NIO中几个非常重要的技术点

参考：http://ifeve.com/selectors/

参考：https://www.ibm.com/developerworks/cn/education/java/j-nio/j-nio.html

netty的NioEventLoop类的实现也是类似

这些都是在实践中踩过雷的，今天某应用再次踩雷，把遇到的几个雷都收集一下，给后来者参考。

1.即使是accept事件，没有真正的read和write,Channel也要关闭，否则unix domain socket会被泄漏（WINDOWS更可怕），因为NIO的每个

Channel上都有两个FD用来监听事件（接收和发送走不同的FD）。

2.cancel事件导致CPU占用100%，http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6403933

其原因就是调用key.cancel()时底层在下一次seelect前并没有真正的取消。导致等待select事件返回却又没有返回我们注册的key.这个事件不断地

循环触发，CPU一直处理返回 key为0的select()调用。解决方法有两种，一是在key.cancel()后立即selectNow();但是如果是多线程并发操作，有

可能这两行语句中间线程被切换，使得key.cancel()后没有立即执行 selectNow().这在多Selector情况下是可能的。另一种就是jetty处理方式，如果

select()返回0且连续几次出现这样的情况（有事件触发返回，却不是返回我们注册的KEY），就将有效的key重新注册到一个新的selector上。其实

glassfish在处理多次次次次write返回为0的情况时也是这种策略。

示例代码：（真实的项目中）

int selectTimeout = connectionConfig.getSelectTimeout();
int allProcessMaxTime = connectionConfig.getAllProcessMaxTime();
//selector在实现时有bug,epool底层可能会发送一个错误的信号导致select方法提前返回，但没有
//返回注册的事件，而且不断循环造成CPU100％
int slelectZeroCount = 0;
int maxZeroCount = 20;
int fixed = 0;  

while (selector.isOpen() && selector.keys().size() != 0 && allProcessMaxTime > 0) {
    long start = System.currentTimeMillis();
    // 查询看是否有已经准备好的通道,指定超时时间
    int count = selector.select(selectTimeout);  

    if (count == 0) {
        slelectZeroCount++;
    } else {
        slelectZeroCount = 0;
        //保证是连续的count==0时才将slelectZeroCount++,如果其中有一次返回注册事件测已经正常
    }
    if (slelectZeroCount > maxZeroCount && fixed == 0) {
        //没有尝试修复动作，则先进行修复干预
        for (SelectionKey key : selector.keys()) {
            if (key.isValid() && key.interestOps() == 0) {
                key.cancel();
            }
        }
        fixed = 1;
    } else if (slelectZeroCount > maxZeroCount && fixed == 1) {
        //如果已经干预过仍然连续返回0，注意如果不返回0的话slelectZeroCount就被置0.
        //重新获取一个selector,将当前事件重新注册到新的selector上。并销毁当前selector
        Selector newSelector = this.getSelector();
        this.changeSelector(selector, newSelector);
        selector = newSelector;
    }
    //对channel进行正常处理

重新注册的代码：

private synchronized void changeSelector(Selector oldSelector, Selector newSelector) {
        for (SelectionKey key : oldSelector.keys()) {
            if (!key.isValid() || key.interestOps() == 0) {
                continue;
            }
            Object att = key.attachment();
            try {
                if (att == null) {
                    key.channel().register(newSelector, key.interestOps());
                } else {
                    key.channel().register(newSelector, key.interestOps(), att);
                }
            } catch (ClosedChannelException e) {
                SocketChannel sc = (SocketChannel) key.channel();
                sc.close();
            }
        }
        try {
            oldSelector.close();
        } catch (IOException e) {
            logger.error(e.getMessage());
        }  

