java - 如何知道锁已被获取

Question

我有一个由多个线程访问的通用方法，

     public void m(String fileName)
     {         
       FileOutputStream fos= new FileOutputStream(fileName);
       FileChannel fc = fos.getChannel();
       fc.tryLock();
       ....

有时，两个或多个线程中的文件名可能相同，并且当我使用 tryLock() 或 lock() 时会出现异常。我如何知道锁是否已被获取？

更新:

import java.io.FileOutputStream;
import java.nio.channels.FileChannel;

class A implements Runnable{
    @Override
    public void run()
    {
        try
        {   
            FileOutputStream fos= new FileOutputStream("file.2txt");
            FileChannel fc = fos.getChannel();
            fc.tryLock();                                

            Thread.sleep(4000);

            fc.close();            
            System.out.println("done");
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }        
    }
}

public class Test 
{
    public static void main(String[] args) throws Exception 
    {
        new Thread(new A()).start();
        Thread.sleep(2000);
        new Thread(new A()).start();  
    }
}

我遇到异常:

java.nio.channels.OverlappingFileLockException
    at sun.nio.ch.FileChannelImpl$SharedFileLockTable.checkList(Unknown Source)
    at sun.nio.ch.FileChannelImpl$SharedFileLockTable.add(Unknown Source)
    at sun.nio.ch.FileChannelImpl.tryLock(Unknown Source)
    at java.nio.channels.FileChannel.tryLock(Unknown Source)
    at run.A.run(Test.java:14)
    at java.lang.Thread.run(Unknown Source)

Answer 1

How can I know if a lock has already been acquired?

最好的答案是使用fc.tryLock();。但是，它不应该抛出异常。如果无法获取锁，它将返回null。这是查看锁是否未锁定的最佳方法。

引用javadocs for FileChannel.tryLock() :

This method does not block. An invocation always returns immediately, either having acquired a lock on the requested region or having failed to do so. If it fails to acquire a lock because an overlapping lock is held by another program then it returns null. If it fails to acquire a lock for any other reason then an appropriate exception is thrown.