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Lucene example source code file (NativeFSLockFactory.java)

This example Lucene source code file (NativeFSLockFactory.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Lucene tags/keywords

cannot, file, file, hashset, io, ioexception, ioexception, lock, nativefslock, nativefslock, nativefslockfactory, override, override, randomaccessfile, string, util

The Lucene NativeFSLockFactory.java source code

package org.apache.lucene.store;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;
import java.io.File;
import java.io.RandomAccessFile;
import java.io.IOException;
import java.util.HashSet;

/**
 * <p>Implements {@link LockFactory} using native OS file
 * locks.  Note that because this LockFactory relies on
 * java.nio.* APIs for locking, any problems with those APIs
 * will cause locking to fail.  Specifically, on certain NFS
 * environments the java.nio.* locks will fail (the lock can
 * incorrectly be double acquired) whereas {@link
 * SimpleFSLockFactory} worked perfectly in those same
 * environments.  For NFS based access to an index, it's
 * recommended that you try {@link SimpleFSLockFactory}
 * first and work around the one limitation that a lock file
 * could be left when the JVM exits abnormally.</p>
 *
 * <p>The primary benefit of {@link NativeFSLockFactory} is
 * that lock files will be properly removed (by the OS) if
 * the JVM has an abnormal exit.</p>
 * 
 * <p>Note that, unlike {@link SimpleFSLockFactory}, the existence of
 * leftover lock files in the filesystem on exiting the JVM
 * is fine because the OS will free the locks held against
 * these files even though the files still remain.</p>
 *
 * <p>If you suspect that this or any other LockFactory is
 * not working properly in your environment, you can easily
 * test it by using {@link VerifyingLockFactory}, {@link
 * LockVerifyServer} and {@link LockStressTest}.</p>
 *
 * @see LockFactory
 */

public class NativeFSLockFactory extends FSLockFactory {

  /**
   * Create a NativeFSLockFactory instance, with null (unset)
   * lock directory. When you pass this factory to a {@link FSDirectory}
   * subclass, the lock directory is automatically set to the
   * directory itself. Be sure to create one instance for each directory
   * your create!
   */
  public NativeFSLockFactory() throws IOException {
    this((File) null);
  }

  /**
   * Create a NativeFSLockFactory instance, storing lock
   * files into the specified lockDirName:
   *
   * @param lockDirName where lock files are created.
   */
  public NativeFSLockFactory(String lockDirName) throws IOException {
    this(new File(lockDirName));
  }

  /**
   * Create a NativeFSLockFactory instance, storing lock
   * files into the specified lockDir:
   * 
   * @param lockDir where lock files are created.
   */
  public NativeFSLockFactory(File lockDir) throws IOException {
    setLockDir(lockDir);
  }

  @Override
  public synchronized Lock makeLock(String lockName) {
    if (lockPrefix != null)
      lockName = lockPrefix + "-" + lockName;
    return new NativeFSLock(lockDir, lockName);
  }

  @Override
  public void clearLock(String lockName) throws IOException {
    // Note that this isn't strictly required anymore
    // because the existence of these files does not mean
    // they are locked, but, still do this in case people
    // really want to see the files go away:
    if (lockDir.exists()) {
      
      // Try to release the lock first - if it's held by another process, this
      // method should not silently fail.
      // NOTE: makeLock fixes the lock name by prefixing it w/ lockPrefix.
      // Therefore it should be called before the code block next which prefixes
      // the given name.
      makeLock(lockName).release();

      if (lockPrefix != null) {
        lockName = lockPrefix + "-" + lockName;
      }
      
      // As mentioned above, we don't care if the deletion of the file failed.
      new File(lockDir, lockName).delete();
    }
  }
}

class NativeFSLock extends Lock {

  private RandomAccessFile f;
  private FileChannel channel;
  private FileLock lock;
  private File path;
  private File lockDir;

  /*
   * The javadocs for FileChannel state that you should have
   * a single instance of a FileChannel (per JVM) for all
   * locking against a given file (locks are tracked per 
   * FileChannel instance in Java 1.4/1.5). Even using the same 
   * FileChannel instance is not completely thread-safe with Java 
   * 1.4/1.5 though. To work around this, we have a single (static) 
   * HashSet that contains the file paths of all currently 
   * locked locks.  This protects against possible cases 
   * where different Directory instances in one JVM (each 
   * with their own NativeFSLockFactory instance) have set 
   * the same lock dir and lock prefix. However, this will not 
   * work when LockFactorys are created by different 
   * classloaders (eg multiple webapps). 
   * 
   * TODO: Java 1.6 tracks system wide locks in a thread safe manner 
   * (same FileChannel instance or not), so we may want to 
   * change this when Lucene moves to Java 1.6.
   */
  private static HashSet<String> LOCK_HELD = new HashSet();

  public NativeFSLock(File lockDir, String lockFileName) {
    this.lockDir = lockDir;
    path = new File(lockDir, lockFileName);
  }

  private synchronized boolean lockExists() {
    return lock != null;
  }

  @Override
  public synchronized boolean obtain() throws IOException {

    if (lockExists()) {
      // Our instance is already locked:
      return false;
    }

