submit() vs execute() 異常處理差異

通常，execute() 命令用於 fire 和 forget 呼叫（不需要分析結果），submit() 命令用於分析 Future 物件的結果。

我們應該知道這兩個命令之間的異常處理機制的關鍵區別。

如果你沒有捕獲它們，框架就會吞下來自 submit() 的異常。

程式碼示例瞭解差異：

情況 1：使用 execute() 命令提交 Runnable，該命令報告異常

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);
   }
 }

輸出：

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)

情況 2：用 submit() 替換 execute()： service.submit(new Runnable(){ 在這種情況下，框架吞下了異常，因為 run() 方法沒有明確地捕獲它們。

輸出：

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

案例 3：將 newFixedThreadPool 更改為 ExtendedExecutor

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

輸出：

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

我已經演示了這個示例以涵蓋兩個主題：使用自定義 ThreadPoolExecutor 並使用自定義 ThreadPoolExecutor 處理 Exectpion。

針對上述問題的其他簡單解決方案： 當你使用普通的 ExecutorService＆submit 命令時，從 Future() 命令獲取 Future 物件，呼叫 Future 上的 get()API。捕獲已在 afterExecute 方法實現中引用的三個異常。自定義 ThreadPoolExecutor 優於此方法：你必須只在一個地方處理異常處理機制 - 自定義 ThreadPoolExecutor。