ByteBuf 内部结构设计

小灰灰 2022-12-27 09:06 28阅读 0赞

一、首先介绍下 ReferenceCounted 接口

public interface ReferenceCounted {
        /**
         * 返回对象的引用数量，如果返回0，表示这个对象已经被释放
         */
        int refCnt();
    
        /**
         * 引用计数加 1
         */
        ReferenceCounted retain();
    
        /**
         * 引用计数加 increment
         */
        ReferenceCounted retain(int increment);
    
        /**
         * 引用计数减 1，如果引用计数为0，则会释放这个对象
         *
         * @return 当且仅当引用计数变为0，并且这个对象已经被释放时返回true
         */
        boolean release();
    
        /**
         * 引用计数减 decrement，如果引用计数为0，则会释放这个对象
         * @return 当且仅当引用计数变为0，并且这个对象已经被释放时返回true
         */
        boolean release(int decrement);
    }

二、ByteBuf介绍

1、ByteBuf实现了ReferenceCounted接口，ByteBuf提供**随机和顺序**两种方式读取一个或者多个字节序列。

> **（1）Random Access Indexing**
> 
> ByteBuf提供类似于原始数组的方式，起始索引是0，结束索引是capacity-1。

> **（2）Sequential Access Indexing** 
> 
> ByteBuf 提供两个指针来支持顺序读和顺序写操作。用readIndex来支持读操作，writeIndex来支持写操作。下图展示readIndex 和 writeIndex对ByteBuf进行分段

![20201209232756197.png][]

> **discardable bytes**
> 
> 这部分表示已经读过的区域，初始为0，最大值为writeIndex，执行读操作时这部分内容会相应的增加。可以通过调用 **discardReadBytes** 方法来回收这部分区域，达到获取更多writeable byte空间的效果

**执行discardReadBytes之前如下图**

![20201209232756197.png][]

**执行discardReadBytes之后如下图**

![2020120923404850.png][]

> **readable bytes**
> 
> 内容实际存储区域。执行以start或者skip开始的方法时，如果没有足够的内容供读取，会抛出 **IndexOutOfBoundsException 异常**

> **writeable bytes**
> 
> 可写区间

> **（3）Clearing the buffer index**
> 
> 可以调用clear方法来把readIndex 和 writeIndex的值重置为0，但是clear方法不会清空内容。

![20201209234654205.png][]

![20201209234706180.png][]

ByteBuf顺序读和顺序写、随机读和随机写的API都比较简单，此处不介绍。介绍下 ByteBuf 与 java nio 原生 ByteBuffer 转换

/**
         * Returns the maximum number of NIO {@link ByteBuffer}s that consist this buffer.  Note that {@link #nioBuffers()}
         * or {@link #nioBuffers(int, int)} might return a less number of {@link ByteBuffer}s.
         *
         * @return {@code -1} if this buffer has no underlying {@link ByteBuffer}.
         *         the number of the underlying {@link ByteBuffer}s if this buffer has at least one underlying
         *         {@link ByteBuffer}.  Note that this method does not return {@code 0} to avoid confusion.
         *
         * @see #nioBuffer()
         * @see #nioBuffer(int, int)
         * @see #nioBuffers()
         * @see #nioBuffers(int, int)
         */
        public abstract int nioBufferCount();
     
        /**
         * Exposes this buffer's readable bytes as an NIO {@link ByteBuffer}.  The returned buffer
         * shares the content with this buffer, while changing the position and limit of the returned
         * NIO buffer does not affect the indexes and marks of this buffer.  This method is identical
         * to {@code buf.nioBuffer(buf.readerIndex(), buf.readableBytes())}.  This method does not
         * modify {@code readerIndex} or {@code writerIndex} of this buffer.  Please note that the
         * returned NIO buffer will not see the changes of this buffer if this buffer is a dynamic
         * buffer and it adjusted its capacity.
         *
         * @throws UnsupportedOperationException
         *         if this buffer cannot create a {@link ByteBuffer} that shares the content with itself
         *
         * @see #nioBufferCount()
         * @see #nioBuffers()
         * @see #nioBuffers(int, int)
         */
        public abstract ByteBuffer nioBuffer();
     
        /**
         * Exposes this buffer's sub-region as an NIO {@link ByteBuffer}.  The returned buffer
         * shares the content with this buffer, while changing the position and limit of the returned
         * NIO buffer does not affect the indexes and marks of this buffer.  This method does not
         * modify {@code readerIndex} or {@code writerIndex} of this buffer.  Please note that the
         * returned NIO buffer will not see the changes of this buffer if this buffer is a dynamic
         * buffer and it adjusted its capacity.
         *
         * @throws UnsupportedOperationException
         *         if this buffer cannot create a {@link ByteBuffer} that shares the content with itself
         *
         * @see #nioBufferCount()
         * @see #nioBuffers()
         * @see #nioBuffers(int, int)
         */
        public abstract ByteBuffer nioBuffer(int index, int length);
     
        /**
         * Internal use only: Exposes the internal NIO buffer.
         */
        public abstract ByteBuffer internalNioBuffer(int index, int length);
     
        /**
         * Exposes this buffer's readable bytes as an NIO {@link ByteBuffer}'s.  The returned buffer
         * shares the content with this buffer, while changing the position and limit of the returned
         * NIO buffer does not affect the indexes and marks of this buffer. This method does not
         * modify {@code readerIndex} or {@code writerIndex} of this buffer.  Please note that the
         * returned NIO buffer will not see the changes of this buffer if this buffer is a dynamic
         * buffer and it adjusted its capacity.
         *
         *
         * @throws UnsupportedOperationException
         *         if this buffer cannot create a {@link ByteBuffer} that shares the content with itself
         *
         * @see #nioBufferCount()
         * @see #nioBuffer()
         * @see #nioBuffer(int, int)
         */
        public abstract ByteBuffer[] nioBuffers();
     
        /**
         * Exposes this buffer's bytes as an NIO {@link ByteBuffer}'s for the specified index and length
         * The returned buffer shares the content with this buffer, while changing the position and limit
         * of the returned NIO buffer does not affect the indexes and marks of this buffer. This method does
         * not modify {@code readerIndex} or {@code writerIndex} of this buffer.  Please note that the
         * returned NIO buffer will not see the changes of this buffer if this buffer is a dynamic
         * buffer and it adjusted its capacity.
         *
         * @throws UnsupportedOperationException
         *         if this buffer cannot create a {@link ByteBuffer} that shares the content with itself
         *
         * @see #nioBufferCount()
         * @see #nioBuffer()
         * @see #nioBuffer(int, int)
         */
        public abstract ByteBuffer[] nioBuffers(int index, int length);

ByteBuf中不存在 java.nio.ByteBuffer的 allocateDirect(int capacity) 等方法。原因是Netty 对 ByteBuf 的创建单独抽象成一个体系 ByteBufAllocator，包括了直接内存、堆内存，池化的 ByteBuf，后续不断学习，不断补充。

[20201209232756197.png]: /images/20221120/cce2c7cb891248d0bfbcc383e5bd33bc.png
[2020120923404850.png]: /images/20221120/8c76706511634ade965e3e6c026a93cf.png
[20201209234654205.png]: /images/20221120/e8a28306c1004e0fa31e8d9c43988094.png
[20201209234706180.png]: /images/20221120/06a9091c1f9c4de29c7391f8c7ecf378.png