    // Ensure that lockDir exists and is a directory.
    if (!lockDir.exists()) {
      if (!lockDir.mkdirs())
        throw new IOException("Cannot create directory: " +
                              lockDir.getAbsolutePath());
    } else if (!lockDir.isDirectory()) {
      throw new IOException("Found regular file where directory expected: " + 
                            lockDir.getAbsolutePath());
    }

    String canonicalPath = path.getCanonicalPath();

    boolean markedHeld = false;

    try {

      // Make sure nobody else in-process has this lock held
      // already, and, mark it held if not:

      synchronized(LOCK_HELD) {
        if (LOCK_HELD.contains(canonicalPath)) {
          // Someone else in this JVM already has the lock:
          return false;
        } else {
          // This "reserves" the fact that we are the one
          // thread trying to obtain this lock, so we own
          // the only instance of a channel against this
          // file:
          LOCK_HELD.add(canonicalPath);
          markedHeld = true;
        }
      }

      try {
        f = new RandomAccessFile(path, "rw");
      } catch (IOException e) {
        // On Windows, we can get intermittent "Access
        // Denied" here.  So, we treat this as failure to
        // acquire the lock, but, store the reason in case
        // there is in fact a real error case.
        failureReason = e;
        f = null;
      }

      if (f != null) {
        try {
          channel = f.getChannel();
          try {
            lock = channel.tryLock();
          } catch (IOException e) {
            // At least on OS X, we will sometimes get an
            // intermittent "Permission Denied" IOException,
            // which seems to simply mean "you failed to get
            // the lock".  But other IOExceptions could be
            // "permanent" (eg, locking is not supported via
            // the filesystem).  So, we record the failure
            // reason here; the timeout obtain (usually the
            // one calling us) will use this as "root cause"
            // if it fails to get the lock.
            failureReason = e;
          } finally {
            if (lock == null) {
              try {
                channel.close();
              } finally {
                channel = null;
              }
            }
          }
        } finally {
          if (channel == null) {
            try {
              f.close();
            } finally {
              f = null;
            }
          }
        }
      }

    } finally {
      if (markedHeld && !lockExists()) {
        synchronized(LOCK_HELD) {
          if (LOCK_HELD.contains(canonicalPath)) {
            LOCK_HELD.remove(canonicalPath);
          }
        }
      }
    }
    return lockExists();
  }

  @Override
  public synchronized void release() throws IOException {
    if (lockExists()) {
      try {
        lock.release();
      } finally {
        lock = null;
        try {
          channel.close();
        } finally {
          channel = null;
          try {
            f.close();
          } finally {
            f = null;
            synchronized(LOCK_HELD) {
              LOCK_HELD.remove(path.getCanonicalPath());
            }
          }
        }
      }
      // LUCENE-2421: we don't care anymore if the file cannot be deleted
      // because it's held up by another process (e.g. AntiVirus). NativeFSLock
      // does not depend on the existence/absence of the lock file
      path.delete();
    } else {
      // if we don't hold the lock, and somebody still called release(), for
      // example as a result of calling IndexWriter.unlock(), we should attempt
      // to obtain the lock and release it. If the obtain fails, it means the
      // lock cannot be released, and we should throw a proper exception rather
      // than silently failing/not doing anything.
      boolean obtained = false;
      try {
        if (!(obtained = obtain())) {
          throw new LockReleaseFailedException(
              "Cannot forcefully unlock a NativeFSLock which is held by another indexer component: "
                  + path);
        }
      } finally {
        if (obtained) {
          release();
        }
      }
    }
  }

  @Override
  public synchronized boolean isLocked() {
    // The test for is isLocked is not directly possible with native file locks:
    
    // First a shortcut, if a lock reference in this instance is available
    if (lockExists()) return true;
    
    // Look if lock file is present; if not, there can definitely be no lock!
    if (!path.exists()) return false;
    
    // Try to obtain and release (if was locked) the lock
    try {
      boolean obtained = obtain();
      if (obtained) release();
      return !obtained;
    } catch (IOException ioe) {
      return false;
    }    
  }

  @Override
  public String toString() {
    return "NativeFSLock@" + path;
  }
}

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