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Java ThreadPoolExecutor的参数深入理解

2019年07月22日  | 移动技术网IT编程  | 我要评论

java threadpoolexecutor的参数深入理解

一、使用executors创建线程池   

        之前创建线程的时候都是用的executors的newfixedthreadpool(),newsinglethreadexecutor(),newcachedthreadpool()这三个方法。当然executors也是用不同的参数去new threadpoolexecutor

    1. newfixedthreadpool()

    创建线程数固定大小的线程池。 由于使用了linkedblockingqueue所以maximumpoolsize 没用，当corepoolsize满了之后就加入到linkedblockingqueue队列中。每当某个线程执行完成之后就从linkedblockingqueue队列中取一个。所以这个是创建固定大小的线程池。

 public static executorservice newfixedthreadpool(int nthreads) {
    return new threadpoolexecutor(nthreads, nthreads,
                   0l, timeunit.milliseconds,
                   new linkedblockingqueue<runnable>());
  }
 public threadpoolexecutor(int corepoolsize,
               int maximumpoolsize,
               long keepalivetime,
               timeunit unit,
               blockingqueue<runnable> workqueue) {
   this(corepoolsize, maximumpoolsize, keepalivetime, unit, workqueue,
       executors.defaultthreadfactory(), defaulthandler);
  }

   2.newsinglethreadpool()

    创建线程数为1的线程池，由于使用了linkedblockingqueue所以maximumpoolsize 没用，corepoolsize为1表示线程数大小为1,满了就放入队列中，执行完了就从队列取一个。

 public static executorservice newsinglethreadexecutor() {
    return new finalizabledelegatedexecutorservice
      (new threadpoolexecutor(1, 1,
                  0l, timeunit.milliseconds,
                  new linkedblockingqueue<runnable>()));
  }

    3.newcachedthreadpool()

    创建可缓冲的线程池。没有大小限制。由于corepoolsize为0所以任务会放入synchronousqueue队列中，synchronousqueue只能存放大小为1，所以会立刻新起线程，由于maxumumpoolsize为integer.max_value所以可以认为大小为2147483647。受内存大小限制。

public static executorservice newcachedthreadpool() {
    return new threadpoolexecutor(0, integer.max_value,
                   60l, timeunit.seconds,
                   new synchronousqueue<runnable>());
  }
public threadpoolexecutor(int corepoolsize,
             int maximumpoolsize,
             long keepalivetime,
             timeunit unit,
             blockingqueue<runnable> workqueue) {
    this(corepoolsize, maximumpoolsize, keepalivetime, unit, workqueue,
       executors.defaultthreadfactory(), defaulthandler);
  }

二、使用threadpoolexecutor创建线程池

threadpoolexecutor的构造函数

 public threadpoolexecutor(int corepoolsize,
               int maximumpoolsize,
               long keepalivetime,
               timeunit unit,
               blockingqueue<runnable> workqueue,
               threadfactory threadfactory,
               rejectedexecutionhandler handler) {
    if (corepoolsize < 0 ||
      maximumpoolsize <= 0 ||
      maximumpoolsize < corepoolsize ||
      keepalivetime < 0)
      throw new illegalargumentexception();
    if (workqueue == null || threadfactory == null || handler == null)
      throw new nullpointerexception();
    this.corepoolsize = corepoolsize;
    this.maximumpoolsize = maximumpoolsize;
    this.workqueue = workqueue;
    this.keepalivetime = unit.tonanos(keepalivetime);
    this.threadfactory = threadfactory;
    this.handler = handler;
  }

参数：

        1、corepoolsize核心线程数大小，当线程数<corepoolsize ，会创建线程执行runnable

        2、maximumpoolsize 最大线程数， 当线程数 >= corepoolsize的时候，会把runnable放入workqueue中

        3、keepalivetime  保持存活时间，当线程数大于corepoolsize的空闲线程能保持的最大时间。

        4、unit 时间单位

        5、workqueue 保存任务的阻塞队列

        6、threadfactory 创建线程的工厂

        7、handler 拒绝策略

任务执行顺序：

        1、当线程数小于corepoolsize时，创建线程执行任务。

        2、当线程数大于等于corepoolsize并且workqueue没有满时，放入workqueue中

        3、线程数大于等于corepoolsize并且当workqueue满时，新任务新建线程运行，线程总数要小于maximumpoolsize

        4、当线程总数等于maximumpoolsize并且workqueue满了的时候执行handler的rejectedexecution。也就是拒绝策略。

threadpoolexecutor默认有四个拒绝策略：

        1、threadpoolexecutor.abortpolicy()   直接抛出异常rejectedexecutionexception

        2、threadpoolexecutor.callerrunspolicy()    直接调用run方法并且阻塞执行

        3、threadpoolexecutor.discardpolicy()   直接丢弃后来的任务

        4、threadpoolexecutor.discardoldestpolicy()  丢弃在队列中队首的任务

当然可以自己继承rejectedexecutionhandler来写拒绝策略.

int corepoolsize = 1;
 int maximumpoolsize = 2;
 int keepalivetime = 10;
// blockingqueue<runnable> workqueue = new linkedblockingqueue<runnable>();
 blockingqueue<runnable> workqueue = new arrayblockingqueue<runnable>(5);
 threadfactory threadfactory = executors.defaultthreadfactory();
 //线程池和队列满了之后的处理方式
 //1.跑出异常
 rejectedexecutionhandler handler = new threadpoolexecutor.abortpolicy(); 
 rejectedexecutionhandler handler2 = new threadpoolexecutor.callerrunspolicy();
 rejectedexecutionhandler handler3 = new threadpoolexecutor.discardpolicy();
 rejectedexecutionhandler handler4 = new threadpoolexecutor.discardoldestpolicy();

 
 threadpoolexecutor threadpoolexecutor = new threadpoolexecutor(corepoolsize, maximumpoolsize, keepalivetime, timeunit.seconds, workqueue, threadfactory, handler2);
 
 
 for (int j = 1; j < 15; j++) {
  threadpoolexecutor.execute(new runnable() {
  
  public void run() {
   
   try {
   system.out.println(thread.currentthread().getname());
   timeunit.seconds.sleep(1);
   } catch (interruptedexception e) {
   e.printstacktrace();
   }
   
   
  }
  });
 }
 
 system.out.println(threadpoolexecutor);
 
 }

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