Java中的FileFlock

最近用到FileLock知識點，對其中的幾點做了一些實驗發現：

1.這是一個進程之間的鎖，對於同一個進程不同的線程你是不能夠同時去競爭一把鎖的。如下實驗：

package springtest.trigger;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.channels.Channel;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;
import java.util.Calendar;
import java.util.TimeZone;

public class Demo {

	public static int i = 0 ;
	private subthread sb  = new subthread();
	public void init()
	{
<span style="color:#ff0000;">		sb.start();
		new subthread().start();
</span>	}
	
	public static void main(String args[]){
		while(true)
		{
			Demo demo = new Demo();
			demo.init();
			//這裏是爲了不要讓這個主線程死掉
			try {
				Thread.sleep(30000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	
	
	class subthread extends Thread{
		private RandomAccessFile ra = null ;
		private FileChannel channel  = null ;
		private FileLock fileLock  = null ;
		public void run()
		{
			while(true)
			{
				System.out.println(" doing ");
				File file = new File("process.lck");
		        try {
					 ra = new RandomAccessFile(file, "rw");
					 channel = ra.getChannel();
					 System.out.println("there are going to get lock...");
					 fileLock = channel.lock(); //這裏面得到鎖了，且是在線程中得到鎖
					 System.out.println("geted the lock!");
				        try {
							Thread.sleep(10000);
						} catch (InterruptedException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
						}
					 //下面我想要這個地方拋出一個沒有抓住的異常
					 String str=null;
					 str.charAt(0);
				} catch (FileNotFoundException e) {
					e.printStackTrace();
				} catch (IOException e) {
					e.printStackTrace();
				}finally{
					if(fileLock != null )
					{
						try {
							fileLock.release();
							fileLock = null ;
						} catch (IOException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
						}
					}
					if(channel != null)
					{
						try {
							channel.close();
							channel = null ;
						} catch (IOException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
						}
					}
				}
		        
		 
			}
			
			
		}
	}
}

這上面會報異常的：

 doing 
 doing 
Exception in thread "Thread-0" java.nio.channels.OverlappingFileLockException
	at sun.nio.ch.FileChannelImpl$SharedFileLockTable.checkList(FileChannelImpl.java:1166)
	at sun.nio.ch.FileChannelImpl$SharedFileLockTable.add(FileChannelImpl.java:1068)
	at sun.nio.ch.FileChannelImpl.lock(FileChannelImpl.java:824)
	at java.nio.channels.FileChannel.lock(FileChannel.java:860)
	at springtest.trigger.Demo$subthread.run(Demo.java:89)
there are going to get lock...
geted the lock!

 

2.多進程間，是可以安然的鎖上的。當然如果你在一個進程中的線程中沒有釋放鎖，而此時這個線程由於異常而退出，但是進程沒有退出，那麼你就會發現這個鎖會一直被拿着，而沒有釋放。所以一定要將鎖釋放寫在finally裏面。