MP4可以说是当前最流行的视频格式，要播放一个MP4文件需要首先将其结构给解析出来。MP4的结构往简单了说就是类似于俄罗斯套娃一样的很多box套box，往复杂了说就是很多种类的box，而且还需要做一些解析和计算的操作，下面就按照其结构来分析一下MP4文件里的主要的box.左侧的目录可以清晰地展示出各种box之间的关系。需要注意的是在ISO标准中box的种类非常多，这里只是列举分析了一些比较重要的box.

Mp4Box

首先我们需要先了解一下box的结构。在Mp4中所有的box都包含了一个BoxHeader和BoxData.其中BoxHeader包含了box的类型和整个box的长度，BoxData里面包含了子box或者各种数据。基于这种结构的考虑，我就可以设计出所有box的父类，还需要注意的是Media Data Box的长度有可能超出32位int的范围，在这种情况下，size的大小就被设置为1，然后使用接下来的64位来存储其长度。如果box的type为”uuid”的话，那接下来的16byte就用来存储uuid.

public class Mp4Box {
    public int headSize = 8;
    public int size;
    public String type;
    public long largeSize;
    public boolean hasSubBox = true;
    public int index;
    public long end;

    public Mp4Box(byte[] byteBuffer, int start) {
        this.index = start;
        size = getSize(byteBuffer);
        end = start + size;
        type = getType(byteBuffer);
        if (type.equals("uuid")) {
            headSize += 16;
        }

        if (size == 1) {
            largeSize = getLongFromBuffer(byteBuffer);
            end = start + largeSize;
            headSize += 8;
        } else if (size == 0) {
            //box extends to the end of file
        }
}

Fullbox

在上面我们定义了父类Mp4box，一般情况下box头部的长度都是8byte, 但是还有一种box在头部还有1byte的version和3byte的flags。所以我们需要创建一个Fullbox父类，让所有这种类型的box来继承它。

public class FullBox extends Mp4Box {
    public int version;
    public int flag;

    public FullBox(byte[] byteBuffer, int start) {
        super(byteBuffer, start);
        version = getIntFromBuffer(byteBuffer, 1);
        flag = getIntFromBuffer(byteBuffer, 3);
        headSize = 12;
    }
}

ftyp

代表了File type box. 一般在文件的开头处（只有固定大小的文件签名可以在其前面），主要包含了该文件brand等，其定义方式如下：

public class FtypBox extends FullBox {
    public int majorBrand;
    public int minor_version;
    public int[] compatibleBrands;

    public FtypBox(byte[] byteBuffer, int start) {
        super(byteBuffer, start);
        hasSubBox = false;
        majorBrand = getIntFromBuffer(byteBuffer, 4);
        minor_version = getIntFromBuffer(byteBuffer, 4);

        int num = (size - index) / 4;
        compatibleBrands = new int[num];
        for (int i = 0; i < num; i++) {
            compatibleBrands[i] = getIntFromBuffer(byteBuffer, 4);
        }

    }
}

moov

代表了Movie box,这个box包含了一个mvhd box 和多个trak box(如video,audio等），一般在文件的开头处仅次于ftyp,但是也有放在文件末尾的。

public class MoovBox extends Mp4Box {

    public MvhdBox mvhdBox;
    public ArrayList<TrakBox> trakBox = new ArrayList<>();

    public MoovBox(byte[] byteBuffer, int start) {
        super(byteBuffer, start);
        parseSub(byteBuffer);
    }

    @Override
    public void parseSub(byte[] byteBuffer) {
        int subStart = index;
        do {
            int size = getSize(byteBuffer);
            String type = getType(byteBuffer);
            if (size > 8) {
                if (type.equals("mvhd")) {
                    mvhdBox = new MvhdBox(byteBuffer, subStart);
                } else if (type.equals("trak")) {
                    trakBox.add(new TrakBox(byteBuffer, subStart));
                }
                subStart += size;
                index = subStart;
            } else {
                break;
            }
        } while (subStart < end);
    }
}

mvhd

代表了Movie Header box。包含了整个媒体问题的信息。

public class MvhdBox extends FullBox {

    public long creationTime;
    public long modificationTime;
    public int timescale;
    public long duration;

    public int rate;
    public int volume;
    public int reserved = 0;
    public int[] matrix = new int[9];
    public int[] preDefined = new int[6];
    public int nextTrackId;

