Netty ChannelBuffer 简介

ChannelBuffer是Netty中比较常用的一个类，其功能类似于字符数组，可以对其进行读写操作。

ChannelBuffer的模型图如下：

 +-------------------+------------------+------------------+
 | discardable bytes |  readable bytes  |  writable bytes  |
 |                   |     (CONTENT)    |                  |
 +-------------------+------------------+------------------+
 |                   |                  |                  |

 0      <=      readerIndex   <=   writerIndex    <=    capacity

如上图所示，一个ChannelBuffer被划分为三个部分：

1.discardable：表示已经读过的内容缓冲区

2.readable：表示可读的内容缓冲区

3.writable：表示可写的内容缓冲区

ChannelBuffer的这三个缓冲区由2个内部控制指针来控制：

readerIndex：控制读缓冲区首地址

writerIndex：控制写缓冲区首地址

因此，ChannelBuffer提供的大部分操作都是围绕readerIndex和writerIndex来进行的。

ChannelBuffer的常用方法如下：

1、

read()/skip()

从readerIndex读出或跳过指定数目的字节，同时readerIndex = readerIndex + byteNumber.如果readerIndex > capacity，表示读取下标越界，会抛出

IndexOutOfBoundsException异常

readable()：boolean

如果buffer有可读内容（此时writerIndex > readerIndex）,则返回true，否则返回false

readerIndex():int

返回readerIndex

readableBytes():int

返回可读的字节数目(writerIndex - readerIndex)

2、

write();

写入指定数目的字节，同时writerIndex = writerIndex + byteNumber. 如果writerIndex > capacity，表示写入下标越界，会抛出

IndexOutOfBoundsException异常

writable():boolean

如果buffer有可写入空间（此时writerIndex < capacity），则返回true，否则返回false。

writerIndex(): int

返回writerIndex

writeableIndex():int
返回可写入的字节数（capacity - writerIndex）。

3、discardReadBytes()

丢弃已读的内容。其执行过程如下：

调用discardReadBytes()之前：

+-------------------+------------------+------------------+
| discardable bytes |  readable bytes  |  writable bytes  |
+-------------------+------------------+------------------+
|                   |                  |                  |
0      <=      readerIndex   <=   writerIndex    <=    capacity

调用 discardReadBytes()方法后

+------------------+--------------------------------------+
|  readable bytes  |    writable bytes (got more space)   |
+------------------+--------------------------------------+
|                  |                                      |
readerIndex (0) <= writerIndex (decreased)    <=      capacity

4、clear()
丢弃所有的数据，并将readerIndex和writerIndex重置为0。

调用clear()之前
 +-------------------+------------------+------------------+
 | discardable bytes |  readable bytes  |  writable bytes  |
 +-------------------+------------------+------------------+
 |                   |                  |                  |
 0      <=      readerIndex   <=   writerIndex    <=    capacity

 

 调用clear()之后

 +---------------------------------------------------------+ |             
                 writable bytes (got more space)             |
 +---------------------------------------------------------+ |                                                         |
0 = readerIndex = writerIndex            <=            capacity

5、

markReaderIndex()
markWriterIndex()

保存readerIndex或writerIndex的状态

6、

resetReaderIndex()

resetWriterIndex()

重置readerIndex或writerIndex为最后一次保存的状态，如果没有保存过，则置为0

7、

duplicate()

slice()

拷贝和源目标共享buffer的数据区域，但是拷贝有自己的readerIndex和writerIndex以及markIndex，实际上只是拷贝了控制指针，数据区还是与源buffer共享。

8、

copy()

拷贝整个buffer，包括控制指针和数据区

测试代码如下：

package test;

import org.jboss.netty.buffer.ChannelBuffer;
import org.jboss.netty.buffer.ChannelBuffers;

public class ChannelBufferTest {

    /**
     *
     * @author lengxue
     * @date 11 Jul 2012 09:05:10
     * @param args
     * @note
     */
    public static void main( String[] args ) {
        // TODO Auto-generated method stub
        ChannelBuffer buffer = ChannelBuffers.buffer( 10 );
        System.out.println("readable bytes: " + buffer.readableBytes( ));
        System.out.println("readable index: " + buffer.readerIndex( ));
        System.out.println("writable bytes: " + buffer.writableBytes( ));
        System.out.println("writable index: " + buffer.writerIndex( ));
        
        buffer.writeInt( 10 );
        System.out.println("after write one integer");
        System.out.println("readable bytes: " + buffer.readableBytes( ));
        System.out.println("readable index: " + buffer.readerIndex( ));
        System.out.println("writable bytes: " + buffer.writableBytes( ));
        System.out.println("writable index: " + buffer.writerIndex( ));

        buffer.writeInt( 10 );
        System.out.println("after write two integer");
        System.out.println("readable bytes: " + buffer.readableBytes( ));
        System.out.println("readable index: " + buffer.readerIndex( ));
        System.out.println("writable bytes: " + buffer.writableBytes( ));
        System.out.println("writable index: " + buffer.writerIndex( ));
        
        int i = buffer.readInt( );
        System.out.println("after read one integer: " + i);
        System.out.println("readable bytes: " + buffer.readableBytes( ));
        System.out.println("readable index: " + buffer.readerIndex( ));
        System.out.println("writable bytes: " + buffer.writableBytes( ));
        System.out.println("writable index: " + buffer.writerIndex( ));
        
        buffer.discardReadBytes( );
        System.out.println("after discard read bytes");
        System.out.println("readable bytes: " + buffer.readableBytes( ));
        System.out.println("readable index: " + buffer.readerIndex( ));
        System.out.println("writable bytes: " + buffer.writableBytes( ));
        System.out.println("writable index: " + buffer.writerIndex( ));
        
        buffer.resetReaderIndex( );
        System.out.println("after reset reader index");
        System.out.println("readable bytes: " + buffer.readableBytes( ));
        System.out.println("readable index: " + buffer.readerIndex( ));
        System.out.println("writable bytes: " + buffer.writableBytes( ));
        System.out.println("writable index: " + buffer.writerIndex( ));
        
        buffer.resetWriterIndex( );
        System.out.println("after reset writer index");
        System.out.println("readable bytes: " + buffer.readableBytes( ));
        System.out.println("readable index: " + buffer.readerIndex( ));
        System.out.println("writable bytes: " + buffer.writableBytes( ));
        System.out.println("writable index: " + buffer.writerIndex( ));
    }

}

结果输出如下：

readable bytes: 0
readable index: 0
writable bytes: 10
writable index: 0
after write one integer
readable bytes: 4
readable index: 0
writable bytes: 6
writable index: 4
after write two integer
readable bytes: 8
readable index: 0
writable bytes: 2
writable index: 8
after read one integer: 10
readable bytes: 4
readable index: 4
writable bytes: 2
writable index: 8
after discard read bytes
readable bytes: 4
readable index: 0
writable bytes: 6
writable index: 4
after reset reader index
readable bytes: 4
readable index: 0
writable bytes: 6
writable index: 4
after reset writer index
readable bytes: 0
readable index: 0
writable bytes: 10
writable index: 0