EasyDarwin开源音频解码项目EasyAudioDecoder：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：http://blog.csdn.net/xiejiashu/article/details/52524099，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
");
        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
");
        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 := libavcodec
LOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libavcodec.a
include $(PREBUILT_STATIC_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := libavdevice
LOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libavdevice.a
include $(PREBUILT_STATIC_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := libavfilter
LOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libavfilter.a
include $(PREBUILT_STATIC_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := libavformat
LOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libavformat.a
include $(PREBUILT_STATIC_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := libavutil
LOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libavutil.a
include $(PREBUILT_STATIC_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := libswresample
LOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libswresample.a
include $(PREBUILT_STATIC_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := libswscale
LOCAL_SRC_FILES := $(LOCAL_PATH)/aacdec/lib/libswscale.a
include $(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_MACROS

LOCAL_MODULE    := AudioCodecer

LOCAL_LDLIBS += -L$(LOCAL_PATH)/aacdec/lib -lavcodec -lswscale -lswresample -lavutil -lavformat -lavfilter -lavdevice -llog -lz

CFLAGS += -mfpu=neon

LOCAL_STATIC_LIBRARIES := libavcodec libswscale libswresample libavutil libavformat libavfilter libavdevice

include $(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

EasyDarwin开源音频解码项目EasyAudioDecoder：EasyPlayer Android音频解码库（第二部分，封装解码器接口）

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