java.nio.Buffer分析

在研究Apache Mina源代码时，在IoFilter中使用IoBuffer做为数据缓冲对象，而IoBuffer的实现来自于java.nio.Buffer。Buffer中的flip()、clear()、reset()、mark()等概念让我有点糊涂，仔细看了jdk的文档说明，才对Buffer对象的概念了然于胸。

在Buffer类当中有以下四个重要的属性：

mark：标记位，用于reset（）时把position恢复到原来的位置，调用mark（）方法使position的值赋与mark

position：表示Buffer中第一个可以被读取或写入的数据的位置，每次调用put（）方法时,把写入的数据放到position位置，然后position的值就会加1

limit：表示buffer中第一个不可被读取或写入的数据的位置，也即停止位，数据操作到此为止

capacity：初始化时调用allocate(int size)为buffer分配的空间大小，不可变

演示代码如下：

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import java.nio.ByteBuffer;   
public class ByteBufferTest {   
    
    public static void main(String[] args) {    
        ByteBuffer bb=ByteBuffer.allocate(10);   
        for(int i=1;i<9;i++){   
            bb.put((byte)i);   
        }   
           
        System.out.println("pos:"+bb.position());   
        System.out.println("limit:"+bb.limit());   
        System.out.println("cap:"+bb.capacity());   
           
        bb.flip();   
        System.out.println("/nafter flip");    
        System.out.println("pos:"+bb.position());   
        System.out.println("limit:"+bb.limit());   
        System.out.println("cap:"+bb.capacity());   
           
             
        bb.mark();    
        System.out.println("/nafter mark");    
        System.out.println("pos:"+bb.position());   
        System.out.println("limit:"+bb.limit());   
        System.out.println("cap:"+bb.capacity());   
           
           
        bb.reset();   
        System.out.println("/nafter reset");    
        System.out.println("pos:"+bb.position());   
        System.out.println("limit:"+bb.limit());   
        System.out.println("cap:"+bb.capacity());   
           
        bb.clear();   
        System.out.println("/nafter clear");    
        System.out.println("pos:"+bb.position());   
        System.out.println("limit:"+bb.limit());   
        System.out.println("cap:"+bb.capacity());    
           
        bb.limit(1);    
        bb.put((byte)9);   
        bb.put((byte)10);//超出limit范围，抛出java.nio.BufferOverflowException异常   
        bb.put((byte)11);   
    }   
}  
import java.nio.ByteBuffer;
public class ByteBufferTest {
 
	public static void main(String[] args) { 
		ByteBuffer bb=ByteBuffer.allocate(10);
		for(int i=1;i<9;i++){
			bb.put((byte)i);
		}
		
		System.out.println("pos:"+bb.position());
		System.out.println("limit:"+bb.limit());
		System.out.println("cap:"+bb.capacity());
		
		bb.flip();
		System.out.println("/nafter flip"); 
		System.out.println("pos:"+bb.position());
		System.out.println("limit:"+bb.limit());
		System.out.println("cap:"+bb.capacity());
		
		  
		bb.mark(); 
		System.out.println("/nafter mark"); 
		System.out.println("pos:"+bb.position());
		System.out.println("limit:"+bb.limit());
		System.out.println("cap:"+bb.capacity());
		
		
		bb.reset();
		System.out.println("/nafter reset"); 
		System.out.println("pos:"+bb.position());
		System.out.println("limit:"+bb.limit());
		System.out.println("cap:"+bb.capacity());
		
		bb.clear();
		System.out.println("/nafter clear"); 
		System.out.println("pos:"+bb.position());
		System.out.println("limit:"+bb.limit());
		System.out.println("cap:"+bb.capacity()); 
		
		bb.limit(1); 
		bb.put((byte)9);
		bb.put((byte)10);//超出limit范围，抛出java.nio.BufferOverflowException异常
		bb.put((byte)11);
	}
}
 

输出结果：

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pos:8  
limit:10  
cap:10  
after flip   
pos:0  
limit:8  
cap:10  
after mark   
pos:0  
limit:8  
cap:10  
after reset   
pos:0  
limit:8  
cap:10  
after clear   
pos:0  
limit:10  
cap:10  
Exception in thread "main" java.nio.BufferOverflowException   
    at java.nio.Buffer.nextPutIndex(Buffer.java:419)   
    at java.nio.HeapByteBuffer.put(HeapByteBuffer.java:145)   
    at com.gftech.mytool.mina.ByteBufferTest.main(ByteBufferTest.java:45)