java - 是什么阻止程序退出？

Question

我是这样写的。

public static void main(String[] args){
    Thread thread = new Thread(() -> {
        while(true){
            try{
                // do something
                Thread.sleep(10);
            }catch(InterruptedException ex){
                System.out.println("ABC");
                break;
            }
        }
    });
    JFrame frame = new JFrame();
    frame.setSize(1280,720);
    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    frame.addWindowListener(new WindowAdapter(){
        @Override
        public void windowClosed(WindowEvent e){
            thread.interrupt();
        }
    });
    thread.start();
    frame.setVisible(true);
}

当我点击窗口的“关闭”按钮时，
程序花了几秒钟才结束。
是什么阻止程序退出？
以及如何不使用JFrame.EXIT_ON_CLOSE 立即关闭程序？

我认为我的线程(while(true) 线程)立即结束
因为在我点击“关闭”按钮后很快就会显示“ABC”。

谢谢。

编辑

没有我的线程也发生了同样的事情。

public static void main(String[] args){
    JFrame frame = new JFrame();
    frame.setSize(1280,720);
    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    frame.setVisible(true);
}

编辑2

public static void main(String[] args){
    Runtime.getRuntime().addShutdownHook(new Thread(() -> {
        Thread[] threads = new Thread[5];
        Thread.enumerate(threads);
        System.out.println(Arrays.toString(threads));
        System.err.println(LocalDateTime.now() + " SHUTDOWN");
    }
    ));
    JFrame frame = new JFrame();
    frame.setSize(1280,720);
    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    frame.addWindowListener(new WindowAdapter(){
            @Override
            public void windowClosed(WindowEvent e){
                Thread[] threads = new Thread[5];
                Thread.enumerate(threads);
                System.out.println(Arrays.toString(threads));
                System.err.println(LocalDateTime.now() + " CLOSE");
            }
        });
    frame.setVisible(true);
}  

输出是:

[Thread[AWT-EventQueue-0,6,main], Thread[DestroyJavaVM,5,main], null, null, null]
2016-08-02T19:04:50.465 CLOSE
[Thread[DestroyJavaVM,5,main], Thread[Thread-0,5,main], null, null, null]
2016-08-02T19:04:51.762 SHUTDOWN

Answer 1

虽然通常应谨慎使用 System.exit()，但 Swing documentation明确指出在问题中所示的场景中这是正确的方法:

When the last displayable window within the Java virtual machine (VM) is disposed of, the VM may terminate. Note, however, that there can be a delay before the program exits automatically, and that under some circumstances the program might keep running. It is quicker and safer to explicitly exit the program using System.exit(int).

以下示例未显示关闭操作和关闭 Hook 之间的延迟。删除 System.exit(0) 调用时，大约延迟。可以观察到 1.3 秒:

package com.example.swing;

import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.time.LocalDateTime;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public class ExitDelay {
    public static void main(String[] args){

        Runtime.getRuntime().addShutdownHook(new Thread(() -> System.err.println(LocalDateTime.now() + " SHUTDOWN") ));

        SwingUtilities.invokeLater(() -> {

            JFrame frame = new JFrame();
            frame.setSize(1280,720);
            frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
            frame.addWindowListener(new WindowAdapter(){
                @Override
                public void windowClosed(WindowEvent e){
                    System.err.println(LocalDateTime.now() + " CLOSE");

                    // Do all required cleanup stuff.
                    // check that threads are done, close files, commit transactions etc.
                    // ...

                    System.exit(0);
                }
            });
            frame.setVisible(true);

        });
    }
}