java 守护线程

java守护线程的定义：

     1.所谓守护线程就是运行在程序后台的线程，程序的主线程Main（比方java程序一开始启动时创建的那个线程）不会是守护线程.

 

      2.Daemon thread在Java里面的定义是，如果虚拟机中只有Daemon thread 在运行，则虚拟机退出。 虚拟机中可能会同时有很多个线程在运行，只有当所有的非守护线程都结束的时候，虚拟机的进程才会结束，不管在运行的线程是不是main()线程。

 

      3.Main主线程结束了（Non-daemon thread）,如果此时正在运行的其他threads是daemon threads,JVM会使得这个threads停止,JVM也停下.如果此时正在运行的其他threads有Non-daemon threads,那么必须等所有的Non daemon线程结束了，JVM才会停下来.

 

       4.总之,必须等所有的Non-daemon线程都运行结束了，只剩下daemon的时候，JVM才会停下来，注意Main主程序是Non-daemon线程.

 

守护线程实例：

package cn.com.brilliance.atmv.udp;

import java.net.DatagramPacket;
import java.net.DatagramSocket;

import org.apache.log4j.Logger;

public class UDPServer extends UDPObject
{
    public static int MAX_RECEIVE = 1024;
    
    private static Logger logger = Logger.getLogger(UDPServer.class);
    
    private DatagramSocket ds = null;
    private boolean runFlag = false;
    private Thread t = null;
    private boolean isLogin = false;
    
    
    /**
     * default serve port
     */
    private int port = 10001;
    
    public UDPServer(int port)
    {
        super();
        this.port = port;
    }
    
    protected UDPServer()
    {
        super();
    }
    
    class ServerThread implements Runnable
    {

        DatagramSocket ds = null;
        UDPService service = null;
        
        ServerThread(DatagramSocket ds, UDPService service)
        {
            this.ds = ds;
            this.service = service;
        }
        
        public void run()
        {
            byte buffer[] = new byte[MAX_RECEIVE];
            DatagramPacket dp = new DatagramPacket(buffer, buffer.length);
            logger.info("服务线程开始运行");
            for(;;)
            {
                try
                {
                    
                    ds.receive(dp);
                    if(dp.getLength() > 0)
                    {
                    	System.out.println("rece data len:" + dp.getLength());
                        String rev = new String(buffer, 0, dp.getLength());
                        System.out.println("rev is:" + rev);
                        String fromIP = dp.getAddress().getHostAddress();
                        logger.debug("收到报文["  +fromIP + ":" + dp.getPort() +  "]: " + rev);
                        if("#9999#".equals(rev))
                        {
                            logger.info("收到退出指令,线程退出运行");
                            break;
                        }
                        try
                        {
                            service.process(buffer, dp.getLength(), fromIP, dp.getPort());
                        }
                        catch(Exception e)
                        {
                            logger.error("process error:", e);
                        }
                    }
                }
                catch (Exception e)
                {
                    throw new RuntimeException("", e);
                }
            }
            buffer = null;
            dp = null;
        }
        
    }
    
    public void listen(UDPService service) throws UDPException
    {
        if(!runFlag)
        {
            try
            {
                ds = new DatagramSocket(port);
                t = new Thread(new ServerThread(ds, service));
                /*
                 * 程序从main方法进入，如果将t设置为守护线程   这无法执行
                 */
                //t.setDaemon(true);
                t.start();
                runFlag = true;
            }
            catch (Exception e)
            {
                throw new UDPException("创建UDP失败.", e);
            }            
        }
        else
        {
            //do nothing
        }
    }
}