使用System.out.print与println的多线程问题 [英] Multithreading problem using System.out.print vs println

问题描述

我有以下线程，每200ms只打印一个点：

I have the following thread which simply prints a dot every 200ms:

public class Progress {

    private static boolean threadCanRun = true;
    private static Thread progressThread = new Thread(new Runnable() 
    {
        public void run() {
            while (threadCanRun) {
                System.out.print('.');
                System.out.flush();
                try {
                    progressThread.sleep(200);
                } catch (InterruptedException ex) {}
            }
        }
    });

    public static void stop()
    {
        threadCanRun = false;
        progressThread.interrupt();
    }

    public static void start()
    {
        if (!progressThread.isAlive())
        {
            progressThread.start();
        } else
        {
            threadCanRun = true;
        }
    }

}

我开始带有此代码的线程（暂时）：

I start the thread with this code (for now):

 System.out.println("Working.");
 Progress.start();


 try {
        Thread.sleep(10000); //To be replaced with code that does work.
 } catch (InterruptedException ex) {}

 Progress.stop();

这真的很奇怪：

如果我使用 System.out.println（'。'）; ，则代码完全按预期工作。 （除了我每次都不想要新行）。

If I use System.out.println('.'); , the code works exactly as expected. (Apart from the fact that I don't want a new line each time).

使用 System.out.print（'。'） ; ，代码等待十秒钟，然后显示输出。

With System.out.print('.');, the code waits for ten seconds, and then shows the output.

System.out.println ：

     Print dot, wait 200ms, print dot, wait 200ms etc...

System.out.print ：

     Wait 5000ms, Print all dots

发生了什么，我该怎么做才能绕过这种行为？

What is happening, and what can I do to go around this behaviour?

编辑：

我也试过这个：

private static synchronized void printDot()
{
    System.err.print('.');
}

和printDot（）而不是 System.out。 print（'。'）;
它仍然不起作用。

and printDot() instead of System.out.print('.'); It still doesn't work.

EDIT2：

有趣。此代码按预期工作：

Interesting. This code works as expected:

        System.out.print('.');
        System.out.flush();  //Makes no difference with or without
        System.out.println();

这不是：

        System.err.print('.');
        System.err.flush();
        System.out.print('.');
        System.out.flush();

解决方案：问题与netbeans有关。当我将它作为来自java -jar的jar文件运行时，它工作正常。

Solution: The issue was netbeans related. It worked fine when I run it as a jar file from java -jar.

这是我一生中见过的最令人沮丧的错误之一。当我尝试在调试模式下使用断点运行此代码时，一切正常。

This is one of the most frustrating errors I have seen in my life. When I try to run this code with breakpoints in debug mode, everything works correctly.

推荐答案

stdout是行缓冲的。
使用stderr，或在每次打印后刷新PrintStream。

The stdout is line buffered. Use stderr, or flush the PrintStream after each print.

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