Android中的OpenSL ES是如何实现的？

一般理解的Android Audio架构是这样的：
Android中的OpenSL ES是如何实现的？_第1张图片
时间长了大家也就比较认可和熟悉了，但是了解OpenSL ES之后，发现如果使用OpenSL ES，是不使用JAVA API的，那么Android系统中是哪一层实现了OpenSL ES的呢？

只要搜索OpenSL ES就会搜索到这个库如何如何在Android上使用，好的文章一大堆，比如这个 Android音频开发（6）：使用OpenSL ES API上下，还有其它googlesamples/android-ndk中的Audio-echo等等。都是OpenSL ES往上，而不是往下。也就是是哪一层实现的OpenSL ES。我是要用我的方法查找到，并不会贴出详细的代码调用流程。

通过Audio-echo得知，要使用OpenSL ES，那么需要链接的库名称为OpenSLES，在手上的一套Android L源码中以LOCAL_MODULE := libOpenSLES查得其所在的Android.mk是frameworks/wilhelm/src/Android.mk。对应该MODULE完整的代码为

include $(CLEAR_VARS)LOCAL_SRC_FILES := sl_entry.c sl_iid.c assert.cLOCAL_C_INCLUDES:=                                                  \        $(call include-path-for, wilhelm)                           \        frameworks/av/media/libstagefright                        \        frameworks/av/media/libstagefright/include                \        frameworks/native/include/media/openmaxLOCAL_MODULE := libOpenSLESLOCAL_PRELINK_MODULE := falseLOCAL_MODULE_TAGS := optionalLOCAL_CFLAGS += -x c++ -DLI_API= -fvisibility=hidden -UNDEBUG \                -DSL_API='__attribute__((visibility("default")))'LOCAL_SHARED_LIBRARIES := libwilhelm libloginclude $(BUILD_SHARED_LIBRARY)

