线程休眠和线程连接

Question

如果我把一个线程放在一个循环中睡觉,netbeans给我一个警告说循环中调用Thread.sleep会导致性能问题.但是,如果我用连接代替睡眠,则不会给出这样的警告.这两个版本都可以编译和工作.我的代码如下(检查" Thread.sleep()vs t.join()" 的最后几行).

public class Test{

    //Display a message, preceded by the name of the current thread
    static void threadMessage(String message) {
        String threadName = Thread.currentThread().getName();
        System.out.format("%s: %s%n", threadName, message);
    }

    private static class MessageLoop implements Runnable {
        public void run() {
            String importantInfo[] = {
                "Mares eat oats",
                "Does eat oats",
                "Little lambs eat ivy",
                "A kid will eat ivy too"
            };
            try {
                for (int i = 0; i < importantInfo.length; i++) {
                    //Pause for 4 seconds
                    Thread.sleep(4000);
                    //Print a message
                    threadMessage(importantInfo[i]);
                }
            } catch (InterruptedException e) {
                threadMessage("I wasn't done!");
            }
        }
    }

    public static void main(String args[]) throws InterruptedException {


        //Delay, in milliseconds before we interrupt MessageLoop
        //thread (default one hour).
        long patience = 1000 * 60 * 60;

        //If command line argument present, gives patience in seconds.
        if (args.length > 0) {
            try {
                patience = Long.parseLong(args[0]) * 1000;
            } catch (NumberFormatException e) {
                System.err.println("Argument must be an integer.");
                System.exit(1);
            }

        }

        threadMessage("Starting MessageLoop thread");
        long startTime = System.currentTimeMillis();
        Thread t = new Thread(new MessageLoop());
        t.start();

        threadMessage("Waiting for MessageLoop thread to finish");
        //loop until MessageLoop thread exits
        while (t.isAlive()) {
            threadMessage("Still waiting...");
            //Wait maximum of 1 second for MessageLoop thread to
            //finish.
            /*******LOOK HERE**********************/
            Thread.sleep(1000);//issues caution unlike t.join(1000)
            /**************************************/
            if (((System.currentTimeMillis() - startTime) > patience) &&
                    t.isAlive()) {
                threadMessage("Tired of waiting!");
                t.interrupt();
                //Shouldn't be long now -- wait indefinitely
                t.join();
            }

        }
        threadMessage("Finally!");
    }
}

据我了解,join等待其他线程完成,但在这种情况下,不是睡觉和加入做同样的事情？那为什么netbeans会引起注意呢？

Answer 1

join()和sleep()之间存在差异.join()将等待超时到期或线程完成.sleep()只会等待指定的时间,除非被打断.因此,join()返回的速度比指定时间快得多.

Netbeans警告你关于sleep()而不是关于join()的原因正是这种差异.join()等待一些有意义的事情,而sleep()只是坐在那里什么都不做.如果你不等待什么,那你为什么要等待呢？它很少有意义,因此警告.

Answer 2

他们可以用来实现同样的事情并不意味着他们以同样的方式被滥用.Thread.sleep()当人们真正使用锁或阻挡的东西时,人们经常会滥用:

 // Allow others to notify me when there's work to do
 synchronized (lock) {
     try {
         lock.wait(); // Much better than a loop with sleep
     } catch(InterruptedException e) {
         // someone killed me
         return;
     }
 }


 // or use a BlockingQueue or similar
 Job job = queue.get();
 if (job instanceof ExitJob) return;