序

并发中的锁文件模式是Java企业设计模式中的一种。可以是本地锁，也可以分布式锁，看文件系统是本地还是分布式的，算是一种比较古老的方式。利用zk实现分布式锁，其实跟这个也比较类似。zk其本质是个树形结构。

代码

import java.io.File;import java.io.IOException;import java.io.RandomAccessFile;/** * Subclass of RandomAccessFile that always ensures that it * has exclusive access to a file before opening it. */public class ExclusiveRandomAccessFile extends RandomAccessFile {    private static final String LOCK_FILE_SUFFIX = ".lck";    private File lockFile;    /**     * Open the named file using a lock file to ensure     * exclusive access.     * @param fileName The name of the file to open.     * @param mode This should either be "r" for read-only     *             access or "rw" for read-write access.     * @exception FileSharingException     *            If there is already a lock file for the named     *            file.     * @exception IOException     *            If there is a problem opening the file     */    public static ExclusiveRandomAccessFile openExclusive(String fileName,                                            String mode)            throws IOException {        File lockFile = new File(fileName+LOCK_FILE_SUFFIX);        //createNewFile是原子操作        if (!lockFile.createNewFile()) {            // lock file already exists            throw new FileSharingException(fileName);        } // if        return new ExclusiveRandomAccessFile(fileName,                mode,                lockFile);    } // openExclusive(String)    /**     * Construct a RandomAccessFile that has exclusive access     * to the given file.     * @param fileName The name of the file to open.     * @param mode This should either be "r" for read-only     *             access or "rw" for read-write access.     * @param lockFile A file object that identifies the lock     *                 file.     */    private ExclusiveRandomAccessFile(String fileName,                                      String mode,                                      File lockFile)            throws IOException {        super(fileName, mode);        this.lockFile = lockFile;    } // constructor(String, String)    /**     * Close this file.     */    public synchronized void close() throws IOException {        if (lockFile!=null) {   // If file is still open            lockFile.delete();            lockFile = null;            super.close();        } // if    } // close()}class FileSharingException extends IOException {    public FileSharingException(String msg) {        super(msg);    } // constructor(String)} // class FileSharingException

要点

1、检查文件如果不存在则创建，要是原子操作，使用File.createNewFile即可实现