netty学习之二:ByteBuf解读

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ByteBuf原理跟之前的ByteBuffer原理类似，只是扩展和弥补了ByteBuffer的不足，设计原理：

1、初始化writerIndex，readerIndex为0

2、写入N个字节后：

3、读入M<N字节后：

4、discardReadBytes之后：

5、discardReadBytes前后操作：

源码：

 public ByteBuf discardReadBytes() {        ensureAccessible();        if (readerIndex == 0) {            return this;        }        if (readerIndex != writerIndex) {            setBytes(0, this, readerIndex, writerIndex - readerIndex);            writerIndex -= readerIndex;            adjustMarkers(readerIndex);            readerIndex = 0;        } else {            adjustMarkers(readerIndex);            writerIndex = readerIndex = 0;        }        return this;    }

discardReadBytes操作如同compact操作压缩空间，将已经读得数据释放，但是频繁调用discardReadBytes操作会导致性能下降，因为发生了数组字节内存复制

6、clear之后：

7、clear前后：

8、 ByteBuf实现动态空间扩展，看它的实现类AbstractByteBuf代码：

a、写操作之前进行ensureAccessible，ensureWritable校验

  @Override    public ByteBuf writeByte(int value) {        ensureAccessible();        ensureWritable(1);        _setByte(writerIndex++, value);        return this;    }  /**     * Should be called by every method that tries to access the buffers content to check     * if the buffer was released before.     */    protected final void ensureAccessible() {        if (refCnt() == 0) {            throw new IllegalReferenceCountException(0);        }    }   @Override    public ByteBuf ensureWritable(int minWritableBytes) {        if (minWritableBytes < 0) {            throw new IllegalArgumentException(String.format(                    "minWritableBytes: %d (expected: >= 0)", minWritableBytes));        }        if (minWritableBytes <= writableBytes()) {            return this;        }        if (minWritableBytes > maxCapacity - writerIndex) {            throw new IndexOutOfBoundsException(String.format(                    "writerIndex(%d) + minWritableBytes(%d) exceeds maxCapacity(%d): %s",                    writerIndex, minWritableBytes, maxCapacity, this));        }        // Normalize the current capacity to the power of 2.        int newCapacity = alloc().calculateNewCapacity(writerIndex + minWritableBytes, maxCapacity);        // Adjust to the new capacity.        capacity(newCapacity);        return this;    }

重新分配了newCapacity

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