Java 多线程之-----守护进程


守护进程，顾名思义就是默默在后台运行的进程，当JVM上没有其他进程运行时运行的进程，最典型的一个列子就是JVM自带的垃圾清理机，他就是在其他程序不运行的时候占用CPU来进行清理内存垃圾，因此他具有很强的不确定性，因为你不知道啥时候他就会运行，也不知道啥时候他不会运行，因此适合做一个不太重要的清理工作或者服务器上的监听工作。

Java多线程从简单到复杂

Java多线程经典案例

Java多线程：ReentrantReadWriteLock读写锁的使用

Java内存映射文件实现多线程下载

Java多线程：一道阿里面试题的分析与应对

Java中两种实现多线程方式的对比分析

下面我们使用JVM的daemon来写一个小例子，实现的效果如下

In this recipe, we will learn how to create a daemon thread developing an example with two threads; one user thread that writes events on a queue and a daemon one that cleans that queue, removing the events which were generated more than 10 seconds ago。

这里的队列是一个双向列表。代码如下

package com.bird.concursey;

import java.util.Date;
import java.util.Deque;
import java.util.concurrent.TimeUnit;

public class WriterTask implements Runnable {

 public WriterTask(Deque<Event> deque) {
  this.deque = deque;
 }

 // 这个为双向队列
 private Deque<Event> deque;

 public Deque<Event> getDeque() {
  return deque;
 }

 public void setDeque(Deque<Event> deque) {
  this.deque = deque;
 }

 @Override
 public void run() {
  for(int i = 0; i < 100; i++) {
   Event event = new Event();
   event.setDate(new Date());
   event.setEvent("The Thread " + Thread.currentThread().getId() + " has generated a event");
   deque.addFirst(event);
   try {
    TimeUnit.SECONDS.sleep(1);
   } catch (InterruptedException e) {
    e.printStackTrace();
   }
  }
 }

}

 

package com.bird.concursey;

import java.util.Date;
import java.util.Deque;

public class CleanerTask extends Thread {

 private Deque<Event> deque;

 public CleanerTask(Deque<Event> deque) {
  this.deque = deque;
  //设置为守护进程
  setDaemon(true);
 }
 
 @Override
 public void run() {
  while(true) {
   Date date = new Date();
   clean(date);
  }
 }

 private void clean(Date date) {
  long difference = 0;
  boolean delete = false;
 
  if(deque.size() == 0) {
   return;
  }
 
  do {
   Event e = deque.getLast();
   difference = date.getTime() - e.getDate().getTime();
   if(difference > 10000) {
    System.out.println("cleaner " + e.getEvent());
    deque.removeLast();
    delete = true;
   }
  }while(difference > 10000);
 
  if(delete) {
   System.out.println("cleaner : the size of the deque " + deque.size());
  }
 }

 public Deque<Event> getDeque() {
  return deque;
 }

 public void setDeque(Deque<Event> deque) {
  this.deque = deque;
 }
 
 
}

 

package com.bird.concursey;

import java.util.ArrayDeque;
import java.util.Date;
import java.util.Deque;

public class Event {

 private Date date;
 private String event;

 public Date getDate() {
  return date;
 }

 public void setDate(Date date) {
  this.date = date;
 }

 public String getEvent() {
  return event;
 }

 public void setEvent(String event) {
  this.event = event;
 }
 
 
 public static void main(String[] args) {
  Deque<Event> deque = new ArrayDeque<Event>();
  WriterTask writer = new WriterTask(deque);
  for(int i = 0; i < 3; i++) {
   Thread thread = new Thread(writer);
   thread.start();
  }
  CleanerTask cleaner = new CleanerTask(deque);
  cleaner.start();
 }
}

运行结果就是

cleaner The Thread 12 has generated a event
cleaner The Thread 11 has generated a event
cleaner : the size of the deque 27
cleaner The Thread 13 has generated a event
cleaner : the size of the deque 26
cleaner The Thread 13 has generated a event
cleaner The Thread 12 has generated a event
cleaner The Thread 11 has generated a event
cleaner : the size of the deque 26
cleaner The Thread 12 has generated a e

这个队列总是维护在25-30左右，因为上来三个线程创建了30个事件，然后等创建事件的线程休息的时候，daemon线程就会占用CPU time 来运行清理建立好的事件，这样就会让队列一直维护在25-30左右，最关键的一句//设置为守护进程 setDaemon(true);必须在start之前设置，因为线程一旦运行起来了就不能更改他的状态，这样就开始了监控的状态，守护进程就OK了.

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