Java中关闭钩子的有用示例？

Question

我正在努力确保我的Java应用程序采取合理的步骤来保持健壮,其中一部分涉及优雅地关闭.我正在阅读有关关闭钩子的事情,我实际上并没有在实践中如何使用它们.

那里有一个实际的例子吗？

Let's say I had a really simple application like this one below, which writes numbers to a file, 10 to a line, in batches of 100, and I want to make sure a given batch finishes if the program is interrupted. I get how to register a shutdown hook but I have no idea how to integrate that into my application. Any suggestions?

package com.example.test.concurrency;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.PrintWriter;

public class GracefulShutdownTest1 {
    final private int N;
    final private File f;
    public GracefulShutdownTest1(File f, int N) { this.f=f; this.N = N; }

    public void run()
    {
        PrintWriter pw = null;
        try {
            FileOutputStream fos = new FileOutputStream(this.f);
            pw = new PrintWriter(fos);
            for (int i = 0; i < N; ++i)
                writeBatch(pw, i);
        }
        catch (FileNotFoundException e) {
            e.printStackTrace();
        }
        finally
        {
            pw.close();
        }       
    }

    private void writeBatch(PrintWriter pw, int i) {
        for (int j = 0; j < 100; ++j)
        {
            int k = i*100+j;
            pw.write(Integer.toString(k));
            if ((j+1)%10 == 0)
                pw.write('\n');
            else
                pw.write(' ');
        }
    }

    static public void main(String[] args)
    {
        if (args.length < 2)
        {
            System.out.println("args = [file] [N] "
                    +"where file = output filename, N=batch count");
        }
        else
        {
            new GracefulShutdownTest1(
                    new File(args[0]), 
                    Integer.parseInt(args[1])
            ).run();
        }
    }
}

Answer 1

您可以执行以下操作:

• 让shutdown hook将一些AtomicBoolean(或volatile boolean)"keepRunning"设置为false
• (可选地,.interrupt工作线程,如果他们在某些阻塞调用中等待数据)
• writeBatch通过调用Thread.join()工作线程上的方法,等待工作线程(在您的情况下执行)完成.
• 终止程序

一些粗略的代码:

• 添加一个 static volatile boolean keepRunning = true;

• 在run()中,您将更改为

  for (int i = 0; i < N && keepRunning; ++i)
      writeBatch(pw, i);
  

• 在main()中添加:

  final Thread mainThread = Thread.currentThread();
  Runtime.getRuntime().addShutdownHook(new Thread() {
      public void run() {
          keepRunning = false;
          mainThread.join();
      }
  });
  

这大致是我在终端中如何优雅地"在击中Control-C时拒绝所有客户".

来自文档:

当虚拟机开始其关闭序列时,它将以某种未指定的顺序启动所有已注册的关闭挂钩,并让它们同时运行.当所有挂钩都完成后,如果启用了finalization-on-exit,它将运行所有未读取的终结器.最后,虚拟机将停止.

也就是说,一个关闭钩子使JVM保持运行,直到钩子终止(从run() -method 返回).