Question

对于家庭作业，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（）应该是1.向命令队列提交一个停止命令，并且2.在所有线程上调用join（），以便交易有机会完成。

但是，join（）未按预期工作。当我尝试调用join（）时，我得到一个NullPointerException。我修改了服务器线程以便在我提交停止命令后等待一秒钟，但是当我应该使用join（）时，这是一个糟糕的替代品。

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() 

}

来自commandQueue的CommandExecutionThreads poll（）命令，当遇到stop命令时，它返回：

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。线程本身在可用时将命令从队列中拉出。出于诊断目的，我已经打印出thread.isAlive（）并且可以正常工作。当我在stop方法中时，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，永远不要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阵列。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()); 
}

调用join（）时，java threads为null

1 个答案: