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
}
}
}