java 调用 join() 时线程为空

Question

作为家庭作业，我正在为管理多个执行事务的线程的服务器编写一个 stop() 方法。 Stop() 应该 1. 向命令队列提交停止命令和 2. 在所有线程上调用 join() 以便事务有机会完成。

但是，join() 没有按预期工作。当我尝试调用 join() 时，出现 NullPointerException。我将服务器线程修改为在提交停止命令后等待一秒钟，但当我应该使用 join() 时，这是一个糟糕的替代方法。

public void stop() throws InterruptedException {
    // TODO Auto-generated method stub

    System.out.println("server stop()");
    for (CommandExecutionThread thread : threads) { 
        System.out.println(threads.length);
    }

    for (CommandExecutionThread thread : threads) { 

        Command e = new CommandStop();
        queue.add(e);

    }   

    for (CommandExecutionThread thread : threads) { 
        if (thread != null) 
        {
            thread.join(); 
        } 
        if (thread == null)
        {
            System.out.println("NULL thread"); //threads are always null at this point 
        }
    }

    for (CommandExecutionThread thread : threads) { 
        System.out.println(threads.length);
    }
    wait(1000);//simulate the wait that I would have had from join() 

}

CommandExecutionThreads poll() 命令来自 commandQueue，当它遇到停止命令时，它 returns:

public void run() {
    while (true) { 
        synchronized (commandQueue) {
            while (commandQueue.isEmpty()) { 
                try {
                    commandQueue.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }//end while(commandQueue.isEmpty())
        }//end synchronized(commandQueue)

        Command c; 

        synchronized (commandQueue) { 

            c = commandQueue.poll(); 

            if (c.isStop()==true)
            { 
                System.out.println("\tSTOP");
                return;
            } 
            else 
            { 
                //System.out.println("\tTRANSACTION"); 
            } 

            if (executeCommandInsideMonitor) { 
                try {
                    c.execute(bank);
                } catch (InsufficientFundsException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }

            commandQueue.notifyAll();
        } // end sync commandQueue 
    } //end while(true)
}//end run()

我在创建银行服务器时更早地初始化了我的线程数组。当命令可用时，线程自己将命令从队列中拉出。出于诊断目的，我打印了 thread.isAlive() 并且有效。当我进入停止方法时，thread.isAlive() 抛出一个空指针错误：

public class BankServerImpl implements BankServer {
Queue<Command> queue = new LinkedList<Command>(); 
CommandExecutionThread[] threads; 
boolean executeCommandInsideMonitor; 
Bank bank; 

/**
 * Constructor for BankServerImpl, which implements BankServer
 * 
 * @param bank
 * @param serverThreads
 * @param executeCommandInsideMonitor
 */
public BankServerImpl(Bank bank, int serverThreads, boolean executeCommandInsideMonitor) {

    this.bank = bank; 
    this.executeCommandInsideMonitor = executeCommandInsideMonitor;
    threads = new CommandExecutionThread[serverThreads]; 

    for(CommandExecutionThread thread : threads) { 
        thread = new CommandExecutionThread(bank, queue, executeCommandInsideMonitor);
        thread.start(); 
        System.out.println(thread.isAlive());   

    }
}

Answer 1

创建线程时，永远不会将它们插入到数组中。

for(CommandExecutionThread thread : threads) { 
    thread = new CommandExecutionThread(bank, queue, executeCommandInsideMonitor);
    thread.start(); 
    System.out.println(thread.isAlive());   
}

此代码不会导致更新数组。你需要这样的东西：

for (int i = 0; i < serverThreads; i += 1) {
    CommandExecutionThread newThread = new CommandExecutionThread(bank, queue, executeCommandInsideMonitor);
    threads[i] = newThread;
    newThread.start();
    System.out.println(newThread.isAlive()); 
}

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