    public MvhdBox(byte[] byteBuffer, int start) {
        super(byteBuffer, start);
        if (version == 1) {
            creationTime = getLongFromBuffer(byteBuffer);
            modificationTime = getLongFromBuffer(byteBuffer);
            timescale = getIntFromBuffer(byteBuffer, 4);
            duration = getLongFromBuffer(byteBuffer);
        } else {
            creationTime = getIntFromBuffer(byteBuffer, 4);
            modificationTime = getIntFromBuffer(byteBuffer, 4);
            timescale = getIntFromBuffer(byteBuffer, 4);
            duration = getIntFromBuffer(byteBuffer, 4);
        }
        rate = getIntFromBuffer(byteBuffer, 4);
        volume = getIntFromBuffer(byteBuffer, 2);
        //skip reserved
        index += 10;

        for (int i = 0; i < matrix.length; i++) {
            matrix[i] = getIntFromBuffer(byteBuffer, 4);
        }
        for (int i = 0; i < preDefined.length; i++) {
            preDefined[i] = getIntFromBuffer(byteBuffer, 4);

        }
        nextTrackId = getIntFromBuffer(byteBuffer, 4);
    }
}

creationTime: 一个int型的数据代表了文件的创建时间（从1904年1月1日凌晨开始的秒数）
modificationTime：修改时间，定义同上

timescale：1秒内包含的时间单位，和下面的duration结合可以得到媒体的时长。

duration：媒体的时长,以时长最长的track为准
rate：播放的速度，1.0为正常速度
volume：播放的音量

matrix：视频的转换矩阵

nextTrackId：下一个track的id

trak

代表了 Track box。包含了一条track。通常情况下一个视频会包含video和audio两条track，还有一条hint track是为streaming准备的。
一个trak box内包含了tkdh和mdia两种box。

public class TrakBox extends Mp4Box {
    public TkhdBox mTkhdBox;
    public MdiaBox mMdiaBox;

    public TrakBox(byte[] byteBuffer, int start) {
        super(byteBuffer, start);
        parseSub(byteBuffer);
    }

    @Override
    public void parseSub(byte[] byteBuffer) {
        int subStart = index;
        do {
            int size = getSize(byteBuffer);
            String type = getType(byteBuffer);
            if (size > 8) {
                if (type.equals("tkhd")) {
                    mTkhdBox = new TkhdBox(byteBuffer, subStart);
                } else if (type.equals("mdia")) {
                    mMdiaBox = new MdiaBox(byteBuffer, subStart);
                }
                subStart += size;
                index = subStart;
            } else {
                break;
            }
        } while (subStart < end);
    }
}

tkhd

代表了Track Header Box。每个trck box会包含一个tkhd box来存储这条track的信息。

public class TkhdBox extends FullBox {
    public long creationTime;
    public long modificationTime;
    public int trackId;
    public long duration;

    public int layer;
    public int alternateGroup;
    public int volume;
    public int[] matrix = new int[9];
    public int width;
    public int height;

    public TkhdBox(byte[] byteBuffer, int start) {
        super(byteBuffer, start);
        if (version == 1) {
            creationTime = getLongFromBuffer(byteBuffer);
            modificationTime = getLongFromBuffer(byteBuffer);
            trackId = getIntFromBuffer(byteBuffer, 4);
            index += 4;
            duration = getLongFromBuffer(byteBuffer);
        } else {
            creationTime = getIntFromBuffer(byteBuffer, 4);
            modificationTime = getIntFromBuffer(byteBuffer, 4);
            trackId = getIntFromBuffer(byteBuffer, 4);
            index += 4;
            duration = getIntFromBuffer(byteBuffer, 4);
        }
        //skip reserved
        index += 8;

        layer = getIntFromBuffer(byteBuffer, 2);
        alternateGroup = getIntFromBuffer(byteBuffer, 2);
        volume = getIntFromBuffer(byteBuffer, 2);

        index += 2;
        for (int i = 0; i < matrix.length; i++) {
            matrix[i] = getIntFromBuffer(byteBuffer, 4);
        }
        width = getIntFromBuffer(byteBuffer, 4);
        height = getIntFromBuffer(byteBuffer, 4);
        Logger.i(toString());
    }
}
`

creationTime: 这条track的创建时间（同mvhd)
modificationTime: 这条track的更改时间（同nvhd)
trackId: 当前track的id，不能为0
duration: 当前track的时长
layer: 播放时video track的前后顺序，数字越小的越在上层。
alternateGroup: 对track进行分组，同一个组内同时只能播放一个track
volume: 音量大小，对于video track 会是0
matrix：video的转换矩阵
width/height 视频的宽度和长度，以像素为单位

mdia

代表了Media Box, 用来包含当前track的信息。mdia baox主要有三个子box，分别是hdlr、mdhd和minf,我们会在接下来分析这几个子box。
其中componentName并不是mdia box需要解析的内容，在这里我们用来存储当前track的类型，如是vedio还是audio。

public class MdiaBox extends Mp4Box {
    public HdlrBox mHdlrBox;
    public MdhdBox mMdhdBox;
    public MinfBox mMinfBox;
    public String componentName;

    public MdiaBox(byte[] byteBuffer, int start) {
        super(byteBuffer, start);
        parseSub(byteBuffer);
    }

