NIO组件Selector调用实例

*对于nio的非阻塞I/O操作,使用Selector获取哪些I/O准备就绪,注册的SelectionKey集合记录关联的Channel这些信息.SelectionKey记录Channel对buffer的操作方式.

---SelectableChannel,Selector,SelectionKey是nio中Channel操作的3个主要部件.

---对应关系,

一个SelectableChannel,记录了一组注册的SelectionKey[]

一个SelectionKey,关联了一个Channel和一个Selector.

一个Selector,维护着注册的一组SelectionKey

*使用示例

---注,这段代码来自"TCP/IP Sockets in Java",典型示例,我做了注解

package chapter5;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.util.Iterator;

public class TCPServerSelector {
	private static final int BUFSIZE = 256;  //Buffer,EchoSelectorProtocol
	private static final int TIMEOUT = 3000; //Selector.select(long timeout) 

	public static void main(String[] args) throws IOException {
		if(args.length<1){
			throw new IllegalArgumentException("Parameter(s): <Port> ...");
		}

		Selector selector =Selector.open(); //创建Selector实例
		for(String arg:args){
			ServerSocketChannel listnChannel =ServerSocketChannel.open();
			listnChannel.socket().bind(new InetSocketAddress(Integer.parseInt(arg)));
			listnChannel.configureBlocking(false); //nonblocking
			listnChannel.register(selector, SelectionKey.OP_ACCEPT); //Channel注册selector,并告知channel感兴趣的操作
		}

		TCPProtocol protocol =new EchoSelectorProtocol(BUFSIZE);
		while(true){ //循环
			if(selector.select(TIMEOUT)==0){ //返回准备就绪I/O的SelectionKey数
				System.out.println("."); //to do others
				continue;
			}

			Iterator<SelectionKey> keyIter =selector.selectedKeys().iterator();//获取已选的SelectionKey集合
			while(keyIter.hasNext()){ //遍历key,根据key的类型做相应处理
				SelectionKey key =keyIter.next();
				if(key.isAcceptable())
					protocol.handleAccept(key);

				if(key.isReadable())
					protocol.handleRead(key);

				//SelectionKey is invalid if it is cancelled, its channel is closed, or its selector is closed.
				if(key.isValid() && key.isWritable())
					protocol.handleWrite(key);

				keyIter.remove(); //手动清空,因为Selector只会在已选的SelectionKey集中添加
			}
		}

	}
}

package chapter5;

import java.io.IOException;
import java.nio.channels.SelectionKey;

public interface TCPProtocol {
	void handleAccept(SelectionKey key) throws IOException;
	void handleRead(SelectionKey key) throws IOException;
	void handleWrite(SelectionKey key) throws IOException;
}

package chapter5;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;

public class EchoSelectorProtocol implements TCPProtocol {

	private int bufSize;

	public EchoSelectorProtocol(int bufSize){
		this.bufSize =bufSize;
	}

	@Override
	public void handleAccept(SelectionKey key) throws IOException {
		SocketChannel clntChan = ((ServerSocketChannel)key.channel()).accept();
		clntChan.configureBlocking(false);
		//对于已注册Selector的Channel,再次调用就更新注册信息,这里更新了SelectionKey的类型和附件,附件是需要操作的buffer.
		clntChan.register(key.selector(), SelectionKey.OP_READ,ByteBuffer.allocate(bufSize));
	}

	@Override
	public void handleRead(SelectionKey key) throws IOException {
		SocketChannel clntChan =(SocketChannel)key.channel();
		ByteBuffer buf =(ByteBuffer) key.attachment(); //获取附件
		long bytesRead = clntChan.read(buf);
		if(bytesRead==-1) //channel已读到结束位置
			clntChan.close();
		else if(bytesRead > 0)
			key.interestOps(SelectionKey.OP_READ|SelectionKey.OP_WRITE);
	}

	@Override
	public void handleWrite(SelectionKey key) throws IOException {
		ByteBuffer buf =(ByteBuffer) key.attachment();
		buf.flip();
		SocketChannel clntChan =(SocketChannel) key.channel();
		clntChan.write(buf);
		if(!buf.hasRemaining()){
			key.interestOps(SelectionKey.OP_READ); //设置Key的兴趣集
		}
		buf.compact();
	}

}

看了代码,如何使用就清楚了吧:).

下面再做些细节说明

*Selector

---更新准备好的SelectionKey,移除isValid()为false的SelectionKey

select() //阻塞等待,直至一个channel准备好或调用wakeup()才返回

select(long timeout) //如上,返回条件多了个超时时间

selectNow() //非阻塞,会立刻返回,没有时返回值=0

wakeup() //使得Selector返回

注意,select()会在上次已选择的键集中添加这次的可用键,故在2次select之间,手动移除已处理的SelectionKey.

*SelectionKey

---兴趣操作集,通过它就可以知道channel可以去做哪些事了.有4种类型,如下:

public static final int OP_READ = 1 << 0;

public static final int OP_ACCEPT = 1 << 4;

public static final int OP_WRITE = 1 << 2;

public static final int OP_CONNECT = 1 << 3;

通过SelectionKey.interestOps(int ops)就可以配置这些值

---附件,主要作用是为channel处理提供辅助信息,如上面示例中att为ByteBuffer

SelectionKey.attach(Object ob) //添加附件,另一种方式SelectableChannel.register(Selector sel, int ops, Object att)

SelectionKey.attachment()  //获取附件

---SelectionKey.cancel(),永久的注销键,加入Selector的注销集中,在下次select()时被移除