在数据库系统中，为了确保数据的一致性和完整性，需要一种机制来防止多个进程同时写入数据库，这就需要使用文件锁。这里提到的“它使用合作式锁定协议”，意味着系统中的各个进程需要相互合作，遵守同一套规则来决定哪个进程可以持有锁，进而执行写操

具体来说，当一个进程想要对数据库进行写操作时，它必须首先请求锁。如果锁可用，该进程将获得锁并执行写操作。操作完成后，进程必须释放锁，以便其他进程可以请求并获取锁，执行它们自己的写操作。

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合作式原理图

合作式锁定协议的实现：

import java.io.*;
import java.nio.channels.*;
import java.nio.file.*;

public class FileLockManager {

    private final Path lockFilePath;
    private FileChannel lockFileChannel;
    private FileLock fileLock;

    public FileLockManager(String lockFileName) throws IOException {
        this.lockFilePath = Paths.get(lockFileName);
        this.lockFileChannel = FileChannel.open(lockFilePath, StandardOpenOption.CREATE, StandardOpenOption.WRITE);
    }

    public boolean tryLock() throws IOException, InterruptedException {
        fileLock = lockFileChannel.tryLock();
        return fileLock != null;
    }

    public void unlock() throws IOException {
        if (fileLock != null) {
            fileLock.release();
        }
        try {
            lockFileChannel.close();
        } catch (IOException e) {
            // Handle exception or log it
        }
    }

    public static void main(String[] args) {
        FileLockManager lockManager = new FileLockManager("database.lock");
        try {
            if (lockManager.tryLock()) {
                // Perform database operations
            } else {
                System.err.println("Could not acquire lock, database may be in use by another process.");
            }
        } catch (IOException | InterruptedException e) {
            // Handle or log the exception
        } finally {
            lockManager.unlock();
        }
    }
}

套接字锁定原理图

基于套接字的锁定机制的实现：

import java.io.IOException;
import java.net.*;

public class SocketLockManager {

    private ServerSocket serverSocket;

    public SocketLockManager(int port) throws IOException {
        this.serverSocket = new ServerSocket(port);
    }

    public boolean tryLock() throws IOException {
        try {
            serverSocket.bind(new InetSocketAddress("localhost", serverSocket.getLocalPort()));
            return true;
        } catch (BindException e) {
            return false; // Port is already in use, another instance might be running
        }
    }

    public void unlock() {
        if (serverSocket != null) {
            try {
                serverSocket.close();
            } catch (IOException e) {
                // Handle exception or log it
            }
        }
    }

    public static void main(String[] args) {
        SocketLockManager lockManager = null;
        try {
            lockManager = new SocketLockManager(12345);
            if (lockManager.tryLock()) {
                // Perform database operations
            } else {
                System.err.println("Could not acquire socket lock, port may be in use by another process.");
            }
        } catch (IOException e) {
            // Handle or log the exception
        } finally {
            if (lockManager != null) {
                lockManager.unlock();
            }
        }
    }
}