nio~Selector

*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