EasyPlayer Android音频解码库（第二部分，封装解码器接口）

上一节我们讲了如何基于ffmpeg-android工程编译安卓上的支持音频的ffmpeg静态库，这篇文章我们将介绍如何封装安卓的解码器。

首先，为了能让我们的app调用调用，我们需要定义一套Java的native接口，解码器主要有三个接口：

• create 创建解码器，参数分别为：
  
  • codec:音频编码格式，参考EasyTypes.h里面的EASY_SDK_AUDIO_CODEC_*宏定义；
  • sample_rate:采样率，通常为8000、44000等等；
  • channels：通道数，1、2分别表示单通道、双通道；
  • sample_bit：采样精度，通常为16bit;
    返回解码器句柄，即后续接口里面用到的handle
• decode 解码，参数为:
  
  • handle :解码器句柄
  • buffer：要解码的buffer（编码后的音频数据）
  • offset：编码数据在buffer里的起始位置
  • length：编码数据的长度
  • pcm：解码后的pcm数据
  • outLen 长度至少为1的int数组，如果解码成功，那outLen[0]被置为pcm的数据长度
• close 关闭解码器，参数为解码器的句柄。关闭后句柄无效，应该置为0.

package org.easydarwin.audio;/** * Created by John on 2016/3/18. */public class AudioCodec {    static {        System.loadLibrary("AudioCodecer");    }    public static native int create(int codec, int sample_rate, int channels, int sample_bit);    public static native int decode(int handle, byte[] buffer, int offset, int length, byte[] pcm, int[] outLen);    public static native void close(int handle);}

接下来我们要实现相应的native接口，头文件可通过调用javah命令（这里不再详述）来生成。以AAC解码来说明，create代码片段如下：

    // 创建获取解码器    AVCodec *pCodec = avcodec_find_decoder(AV_CODEC_ID_AAC);    if (pCodec == NULL)    {        LOGI("find aac decoder error");        printf("find aac decoder error\r\n");        return 0;    }    // 创建解码Context并open解码器    pCodecCtx = avcodec_alloc_context3(pCodec);    pCodecCtx->channels = channels;    pCodecCtx->sample_rate = sample_rate;    pCodecCtx->bit_rate = bit_rate;    if(avcodec_open2(pCodecCtx, pCodec, NULL) < 0)    {        printf("open codec error\r\n");        return 0;    }    // 分配内存，存放解码后的数据    pFrame = av_frame_alloc();    

decode：

    // 源码流赋值    packet.size = length;    packet.data = buffer;    int got_frame = 0;    // 解码、数据拷贝    while (pkt_pos < nLen)    {    //  pkt_pos = 0;        int got_frame = 0;        src_len = avcodec_decode_audio4(pAACD->pCodecCtx, pAACD->pFrame, &got_frame, &packet);        if (src_len < 0)        {            return -3;        }        data_len += src_len        if (got_frame)        {            memcpy(pAACD->pFrame, pAACD->audio_buf, len);            dst_len += len;        }        pkt_pos += src_len;        packet.data = pData + pkt_pos;        packet.size = nLen - pkt_pos;    }    if (NULL != outLen)         *outLen = dst_len;    // 释放    av_free_packet(&packet);

close:

    av_frame_free(&pFrame);    avcodec_close(pComponent->pCodecCtx);    avcodec_free_context(&pCodecCtx);

对于其它格式的音频算法，只要在avcodec_find_decoder时传入不同的算法ID即可。

接下来，我们需要使用NDK编译出ANDROID上可以使用的动态库，为此我们需要编辑Android.mk文件，其内容如下：

# Copyright (C) 2009 The Android Open Source Project## Licensed under the Apache License, Version 2.0 (the "License");# you may not use this file except in compliance with the License.# You may obtain a copy of the License at##      http://www.apache.org/licenses/LICENSE-2.0## Unless required by applicable law or agreed to in writing, software# distributed under the License is distributed on an "AS IS" BASIS,# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.# See the License for the specific language governing permissions and# limitations under the License.#LOCAL_PATH:= $(call my-dir)SRC_ROOT_PATH := $(call my-dir)LOCAL_INCLUDE := $(LOCAL_PATH)/aacdec/include# 加载预编译的静态库include $(CLEAR_VARS)LOCAL_MODULE := libavcodecLOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libavcodec.ainclude $(PREBUILT_STATIC_LIBRARY)include $(CLEAR_VARS)LOCAL_MODULE := libavdeviceLOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libavdevice.ainclude $(PREBUILT_STATIC_LIBRARY)include $(CLEAR_VARS)LOCAL_MODULE := libavfilterLOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libavfilter.ainclude $(PREBUILT_STATIC_LIBRARY)include $(CLEAR_VARS)LOCAL_MODULE := libavformatLOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libavformat.ainclude $(PREBUILT_STATIC_LIBRARY)include $(CLEAR_VARS)LOCAL_MODULE := libavutilLOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libavutil.ainclude $(PREBUILT_STATIC_LIBRARY)include $(CLEAR_VARS)LOCAL_MODULE := libswresampleLOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libswresample.ainclude $(PREBUILT_STATIC_LIBRARY)include $(CLEAR_VARS)LOCAL_MODULE := libswscaleLOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libswscale.ainclude $(PREBUILT_STATIC_LIBRARY)include $(CLEAR_VARS)LOCAL_C_INCLUDES += $(LOCAL_PATH)LOCAL_C_INCLUDES += $(LOCAL_INCLUDE)PROJECT_FILES := $(wildcard $(SRC_ROOT_PATH)/*.cpp)PROJECT_FILES += $(wildcard $(SRC_ROOT_PATH)/*.c)PROJECT_FILES += $(wildcard $(SRC_ROOT_PATH)/aacdec/*.cpp)$(warning $(PROJECT_FILES))PROJECT_FILES := $(PROJECT_FILES:$(LOCAL_PATH)/%=%)$(warning $(PROJECT_FILES))LOCAL_SRC_FILES := $(PROJECT_FILES)LOCAL_CFLAGS := -D__unix__ -DANDROID_OS -D__arm__ -D__STDC_CONSTANT_MACROSLOCAL_MODULE    := AudioCodecerLOCAL_LDLIBS += -L$(LOCAL_PATH)/aacdec/lib -lavcodec -lswscale -lswresample -lavutil -lavformat -lavfilter -lavdevice -llog -lzCFLAGS += -mfpu=neonLOCAL_STATIC_LIBRARIES := libavcodec libswscale libswresample libavutil libavformat libavfilter libavdeviceinclude $(BUILD_SHARED_LIBRARY)

我们这里需要把之前编译出来的静态库链接起来，生成一个动态库。其中，静态库lib文件和头文件分别放在/aacdec/lib/ 和 /aacdec/include/ 下。

接下来，cd到jni目录，执行ndk-build，如果顺利的话，我们要的动态库文件就会生成。

整个项目源码见Github