细说Lucene源码(一)：索引文件锁机制

大家都知道，在多线程或多进程的环境中，对统一资源的访问需要特别小心，特别是在写资源时，如果不加锁，将会导致很多严重的后果，Lucene的索引也是如此，lucene对索引的读写分为IndexReader和IndexWriter，顾名思义，一个读，一个写，lucene可以对同一个索引文件建立多个IndexReader对象，但是只能有一个IndexWriter对象，这是怎么做到的呢？显而易见是需要加锁的，加锁可以保证一个索引文件只能建立一个IndexWriter对象。下面就细说Lucene索引文件锁机制：

如果我们对同一个索引文件建立多个不同的IndexWriter会怎么样呢？

IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer);

IndexWriter indexWriter = new IndexWriter(dir, indexWriterConfig);

IndexWriterConfig indexWriterConfig2 = new IndexWriterConfig(analyzer);

IndexWriter indexWriter2 = new IndexWriter(dir,indexWriterConfig2);

运行后，控制台输出：

Exception in thread "main" org.apache.lucene.store.LockObtainFailedException: Lock obtain timed out: NativeFSLock@C:\Users\new\Desktop\Lucene\write.lock

    at org.apache.lucene.store.Lock.obtain(Lock.java:89)

    at org.apache.lucene.index.IndexWriter.<init>(IndexWriter.java:755)

    at test.Index.index(Index.java:51)

    at test.Index.main(Index.java:78)

显然是不可以对同一个索引文件开启多个IndexWriter。

上面是一个比较简略的类图，可以看到lucene采用了工厂方法，这样可以方便扩展其他实现，这里只以SimpleFsLock为例说明lucene的锁机制（其他的有兴趣可以看lucene源码）。

Lock类是锁的基类，一个抽象类，源码如下：

public abstract class Lock implements Closeable {

  /** How long {@link #obtain(long)} waits, in milliseconds,
   *  in between attempts to acquire the lock. */
  public static long LOCK_POLL_INTERVAL = 1000;

  /** Pass this value to {@link #obtain(long)} to try
   *  forever to obtain the lock. */
  public static final long LOCK_OBTAIN_WAIT_FOREVER = -1;

  /** Attempts to obtain exclusive access and immediately return
   *  upon success or failure.  Use {@link #close} to
   *  release the lock.
   * @return true iff exclusive access is obtained
   */
  public abstract boolean obtain() throws IOException;

  /**
   * If a lock obtain called, this failureReason may be set
   * with the "root cause" Exception as to why the lock was
   * not obtained.
   */
  protected Throwable failureReason;

  /** Attempts to obtain an exclusive lock within amount of
   *  time given. Polls once per {@link #LOCK_POLL_INTERVAL}
   *  (currently 1000) milliseconds until lockWaitTimeout is
   *  passed.
   * @param lockWaitTimeout length of time to wait in
   *        milliseconds or {@link
   *        #LOCK_OBTAIN_WAIT_FOREVER} to retry forever
   * @return true if lock was obtained
   * @throws LockObtainFailedException if lock wait times out
   * @throws IllegalArgumentException if lockWaitTimeout is
   *         out of bounds
   * @throws IOException if obtain() throws IOException
   */
  public final boolean obtain(long lockWaitTimeout) throws IOException {
    failureReason = null;
    boolean locked = obtain();
    if (lockWaitTimeout < 0 && lockWaitTimeout != LOCK_OBTAIN_WAIT_FOREVER)
      throw new IllegalArgumentException("lockWaitTimeout should be LOCK_OBTAIN_WAIT_FOREVER or a non-negative number (got " + lockWaitTimeout + ")");

    long maxSleepCount = lockWaitTimeout / LOCK_POLL_INTERVAL;
    long sleepCount = 0;
    while (!locked) {
      if (lockWaitTimeout != LOCK_OBTAIN_WAIT_FOREVER && sleepCount++ >= maxSleepCount) {
        String reason = "Lock obtain timed out: " + this.toString();
        if (failureReason != null) {
          reason += ": " + failureReason;
        }
        throw new LockObtainFailedException(reason, failureReason);
      }
      try {
        Thread.sleep(LOCK_POLL_INTERVAL);
      } catch (InterruptedException ie) {
        throw new ThreadInterruptedException(ie);
      }
      locked = obtain();
    }
    return locked;
  }

  /** Releases exclusive access. */
  public abstract void close() throws IOException;

  /** Returns true if the resource is currently locked.  Note that one must
   * still call {@link #obtain()} before using the resource. */
  public abstract boolean isLocked() throws IOException;

  /** Utility class for executing code with exclusive access. */
  public abstract static class With {
    private Lock lock;
    private long lockWaitTimeout;

    /** Constructs an executor that will grab the named lock. */
    public With(Lock lock, long lockWaitTimeout) {
      this.lock = lock;
      this.lockWaitTimeout = lockWaitTimeout;
    }

    /** Code to execute with exclusive access. */
    protected abstract Object doBody() throws IOException;

    /** Calls {@link #doBody} while <i>lock</i> is obtained.  Blocks if lock
     * cannot be obtained immediately.  Retries to obtain lock once per second
     * until it is obtained, or until it has tried ten times. Lock is released when
     * {@link #doBody} exits.
     * @throws LockObtainFailedException if lock could not
     * be obtained
     * @throws IOException if {@link Lock#obtain} throws IOException
     */
    public Object run() throws IOException {
      boolean locked = false;
      try {
         locked = lock.obtain(lockWaitTimeout);
         return doBody();
      } finally {
        if (locked) {
          lock.close();
        }
      }
    }
  }

