Android 多媒体框架（2）—— android_media_MediaCodec.cpp源码分析
android_media_MediaCodec.cpp源码分析
这里我们来分析和MediaCodec.java对应的native层类：android_media_MediaCodec.cpp的源代码。
在该类的最后，我们会看到这样的代码：
static const JNINativeMethod gMethods[] = {
1881 { "native_release", "()V", (void *)android_media_MediaCodec_release },
1882
1883 { "native_reset", "()V", (void *)android_media_MediaCodec_reset },
1884
1885 { "native_releasePersistentInputSurface",
1886 "(Landroid/view/Surface;)V",
1887 (void *)android_media_MediaCodec_releasePersistentInputSurface},
1888
1889 { "native_createPersistentInputSurface",
1890 "()Landroid/media/MediaCodec$PersistentSurface;",
1891 (void *)android_media_MediaCodec_createPersistentInputSurface },
1892
1893 { "native_setInputSurface", "(Landroid/view/Surface;)V",
1894 (void *)android_media_MediaCodec_setInputSurface },
1895
1896 { "native_enableOnFrameRenderedListener", "(Z)V",
1897 (void *)android_media_MediaCodec_native_enableOnFrameRenderedListener },
1898
1899 { "native_setCallback",
1900 "(Landroid/media/MediaCodec$Callback;)V",
1901 (void *)android_media_MediaCodec_native_setCallback },
1902
1903 { "native_configure",
1904 "([Ljava/lang/String;[Ljava/lang/Object;Landroid/view/Surface;"
1905 "Landroid/media/MediaCrypto;I)V",
1906 (void *)android_media_MediaCodec_native_configure },
1907
1908 { "native_setSurface",
1909 "(Landroid/view/Surface;)V",
1910 (void *)android_media_MediaCodec_native_setSurface },
1911
1912 { "createInputSurface", "()Landroid/view/Surface;",
1913 (void *)android_media_MediaCodec_createInputSurface },
1914
1915 { "native_start", "()V", (void *)android_media_MediaCodec_start },
1916 { "native_stop", "()V", (void *)android_media_MediaCodec_stop },
1917 { "native_flush", "()V", (void *)android_media_MediaCodec_flush },
1918
1919 { "native_queueInputBuffer", "(IIIJI)V",
1920 (void *)android_media_MediaCodec_queueInputBuffer },
1921
1922 { "native_queueSecureInputBuffer", "(IILandroid/media/MediaCodec$CryptoInfo;JI)V",
1923 (void *)android_media_MediaCodec_queueSecureInputBuffer },
1924
1925 { "native_dequeueInputBuffer", "(J)I",
1926 (void *)android_media_MediaCodec_dequeueInputBuffer },
1927
1928 { "native_dequeueOutputBuffer", "(Landroid/media/MediaCodec$BufferInfo;J)I",
1929 (void *)android_media_MediaCodec_dequeueOutputBuffer },
1930
1931 { "releaseOutputBuffer", "(IZZJ)V",
1932 (void *)android_media_MediaCodec_releaseOutputBuffer },
1933
1934 { "signalEndOfInputStream", "()V",
1935 (void *)android_media_MediaCodec_signalEndOfInputStream },
1936
1937 { "getFormatNative", "(Z)Ljava/util/Map;",
1938 (void *)android_media_MediaCodec_getFormatNative },
1939
1940 { "getOutputFormatNative", "(I)Ljava/util/Map;",
1941 (void *)android_media_MediaCodec_getOutputFormatForIndexNative },
1942
1943 { "getBuffers", "(Z)[Ljava/nio/ByteBuffer;",
1944 (void *)android_media_MediaCodec_getBuffers },
1945
1946 { "getBuffer", "(ZI)Ljava/nio/ByteBuffer;",
1947 (void *)android_media_MediaCodec_getBuffer },
1948
1949 { "getImage", "(ZI)Landroid/media/Image;",
1950 (void *)android_media_MediaCodec_getImage },
1951
1952 { "getName", "()Ljava/lang/String;",
1953 (void *)android_media_MediaCodec_getName },
1954
1955 { "setParameters", "([Ljava/lang/String;[Ljava/lang/Object;)V",
1956 (void *)android_media_MediaCodec_setParameters },
1957
1958 { "setVideoScalingMode", "(I)V",
1959 (void *)android_media_MediaCodec_setVideoScalingMode },
1960
1961 { "native_init", "()V", (void *)android_media_MediaCodec_native_init },
1962
1963 { "native_setup", "(Ljava/lang/String;ZZ)V",
1964 (void *)android_media_MediaCodec_native_setup },
1965
1966 { "native_finalize", "()V",
1967 (void *)android_media_MediaCodec_native_finalize },
1968};
这些就是在java层和native层代码的一个对应列表。通过下面这个函数，实现natvie方法的注册：
int register_android_media_MediaCodec(JNIEnv *env) {
1971 return AndroidRuntime::registerNativeMethods(env,
1972 "android/media/MediaCodec", gMethods, NELEM(gMethods));
1973}
接下来我们逐次分析MediaCodec各个阶段对应的native方法。首先要看的就是configure方法。
- configure
static void android_media_MediaCodec_native_configure(
956 JNIEnv *env,
