一個(gè)構(gòu)造體

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

name：就是在java層辦法名稱(chēng)
signature：就是辦法在簽名
fnPtr：在jni層對(duì)應(yīng)的函數(shù)名稱(chēng)
，那么我們找到native_init在gMethods對(duì)應(yīng)的值吧

static JNINativeMethod gMethods[] = {
    {
        "_setDataSource",
        "(Ljava/lang/String;[Ljava/lang/String;[Ljava/lang/String;)V",
        (void *)android_media_MediaPlayer_setDataSourceAndHeaders
    },

	....
    {"native_init",         "()V",                              (void *)android_media_MediaPlayer_native_init},
    ...
};

接下來(lái)，我們看看AndroidRuntime中的registerNativeMethods做了什么吧

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

調(diào)用了jniRegisterNativeMethods

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;
}

最終調(diào)用了env的RegisterNativers完成了注冊(cè)。
其實(shí)寫(xiě)到這里，我們已經(jīng)知道了java層和jni是若何接洽起來(lái)的，接下來(lái)我想說(shuō)的是jni是若何將java層和native接洽起來(lái)的，照樣用MediaPlayer為例吧，我們進(jìn)入MediaPlayer的構(gòu)造函數(shù)。

    public MediaPlayer() {

        Looper looper;
        if ((looper = Looper.myLooper()) != null) {
            mEventHandler = new EventHandler(this, looper);
        } else if ((looper = Looper.getMainLooper()) != null) {
            mEventHandler = new EventHandler(this, looper);
        } else {
            mEventHandler = null;
        }

        /* Native setup requires a weak reference to our object.
         * It's easier to create it here than in C++.
         */
        native_setup(new WeakReference<MediaPlayer>(this));
    }

這里創(chuàng)建了一個(gè)mEventHandler對(duì)象，并調(diào)用了native_setup辦法，我們進(jìn)入到android_media_MediaPlayer.cpp的對(duì)應(yīng)辦法看看

static void
android_media_MediaPlayer_native_setup(JNIEnv *env, jobject thiz, jobject weak_this)
{
    ALOGV("native_setup");
    sp<MediaPlayer> mp = new MediaPlayer();
    if (mp == NULL) {
        jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
        return;
    }

    // create new listener and give it to MediaPlayer
    sp<JNIMediaPlayerListener> listener = new JNIMediaPlayerListener(env, thiz, weak_this);
    mp->setListener(listener);

    // Stow our new C++ MediaPlayer in an opaque field in the Java object.
    setMediaPlayer(env, thiz, mp);
}

這里創(chuàng)建了一個(gè)本地MediaPlayer對(duì)象，并且設(shè)置了listener,(如不雅做過(guò)播放器的同窗應(yīng)當(dāng)知道這個(gè)listener應(yīng)當(dāng)知道干啥，不知道也沒(méi)緊要)，最后調(diào)用了setMediaPlayer辦法，這個(gè)才是我們須要存眷的。

static sp<MediaPlayer> setMediaPlayer(JNIEnv* env, jobject thiz, const sp<MediaPlayer>& player)
{
    Mutex::Autolock l(sLock);
    sp<MediaPlayer> old = (MediaPlayer*)env->GetIntField(thiz, fields.context);
    if (player.get()) {
        player->incStrong(thiz);
    }
    if (old != 0) {
        old->decStrong(thiz);
    }
    env->SetIntField(thiz, fields.context, (int)player.get());
    return old;
}

其實(shí)就是先拿到fields.context的對(duì)應(yīng)的值，還記得這個(gè)這個(gè)值是什么嗎，不記得的可以回到膳綾擎看看

fields.context = env->GetFieldID(clazz, "mNativeContext", "I");

其實(shí)就是java層mNativeContext對(duì)應(yīng)的值，就是將本地MediaPlayer的地址存放到mNativeContext中。
如今參加我們要播放一個(gè)本地Mp4視頻，那么應(yīng)用如下代碼即可

mediaPlayer.setDataSource("/mnt/sdcard/a.mp4");   
mediaPlayer.setDisplay(surface1.getHolder());  
mediaPlayer.prepare();  
mediaPlayer.start();  