}

里面最重要的方法就是obtain()，这个方法用来维持锁，建立锁之后，维持时间为LOCK_POLL_INTERVAL，之后需要重新申请维持锁，这样做是为了支持多线程读写。当然也可以将lockWaitTimeout设置为-1，这样就是一直维持写锁。

抽象基类LockFactory，只定义了一个抽象方法makeLock，返回Lock对象的一个实例。

public abstract class LockFactory {

  /**
   * Return a new Lock instance identified by lockName.
   * @param lockName name of the lock to be created.
   */
  public abstract Lock makeLock(Directory dir, String lockName);

}

抽象类FSLockFactory继承Lock：

public abstract class FSLockFactory extends LockFactory {

  /** Returns the default locking implementation for this platform.
   * This method currently returns always {@link NativeFSLockFactory}.
   */
  public static final FSLockFactory getDefault() {
    return NativeFSLockFactory.INSTANCE;
  }

  @Override
  public final Lock makeLock(Directory dir, String lockName) {
    if (!(dir instanceof FSDirectory)) {
      throw new UnsupportedOperationException(getClass().getSimpleName() + " can only be used with FSDirectory subclasses, got: " + dir);
    }
    return makeFSLock((FSDirectory) dir, lockName);
  }

  /** Implement this method to create a lock for a FSDirectory instance. */
  protected abstract Lock makeFSLock(FSDirectory dir, String lockName);

}

可以看到

public static final FSLockFactory getDefault() {

return NativeFSLockFactory.INSTANCE;

}

这个方法默认返回NativeFSLockFactory，和SimpleFSLockFactory一样是一个具体实现，NativeFSLockFactory使用的是nio中FileChannel.tryLock方法，这里不展开讨论，有兴趣的读者可以去看jdk nio的源码（好像现在oracle不提供FileChannel实现类的源码了，需要去jvm里找）。

下面就是本篇文章的重头戏，SimpleFSLockFactory

public final class SimpleFSLockFactory extends FSLockFactory {

  /**
   * Singleton instance
   */
  public static final SimpleFSLockFactory INSTANCE = new SimpleFSLockFactory();

  private SimpleFSLockFactory() {}

  @Override
  protected Lock makeFSLock(FSDirectory dir, String lockName) {
    return new SimpleFSLock(dir.getDirectory(), lockName);
  }

  static class SimpleFSLock extends Lock {

    Path lockFile;
    Path lockDir;

    public SimpleFSLock(Path lockDir, String lockFileName) {
      this.lockDir = lockDir;
      lockFile = lockDir.resolve(lockFileName);
    }

    @Override
    public boolean obtain() throws IOException {
      try {
        Files.createDirectories(lockDir);
        Files.createFile(lockFile);
        return true;
      } catch (IOException ioe) {
        // On Windows, on concurrent createNewFile, the 2nd process gets "access denied".
        // In that case, the lock was not aquired successfully, so return false.
        // We record the failure reason here; the obtain with timeout (usually the
        // one calling us) will use this as "root cause" if it fails to get the lock.
        failureReason = ioe;
        return false;
      }
    }

    @Override
    public void close() throws LockReleaseFailedException {
      // TODO: wierd that clearLock() throws the raw IOException...
      try {
        Files.deleteIfExists(lockFile);
      } catch (Throwable cause) {
        throw new LockReleaseFailedException("failed to delete " + lockFile, cause);
      }
    }

    @Override
    public boolean isLocked() {
      return Files.exists(lockFile);
    }

    @Override
    public String toString() {
      return "SimpleFSLock@" + lockFile;
    }
  }

}

在SimpleFSLockFactory定义了一个内部类SimpleFSLock继承Lock，我们还是主要看SimpleFSLockFactory的obtain方法，这里就是SimpleFSLock具体实现文件锁的代码。

Files.createDirectories(lockDir);

Files.createFile(lockFile);

可以看着两行代码，createDirectories建立write.lock（可以是别的文件名，lucene默认使用write.lock）文件所在的文件夹及父文件夹。createFile则是创建write.lock文件，这里有一个精妙的地方，如果write.lock已经存在，那么createFile则会抛出异常，如果抛出异常，则表明SimpleFSLockFactory维持文件锁失败，也即意味着别的进程正在写索引文件。

看到close()方法中Files.deleteIfExists(lockFile); 就表示如果每次关闭IndexWriter，则会删除write.lock文件。

总结一下，SimpleFSLockFactory加文件锁的机制可以通俗的理解为，在索引文件所在的目录下，创建一个write.lock文件，如果此文件夹下已经有write.lock文件，则表明已经有其他进程在写当前的索引目录，所以此次添加文件锁失败，也即不能像索引文件中添加信息。每次添加完信息后，则会删除write.lock文件，释放文件锁。也即如果write.lock文件存在，就表明已经有进程在写索引文件，如果write.lock不存在就创建文件并添加了文件锁，别的进程不能写文件。

这是一个非常精妙的方式去实现写文件锁，当然可能有些读者会疑惑为什么自己在Demo中，创建完索引，close后还有write.lock文件存在，因为现在lucene的默认实现是NativeFSLockFactory，也是上文提及的使用nio调用本地方法去实现的lock。