Merge pull request #8688 from psoares/master

[BAEL-3601] How to Lock Files in Java

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

@@ -0,0 +1,148 @@
+package com.baeldung.lock;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+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.channels.NonReadableChannelException;
+import java.nio.channels.NonWritableChannelException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardOpenOption;
+
+public class FileLocks {
+
+    private static final Logger LOG = LoggerFactory.getLogger(FileLocks.class);
+
+    // Write locks
+
+    /**
+     * Trying to get an exclusive lock on a read-only FileChannel won't work.
+     */
+    static void getExclusiveLockFromInputStream() throws IOException {
+        Path path = Files.createTempFile("foo", "txt");
+        try (FileInputStream fis = new FileInputStream(path.toFile());
+            FileLock lock = fis.getChannel()
+                .lock()) {
+            LOG.debug("This won't happen");
+        } catch (NonWritableChannelException e) {
+            LOG.error("The channel obtained through a FileInputStream isn't writable. You can't obtain an exclusive lock on it!");
+            throw e;
+        }
+    }
+
+    /**
+     * Gets an exclusive lock from a RandomAccessFile. Works because the file is writable.
+     * @param from beginning of the locked region
+     * @param size how many bytes to lock
+     * @return A lock object representing the newly-acquired lock
+     * @throws IOException if there is a problem creating the temporary file
+     */
+    static FileLock getExclusiveLockFromRandomAccessFile(long from, long size) throws IOException {
+        Path path = Files.createTempFile("foo", "txt");
+        try (RandomAccessFile file = new RandomAccessFile(path.toFile(), "rw");
+            FileLock lock = file.getChannel()
+                .lock(from, size, false)) {
+            if (lock.isValid()) {
+                LOG.debug("This is a valid exclusive lock");
+                return lock;
+            }
+            return null;
+        } catch (Exception e) {
+            LOG.error(e.getMessage());
+        }
+        return null;
+    }
+
+    /**
+     * Acquires an exclusive lock on a file region.
+     * @param from beginning of the locked region
+     * @param size how many bytes to lock
+     * @return A lock object representing the newly-acquired lock
+     * @throws IOException if there is a problem creating the temporary file
+     */
+    static FileLock getExclusiveLockFromFileChannelOpen(long from, long size) throws IOException {
+        Path path = Files.createTempFile("foo", "txt");
+        try (FileChannel channel = FileChannel.open(path, StandardOpenOption.APPEND); FileLock lock = channel.lock(from, size, false)) {
+            String text = "Hello, world.";
+            ByteBuffer buffer = ByteBuffer.allocate(text.length() + System.lineSeparator()
+                .length());
+            buffer.put((text + System.lineSeparator()).getBytes(StandardCharsets.UTF_8));
+            buffer.flip();
+            while (buffer.hasRemaining()) {
+                channel.write(buffer, channel.size());
+            }
+            LOG.debug("This was written to the file");
+            Files.lines(path)
+                .forEach(LOG::debug);
+            return lock;
+        }
+    }
+
+    // Read locks
+
+    /**
+     * Trying to get a shared lock on a write-only FileChannel won't work.
+     */
+    static void getReadLockFromOutputStream() throws IOException {
+        Path path = Files.createTempFile("foo", "txt");
+        try (FileOutputStream fis = new FileOutputStream(path.toFile());
+            FileLock lock = fis.getChannel()
+                .lock(0, Long.MAX_VALUE, true)) {
+            LOG.debug("This won't happen");
+        } catch (NonReadableChannelException e) {
+            LOG.error("The channel obtained through a FileOutputStream isn't readable. " + "You can't obtain an shared lock on it!");
+            throw e;
+        }
+    }
+
+    /**
+     * Gets a lock from an <tt>InputStream</tt>.
+     * @param from beginning of the locked region
+     * @param size how many bytes to lock
+     * @return A lock object representing the newly-acquired lock
+     * @throws IOException if there is a problem creating the temporary file
+     */
+    static FileLock getReadLockFromInputStream(long from, long size) throws IOException {
+        Path path = Files.createTempFile("foo", "txt");
+        try (FileInputStream fis = new FileInputStream(path.toFile());
+            FileLock lock = fis.getChannel()
+                .lock(from, size, true)) {
+            if (lock.isValid()) {
+                LOG.debug("This is a valid shared lock");
+                return lock;
+            }
+            return null;
+        }
+    }
+
+    /**
+     * Gets an exclusive lock from a RandomAccessFile. Works because the file is readable.
+     * @param from beginning of the locked region
+     * @param size how many bytes to lock
+     * @return A lock object representing the newly-acquired lock
+     * @throws IOException if there is a problem creating the temporary file
+     */
+    static FileLock getReadLockFromRandomAccessFile(long from, long size) throws IOException {
+        Path path = Files.createTempFile("foo", "txt");
+        try (RandomAccessFile file = new RandomAccessFile(path.toFile(), "r"); // could also be "rw", but "r" is sufficient for reading
+            FileLock lock = file.getChannel()
+                .lock(from, size, true)) {
+            if (lock.isValid()) {
+                LOG.debug("This is a valid shared lock");
+                return lock;
+            }
+        } catch (Exception e) {
+            LOG.error(e.getMessage());
+        }
+        return null;
+    }
+
+}

core-java-modules/core-java-nio-2/src/test/java/com/baeldung/lock/FileLocksUnitTest.java

@@ -0,0 +1,54 @@
+package com.baeldung.lock;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.io.IOException;
+import java.nio.channels.FileLock;
+import java.nio.channels.NonReadableChannelException;
+import java.nio.channels.NonWritableChannelException;
+
+import org.junit.jupiter.api.Test;
+
+class FileLocksUnitTest {
+
+    @Test
+    void givenAnInputStream_whenGetWriteLock_thenThrowNonWritableChannelException() {
+        assertThrows(NonWritableChannelException.class, () -> FileLocks.getExclusiveLockFromInputStream());
+    }
+
+    @Test
+    void givenARandomAccessFileWithReadWriteAccess_whenGetWriteLock_thenLock() throws IOException {
+        FileLock lock = FileLocks.getExclusiveLockFromRandomAccessFile(0, 10);
+        assertNotNull(lock);
+        assertFalse(lock.isShared());
+    }
+
+    @Test
+    void givenAPath_whenGetExclusiveLock_thenLock() throws IOException {
+        FileLock lock = FileLocks.getExclusiveLockFromFileChannelOpen(0, 10);
+        assertNotNull(lock);
+        assertFalse(lock.isShared());
+    }
+
+    @Test
+    void givenAFileOutputStream_whenGetSharedLock_thenThrowNonReadableChannelException() {
+        assertThrows(NonReadableChannelException.class, FileLocks::getReadLockFromOutputStream);
+    }
+
+    @Test
+    void givenAnInputStream_whenGetSharedLock_thenLock() throws IOException {
+        FileLock lock = FileLocks.getReadLockFromInputStream(0, 10);
+        assertNotNull(lock);
+        assertTrue(lock.isShared());
+    }
+
+    @Test
+    void givenARandomAccessFile_whenGetSharedLock_thenLock() throws IOException {
+        FileLock lock = FileLocks.getReadLockFromRandomAccessFile(0, 10);
+        assertNotNull(lock);
+        assertTrue(lock.isShared());
+    }
+}