Make sure only single/one instance running – with File Lock and ShutdownHook

http://jimlife.wordpress.com/2008/07/21/java-application-make-sure-only-singleone-instance-running-with-file-lock-ampampampampamp-shutdownhook/

 应该说，这篇文章讲述的是启动、关闭一个程序的最佳实践或常用方案。

涉及的方面是：

1，当程序已经运行时，使用lock文件，防止有人试图再次启动该程序。

2， 程序关闭时，使用shutdown hook保证一些收尾工作被执行。

代码如下，非常非常的简单~

package test.jimmy.filelock;import java.io.File;import java.io.IOException;import java.io.RandomAccessFile;import java.nio.channels.FileChannel;import java.nio.channels.FileLock;public class MyApp {    private static File f;    private static FileChannel channel;    private static FileLock lock;    public static void main(String[] args) {        try {            f = new File("RingOnRequest.lock");            // Check if the lock exist            if (f.exists()) {                // if exist try to delete it                //如果抛出IO异常，说明有其他process占用，则停止运行。               //这里不仅仅是检查lock文件是否存在，而且检查是否被占用。               //这样就避免了由于特殊原因导致lock未被删除，进而导致程序无法再次启动的问题。               f.delete();            }            // Try to get the lock            //这个用法没见过，学习一下           //只有通过channel才能get一个FileLock            channel = new RandomAccessFile(f, "rw").getChannel();            lock = channel.tryLock();            if(lock == null)            {                // File is lock by other application               channel.close();                throw new RuntimeException("Only 1 instance of MyApp can run.");            }            // Add shutdown hook to release lock when application shutdown          // 另外一个用法就是使用shutdownhook保证收尾的工作一定会被执行            ShutdownHook shutdownHook = new ShutdownHook();            Runtime.getRuntime().addShutdownHook(shutdownHook);           //Your application tasks here..           System.out.println("Running");            try {                Thread.sleep(10000);            } catch (InterruptedException e) {                e.printStackTrace();            }        }        catch(IOException e)        {            throw new RuntimeException("Could not start process.", e);        }    }    public static void unlockFile() {        // release and delete file lock        try {            if(lock != null) {                lock.release();                channel.close();                f.delete();           }        } catch(IOException e) {            e.printStackTrace();        }    }    static class ShutdownHook extends Thread {        public void run() {            unlockFile();        }    }}