《Java Concurrency in Practice》ch6 Task Execution

最新推荐文章于 2020-05-29 22:55:01 发布

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#concurrency #java #thread #timer #asynchronous #exception

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本文介绍了并发编程中的三个实用建议：使用Executor而非Thread来提高任务执行的灵活性；使用ScheduledThreadPoolExecutor代替Timer避免线程泄露；以及使用Callable与Future替代Runnable以更好地管理和获取任务结果。

3个notes，

1. don't use Thread, use Executor

Executor may be a simple interface, but it forms the basis for a flexible and powerful framework for asynchronous task execution that supports a wide variety of task execution policies. It provides a standard means of decoupling task submission from task execution, describing tasks with Runnable. The Executor implementations also provide lifecycle support and hooks for adding statistics gathering, application management, and monitoring.

Executor is based on the producer-consumer pattern, where activities that submit tasks are the producers (producing units of work to be done) and the threads that execute tasks are the consumers (consuming those units of work).

2. don't use Timer, use ScheduledThreadPoolExecutor (thread leakage)

thread leakage 例子，

import java.util.Timer;
import java.util.TimerTask;


/**
 * thread leakage
 *
 */
public class OutOfTime {

	public static void main(String [] args) throws InterruptedException{
		Timer timer = new Timer();
		timer.schedule(new ThrowTask(), 1);
		Thread.sleep(1000);
		timer.schedule(new ThrowTask(), 1);
		Thread.sleep(50000);
	}
	
	static class ThrowTask extends TimerTask {
		public void run() {
			throw new RuntimeException();
		}
	}
}

Question: main will exit after how many seconds?

1) 1

2) 51

Run Result:

Exception in thread "Timer-0" java.lang.RuntimeException
	at OutOfTime$ThrowTask.run(OutOfTime.java:21)
	at java.util.TimerThread.mainLoop(Timer.java:512)
	at java.util.TimerThread.run(Timer.java:462)
Exception in thread "main" java.lang.IllegalStateException: Timer already cancelled.
	at java.util.Timer.sched(Timer.java:354)
	at java.util.Timer.schedule(Timer.java:170)
	at OutOfTime.main(OutOfTime.java:15)

Answer: 1 second

3. don't user Runnable, use Callable/Future

Finding Exploitable Parallelism 找出可以利用的并行性

The real performance payoff of dividing a program's workload into tasks comes when there are a large number of independent, homogeneous tasks that can be processed concurrently.

在异构任务并行化中存在的局限:只有当大量相互独立且同构的任务可以并发进行处理时，才能体现出将程序的工作负载分配到多个任务中带来的真正性能提升。