ExcectorService中提交（可运行）和提交（可调用）之间的区别？

Question

Bellow是实现Runnable类的代码

public class ExecutorServiceWithRunnable {

    public static void main(String[] args) {
        ExecutorService pool = Executors.newFixedThreadPool(10);
        Future<emp> submit =null;
        emp emp = new emp();
        for(int i=0;i<5;i++) {
             submit = (Future<emp>) pool.submit(new RunnableClass(emp));
             try {
                    System.out.println(submit.get().getId());
                } catch (InterruptedException | ExecutionException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
        }


    }
}
class RunnableClass implements Runnable{

    emp e=null;
    public RunnableClass(emp emp) {
        // TODO Auto-generated constructor stub
        this.e=emp;
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub
        System.out.println(Thread.currentThread().getName()+"...."+e.getId());
        e.setId(e.getId()+1);

    }

}
class emp{
    int id;
    String status;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getStatus() {
        return status;
    }
    public void setStatus(String status) {
        this.status = status;
    }
}

得到的输出是：pool-1-thread-1…..0线程“main”java.lang.NullPointerException中的ExecutorServiceWithRunnableCallable.main（ExecutorSServiceWithRunnable.java:15）

以下是Callable的代码

public class ExecutorServiceWithRunnableCallable2 {

    public static void main(String[] args) {
        ExecutorService pool = Executors.newFixedThreadPool(10);
        Future<emp> submit =null;
        emp emp = new emp();
        for(int i=0;i<5;i++) {
             submit = pool.submit(new CallableClass(emp));
             try {
                System.out.println(submit.get().getId());
            } catch (InterruptedException | ExecutionException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }


    }
}
class CallableClass implements Callable<emp>{
    emp e=null;
    public CallableClass(emp emp) {
        // TODO Auto-generated constructor stub
        this.e=emp;
    }

    @Override
    public emp call() {
        // TODO Auto-generated method stub
        System.out.println(Thread.currentThread().getName()+"...."+e.getId());
        e.setId(e.getId()+1);
        return e;
    }

}

**获得的输出为：**池-1-线程-1….0 1池-1-线程-2….1 2池-1-线程-3….2 3池-1-线程-4….3 4池-1-线程-5….4 5

所以在两种情况下，请澄清提交方法返回的未来对象。

Answer 1

< code>Runnable不返回结果。所以< code>Future::get仍然会阻塞，直到runnable完成，但随后只返回< code>null。

submit = (Future<emp>) pool.submit(new RunnableClass(emp));

此类强制转换将产生警告，因为您实际上并没有得到Future

正如Javadoc所说，你真的得到了一个未来

submit = pool.submit(new CallableClass(emp));

如果你想要一个结果，提交一个可调用的。请注意，没有必要进行类强制转换，它可以从可调用的类型中推断出未来的泛型类型。