其拭魅這里調(diào)用的 幾個(gè)都是本處所法，這里我就是用prepare辦法為例，講解MediaPlaeyr.java和MediaPlayer.cpp的交互
當(dāng)在java層調(diào)用prepare辦法時(shí)，在jni層會(huì)調(diào)用如下辦法

static void
android_media_MediaPlayer_prepare(JNIEnv *env, jobject thiz)
{
    sp<MediaPlayer> mp = getMediaPlayer(env, thiz);
    if (mp == NULL ) {
        jniThrowException(env, "java/lang/IllegalStateException", NULL);
        return;
    }

    // Handle the case where the display surface was set before the mp was
    // initialized. We try again to make it stick.
    sp<ISurfaceTexture> st = getVideoSurfaceTexture(env, thiz);
    mp->setVideoSurfaceTexture(st);

    process_media_player_call( env, thiz, mp->prepare(), "java/io/IOException", "Prepare failed." );
}

這里經(jīng)由過(guò)程getMediaPlayer辦法拿到本地的MediaPlayer對(duì)象，調(diào)用調(diào)用本處所法process_media_player_call，并將本地MediaPlayer調(diào)用parepare辦法的結(jié)不雅傳遞給此辦法。

static void process_media_player_call(JNIEnv *env, jobject thiz, status_t opStatus, const char* exception, const char *message)
{
    if (exception == NULL) {  // Don't throw exception. Instead, send an event.
        if (opStatus != (status_t) OK) {
            sp<MediaPlayer> mp = getMediaPlayer(env, thiz);
            if (mp != 0) mp->notify(MEDIA_ERROR, opStatus, 0);
        }
    } else {  // Throw exception!
        if ( opStatus == (status_t) INVALID_OPERATION ) {
            jniThrowException(env, "java/lang/IllegalStateException", NULL);
        } else if ( opStatus == (status_t) PERMISSION_DENIED ) {
            jniThrowException(env, "java/lang/SecurityException", NULL);
        } else if ( opStatus != (status_t) OK ) {
            if (strlen(message) > 230) {
               // if the message is too long, don't bother displaying the status code
               jniThrowException( env, exception, message);
            } else {
               char msg[256];
                // append the status code to the message
               sprintf(msg, "%s: status=0x%X", message, opStatus);
               jniThrowException( env, exception, msg);
            }
        }
    }
}

在這個(gè)瑯綾擎根據(jù)prepare返回的狀況，如不雅exception==null 并且prepare履行掉敗，測(cè)試不拋異常，而是調(diào)用本地MediaPlayer的notify辦法。

void MediaPlayer::notify(int msg, int ext1, int ext2, const Parcel *obj)
{
    ALOGV("message received msg=%d, ext1=%d, ext2=%d", msg, ext1, ext2);
    bool send = true;
    bool locked = false;

   ...

    switch (msg) {
    case MEDIA_NOP: // interface test message
        break;
    case MEDIA_PREPARED:
        ALOGV("prepared");
        mCurrentState = MEDIA_PLAYER_PREPARED;
        if (mPrepareSync) {
            ALOGV("signal application thread");
            mPrepareSync = false;
            mPrepareStatus = NO_ERROR;
            mSignal.signal();
        }
        break;
    case MEDIA_PLAYBACK_COMPLETE:
        ALOGV("playback complete");
        if (mCurrentState == MEDIA_PLAYER_IDLE) {
            ALOGE("playback complete in idle state");
        }
        if (!mLoop) {
            mCurrentState = MEDIA_PLAYER_PLAYBACK_COMPLETE;
        }
        break;
    case MEDIA_ERROR:
        // Always log errors.
        // ext1: Media framework error code.
        // ext2: Implementation dependant error code.
        ALOGE("error (%d, %d)", ext1, ext2);
        mCurrentState = MEDIA_PLAYER_STATE_ERROR;
        if (mPrepareSync)
        {
            ALOGV("signal application thread");
            mPrepareSync = false;
            mPrepareStatus = ext1;
            mSignal.signal();
            send = false;
        }
        break;
    case MEDIA_INFO:
        // ext1: Media framework error code.
        // ext2: Implementation dependant error code.
        if (ext1 != MEDIA_INFO_VIDEO_TRACK_LAGGING) {
            ALOGW("info/warning (%d, %d)", ext1, ext2);
        }
        break;
    case MEDIA_SEEK_COMPLETE:
        ALOGV("Received seek complete");
        if (mSeekPosition != mCurrentPosition) {
            ALOGV("Executing queued seekTo(%d)", mSeekPosition);
            mSeekPosition = -1;
            seekTo_l(mCurrentPosition);
        }
        else {
            ALOGV("All seeks complete - return to regularly scheduled program");
            mCurrentPosition = mSeekPosition = -1;
        }
        break;
    case MEDIA_BUFFERING_UPDATE:
        ALOGV("buffering %d", ext1);
        break;
    case MEDIA_SET_VIDEO_SIZE:
        ALOGV("New video size %d x %d", ext1, ext2);
        mVideoWidth = ext1;
        mVideoHeight = ext2;
        break;
    case MEDIA_TIMED_TEXT:
        ALOGV("Received timed text message");
        break;
    default:
        ALOGV("unrecognized message: (%d, %d, %d)", msg, ext1, ext2);
        break;
    }

