ScheduledExecutorService异常分析

一、源码解析

关注一块代码：

/**
     * Creates and executes a periodic action that becomes enabled first
     * after the given initial delay, and subsequently with the given
     * period; that is executions will commence after
     * {@code initialDelay} then {@code initialDelay+period}, then
     * {@code initialDelay + 2 * period}, and so on.
     * If any execution of the task
     * encounters an exception, subsequent executions are suppressed.
     * Otherwise, the task will only terminate via cancellation or
     * termination of the executor.  If any execution of this task
     * takes longer than its period, then subsequent executions
     * may start late, but will not concurrently execute.
     *
     * @param command the task to execute
     * @param initialDelay the time to delay first execution
     * @param period the period between successive executions
     * @param unit the time unit of the initialDelay and period parameters
     * @return a ScheduledFuture representing pending completion of
     *         the task, and whose {@code get()} method will throw an
     *         exception upon cancellation
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if command is null
     * @throws IllegalArgumentException if period less than or equal to zero
     */
    public ScheduledFuture scheduleAtFixedRate(Runnable command,
                                                  long initialDelay,
                                                  long period,
                                                  TimeUnit unit);

二、说明

由scheduleAtFixedRate产生的周期工作线程，如果遇到任何意外则会停止后续执行；
如果执行的Runnable线程任务没有加异常，则会出现只执行一次则线程停止的问题！特别引起重视！

三、总结

1、获取一个线程池定时任务异常关闭的小tip，注意任何心跳类希望一直保持线程定时任务，为保证线程可靠性，最好捕获异常进行相关保护；
2、灵感源于代码，因此切忌遇到问题要关注代码底层实现，学会看jdk文档接口的详细说明，可以少走很多弯路；
3、学习ScheduledFuture这个返回：

• 继承了Delayed, Future两个接口
• long getDelay(TimeUnit unit);支持返回线程任务的最近间隔时间
• future提供的cancel、get等方法用于任务取消或者获取，作用也很重要，注意学习用法

四、测试代码

package FutureScheuledTest;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;

/**
 * Created by fwding on 2018/3/21 0021.
 */
public class FutureScheduledTest {
    private static ScheduledFuture global_sfd;
    public static void main(String[] args) throws Exception {
        ScheduledExecutorService ses = Executors.newScheduledThreadPool(10);
        ScheduledFuture sfa = ses.scheduleAtFixedRate(new TimerTask("a"), 200,
                1000, TimeUnit.MILLISECONDS);
        ScheduledFuture sfb = ses.scheduleAtFixedRate(new TimerTask("b"), 400,
                1000, TimeUnit.MILLISECONDS);
        ScheduledFuture sfc = ses.scheduleAtFixedRate(new TimerTask("c"), 600,
                1000, TimeUnit.MILLISECONDS);
        global_sfd = ses.scheduleAtFixedRate(new TimerTask("d"), 800,
                1000, TimeUnit.MILLISECONDS);
        Thread.sleep(5000);
        sfa.cancel(true);
        sfb.cancel(true);
        sfc.cancel(true);
        global_sfd.cancel(true);

        Thread.sleep(5000);
        ses.shutdown();
    }

    static class TimerTask implements Runnable{
        private String id;
        public TimerTask(String id){
            this.id = id;
        }
        @Override
        public void run(){
            System.out.println(id);
            //
            System.out.println("ID:" + id +"------------THREAD_D:TIME DELAY:" +
                    global_sfd.getDelay(TimeUnit.MILLISECONDS));
            if (!id.equals("d") && global_sfd.getDelay(TimeUnit.MILLISECONDS) < 400) {
                global_sfd.cancel(true);
                System.out.println("ID:" + id +"------------stop the THREAD_D!!!!!!");
                Thread.currentThread().stop();
            }

            //异常测试
            /*
            try {
                int[] s = new int[2];
                s[0] = 1;
                s[1] = 2;
                s[2] = 3;
            } catch (Exception e) {
                e.printStackTrace();
            }
            */
        }
    }
}