在 call() 方法的 return 语句执行之前具有对象引用的未来对象

Question

以下是我的程序和输出。

甚至在执行 call() 方法的 return 语句之前（第 1 行）for each 循环（第 2 行) 打印非空的 Future 变量 f 的引用。

我的问题是，在执行 return 语句并将对象分配给 f 之前，future 变量怎么可能指向某些对象而不是 null，这在内部是如何工作的。

代码：-

import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class ObjectPool2
{
    public static void main(String[] args) 
    {
        Pool[] pObject = {new Pool(),new Pool(),new Pool(),new Pool(),new Pool(),new Pool(),new Pool(),new Pool()};

    ExecutorService es = Executors.newFixedThreadPool(8);

    Future f = null;

    for(Pool p : pObject)                         **// line 2**
    {
        f = es.submit(p);
        System.out.println(f.toString());
        System.out.println(f);
        System.out.println(f);
        System.out.println(f);
        System.out.println("---------------");
        }

        System.out.println(f);
    }
}

class Pool implements Callable
{
    public static ConcurrentLinkedQueue unlocked_queue2 = new ConcurrentLinkedQueue();
    public static ConcurrentLinkedQueue locked_queue2 = new ConcurrentLinkedQueue();

static
{
    int i = 1;
    int i2 = 2;
    int i3 = 3;
    int i4 = 4;
    unlocked_queue2.add(i);
    unlocked_queue2.add(i2);
    unlocked_queue2.add(i3);
    unlocked_queue2.add(i4);
}

public Object get() throws InterruptedException
{
    synchronized(getClass())
    {
    while(unlocked_queue2.isEmpty())
    {
        System.out.println(Thread.currentThread().getName() + " waiting ");
        this.getClass().wait();
    }
    int op = (int)unlocked_queue2.poll();
    locked_queue2.add(op);
    System.out.println(Thread.currentThread().getName() + " got lock of : "+ op +" now going to sleep");
    Thread.currentThread().sleep(5000);
    return op;
    }
}

public void leave()
{
    synchronized(getClass())
    {
    int ol = (int)locked_queue2.poll();
    unlocked_queue2.add(ol);
    System.out.println(Thread.currentThread().getName() + " leaving lock of : "+ ol);
    this.getClass().notifyAll();
    }
}

@Override
public Object call() throws Exception
{
    Object lock = get();
    return lock;                                 **//line 1**
}

}

输出：-

java.util.concurrent.FutureTask@55f96302
java.util.concurrent.FutureTask@55f96302
java.util.concurrent.FutureTask@55f96302
java.util.concurrent.FutureTask@55f96302
---------------
pool-1-thread-1 got lock of : 1 now going to sleep
java.util.concurrent.FutureTask@42a57993
java.util.concurrent.FutureTask@42a57993
java.util.concurrent.FutureTask@42a57993
java.util.concurrent.FutureTask@42a57993
---------------
java.util.concurrent.FutureTask@6bc7c054
java.util.concurrent.FutureTask@6bc7c054
java.util.concurrent.FutureTask@6bc7c054
java.util.concurrent.FutureTask@6bc7c054
---------------
java.util.concurrent.FutureTask@4aa298b7
java.util.concurrent.FutureTask@4aa298b7
java.util.concurrent.FutureTask@4aa298b7
java.util.concurrent.FutureTask@4aa298b7
---------------
java.util.concurrent.FutureTask@28d93b30
java.util.concurrent.FutureTask@28d93b30
java.util.concurrent.FutureTask@28d93b30
java.util.concurrent.FutureTask@28d93b30
---------------
java.util.concurrent.FutureTask@4554617c
java.util.concurrent.FutureTask@4554617c
java.util.concurrent.FutureTask@4554617c
java.util.concurrent.FutureTask@4554617c
---------------
java.util.concurrent.FutureTask@1540e19d
java.util.concurrent.FutureTask@1540e19d
java.util.concurrent.FutureTask@1540e19d
java.util.concurrent.FutureTask@1540e19d
---------------
java.util.concurrent.FutureTask@14ae5a5
java.util.concurrent.FutureTask@14ae5a5
java.util.concurrent.FutureTask@14ae5a5
java.util.concurrent.FutureTask@14ae5a5
---------------
java.util.concurrent.FutureTask@14ae5a5
pool-1-thread-8 got lock of : 2 now going to sleep
pool-1-thread-7 got lock of : 3 now going to sleep
pool-1-thread-6 got lock of : 4 now going to sleep
pool-1-thread-5 waiting 
pool-1-thread-4 waiting 
pool-1-thread-3 waiting 
pool-1-thread-2 waiting

Answer 1

Future 的创建是同步的，在主调用线程中实现。当您将 Callable 提交给 ThreadPoolExecutor 时，它的工作方式如下：

public Future submit(Callable callable) {
    Future future = new FutureTask(callable);
    if (currentThread < coreThread) {
        createNewThreadWhichWillExecuteTheFutureAutomically(); // this will not get blocked, the thread will execute the future itself
    } else if (queueIsNotFull) {
        putTheFutureInQueue(future);
    } else if (currentThread < maxThread) {
        createNewThreadWhichWillExecuteTheFutureAutomically(); // this will not get blocked, the thread will execute the future itself
    } else {
       throw RejectedExecutionException();
    }
    return future;
}

而future.get()的执行不会return直到任务执行完或者中断或超时。

Answer 2

实际上这是你给f赋值的地方

f = es.submit(p);

之后 f 行不再是空行。它是一个未来类型的对象。你可以从系统中看到 - 它打印出来 java.util.concurrent.FutureTask@135fbaa4 基本上是 FutureTask 类型的对象。

当您的线程完成时，它不会更改 f 的值 -> 它仍然是 future 类型的对象，但它已完成执行（基本上 isDone() 变为 true）。完成后，您可以从以下位置获得实际结果：

f.get();

您可以检查它是否已经完成：

f.isDone()

在 call() 方法的 return 语句执行之前具有对象引用的未来对象

Future Object having Object reference before return statement of call() method executes

java

multithreading

future

callable

executorservice