    @Override
    public void parseSub(byte[] byteBuffer) {
        int subStart = index;
        do {
            int size = getSize(byteBuffer);
            String type = getType(byteBuffer);
            if (size > 8) {
                if (type.equals("hdlr")) {
                    mHdlrBox = new HdlrBox(byteBuffer, subStart);
                    componentName = mHdlrBox.componentName;
                } else if (type.equals("mdhd")) {
                    mMdhdBox = new MdhdBox(byteBuffer, subStart);
                } else if (type.equals("minf")) {
                    mMinfBox = new MinfBox(byteBuffer, subStart, componentName);
                }
                subStart += size;
                index = subStart;
            } else {
                break;
            }
        } while (subStart < end);
    }
}

mdhd

代表了Media Header Box。包含了一些当前track的信息，如creationTime等，在上文已经提过了，这里将不再赘述。

public class MdhdBox extends FullBox {
    public long creationTime;
    public long modificationTime;
    public int timescale;
    public long duration;
    public int language;
    public int predefined;
    public MdhdBox(byte[] byteBuffer, int start) {
        super(byteBuffer, start);
        if (version == 1) {
            creationTime = getLongFromBuffer(byteBuffer);
            modificationTime = getLongFromBuffer(byteBuffer);
            timescale = getIntFromBuffer(byteBuffer, 4);
            duration = getLongFromBuffer(byteBuffer);
        } else {
            creationTime = getIntFromBuffer(byteBuffer, 4);
            modificationTime = getIntFromBuffer(byteBuffer, 4);
            timescale = getIntFromBuffer(byteBuffer, 4);
            duration = getIntFromBuffer(byteBuffer, 4);
        }
        language = getIntFromBuffer(byteBuffer, 2);
        predefined = getIntFromBuffer(byteBuffer, 2);
    }
}

- language 当前媒体的语言代码

hdlr

代表了Handler Refrence Box。包含了当前track类型的信息，如这条track是video、sound还是hint。

public class HdlrBox extends FullBox {
    public int componentType;
    public int componentSubType;
    public String componentName;

    public HdlrBox(byte[] byteBuffer, int start) {
        super(byteBuffer, start);

        componentType = getIntFromBuffer(byteBuffer, 4);
        componentSubType = getIntFromBuffer(byteBuffer, 4);
        index += 12;
        componentName = getStringFromBuffer(byteBuffer, 12);
    }
}

minf

代表了Media Information Box。内部主要包含了一个stbl box。

public class MinfBox extends Mp4Box {
    StblBox mStblBox;
    private String mType;

    public MinfBox(byte[] byteBuffer, int start, String type) {
        super(byteBuffer, start);
        mType = type;
        parseSub(byteBuffer);
    }

    @Override
    public void parseSub(byte[] byteBuffer) {
        int subStart = index;
        do {
            int size = getSize(byteBuffer);
            String type = getType(byteBuffer);
            if (size > 8) {
                if (type.equals("stbl")) {
                    mStblBox = new StblBox(byteBuffer, subStart,mType);
                }
                subStart += size;
                index = subStart;
            } else {
                break;
            }
        } while (subStart < end);
    }
}

stbl

代表了Sample Table Box。这个box包含了当前track中所有Samples的时间和数据索引,所以根据这些信息就可以定位某个时间点的Sample及其大小等信息。
这个box包含了很多子box，将各个子box的信息结合起来就得得到Samples的详细信息了。具体怎么计算将在最后做一下分析。

public class StblBox extends Mp4Box {
    private final String mType;
    public StsdBox mStsdBox;
    public StssBox mStssBox;
    public SttsBox mSttsBox;
    public StszBox mStszBox;
    public StscBox mStscBox;
    public StcoBox mStcoBox;
    public Co64Box mCo64Box;
    public CttsBox mCttsBox;

    public StblBox(byte[] byteBuffer, int start, String type) {
        super(byteBuffer, start);
        mType = type;
        parseSub(byteBuffer);
    }

