在单个JVM和多个JVM中使用java文件锁

Question

我想我错过了一些东西,但我无法理解文件锁如何在Java中工作.更确切地说 - 它是如何实现的.

我似乎无法获取(甚至无法尝试获取)单个JVM内的同一文件的两个或更多锁.将成功获取第一个锁,所有进一步尝试获取更多锁将导致OverlapingFileLockException.然而,它适用于单独的过程.

我想实现由文件系统支持的数据存储,该数据存储旨在处理多个并发请求(读取和写入).我想使用文件锁来锁定存储中的特定文件.

似乎我必须在JVM级别上引入一个同步(独占),然后才对文件进行同步以避免此异常.

有人这样做过吗？

我准备了简单的测试用例来说明我的问题.我使用的是Mac OS X,Java 6.

import junit.framework.*;

import javax.swing.*;
import java.io.*;
import java.nio.channels.*;

/**
 * Java file locks test.
 */
public class FileLocksTest extends TestCase {
    /** File path (on Windows file will be created under the root directory of the current drive). */
    private static final String LOCK_FILE_PATH = "/test-java-file-lock-tmp.bin";

    /**
     * @throws Exception If failed.
     */
    public void testWriteLocks() throws Exception {
        final File file = new File(LOCK_FILE_PATH);

        file.createNewFile();

        RandomAccessFile raf = new RandomAccessFile(file, "rw");

        System.out.println("Getting lock...");

        FileLock lock = raf.getChannel().lock();

        System.out.println("Obtained lock: " + lock);

        Thread thread = new Thread(new Runnable() {
            @Override public void run() {
                try {
                    RandomAccessFile raf = new RandomAccessFile(file, "rw");

                    System.out.println("Getting lock (parallel thread)...");

                    FileLock lock = raf.getChannel().lock();

                    System.out.println("Obtained lock (parallel tread): " + lock);

                    lock.release();
                }
                catch (Throwable e) {
                    e.printStackTrace();
                }
            }
        });

        thread.start();

        JOptionPane.showMessageDialog(null, "Press OK to release lock.");

        lock.release();

        thread.join();
    }

    /**
     * @throws Exception If failed.
     */
    public void testReadLocks() throws Exception {
        final File file = new File(LOCK_FILE_PATH);

        file.createNewFile();

        RandomAccessFile raf = new RandomAccessFile(file, "r");

        System.out.println("Getting lock...");

        FileLock lock = raf.getChannel().lock(0, Long.MAX_VALUE, true);

        System.out.println("Obtained lock: " + lock);

        Thread thread = new Thread(new Runnable() {
            @Override public void run() {
                try {
                    RandomAccessFile raf = new RandomAccessFile(file, "r");

                    System.out.println("Getting lock (parallel thread)...");

                    FileLock lock = raf.getChannel().lock(0, Long.MAX_VALUE, true);

                    System.out.println("Obtained lock (parallel thread): " + lock);

                    lock.release();

                }
                catch (Throwable e) {
                    e.printStackTrace();
                }
            }
        });

        thread.start();

        JOptionPane.showMessageDialog(null, "Press OK to release lock.");

        lock.release();

        thread.join();
    }
}

Answer 1

来自Javadoc:

文件锁代表整个Java虚拟机.它们不适合控制同一虚拟机中多个线程对文件的访问.

Answer 2

每个文件只能获取一次锁。AFAIK 锁不可重入。

恕我直言：使用文件在进程之间进行通信是一个非常糟糕的主意。也许你能够让它可靠地工作，如果可以的话请告诉我；）

我只会在一个进程中只有一个线程读/写。