详解Java编程中线程同步以及定时启动线程的方法

使用wait()与notify()实现线程间协作
1. wait()与notify()/notifyAll()
调用sleep()和yield()的时候锁并没有被释放,而调用wait()将释放锁。这样另一个任务(线程)可以获得当前对象的锁,从而进入它的synchronized方法中。可以通过notify()/notifyAll(),或者时间到期,从wait()中恢复执行。
只能在同步控制方法或同步块中调用wait()、notify()和notifyAll()。如果在非同步的方法里调用这些方法,在运行时会抛出IllegalMonitorStateException异常。
2.模拟单个线程对多个线程的唤醒
模拟线程之间的协作。Game类有2个同步方法prepare()和go()。标志位start用于判断当前线程是否需要wait()。Game类的实例首先启动所有的Athele类实例,使其进入wait()状态,在一段时间后,改变标志位并notifyAll()所有处于wait状态的Athele线程。
Game.java

package concurrency;

import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

class Athlete implements Runnable {
  private final int id;
  private Game game;

  public Athlete(int id, Game game) {
   this.id = id;
   this.game = game;
  }

  public boolean equals(Object o) {
   if (!(o instanceof Athlete))
    return false;
   Athlete athlete = (Athlete) o;
   return id == athlete.id;
  }

  public String toString() {
   return "Athlete<" + id + ">";
  }

  public int hashCode() {
   return new Integer(id).hashCode();
  }

  public void run() {
   try {
    game.prepare(this);
   } catch (InterruptedException e) {
    System.out.println(this + " quit the game");
   }
  }
 }

public class Game implements Runnable {
  private Set<Athlete> players = new HashSet<Athlete>();
  private boolean start = false;

  public void addPlayer(Athlete one) {
   players.add(one);
  }

  public void removePlayer(Athlete one) {
   players.remove(one);
  }

  public Collection<Athlete> getPlayers() {
   return Collections.unmodifiableSet(players);
  }

  public void prepare(Athlete athlete) throws InterruptedException {
   System.out.println(athlete + " ready!");
   synchronized (this) {
    while (!start)
    wait();
    if (start)
     System.out.println(athlete + " go!");
   }
  }

  public synchronized void go() {
   notifyAll();
  }
  
  public void ready() {
   Iterator<Athlete> iter = getPlayers().iterator();
   while (iter.hasNext())
    new Thread(iter.next()).start();
  }

  public void run() {
   start = false;
   System.out.println("Ready......");
   System.out.println("Ready......");
   System.out.println("Ready......");
   ready();
   start = true;
   System.out.println("Go!");
   go();
  }

  public static void main(String[] args) {
   Game game = new Game();
   for (int i = 0; i < 10; i++)
    game.addPlayer(new Athlete(i, game));
   new Thread(game).start();
  }
}

结果:

Ready......
Ready......
Ready......
Athlete<0> ready!
Athlete<1> ready!
Athlete<2> ready!
Athlete<3> ready!
Athlete<4> ready!
Athlete<5> ready!
Athlete<6> ready!
Athlete<7> ready!
Athlete<8> ready!
Athlete<9> ready!
Go!
Athlete<9> go!
Athlete<8> go!
Athlete<7> go!
Athlete<6> go!
Athlete<5> go!
Athlete<4> go!
Athlete<3> go!
Athlete<2> go!
Athlete<1> go!
Athlete<0> go!

3.模拟忙等待过程
MyObject类的实例是被观察者,当观察事件发生时,它会通知一个Monitor类的实例(通知的方式是改变一个标志位)。而此Monitor类的实例是通过忙等待来不断的检查标志位是否变化。
BusyWaiting.java

import java.util.concurrent.TimeUnit;

class MyObject implements Runnable {
  private Monitor monitor;

  public MyObject(Monitor monitor) {
   this.monitor = monitor;
  }

  public void run() {
   try {
    TimeUnit.SECONDS.sleep(3);
    System.out.println("i'm going.");
    monitor.gotMessage();
   } catch (InterruptedException e) {
    e.printStackTrace();
   }
  }
}

class Monitor implements Runnable {
  private volatile boolean go = false;

  public void gotMessage() throws InterruptedException {
   go = true;
  }

  public void watching() {
   while (go == false)
    ;
   System.out.println("He has gone.");
  }

  public void run() {
   watching();
  }
}

public class BusyWaiting {
  public static void main(String[] args) {
   Monitor monitor = new Monitor();
   MyObject o = new MyObject(monitor);
   new Thread(o).start();
   new Thread(monitor).start();
  }
}

结果:

i'm going.
He has gone.

