callable的future问题

Question

为什么task2无法完成呢？
import java.util.concurrent.Callable;  
import java.util.concurrent.ExecutorService;  
import java.util.concurrent.Executors;  
import java.util.concurrent.Future;  
  
public class FutureExample {  
      
    public static void main(String[] args) throws Exception {  
        ExecutorService executor = Executors.newCachedThreadPool();  
        Runnable task1 = new Runnable() {  
            @Override  
            public void run() {  
                //do something  
                System.out.println("i am task1.....");  
            }  
 };  
 Callable<Integer> task2 = new Callable<Integer>() {  
     @Override  
     public Integer call() throws Exception {  
         //do something  
         return new Integer(100);  
     }  
 };  
 Future<?> f1 = executor.submit(task1);  
 Future<Integer> f2 = executor.submit(task2);  
 System.out.println("task1 is completed? " + f1.isDone());  
 System.out.println("task2 is completed? " + f2.isDone());  
 //waiting task1 completed          while(f1.isDone()){  
            System.out.println("task1 completed.");  
            break;  
        }  
        //waiting task2 completed  
        while(f2.isDone()){  
            System.out.println("return value by task2: " + f2.get());  
            break;  
        }  
    }  
  
}  结果：
i am task1.....
task1 is completed? true
task2 is completed? false
task1 completed.

Answer 1

JDK 1.7，Eclipse Luna么有问题

Answer 2

在你做输出之前加上一句Thread.sleep(5000);试试吧。
所谓的并发，就是无论线程是否完成，主线程都不会等待，主线程动作够快的话你连task1都完成不了。

Answer 3

还真是这样，加了一个Thread.sleep(500);
import java.util.concurrent.Callable;  
import java.util.concurrent.ExecutorService;  
import java.util.concurrent.Executors;  
import java.util.concurrent.Future;  
  
public class FutureExample {  
      
    public static void main(String[] args) throws Exception {  
        ExecutorService executor = Executors.newCachedThreadPool();  
        Runnable task1 = new Runnable() {  
            @Override  
            public void run() {  
                //do something  
                System.out.println("i am task1.....");  
            }  
 };  
 Callable<Integer> task2 = new Callable<Integer>() {  
     @Override  
     public Integer call() throws Exception {  
      System.out.println("i am task2.....");    
         return new Integer(100);  
     }  
 };  
 
 Future<?> f1 = executor.submit(task1); 
 Future<Integer> f2 = executor.submit(task2); 
  
  Thread.sleep(500);
 System.out.println("task1 is completed? " + f1.isDone());  
 System.out.println("task2 is completed? " + f2.isDone());  
 //waiting task1 completed          while(f1.isDone()){  
            System.out.println("task1 completed.");  
            break;  
        }  
        //waiting task2 completed  
        while(f2.isDone()){  
            System.out.println("return value by task2: " + f2.get());  
            break;  
        }  
    }  
  
}  然后输出：
i am task1.....
i am task2.....
task1 is completed? true
task2 is completed? true
task1 completed.
return value by task2: 100

Answer 4

然后又改了一下，一样得到同样的结果
import java.util.concurrent.Callable;  
import java.util.concurrent.ExecutorService;  
import java.util.concurrent.Executors;  
import java.util.concurrent.Future;  
  
public class FutureExample {  
      
    public static void main(String[] args) throws Exception {  
        ExecutorService executor = Executors.newCachedThreadPool();  
        Runnable task1 = new Runnable() {  
            @Override  
            public void run() {  
                //do something  
                System.out.println("i am task1.....");  
            }  
 };  
 Callable<Integer> task2 = new Callable<Integer>() {  
     @Override  
     public Integer call() throws Exception {  
      System.out.println("i am task2.....");    
         return new Integer(100);  
     }  
 };  
 
 Future<?> f1 = executor.submit(task1); 
 Future<Integer> f2 = executor.submit(task2); 
  
 //Thread.sleep(500);
 while(!(f1.isDone()&&f2.isDone())){
 //System.out.println("task1 is completed? " + f1.isDone());  
 //System.out.println("task2 is completed? " + f2.isDone());  
 }        while(f1.isDone()){  
            System.out.println("task1 completed.");  
            break;  
        }  
        //waiting task2 completed  
        while(f2.isDone()){  
          System.out.println("task2 completed.");  
            System.out.println("return value by task2: " + f2.get());  
            break;  
        }  
    }  
  
}  i am task1.....
i am task2.....
task1 completed.
task2 completed.
return value by task2: 100