线程池submit的坑

package com.middle.threadpool;

import java.util.concurrent.*;

/**
 * @author 15510
 * @create 2019-07-31 20:39
 */
public class ThreadPoolDemo {
    public static void main(String[] args) {

        ExecutorService executorService = Executors.newFixedThreadPool(4);

        for (int i = 0; i < 5; i++) {
            int index = i;
            executorService.submit(() -> divTask(100, index));
        }
        executorService.shutdown();
    }

    private static void divTask(int a, int b) {
        double result = a / b;
        System.out.println(result);
    }

}

结果如下

100.0
33.0
50.0
25.0

Process finished with exit code 0

上述代码，可以看出运行结果为4个，应该是有5个的，但是当 i=0 的时候，100/0是会报错的，但是日志信息中没有任何信息，是为什么那？如果使用了 submit(Runnable task) 就会出现这种情况，任何的错误信息都出现不了！

---

这是因为使用 submit(Runnable task) 的时候，错误的堆栈信息跑出来的时候会被内部捕获到，所以打印不出来具体的信息让我们查看，解决的方法有如下两种：

1. 使用 execute() 代替 submit()

package com.middle.threadpool;

import java.util.concurrent.*;

/**
 * @author 15510
 * @create 2019-07-31 20:39
 */
public class ThreadPoolDemo {
    public static void main(String[] args) {

        ExecutorService executorService = Executors.newFixedThreadPool(4);

        for (int i = 0; i < 5; i++) {
            int index = i;
            executorService.execute(() -> divTask(100, index));
        }
        executorService.shutdown();
    }

    private static void divTask(int a, int b) {
        double result = a / b;
        System.out.println(result);
    }

}

结果

Exception in thread "pool-1-thread-1" 100.0
25.0
33.0
50.0
java.lang.ArithmeticException: / by zero
	at com.middle.threadpool.ThreadPoolDemo.divTask(ThreadPoolDemo.java:22)
	at com.middle.threadpool.ThreadPoolDemo.lambda$main$0(ThreadPoolDemo.java:16)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

Process finished with exit code 0

2. 使用Future

package com.middle.threadpool;

import java.util.concurrent.*;

/**
 * @author 15510
 * @create 2019-07-31 20:39
 */
public class ThreadPoolDemo {
    public static void main(String[] args) {

        ExecutorService executorService = Executors.newFixedThreadPool(4);

        for (int i = 0; i < 5; i++) {
            int index = i;
            Future<?> future = executorService.submit(() -> divTask(100, index));

            try {
                future.get();
            } catch (InterruptedException | ExecutionException e) {
                e.printStackTrace();
            }
        }
        executorService.shutdown();
    }

    private static void divTask(int a, int b) {
        double result = a / b;
        System.out.println(result);
    }

}

结果

java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
	at java.util.concurrent.FutureTask.report(FutureTask.java:122)
	at java.util.concurrent.FutureTask.get(FutureTask.java:192)
	at com.middle.threadpool.ThreadPoolDemo.main(ThreadPoolDemo.java:19)
Caused by: java.lang.ArithmeticException: / by zero
	at com.middle.threadpool.ThreadPoolDemo.divTask(ThreadPoolDemo.java:28)
	at com.middle.threadpool.ThreadPoolDemo.lambda$main$0(ThreadPoolDemo.java:16)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)
100.0
50.0
33.0
25.0

Process finished with exit code 0

3. execute 和 submit 的区别

execute() 方法用于提交不需要返回值的任务，所以无法判断任务是否被线程池执行成功。

submit() 方法用于提交需要返回值的任务。线程池会返回一个 future 类型的对象，通过这个 future 对象可以判断任务是否执行成功，并且可以通过 future 的 get() 方法来获取返回值，get() 方法会阻塞当前线程直到任务完成，而使用 get（long timeout，TimeUnit unit）方法则会阻塞当前线程一段时间后立即返回，这时候有可能任务没有执行完。
没有返回值也是可以用submit