4.使用wait()与notify()改写上面的例子
下面的例子通过wait()来取代忙等待机制,当收到通知消息时,notify当前Monitor类线程。
Wait.java

package concurrency.wait;

import java.util.concurrent.TimeUnit;

class MyObject implements Runnable {
  private Monitor monitor;

  public MyObject(Monitor monitor) {
   this.monitor = monitor;
  }

定时启动线程
这里提供两种在指定时间后启动线程的方法。一是通过java.util.concurrent.DelayQueue实现;二是通过java.util.concurrent.ScheduledThreadPoolExecutor实现。
1. java.util.concurrent.DelayQueue
类DelayQueue是一个无界阻塞队列,只有在延迟期满时才能从中提取元素。它接受实现Delayed接口的实例作为元素。
<<interface>>Delayed.java

package java.util.concurrent;
import java.util.*;
public interface Delayed extends Comparable<Delayed> {
  long getDelay(TimeUnit unit);
}

getDelay()返回与此对象相关的剩余延迟时间,以给定的时间单位表示。此接口的实现必须定义一个 compareTo 方法,该方法提供与此接口的 getDelay 方法一致的排序。

DelayQueue队列的头部是延迟期满后保存时间最长的 Delayed 元素。当一个元素的getDelay(TimeUnit.NANOSECONDS) 方法返回一个小于等于 0 的值时,将发生到期。
2.设计带有时间延迟特性的队列
类DelayedTasker维护一个DelayQueue<DelayedTask> queue,其中DelayedTask实现了Delayed接口,并由一个内部类定义。外部类和内部类都实现Runnable接口,对于外部类来说,它的run方法是按定义的时间先后取出队列中的任务,而这些任务即内部类的实例,内部类的run方法定义每个线程具体逻辑。

这个设计的实质是定义了一个具有时间特性的线程任务列表,而且该列表可以是任意长度的。每次添加任务时指定启动时间即可。
DelayedTasker.java

package com.zj.timedtask;

import static java.util.concurrent.TimeUnit.SECONDS;
import static java.util.concurrent.TimeUnit.NANOSECONDS;

import java.util.Collection;
import java.util.Collections;
import java.util.Random;
import java.util.concurrent.DelayQueue;
import java.util.concurrent.Delayed;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class DelayedTasker implements Runnable {
  DelayQueue<DelayedTask> queue = new DelayQueue<DelayedTask>();

  public void addTask(DelayedTask e) {
    queue.put(e);
  }

  public void removeTask() {
    queue.poll();
  }

  public Collection<DelayedTask> getAllTasks() {
    return Collections.unmodifiableCollection(queue);
  }

  public int getTaskQuantity() {
    return queue.size();
  }

  public void run() {
    while (!queue.isEmpty())
      try {
       queue.take().run();
      } catch (InterruptedException e) {
       System.out.println("Interrupted");
      }
    System.out.println("Finished DelayedTask");
  }

  public static class DelayedTask implements Delayed, Runnable {
    private static int counter = 0;
    private final int id = counter++;
    private final int delta;
    private final long trigger;

    public DelayedTask(int delayInSeconds) {
      delta = delayInSeconds;
      trigger = System.nanoTime() + NANOSECONDS.convert(delta, SECONDS);
    }

    public long getDelay(TimeUnit unit) {
      return unit.convert(trigger - System.nanoTime(), NANOSECONDS);
    }

    public int compareTo(Delayed arg) {
      DelayedTask that = (DelayedTask) arg;
      if (trigger < that.trigger)
       return -1;
      if (trigger > that.trigger)
       return 1;
      return 0;
    }

    public void run() {
      //run all that you want to do
      System.out.println(this);
    }

    public String toString() {
      return "[" + delta + "s]" + "Task" + id;
    }
  }

  public static void main(String[] args) {
    Random rand = new Random();
    ExecutorService exec = Executors.newCachedThreadPool();
    DelayedTasker tasker = new DelayedTasker();
    for (int i = 0; i < 10; i++)
      tasker.addTask(new DelayedTask(rand.nextInt(5)));
    exec.execute(tasker);
    exec.shutdown();
  }
}

结果:

[0s]Task 1
[0s]Task 2
[0s]Task 3
[1s]Task 6
[2s]Task 5
[3s]Task 8
[4s]Task 0
[4s]Task 4
[4s]Task 7
[4s]Task 9
Finished DelayedTask

