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File locking example
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‎core-java-modules/core-java-io-2/README.md‎

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- [List Files in a Directory in Java](https://www.baeldung.com/java-list-directory-files)
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- [Java – Append Data to a File](https://www.baeldung.com/java-append-to-file)
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- [How to Copy a File with Java](https://www.baeldung.com/java-copy-file)
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- [Create a Directory in Java](https://www.baeldung.com/java-create-directory)
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- [Create a Directory in Java](https://www.baeldung.com/java-create-directory)
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- [[<-- Prev]](/core-java-modules/core-java-io)

‎core-java-modules/core-java-nio-2/pom.xml‎

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<version>0.0.1-SNAPSHOT</version>
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<relativePath>../../parent-java</relativePath>
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</parent>
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<dependencies>
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<!-- https://mvnrepository.com/artifact/com.github.jnr/jnr-ffi -->
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<dependency>
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<groupId>com.github.jnr</groupId>
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<artifactId>jnr-ffi</artifactId>
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<version>2.1.11</version>
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</dependency>
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<!-- https://mvnrepository.com/artifact/com.github.jnr/jnr-constants -->
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<dependency>
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<groupId>com.github.jnr</groupId>
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<artifactId>jnr-constants</artifactId>
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<version>0.9.14</version>
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</dependency>
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</dependencies>
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</project>

‎core-java-modules/core-java-nio-2/src/main/java/com/baeldung/lock/FileLocks.java‎

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package com.baeldung.lock;
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import java.io.IOException;
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import java.io.RandomAccessFile;
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import java.nio.ByteBuffer;
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import java.nio.channels.FileChannel;
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import java.nio.channels.FileLock;
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import java.nio.charset.StandardCharsets;
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import java.nio.file.Files;
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import java.nio.file.Path;
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import java.nio.file.Paths;
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import jnr.ffi.LibraryLoader;
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import jnr.ffi.Memory;
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import jnr.ffi.Pointer;
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import jnr.ffi.types.pid_t;
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public class FileLocks {
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public static interface LibC {
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public static final int O_NONBLOCK = jnr.constants.platform.OpenFlags.O_NONBLOCK.intValue();
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public static final int O_RDWR = jnr.constants.platform.OpenFlags.O_RDWR.intValue();
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public static final int O_EXLOCK = jnr.constants.platform.OpenFlags.O_EXLOCK.intValue();
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public long write(int fd, Pointer data, long len);
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@pid_t
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long getpid();
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int open(String filename, int flags);
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int close(int fd);
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}
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public static void main(String[] args) throws IOException, InterruptedException {
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Path path = Paths.get("/tmp/foo");
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// Delete the file if it exists
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Files.deleteIfExists(path);
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// Start with a fresh empty file
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Files.createFile(path);
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// Prepare some external libc calls. Will only work on systems that have libc.
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LibC libc = LibraryLoader.create(LibC.class).load("c");
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byte[] bytes = "Hello from C\n".getBytes("UTF-8");
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jnr.ffi.Runtime runtime = jnr.ffi.Runtime.getRuntime(libc);
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Pointer buffer = Memory.allocateDirect(runtime, bytes.length);
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buffer.put(0, bytes, 0, bytes.length);
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// Open the file through a libc call. This returns a file descriptor.
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int fd = libc.open(path.toString(), libc.O_RDWR + libc.O_EXLOCK + libc.O_NONBLOCK);
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System.out.println("Opened the file through a libc call that locks it.");
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// Our java method will see the lock. Itd will be well behaved and won't write to the file.
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// Note that external processes on POSIX systems would still be able to write to this file ignoring any locks.
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writeToRandomAccessFile(path, "I won't write this", 0L);
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// Libc opened the file, it can write to its corresponding file handle.
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libc.write(fd, buffer, bytes.length);
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// Now let's close the file through a libc call, to release its lock.
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libc.close(fd);
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System.out.println("Invoked libc's close() method");
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// This time our method won't see the lock and will write to the file.
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writeToRandomAccessFile(path, "Hello from java", bytes.length);
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System.out.println("Now that all the locks are gone, here are the file contents:");
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System.out.println("------------------------------------------------------------");
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Files.lines(path).forEach(System.out::println);
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}
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public static RandomAccessFile writeToRandomAccessFile(Path path, String data, long position) {
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RandomAccessFile file = null;
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try {
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file = new RandomAccessFile(path.toFile(), "rws");
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FileChannel channel = file.getChannel();
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// Try to acquire a lock
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try (FileLock lock = channel.tryLock()) {
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if (lock == null) {
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System.out.println("Tried to lock through the FileChannel's lock() method. This file is already locked! It's my responsibility to not write to it, even if the OS would let me!");
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} else {
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System.out.println("I don't see a lock on this file anymore. Now I can write to it.");
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int i = 0;
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channel.write(
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ByteBuffer.wrap((data).getBytes(StandardCharsets.UTF_8)), position);
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}
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} catch (Exception e) {
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System.out.println("Error while locking");
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e.printStackTrace();
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}
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} catch (Exception e) {
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System.out.println("Other Error.");
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e.printStackTrace();
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}
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return file;
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}
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}

‎core-java-modules/core-java-nio-2/src/main/java/com/baeldung/lock/com_baeldung_lock_FileLocks.h‎

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