可以看到它链接到了libwilhelm中。查看libwilhelm对应的Android.mk为

include $(CLEAR_VARS)# do not claim support for any OpenSL ES or OpenMAX AL profilesLOCAL_CFLAGS += -DUSE_PROFILES=0# enable API logging; details are set separately by SL_TRACE_DEFAULT belowLOCAL_CFLAGS += -DUSE_TRACE# or -UUSE_TRACE to disable API logging# enable mutex deadlock detectionLOCAL_CFLAGS += -DUSE_DEBUG# or -UUSE_DEBUG for no mutex deadlock detection# enable assert() to do runtime checkingLOCAL_CFLAGS += -UNDEBUG# or -DNDEBUG for no runtime checking# select the level of log messagesLOCAL_CFLAGS += -DUSE_LOG=SLAndroidLogLevel_Info# or -DUSE_LOG=SLAndroidLogLevel_Verbose for verbose logging# log all API entries and exits (also requires Debug or Verbose log level)# LOCAL_CFLAGS += -DSL_TRACE_DEFAULT=SL_TRACE_ALL# (otherwise a warning log on error results only)# API levelLOCAL_CFLAGS += -DPLATFORM_SDK_VERSION=$(PLATFORM_SDK_VERSION)# Reduce size of .so and hide internal global symbolsLOCAL_CFLAGS += -fvisibility=hidden -DLI_API='__attribute__((visibility("default")))'LOCAL_SRC_FILES:=                     \        OpenSLES_IID.c                \        assert.c                      \        classes.c                     \        data.c                        \        devices.c                     \        entry.c                       \        handler_bodies.c              \        trace.c                       \        locks.c                       \        sles.c                        \        sl_iid.c                      \        sllog.c                       \        ThreadPool.c                  \        android/AudioPlayer_to_android.cpp    \        android/AudioRecorder_to_android.cpp  \        android/MediaPlayer_to_android.cpp    \        android/OutputMix_to_android.cpp      \        android/VideoCodec_to_android.cpp     \        android/BufferQueueSource.cpp         \        android/CallbackProtector.cpp         \        android/AacBqToPcmCbRenderer.cpp      \        android/android_AudioSfDecoder.cpp    \        android/android_AudioToCbRenderer.cpp \        android/android_GenericMediaPlayer.cpp\        android/android_GenericPlayer.cpp     \        android/android_LocAVPlayer.cpp       \        android/android_StreamPlayer.cpp      \        android/android_Effect.cpp            \        android/util/AacAdtsExtractor.cpp     \        autogen/IID_to_MPH.c                  \        objects/C3DGroup.c                    \        objects/CAudioPlayer.c                \        objects/CAudioRecorder.c              \        objects/CEngine.c                     \        objects/COutputMix.c                  \        objects/CMediaPlayer.c                \        itf/IAndroidBufferQueue.c         \        itf/IAndroidConfiguration.c       \        itf/IAndroidEffect.cpp            \        itf/IAndroidEffectCapabilities.c  \        itf/IAndroidEffectSend.c          \        itf/IAcousticEchoCancellation.c   \        itf/IAutomaticGainControl.c       \        itf/IBassBoost.c                  \        itf/IBufferQueue.c                \        itf/IDynamicInterfaceManagement.c \        itf/IEffectSend.c                 \        itf/IEngine.c                     \        itf/IEngineCapabilities.c         \        itf/IEnvironmentalReverb.c        \        itf/IEqualizer.c                  \        itf/IMetadataExtraction.c         \        itf/INoiseSuppression.c           \        itf/IMuteSolo.c                   \        itf/IObject.c                     \        itf/IOutputMix.c                  \        itf/IPlay.c                       \        itf/IPlaybackRate.c               \        itf/IPrefetchStatus.c             \        itf/IPresetReverb.c               \        itf/IRecord.c                     \        itf/ISeek.c                       \        itf/IStreamInformation.cpp        \        itf/IVideoDecoderCapabilities.cpp \        itf/IVirtualizer.c                \        itf/IVolume.cEXCLUDE_SRC :=                            \        sync.c                            \        itf/I3DCommit.c                   \        itf/I3DDoppler.c                  \        itf/I3DGrouping.c                 \        itf/I3DLocation.c                 \        itf/I3DMacroscopic.c              \        itf/I3DSource.c                   \        itf/IAudioDecoderCapabilities.c   \        itf/IAudioEncoder.c               \        itf/IAudioEncoderCapabilities.c   \        itf/IAudioIODeviceCapabilities.c  \        itf/IDeviceVolume.c               \        itf/IDynamicSource.c              \        itf/ILEDArray.c                   \        itf/IMIDIMessage.c                \        itf/IMIDIMuteSolo.c               \        itf/IMIDITempo.c                  \        itf/IMIDITime.c                   \        itf/IMetadataTraversal.c          \        itf/IPitch.c                      \        itf/IRatePitch.c                  \        itf/IThreadSync.c                 \        itf/IVibra.c                      \        itf/IVisualization.cLOCAL_C_INCLUDES:=                                                  \        $(call include-path-for, wilhelm)                           \        frameworks/av/media/libstagefright                        \        frameworks/av/media/libstagefright/include                \        frameworks/av/media/libstagefright/http                     \        frameworks/native/include/media/openmax                     \        $(call include-path-for, audio-effects)# __pthread_gettidLOCAL_C_INCLUDES += bionic/libc/privateLOCAL_CFLAGS += -x c++ -Wno-multichar -Wno-invalid-offsetofLOCAL_STATIC_LIBRARIES += \        libopensles_helper        \        libOpenSLESUTLOCAL_SHARED_LIBRARIES :=         \        liblog                    \        libutils                  \        libmedia                  \        libbinder                 \        libstagefright            \        libstagefright_foundation \        libstagefright_http_support \        libcutils                 \        libgui                    \        libdl                     \        libeffectsLOCAL_MODULE := libwilhelmLOCAL_MODULE_TAGS := optionalifeq ($(TARGET_BUILD_VARIANT),userdebug)        LOCAL_CFLAGS += -DUSERDEBUG_BUILD=1endifLOCAL_PRELINK_MODULE := falseinclude $(BUILD_SHARED_LIBRARY)

可见该库实现了OpenSL ES所需要的entry.c。真正的实现就是该库了。这个库又属于哪一层呢？需要进一步分析。

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