    }

　同样对于网络状态不好时，连续写操作返回0的处理：　

　

private void flushData(Selector selector, SocketChannel socketChannel, ByteBuffer byteBuffer)
        throws IOException {  

    int count = 0;
    int maxCount = 20;  

    while (byteBuffer.hasRemaining()) {
        int len = socketChannel.write(byteBuffer);
        if (len < 0) {
            throw new EOFException("write channel is closed.");
        }
        // 如果不对len==0(即当前网络不可用)的情况处理，则while(byteBuffer.hasRemaining())可能一直
        // 循环下去而消耗大量的CPU.
        if (len == 0) {
            count++;
        } else {
            count = 0;
        }
        if (count > maxCount) {
            throw new IOException("can't connect to target.");
        }
    }  

}  

我自己写的代码：

package com.eshore.ismp.hbinterface.crm;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.nio.charset.Charset;
import java.util.Iterator;
import java.util.Set;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.eshore.ismp.hbinterface.service.BizCommonService;

/**
 * @author mercy
 *接收CRM优惠工单信息
 */
public class CrmServerTest {
	private static final Logger logger = LoggerFactory.getLogger(CrmServerTest.class);
	private Selector selector=null;
	private ServerSocketChannel serverSocketChannel=null;
	private int port=10003;
	//private Charset charset=Charset.forName("GBK");//返回一个字符类型对象
	public CrmServerTest() throws IOException{
		serverSocketChannel=ServerSocketChannel.open();
		serverSocketChannel.socket().setReuseAddress(true);
		serverSocketChannel.configureBlocking(false);//设置无阻塞模式
		serverSocketChannel.socket().bind(new InetSocketAddress(port));
		logger.info("服务已启动...");
	}
	public void start(BizCommonService bizCommonService) throws IOException{
		buildSelector();
		this.service(bizCommonService);
	}
	public void buildSelector()throws IOException{
		logger.info("构建selector");
		selector=Selector.open();
		//创建Selector对象
		serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
    }
	//重建selector
	public void rebuildSelector(){
		logger.info("重建selector");
		Selector oldSelector=selector;
		if (oldSelector == null) {
            return;
        }
		try {
			selector=Selector.open();
		} catch (IOException e1) {
			e1.printStackTrace();
		}
		int nChannels = 0;
		for (SelectionKey key: oldSelector.keys()) {
			//返回key的附加对象
			 Object a = key.attachment();
			 try{
				 if (!key.isValid() || key.channel().keyFor(selector) != null) {
	                 continue;
	             }
				 int interestOps = key.interestOps();
	             key.cancel();
	             key.channel().register(selector, interestOps, a);
	             //就不再更新通道的selectKey
	             nChannels ++;
	         } catch (Exception e) {
	             logger.warn("Failed to re-register a Channel to the new Selector.", e);
	         }
		}
		try {
            // time to close the old selector as everything else is registered to the new one
            oldSelector.close();
        } catch (Throwable t) {
            if (logger.isWarnEnabled()) {
                logger.warn("Failed to close the old Selector.", t);
            }
        }
		  logger.info("Migrated " + nChannels + " channel(s) to the new Selector.");
    }
	public void service(BizCommonService bizCommonService) throws IOException{
		int start=0;
		while(true){
			if(selector.select()==0){
				start++;
				logger.info("continue....");
				continue;
			}
			if(start==5000){
				start=0;
				logger.info("重建selector");
				rebuildSelector();
			}
			Set readyKeys=selector.selectedKeys();
			Iterator it=readyKeys.iterator();
			while(it.hasNext()){
				SelectionKey key=null;
				try{
					key=(SelectionKey)it.next();
					it.remove();//删除集合中的key
					if(key.isAcceptable()){//是否可以接收客户端的socket连接
						ServerSocketChannel ssc=(ServerSocketChannel)key.channel();
						SocketChannel socketChannel=ssc.accept();
						//logger.info("接收到的客户端连接,来自："+socketChannel.socket().getInetAddress()+":"+socketChannel.socket().getPort());
						socketChannel.configureBlocking(false);//设置无阻塞模式
						ByteBuffer buffer=ByteBuffer.allocate(6000);//创建一个ByteBuffer对象用于存放数据(数据存放缓冲区)
						socketChannel.register(selector,SelectionKey.OP_READ|SelectionKey.OP_WRITE,buffer);//注册事件,Selector会监控事件是否发生
					}
					if(key.isReadable()){//key的channel是否可读
						receive(key);
					}
					if(key.isWritable()){//key的channel是否可写
						send(key,bizCommonService);
					}
				}catch(IOException e){
					logger.info(" crm exception....");
					e.printStackTrace();
					try{
						if(key!=null){
							key.cancel();
							key.channel().close();
							selector.selectNow();
						}
					}catch(Exception ex){
						ex.printStackTrace();
					}
				}
			}
		}

