File locking example

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

@@ -15,4 +15,21 @@
         <relativePath>../../parent-java</relativePath>
     </parent>
     
+    <dependencies>
+    	<!-- https://mvnrepository.com/artifact/com.github.jnr/jnr-ffi -->
+		<dependency>
+		    <groupId>com.github.jnr</groupId>
+		    <artifactId>jnr-ffi</artifactId>
+		    <version>2.1.11</version>
+		</dependency>
+		    	
+		<!-- https://mvnrepository.com/artifact/com.github.jnr/jnr-constants -->
+		<dependency>
+		    <groupId>com.github.jnr</groupId>
+		    <artifactId>jnr-constants</artifactId>
+		    <version>0.9.14</version>
+		</dependency>
+				
+    </dependencies>
+
 </project>

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

@@ -0,0 +1,105 @@
+package com.baeldung.lock;
+
+import java.io.IOException;
+import java.io.RandomAccessFile;
+import java.nio.ByteBuffer;
+import java.nio.channels.FileChannel;
+import java.nio.channels.FileLock;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+import jnr.ffi.LibraryLoader;
+import jnr.ffi.Memory;
+import jnr.ffi.Pointer;
+import jnr.ffi.types.pid_t;
+
+public class FileLocks {
+
+	public static interface LibC {
+
+		public static final int O_NONBLOCK = jnr.constants.platform.OpenFlags.O_NONBLOCK.intValue();
+		public static final int O_RDWR = jnr.constants.platform.OpenFlags.O_RDWR.intValue();
+		public static final int O_EXLOCK = jnr.constants.platform.OpenFlags.O_EXLOCK.intValue();
+
+		public long write(int fd, Pointer data, long len);
+
+		@pid_t
+		long getpid();
+
+		int open(String filename, int flags);
+
+		int close(int fd);
+	}
+
+	public static void main(String[] args) throws IOException, InterruptedException {
+		
+		Path path = Paths.get("/tmp/foo");
+		
+		// Delete the file if it exists
+		Files.deleteIfExists(path);
+		
+		// Start with a fresh empty file
+		Files.createFile(path);
+		
+		// Prepare some external libc calls. Will only work on systems that have libc.
+		LibC libc = LibraryLoader.create(LibC.class).load("c");
+		byte[] bytes = "Hello from C\n".getBytes("UTF-8");
+		jnr.ffi.Runtime runtime = jnr.ffi.Runtime.getRuntime(libc);
+		Pointer buffer = Memory.allocateDirect(runtime, bytes.length);
+		buffer.put(0, bytes, 0, bytes.length);
+		
+		// Open the file through a libc call. This returns a file descriptor.
+		int fd = libc.open(path.toString(), libc.O_RDWR + libc.O_EXLOCK + libc.O_NONBLOCK);
+		System.out.println("Opened the file through a libc call that locks it.");
+		
+		// Our java method will see the lock. Itd will be well behaved and won't write to the file.
+		// Note that external processes on POSIX systems would still be able to write to this file ignoring any locks.
+		writeToRandomAccessFile(path, "I won't write this", 0L);
+		
+		// Libc opened the file, it can write to its corresponding file handle.
+		libc.write(fd, buffer, bytes.length);
+		
+		// Now let's close the file through a libc call, to release its lock.	
+		libc.close(fd);
+		System.out.println("Invoked libc's close() method");
+		
+		// This time our method won't see the lock and will write to the file.
+		writeToRandomAccessFile(path, "Hello from java", bytes.length);
+		
+		System.out.println("Now that all the locks are gone, here are the file contents:");
+		System.out.println("------------------------------------------------------------");
+		Files.lines(path).forEach(System.out::println);
+
+	}
+	
+	public static RandomAccessFile writeToRandomAccessFile(Path path, String data, long position) {
+		RandomAccessFile file = null;
+		try {
+			file = new RandomAccessFile(path.toFile(), "rws");
+			FileChannel channel = file.getChannel();
+			// Try to acquire a lock
+			try (FileLock lock = channel.tryLock()) {
+				if (lock == null) {
+					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!");
+				} else {
+					System.out.println("I don't see a lock on this file anymore. Now I can write to it.");
+					int i = 0;
+					channel.write(
+							ByteBuffer.wrap((data).getBytes(StandardCharsets.UTF_8)), position);
+				}
+			} catch (Exception e) {
+				System.out.println("Error while locking");
+				e.printStackTrace();
+			}
+		} catch (Exception e) {
+			System.out.println("Other Error.");
+			e.printStackTrace();
+		}
+		return file;
+	}
+	
+	
+
+}

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

@@ -0,0 +1,21 @@
+/* DO NOT EDIT THIS FILE - it is machine generated */
+#include <jni.h>
+/* Header for class com_baeldung_lock_FileLocks */
+
+#ifndef _Included_com_baeldung_lock_FileLocks
+#define _Included_com_baeldung_lock_FileLocks
+#ifdef __cplusplus
+extern "C" {
+#endif
+/*
+ * Class:     com_baeldung_lock_FileLocks
+ * Method:    getpid
+ * Signature: ()J
+ */
+JNIEXPORT jlong JNICALL Java_com_baeldung_lock_FileLocks_getpid
+  (JNIEnv *, jclass);
+
+#ifdef __cplusplus
+}
+#endif
+#endif