Netty分布式ByteBuf使用的底层实现方式源码解析

//读指针
int readerIndex;
//写指针
int writerIndex;
//保存读指针
private int markedReaderIndex;
//保存写指针
private int markedWriterIndex;
//最大分配容量
private int maxCapacity;
protected AbstractByteBuf(int maxCapacity) {
    if (maxCapacity < 0) {
        throw new IllegalArgumentException("maxCapacity: " + maxCapacity + " (expected: >= 0)");
    }
    this.maxCapacity = maxCapacity;
}

@Override
public int maxCapacity() {
    return maxCapacity;
}
@Override
public int readerIndex() {
    return readerIndex;
}
@Override
public int writerIndex() {
    return writerIndex;
}

@Override
public ByteBuf writeBytes(ByteBuf src) {
    writeBytes(src, src.readableBytes());
    return this;
}

@Override
public int readableBytes() {
    return writerIndex - readerIndex;
}

@Override
public ByteBuf writeBytes(ByteBuf src, int length) {
    if (length > src.readableBytes()) {
        throw new IndexOutOfBoundsException(String.fORMat(
                "length(%d) exceeds src.readableBytes(%d) where src is: %s", length, src.readableBytes(), src));
    }
    writeBytes(src, src.readerIndex(), length);
    src.readerIndex(src.readerIndex() + length);
    return this;
}

public ByteBuf writeBytes(ByteBuf src, int srcIndex, int length) {
    ensureAccessible();
    ensureWritable(length);
    setBytes(writerIndex, src, srcIndex, length);
    writerIndex += length;
    return this;
}

public ByteBuf ensureWritable(int minWritableBytes) {
    if (minWritableBytes < 0) {
        throw new IllegalArgumentException(String.format(
                "minWritableBytes: %d (expected: >= 0)", minWritableBytes));
    }
    ensureWritable0(minWritableBytes);
    return this;
}

private void ensureWritable0(int minWritableBytes) {
    if (minWritableBytes <= writableBytes()) {
        return;
    }
    if (minWritableBytes > maxCapacity - writerIndex) {
        throw new IndexOutOfBoundsException(String.format(
                "writerIndex(%d) + minWritableBytes(%d) exceeds maxCapacity(%d): %s", 
                writerIndex, minWritableBytes, maxCapacity, this));
    }
    //自动扩容
    int newCapacity = alloc().calculateNewCapacity(writerIndex + minWritableBytes, maxCapacity);
    capacity(newCapacity);
}

int newCapacity = alloc().calculateNewCapacity(writerIndex + minWritableBytes, maxCapacity);

public int calculateNewCapacity(int minNewCapacity, int maxCapacity) {
    //合法性校验
    if (minNewCapacity < 0) {
        throw new IllegalArgumentException("minNewCapacity: " + minNewCapacity + " (expectd: 0+)");
    }
    if (minNewCapacity > maxCapacity) {
        throw new IllegalArgumentException(String.format(
                "minNewCapacity: %d (expected: not greater than maxCapacity(%d)", 
                minNewCapacity, maxCapacity));
    }
    //阈值为4mb
    final int threshold = 1048576 * 4;
    //最小需要扩容内存(总内存) == 阈值
    if (minNewCapacity == threshold) {
        //返回阈值
        return threshold;
    }
    //最小扩容内存>阈值
    if (minNewCapacity > threshold) {
        //newCapacity为需要扩容内存
        int newCapacity = minNewCapacity / threshold * threshold;
        //目标容量+阈值>最大容量
        if (newCapacity > maxCapacity - threshold) {
            //将最大容量作为新容量
            newCapacity = maxCapacity;
        } else {
            //否则, 目标容量+阈值
            newCapacity += threshold;
        }
        return newCapacity;
    }
    //如果小于阈值
    int newCapacity = 64;
    //目标容量<需要扩容的容量
    while (newCapacity < minNewCapacity) {
        //倍增
        newCapacity <<= 1;
    }
    //目标容量和最大容量返回一个最小的
    return Math.min(newCapacity, maxCapacity);
}

public ByteBuf writeBytes(ByteBuf src, int srcIndex, int length) {
    ensureAccessible();
    ensureWritable(length);
    setBytes(writerIndex, src, srcIndex, length);
    writerIndex += length;
    return this;
}

public ByteBuf writeBytes(ByteBuf src, int length) {
    if (length > src.readableBytes()) {
        throw new IndexOutOfBoundsException(String.format(
                "length(%d) exceeds src.readableBytes(%d) where src is: %s", length, src.readableBytes(), src));
    }
    writeBytes(src, src.readerIndex(), length);
    src.readerIndex(src.readerIndex() + length);
    return this;
}