submit() vs execute() 异常处理差异

通常，execute() 命令用于 fire 和 forget 调用（不需要分析结果），submit() 命令用于分析 Future 对象的结果。

我们应该知道这两个命令之间的异常处理机制的关键区别。

如果你没有捕获它们，框架就会吞下来自 submit() 的异常。

代码示例了解差异：

情况 1：使用 execute() 命令提交 Runnable，该命令报告异常

import java.util.concurrent.*;
import java.util.*;

public class ExecuteSubmitDemo {
    public ExecuteSubmitDemo() {
        System.out.println("creating service");
        ExecutorService service = Executors.newFixedThreadPool(2);
        //ExtendedExecutor service = new ExtendedExecutor();
        for (int i = 0; i < 2; i++){
            service.execute(new Runnable(){
                 public void run(){
                    int a = 4, b = 0;
                    System.out.println("a and b=" + a + ":" + b);
                    System.out.println("a/b:" + (a / b));
                    System.out.println("Thread Name in Runnable after divide by zero:"+Thread.currentThread().getName());
                 }
            });
        }
        service.shutdown();
    }
    public static void main(String args[]){
        ExecuteSubmitDemo demo = new ExecuteSubmitDemo();
    }
}

class ExtendedExecutor extends ThreadPoolExecutor {

   public ExtendedExecutor() { 
       super(1, 1, 60, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(100));
   }
   // ...
   protected void afterExecute(Runnable r, Throwable t) {
     super.afterExecute(r, t);
     if (t == null && r instanceof Future<?>) {
       try {
         Object result = ((Future<?>) r).get();
       } catch (CancellationException ce) {
           t = ce;
       } catch (ExecutionException ee) {
           t = ee.getCause();
       } catch (InterruptedException ie) {
           Thread.currentThread().interrupt(); // ignore/reset
       }
     }
     if (t != null)
       System.out.println(t);
   }
 }

输出：

creating service
a and b=4:0
a and b=4:0
Exception in thread "pool-1-thread-1" Exception in thread "pool-1-thread-2" java.lang.ArithmeticException: / by zero
        at ExecuteSubmitDemo$1.run(ExecuteSubmitDemo.java:15)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:744)
java.lang.ArithmeticException: / by zero
        at ExecuteSubmitDemo$1.run(ExecuteSubmitDemo.java:15)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:744)

情况 2：用 submit() 替换 execute()： service.submit(new Runnable(){ 在这种情况下，框架吞下了异常，因为 run() 方法没有明确地捕获它们。

输出：

creating service
a and b=4:0
a and b=4:0

案例 3：将 newFixedThreadPool 更改为 ExtendedExecutor

//ExecutorService service = Executors.newFixedThreadPool(2);
 ExtendedExecutor service = new ExtendedExecutor(); 

输出：

creating service
a and b=4:0
java.lang.ArithmeticException: / by zero
a and b=4:0
java.lang.ArithmeticException: / by zero

我已经演示了这个示例以涵盖两个主题：使用自定义 ThreadPoolExecutor 并使用自定义 ThreadPoolExecutor 处理 Exectpion。

针对上述问题的其他简单解决方案： 当你使用普通的 ExecutorService＆submit 命令时，从 Future() 命令获取 Future 对象，调用 Future 上的 get()API。捕获已在 afterExecute 方法实现中引用的三个异常。自定义 ThreadPoolExecutor 优于此方法：你必须只在一个地方处理异常处理机制 - 自定义 ThreadPoolExecutor。