	}
	/**
	 * @param key
	 * @throws IOException
	 * @author mercy
	 * 根据读取的数据处理完返回给客户端
	 */
	public void send(SelectionKey key,BizCommonService bizCommonService) throws IOException{
		ByteBuffer buffer=(ByteBuffer) key.attachment();
		SocketChannel socketChannel=(SocketChannel) key.channel();
		buffer.flip();
		String data=decode(buffer);//解码客户端发过来的数据
		if(data.length()==0){
			return ;
		}
		String outputData=data;//.substring(0, data.indexOf("\n")+1);
		//logger.info("客户端发送的数据:"+outputData+",length:"+outputData.length());
		//String reply="FFFF02141433570200012400050301IBSS01662       001023CS0214143357*0189086510002001100301420170214143500004003099005007success";
		//ByteBuffer outputBuffer=encode("echo:"+reply);//返回给客户端的数据
		boolean result = bizCommonService.sendOperToCacheAysn(String.valueOf(outputData));
		//创建响应报文
		String res = bizCommonService.createResponseStr(String.valueOf(outputData),result);
		ByteBuffer outputBuffer=encode(res);//返回给客户端的数据
		while(outputBuffer.hasRemaining()){
			//System.out.println("=="+decode(outputBuffer));
			socketChannel.write(outputBuffer);
		}
		ByteBuffer temp=encode(outputData);
		buffer.position(temp.limit());
		buffer.compact();
		if(outputData.length()==0){
			key.cancel();
			socketChannel.close();
			logger.info("关闭与某客户端的连接");
		}
	}
	/**
	 * @param key
	 * @throws IOException
	 * @author mercy
	 * 读取客户端发来的数据
	 */
	public void receive(SelectionKey key) throws IOException{
		ByteBuffer buffer=(ByteBuffer) key.attachment();
		SocketChannel socketChannel=(SocketChannel) key.channel();
		ByteBuffer readBuffer=ByteBuffer.allocate(6000);//创建自定义内存的buffer(存放读到的数据)
		socketChannel.read(readBuffer);
		readBuffer.flip();
		buffer.limit(buffer.capacity());//设置buffer的极限为buffer的容量
		buffer.put(readBuffer);//复制到缓存区
	}
	public String decode(ByteBuffer buffer){//解码
		CharBuffer charBuffer=Charset.forName("GBK").decode(buffer);
		return charBuffer.toString();

	}
	public ByteBuffer encode(String str){//编码
		return Charset.forName("GBK").encode(str);
	}
	public static void main(String[] args) throws IOException {
		final Logger log = LoggerFactory.getLogger(CrmServerTest.class);
		try{
			ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(
					new String[] { "applicationContext.xml" });
			context.start();
			BizCommonService bizCommonService = (BizCommonService) context.getBean("bizCommonService");
			new CrmServerTest().start(bizCommonService);
		}catch(Exception e){
			log.error("start agent interface server error:",e);
			System.exit(-1);
		}
	}
}

一般来说

 while(true){
            if(selector.select()==0){
                start++;
                logger.info("continue....");
                continue;
            }

　　没有

selector.select()>0

好，因为万一　selector.select() 不阻塞了返回0用while(true)会反复执行 if(selector.select()==0) 这一段