    @Override
    public void parseSub(byte[] byteBuffer) {
        int subStart = index;
        do {
            int size = getSize(byteBuffer);
            String type = getType(byteBuffer);
            if (type.equals("stsd")) {
                mStsdBox = new StsdBox(byteBuffer, subStart, mType);
            } else if (type.equals("stts")) {
                mSttsBox = new SttsBox(byteBuffer, subStart);
            } else if (type.equals("stsz")) {
                mStszBox = new StszBox(byteBuffer, subStart);
            } else if (type.equals("stsc")) {
                mStscBox = new StscBox(byteBuffer, subStart);
            } else if (type.equals("stco")) {
                mStcoBox = new StcoBox(byteBuffer, subStart);
            } else if (type.equals("Co64")) {
                mCo64Box = new Co64Box(byteBuffer, subStart);
            } else if (type.equals("ctts")) {
                mCttsBox = new CttsBox(byteBuffer, subStart);
            }
            subStart += size;
            index = subStart;
        } while (subStart < end);
    }
}

stts

代表了Decoding Time to Sample Box。这个box包含了一个表，从这个表里可以根据解码时间来定位sample的序号。

public class SttsBox extends FullBox {
    private final int entryCount;
    public int[] sampleCount;
    public int[] sampleDelta;

    public SttsBox(byte[] byteBuffer, int start) {
        super(byteBuffer, start);
        entryCount = getIntFromBuffer(byteBuffer, 4);
        sampleCount = new int[entryCount];
        sampleDelta = new int[entryCount];
        for (int i = 0; i < entryCount; i++) {
            sampleCount[i] = getIntFromBuffer(byteBuffer, 4);
            sampleDelta[i] = getIntFromBuffer(byteBuffer, 4);
        }
    }
}

sampleCount: 一个时间段内连续的Sample个数
sampleDelta:一个时间段内sample的delta。
ctts
代表了Sample Table Box。包含了decoding time 和composition time之间的偏移量。

public class CttsBox extends FullBox {
    private final int mEntryCount;
    public int[] sampleCount;
    public int[] sampleOffset;

    public CttsBox(byte[] byteBuffer, int start) {
        super(byteBuffer, start);
        mEntryCount = getIntFromBuffer(byteBuffer, 4);
        sampleCount = new int[mEntryCount];
        sampleOffset = new int[mEntryCount];
        for (int i = 0; i < mEntryCount; i++) {
            sampleCount[i] = getIntFromBuffer(byteBuffer, 4);
            sampleOffset[i] = getIntFromBuffer(byteBuffer, 4);
        }
    }
}

sampleCount: 特定偏移量内连续的Sample个数
sampleDelta: CT和DT之间的偏移量，如CT(n)=DT(n)+CTTS(n)

stsd

代表了Sample Description Box，里面包含了一些子box，用来存储编码的信息。根据当前track的类型来决定子box的类型。

public class StsdBox extends FullBox {
    private final String mType;
    public int numberOfEntries;
    public SampleEntry[] entrys;

    public StsdBox(byte[] byteBuffer, int start, String type) {
        super(byteBuffer, start);
        numberOfEntries = getIntFromBuffer(byteBuffer, 4);
        entrys = new SampleEntry[numberOfEntries];
        mType = type;
        parseSub(byteBuffer);
    }

    @Override
    public void parseSub(byte[] byteBuffer) {
        int subStart = index;
        for (int i = 0; i < numberOfEntries; i++) {
            int size = getSize(byteBuffer);
            String type = getType(byteBuffer);
            Logger.i("type:" + type);
            entrys[i] = mType.startsWith("Video") ? new VisualSampleEntry(byteBuffer, subStart) : new AudioSampleEntry(byteBuffer, subStart);
            subStart += size;
        }
    }
}

####### VisualSampleEntry

public class VisualSampleEntry extends SampleEntry {
    private final int mWidth;
    private final int mHeight;
    private final int mHorizeresolution;
    private final int mVertresolution;
    private final int mFrameCount;
    private final String mCompressorName;

    public VisualSampleEntry(byte[] byteBuffer, int start) {
        super(byteBuffer, start);
        index += 16;
        mWidth = getIntFromBuffer(byteBuffer, 2);
        mHeight = getIntFromBuffer(byteBuffer, 2);
        mHorizeresolution = getIntFromBuffer(byteBuffer, 4);
        mVertresolution = getIntFromBuffer(byteBuffer, 4);
        index += 4;
        mFrameCount = getIntFromBuffer(byteBuffer, 2);
        mCompressorName = getStringFromBuffer(byteBuffer, 4);
        Logger.i(toString());
    }
}

//todo