    sp<MediaPlayerListener> listener = mListener;
    if (locked) mLock.unlock();

    // this prevents re-entrant calls into client code
    if ((listener != 0) && send) {
        Mutex::Autolock _l(mNotifyLock);
        ALOGV("callback application");
        listener->notify(msg, ext1, ext2, obj);
        ALOGV("back from callback");
    }
}

做過(guò)播放器的同窗應(yīng)當(dāng)對(duì)膳綾擎幾個(gè)消息都不陌生吧，因?yàn)閯偛耪{(diào)用prepare辦法掉敗了，所以這里應(yīng)當(dāng)履行MEDIA_ERROR分支，最后調(diào)用listener的notify代碼，這個(gè)listener就是在native_setup中設(shè)置的

void JNIMediaPlayerListener::notify(int msg, int ext1, int ext2, const Parcel *obj)
{
    JNIEnv *env = AndroidRuntime::getJNIEnv();
    if (obj && obj->dataSize() > 0) {
        jobject jParcel = createJavaParcelObject(env);
        if (jParcel != NULL) {
            Parcel* nativeParcel = parcelForJavaObject(env, jParcel);
            nativeParcel->setData(obj->data(), obj->dataSize());
            env->CallStaticVoidMethod(mClass, fields.post_event, mObject,
                    msg, ext1, ext2, jParcel);
        }
    } else {
        env->CallStaticVoidMethod(mClass, fields.post_event, mObject,
                msg, ext1, ext2, NULL);
    }
    if (env->ExceptionCheck()) {
        ALOGW("An exception occurred while notifying an event.");
        LOGW_EX(env);
        env->ExceptionClear();
    }
}

還記得fields.post_event保存的是什么嗎

fields.post_event = env->GetStaticMethodID(clazz, "postEventFromNative",
                                               "(Ljava/lang/Object;IIILjava/lang/Object;)V");

就是java層MediaPlayer的postEventFromNative辦法，也就是說(shuō)如不雅播放掉足了，那么就經(jīng)由過(guò)程調(diào)用postEventFromNative辦法來(lái)告訴java層的MediaPlayer。

    private static void postEventFromNative(Object mediaplayer_ref,
                                            int what, int arg1, int arg2, Object obj)
    {
        MediaPlayer mp = (MediaPlayer)((WeakReference)mediaplayer_ref).get();
        if (mp == null) {
            return;
        }

        if (what == MEDIA_INFO && arg1 == MEDIA_INFO_STARTED_AS_NEXT) {
            // this acquires the wakelock if needed, and sets the client side state
            mp.start();
        }
        if (mp.mEventHandler != null) {
            Message m = mp.mEventHandler.obtainMessage(what, arg1, arg2, obj);
            mp.mEventHandler.sendMessage(m);
        }
    }

這個(gè)時(shí)光最終經(jīng)由過(guò)程mEventHandler處理，也就是在我們app過(guò)程中處理這個(gè)缺點(diǎn)。
寫(xiě)到這里，信賴你應(yīng)當(dāng)對(duì)java層和native層的交淮竽暌剮了導(dǎo)致的懂得。