文件锁测试

上读锁，测试写

场景：Java程序上读锁，其他程序不申请锁直接写
结果：写入失败，但是会清空文件

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.SeekableByteChannel;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Arrays;
public class FileChannelDemo {
    public static void writeFile(String filepath) throws IOException {
        Path path = Paths.get(filepath);
        if (Files.notExists(path)) {
            if (Files.notExists(path.getParent())) {
                Files.createDirectories(path.getParent());
            }
            Files.createFile(path);
        }
        FileOutputStream outputStream = new FileOutputStream(path.toFile());
        FileChannel fileChannel = outputStream.getChannel();
        ByteBuffer byteBuffer = ByteBuffer.wrap("hello world".getBytes());
        fileChannel.write(byteBuffer);
        // 重新拿到内容
        byteBuffer.rewind();
        byteBuffer.position(5);
        byte[] bs = " Tom's".getBytes();
        byte[] newBs = Arrays.copyOf(bs, bs.length + byteBuffer.remaining());
        byteBuffer.get(newBs, bs.length, byteBuffer.remaining());
        byteBuffer = ByteBuffer.wrap(newBs);
        fileChannel.position(5);
        fileChannel.write(byteBuffer);
        fileChannel.close();
        outputStream.close();
    }
}

import java.io.FileInputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;
public class FileLockDemo {
    public static final String FILEPATH = "nio-demo/lock.txt";
    public static void main(String[] args) throws IOException, InterruptedException {
        FileChannelDemo.writeFile(FILEPATH);
        FileInputStream inputStream = new FileInputStream(FILEPATH);
        FileChannel channel = inputStream.getChannel();
        FileLock lock = channel.lock(0, Integer.MAX_VALUE, true);
        System.out.println("文件已上锁");
        Thread.sleep(10000);
        ByteBuffer buffer = ByteBuffer.allocateDirect(32);
        channel.read(buffer);
        buffer.flip();
        byte[] bs = new byte[buffer.remaining()];
        buffer.get(bs);
        System.out.println("文件读取完成: " + new String(bs));
        lock.release();
        System.out.println("锁已释放");
        channel.close();
        inputStream.close();
    }
}

import java.io.FileOutputStream;
import java.io.IOException;
public class FileNoLockWriteDemo {
    public static void main(String[] args) throws IOException {
        System.out.println("准备打开文件");
        FileOutputStream outputStream = new FileOutputStream(FileLockDemo.FILEPATH);
        byte[] bs = "This is a joke".getBytes();
        System.out.println("准备写入");
        outputStream.write(bs);
        System.out.println("写入完成");
    }
}

java异常信息：

准备打开文件
准备写入
Exception in thread "main" java.io.IOException: 另一个程序已锁定文件的一部分，进程无法访问。
    at java.io.FileOutputStream.writeBytes(Native Method)
    at java.io.FileOutputStream.write(FileOutputStream.java:313)
    at com.semonx.demoset.io.nio.FileNoLockWriteDemo.main(FileNoLockWriteDemo.java:13)

尝试过使用Notepad++来写文件，结果是一样的：写入失败，文件被清空

上写锁，测试写

场景：Java程序上写锁，其他程序不上锁
结果：在写的过程中，其他进程来写，有可能造成文件数据被破坏

import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;

public class FileWriteLockDemo {
    public static final String FILEPATH = "nio-demo/lock.txt";

    public static void main(String[] args) throws IOException, InterruptedException {
        FileChannelDemo.writeFile(FILEPATH);
        FileOutputStream outputStream = new FileOutputStream(FILEPATH);
        FileChannel channel = outputStream.getChannel();

        FileLock lock = channel.lock();
        System.out.println("文件已上锁");
        channel.write(ByteBuffer.wrap("write lock!\r\n".getBytes()));
        Thread.sleep(10000);
        channel.write(ByteBuffer.wrap("write lock!".getBytes()));

        lock.release();
        System.out.println("锁已释放");

        channel.close();
        outputStream.close();
    }
}

推测

文件锁和普通的内存锁应该差不多，只有正确地使用锁，才能保证临界资源的安全性。如果有进程不申请锁就进行访问，是有可能破坏掉文件的。