957 jobject thiz,
958 jobjectArray keys, jobjectArray values,
959 jobject jsurface,
960 jobject jcrypto,
961 jint flags) {
962 sp codec = getMediaCodec(env, thiz);
963
964 if (codec == NULL) {
965 throwExceptionAsNecessary(env, INVALID_OPERATION);
966 return;
967 }
968
969 sp format;
970 status_t err = ConvertKeyValueArraysToMessage(env, keys, values, &format);
971
972 if (err != OK) {
973 jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
974 return;
975 }
976
977 sp bufferProducer;
978 if (jsurface != NULL) {
979 sp surface(android_view_Surface_getSurface(env, jsurface));
980 if (surface != NULL) {
981 bufferProducer = surface->getIGraphicBufferProducer();
982 } else {
983 jniThrowException(
984 env,
985 "java/lang/IllegalArgumentException",
986 "The surface has been released");
987 return;
988 }
989 }
990
991 sp crypto;
992 if (jcrypto != NULL) {
993 crypto = JCrypto::GetCrypto(env, jcrypto);
994 }
995
996 err = codec->configure(format, bufferProducer, crypto, flags);
997
998 throwExceptionAsNecessary(env, err);
999}
1000
通过getMediaCodec方法，获取一个JMediaCodec的强指针codec。如果codec为NULL，就抛出一个INVALID_OPERATION异常。定义AMessage的强引用format，通过ConvertKeyValueArraysToMessage(env, keys, values, &format)，将keys和values转换为配置信息，并且将format信息，存储到format中。如果方法返回error，就抛出IllegalArgumentException异常，并且从android_media_MediaCodec_native_configure返回。定义IGraphicBufferProducer的强引用bufferProducer，如果参数jsurface不等于NULL，通过android_view_Surface_getSurface(env, jsurface)获取一个Surface的强引用surface，如果surface不等于NULL，调用surface->getIGraphicBufferProducer()，把返回值赋值给bufferProducer否则，如果surface等于NULL，抛出IllegalArgumentException异常。定义ICrypto的强引用crypto，如果参数jcrypto不为NULL，调用JCrypto::GetCrypto(env, jcrypto)，把值返回给crypto。这是针对视频加密的情况。最后，通过codec->configure(format, bufferProducer, crypto, flags)实现对MediaCodec的配置，其中，format，bufferProducer，crypto是之前生成的，flags是传入的参数。上边的调用会返回错误信息，throwExceptionAsNecessary(env, err)会根据返回的具体错误信息，决定是否抛出异常，以及抛出何种异常。
- start
下一个阶段是start。
static void android_media_MediaCodec_start(JNIEnv *env, jobject thiz) {
1168 ALOGV("android_media_MediaCodec_start");
1169
1170 sp codec = getMediaCodec(env, thiz);
1171
1172 if (codec == NULL) {
1173 throwExceptionAsNecessary(env, INVALID_OPERATION);
1174 return;
1175 }
1176
1177 status_t err = codec->start();
1178
1179 throwExceptionAsNecessary(env, err, ACTION_CODE_FATAL, "start failed");
1180}
通过getMediaCodec(env, thiz)获取JMediaCodec的强引用codec。如果codec为NULL，抛出INVALID_OPERATION异常。调用JMediaCodec的startstatus_t err = codec->start()，根据返回的err的状态，决定是否抛出异常，以及抛出何种异常。
- dequeueInputBuffer
下一个状态是dequeueInputBuffer。
static jint android_media_MediaCodec_dequeueInputBuffer(
1426 JNIEnv *env, jobject thiz, jlong timeoutUs) {
1427 ALOGV("android_media_MediaCodec_dequeueInputBuffer");
1428
1429 sp codec = getMediaCodec(env, thiz);
1430
1431 if (codec == NULL) {
1432 throwExceptionAsNecessary(env, INVALID_OPERATION);
1433 return -1;
1434 }
1435
1436 size_t index;
1437 status_t err = codec->dequeueInputBuffer(&index, timeoutUs);
1438
1439 if (err == OK) {
1440 return (jint) index;
1441 }
1442
1443 return throwExceptionAsNecessary(env, err);
1444}
过程基本和start类似，不再详述。值得注意的是这里会返回input buffer的索引。android_media_MediaCodec_dequeueOutputBuffer方法类似，只是返回output buffer的索引。
- queueInputBuffer
static void android_media_MediaCodec_queueInputBuffer(
1235 JNIEnv *env,
1236 jobject thiz,
1237 jint index,
1238 jint offset,
1239 jint size,
1240 jlong timestampUs,
1241 jint flags) {
1242 ALOGV("android_media_MediaCodec_queueInputBuffer");
1243
1244 sp codec = getMediaCodec(env, thiz);
1245
1246 if (codec == NULL) {
1247 throwExceptionAsNecessary(env, INVALID_OPERATION);
1248 return;
1249 }
1250
1251 AString errorDetailMsg;
1252
1253 status_t err = codec->queueInputBuffer(
1254 index, offset, size, timestampUs, flags, &errorDetailMsg);
1255
1256 throwExceptionAsNecessary(
1257 env, err, ACTION_CODE_FATAL, errorDetailMsg.empty() ? NULL : errorDetailMsg.c_str());
1258}
这里会的调用JMediaCodec的queueInputBuffer，把缓冲区索引index，偏移offset，size，时间戳timestampUs，flags传入，把错误信息回传到errorDetailMsg。
- flush, stop, reset, release
实际上，这些操作，都是通过JMediaCodec中对应的方法实现。