目录
- 参考
- 概述
- MediaPlayer与native层的交互流程
1. 参考
2. 概述
MediaPlayer是Android播放器的组件，先回顾一下音视频播放的流程：
数据获取->数据解析->音/视频解码->同步控制->渲染
image.png
android media的框架图[2]
image.png
- 播放服务是通过在独立进程的MediaPlayerService提供的。
- MediaPlayerService的实现在native层。
本文主要分析MediaPlayer是怎么与native层交互的，不涉及播放流程。
3. MediaPlayer与native的交互流程
MediaPlayer_native_workflow.png
交互流程
- MediaPlayer类加载的时机
(1) 加载media_jni.so
(2) Java和native的方法进行绑定。
//MediaPlayer.java
static {
System.loadLibrary("media_jni");
native_init();
}
//android_media_MediaPlayer.cpp
static const JNINativeMethod gMethods[] = {
{
"nativeSetDataSource",
"(Landroid/os/IBinder;Ljava/lang/String;[Ljava/lang/String;"
"[Ljava/lang/String;)V",
(void *)android_media_MediaPlayer_setDataSourceAndHeaders
},
...
}
static int register_android_media_MediaPlayer(JNIEnv *env)
{
return AndroidRuntime::registerNativeMethods(env,
"android/media/MediaPlayer", gMethods, NELEM(gMethods));
}
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;
}
//android_media_MediaPlayer.cpp
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", "J");
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;
}
...
}
- MediaPlayer创建的时机
(1) 创建C++的MediaPlayer对象，设置listener。
(2) C++保存Java MediaPlayer对象的一个弱引用，调用MediaPlayer的static postEventFromNative方法时使用。
(3) Java层的MediaPlayer增加对新创建的C++的MediaPlayer的强引用sp。
//MediaPlayer.java
public MediaPlayer() {
...
native_setup(new WeakReference<MediaPlayer>(this));
...
}
//android_media_MediaPlayer.cpp
static void
android_media_MediaPlayer_native_setup(JNIEnv *env, jobject thiz, jobject weak_this)
{
sp<MediaPlayer> mp = new MediaPlayer();
if (mp == NULL) {
jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
return;
}
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);
}
static sp<MediaPlayer> setMediaPlayer(JNIEnv* env, jobject thiz, const sp<MediaPlayer>& player)
{
Mutex::Autolock l(sLock);
sp<MediaPlayer> old = (MediaPlayer*)env->GetLongField(thiz, fields.context);
if (player.get()) {
player->incStrong((void*)setMediaPlayer);
}
if (old != 0) {
old->decStrong((void*)setMediaPlayer);
}
env->SetLongField(thiz, fields.context, (jlong)player.get());
return old;
}
- Java层的MediaPlayer的
private long mNativeContext; // accessed by native methods成员保存了native层c++的MediaPlayer对象的指针。
- Native层通知事件给Java层
//android_media_MediaPlayer.cpp
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);
env->DeleteLocalRef(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();
}
}
//MediaPlayer.java
private static void postEventFromNative(Object mediaplayer_ref,
int what, int arg1, int arg2, Object obj)
{
final MediaPlayer mp = (MediaPlayer)((WeakReference)mediaplayer_ref).get();
if (mp == null) {
return;
}
switch (what) {
case MEDIA_INFO:
if (arg1 == MEDIA_INFO_STARTED_AS_NEXT) {
new Thread(new Runnable() {
@Override
public void run() {
// this acquires the wakelock if needed, and sets the client side state
mp.start();
}
}).start();
Thread.yield();
}
break;
...
}
- 其他一些native方法的调用
比如Java MediaPlayer的nativeSetDataSource方法绑定到了native的android_media_MediaPlayer_setDataSourceAndHeaders方法。
(1) 获取Java的MediaPlayer持有的C++ MediaPlayer的引用。
(2) JNI数据结构和native数据结构的转换。
(3) 调用C++ MediaPlayer的对应方法。
(4) 异常抛给Java层。
//android_media_MediaPlayer.cpp
static sp<MediaPlayer> getMediaPlayer(JNIEnv* env, jobject thiz)
{
Mutex::Autolock l(sLock);
MediaPlayer* const p = (MediaPlayer*)env->GetLongField(thiz, fields.context);
return sp<MediaPlayer>(p);
}
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) BAD_VALUE ) {
jniThrowException(env, "java/lang/IllegalArgumentException", 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);
}
}
}
}
static void
android_media_MediaPlayer_setDataSourceAndHeaders(
JNIEnv *env, jobject thiz, jobject httpServiceBinderObj, jstring path,
jobjectArray keys, jobjectArray values) {
sp<MediaPlayer> mp = getMediaPlayer(env, thiz);
if (mp == NULL ) {
jniThrowException(env, "java/lang/IllegalStateException", NULL);
return;
}
if (path == NULL) {
jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
return;
}
const char *tmp = env->GetStringUTFChars(path, NULL);
if (tmp == NULL) { // Out of memory
return;
}
String8 pathStr(tmp);
env->ReleaseStringUTFChars(path, tmp);
tmp = NULL;
// We build a KeyedVector out of the key and val arrays
KeyedVector<String8, String8> headersVector;
if (!ConvertKeyValueArraysToKeyedVector(
env, keys, values, &headersVector)) {
return;
}
sp<IMediaHTTPService> httpService;
if (httpServiceBinderObj != NULL) {
sp<IBinder> binder = ibinderForJavaObject(env, httpServiceBinderObj);
httpService = interface_cast<IMediaHTTPService>(binder);
}
status_t opStatus =
mp->setDataSource(
httpService,
pathStr,
headersVector.size() > 0? &headersVector : NULL);
process_media_player_call(
env, thiz, opStatus, "java/io/IOException",
"setDataSource failed." );
}
