Java File Lock Test

在Java中如何给文件加锁？如何给确认文件是否被加锁？做个测试先。

Test.java

package me.test;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.channels.FileLock;
import java.nio.channels.OverlappingFileLockException;

public class Test {
    public static void main(String[] args) throws InterruptedException {
        // 先启动读取
        R r = new R();
        r.start();

        // 再启动写入
        Thread.sleep(1000 * 2);
        W w = new W();
        w.start();

        Thread.sleep(1000 * 2);
        System.exit(0);
    }

    // 检查文件是否被锁（通过尝试对该文件追加排他锁）
    public static boolean i***clusivelyLocked(File f) throws IOException {
        FileOutputStream out = new FileOutputStream(f);
        FileLock lock = null;
        try {
            lock = out.getChannel().tryLock();
            if (lock != null) {
                lock.release();
                return false; // Has not been locked yet.
            }
        } catch (OverlappingFileLockException e) {
            return true; // Already locked by same JVM
        } finally {
            out.close();
        }

        return true; // Already locked by ohter JVM or applications
    }

    // 写入一个文件并加锁()
    public static class W extends Thread {
        @Override
        public void run() {
            try {
                File f = new File("C:/123.txt");
                FileOutputStream out = new FileOutputStream(f);
                // 加锁（独占）
                FileLock lock = out.getChannel().lock();
                System.out.println("W: lock.isShared() = " + lock.isShared());
                System.out.println("W: lock.isValid() = " + lock.isValid());
                out.write("Hello World~".getBytes());
                while (true) {
                    Thread.sleep(1000);
                }
                // out.close(); // 一旦close，lock.isValid() == false;
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    // 读取一个文件并加锁
    public static class R extends Thread {
        @Override
        public void run() {
            try {
                File f = new File("C:/123.txt");
                while (true) {
                    System.out.println("R: canRead() = " + f.canRead()
                            + ", isLocked() = " + i***clusivelyLocked(f));
                    Thread.sleep(1000);
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

命令行输出结果：

R: canRead() = true, isLocked() = false
R: canRead() = true, isLocked() = false
W: lock.isShared() = false
W: lock.isValid() = true
R: canRead() = true, isLocked() = true
R: canRead() = true, isLocked() = true
R: canRead() = true, isLocked() = true