3. java.util.concurrent.ScheduledThreadPoolExecutor
该类可以另行安排在给定的延迟后运行任务(线程),或者定期(重复)执行任务。在构造子中需要知道线程池的大小。最主要的方法是:

[1] schedule
public ScheduledFuture<?> schedule(Runnable command, long delay,TimeUnit unit)
创建并执行在给定延迟后启用的一次性操作。
指定者:
-接口 ScheduledExecutorService 中的 schedule;
参数:
-command - 要执行的任务 ;
-delay - 从现在开始延迟执行的时间 ;
-unit - 延迟参数的时间单位 ;
返回:
-表示挂起任务完成的 ScheduledFuture,并且其 get() 方法在完成后将返回 null。
 
[2] scheduleAtFixedRate
public ScheduledFuture<?> scheduleAtFixedRate(
Runnable command,long initialDelay,long period,TimeUnit unit)
创建并执行一个在给定初始延迟后首次启用的定期操作,后续操作具有给定的周期;也就是将在 initialDelay 后开始执行,然后在 initialDelay+period 后执行,接着在 initialDelay + 2 * period 后执行,依此类推。如果任务的任何一个执行遇到异常,则后续执行都会被取消。否则,只能通过执行程序的取消或终止方法来终止该任务。如果此任务的任何一个执行要花费比其周期更长的时间,则将推迟后续执行,但不会同时执行。
指定者:
-接口 ScheduledExecutorService 中的 scheduleAtFixedRate;
参数:
-command - 要执行的任务 ;
-initialDelay - 首次执行的延迟时间 ;
-period - 连续执行之间的周期 ;
-unit - initialDelay 和 period 参数的时间单位 ;
返回:
-表示挂起任务完成的 ScheduledFuture,并且其 get() 方法在取消后将抛出异常。
4.设计带有时间延迟特性的线程执行者
类ScheduleTasked关联一个ScheduledThreadPoolExcutor,可以指定线程池的大小。通过schedule方法知道线程及延迟的时间,通过shutdown方法关闭线程池。对于具体任务(线程)的逻辑具有一定的灵活性(相比前一中设计,前一种设计必须事先定义线程的逻辑,但可以通过继承或装饰修改线程具体逻辑设计)。
ScheduleTasker.java

package com.zj.timedtask;

import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class ScheduleTasker {
  private int corePoolSize = 10;
  ScheduledThreadPoolExecutor scheduler;

  public ScheduleTasker() {
    scheduler = new ScheduledThreadPoolExecutor(corePoolSize);
  }

  public ScheduleTasker(int quantity) {
    corePoolSize = quantity;
    scheduler = new ScheduledThreadPoolExecutor(corePoolSize);
  }

  public void schedule(Runnable event, long delay) {
    scheduler.schedule(event, delay, TimeUnit.SECONDS);
  }

  public void shutdown() {
    scheduler.shutdown();
  }

  public static void main(String[] args) {
    ScheduleTasker tasker = new ScheduleTasker();
    tasker.schedule(new Runnable() {
      public void run() {
       System.out.println("[1s]Task 1");
      }
    }, 1);
    tasker.schedule(new Runnable() {
      public void run() {
       System.out.println("[2s]Task 2");
      }
    }, 2);
    tasker.schedule(new Runnable() {
      public void run() {
       System.out.println("[4s]Task 3");
      }
    }, 4);
    tasker.schedule(new Runnable() {
      public void run() {
       System.out.println("[10s]Task 4");
      }
    }, 10);

    tasker.shutdown();
  }
}

结果:

[1s]Task 1
[2s]Task 2
[4s]Task 3
[10s]Task 4
  public void run() {
   try {
    TimeUnit.SECONDS.sleep(3);
    System.out.println("i'm going.");
    monitor.gotMessage();
   } catch (InterruptedException e) {
    e.printStackTrace();
   }
  }
}
class Monitor implements Runnable {
  private volatile boolean go = false;

  public synchronized void gotMessage() throws InterruptedException {
   go = true;
   notify();
  }

  public synchronized void watching() throws InterruptedException {
   while (go == false)
    wait();
   System.out.println("He has gone.");
  }

  public void run() {
   try {
    watching();
   } catch (InterruptedException e) {
    e.printStackTrace();
   }
  }
}

public class Wait {
  public static void main(String[] args) {
   Monitor monitor = new Monitor();
   MyObject o = new MyObject(monitor);
   new Thread(o).start();
   new Thread(monitor).start();
  }
}

结果:

i'm going.
He has gone.

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