細說Lucene源碼(一)：索引文件鎖機制

時間 2019-12-14

標籤細說 lucene 源碼索引文件機制简体版

原文原文鏈接

你們都知道，在多線程或多進程的環境中，對統一資源的訪問須要特別當心，特別是在寫資源時，若是不加鎖，將會致使不少嚴重的後果，Lucene的索引也是如此，lucene對索引的讀寫分爲IndexReader和IndexWriter，顧名思義，一個讀，一個寫，lucene能夠對同一個索引文件創建多個IndexReader對象，可是隻能有一個IndexWriter對象，這是怎麼作到的呢？顯而易見是須要加鎖的，加鎖能夠保證一個索引文件只能創建一個IndexWriter對象。下面就細說Lucene索引文件鎖機制：java

若是咱們對同一個索引文件創建多個不一樣的IndexWriter會怎麼樣呢？apache

IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer);

IndexWriter indexWriter = new IndexWriter(dir, indexWriterConfig);

 

IndexWriterConfig indexWriterConfig2 = new IndexWriterConfig(analyzer);

IndexWriter indexWriter2 = new IndexWriter(dir,indexWriterConfig2);

運行後，控制檯輸出：多線程

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)

顯然是不能夠對同一個索引文件開啓多個IndexWriter。oracle

上面是一個比較簡略的類圖，能夠看到lucene採用了工廠方法，這樣能夠方便擴展其餘實現，這裏只以SimpleFsLock爲例說明lucene的鎖機制（其餘的有興趣能夠看lucene源碼）。jvm

Lock類是鎖的基類，一個抽象類，源碼以下：ide

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();
        }
      }
    }
  }

}

裏面最重要的方法就是obtain()，這個方法用來維持鎖，創建鎖以後，維持時間爲LOCK_POLL_INTERVAL，以後須要從新申請維持鎖，這樣作是爲了支持多線程讀寫。固然也能夠將lockWaitTimeout設置爲-1，這樣就是一直維持寫鎖。ui

抽象基類LockFactory，只定義了一個抽象方法makeLock，返回Lock對象的一個實例。this

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);

}

抽象類FSLockFactory繼承Lock：spa

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);

}

能夠看到線程

public static final FSLockFactory getDefault() {

return NativeFSLockFactory.INSTANCE;

}

這個方法默認返回NativeFSLockFactory，和SimpleFSLockFactory同樣是一個具體實現，NativeFSLockFactory使用的是nio中FileChannel.tryLock方法，這裏不展開討論，有興趣的讀者能夠去看jdk nio的源碼（好像如今oracle不提供FileChannel實現類的源碼了，須要去jvm裏找）。

下面就是本篇文章的重頭戲，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定義了一個內部類SimpleFSLock繼承Lock，咱們仍是主要看SimpleFSLockFactory的obtain方法，這裏就是SimpleFSLock具體實現文件鎖的代碼。

Files.createDirectories(lockDir);

Files.createFile(lockFile);

能夠看着兩行代碼，createDirectories創建write.lock（能夠是別的文件名，lucene默認使用write.lock）文件所在的文件夾及父文件夾。createFile則是建立write.lock文件，這裏有一個精妙的地方，若是write.lock已經存在，那麼createFile則會拋出異常，若是拋出異常，則代表SimpleFSLockFactory維持文件鎖失敗，也即意味着別的進程正在寫索引文件。

看到close()方法中Files.deleteIfExists(lockFile); 就表示若是每次關閉IndexWriter，則會刪除write.lock文件。

總結一下，SimpleFSLockFactory加文件鎖的機制能夠通俗的理解爲，在索引文件所在的目錄下，建立一個write.lock文件，若是此文件夾下已經有write.lock文件，則代表已經有其餘進程在寫當前的索引目錄，因此這次添加文件鎖失敗，也即不能像索引文件中添加信息。每次添加完信息後，則會刪除write.lock文件，釋放文件鎖。也即若是write.lock文件存在，就代表已經有進程在寫索引文件，若是write.lock不存在就建立文件並添加了文件鎖，別的進程不能寫文件。

這是一個很是精妙的方式去實現寫文件鎖，固然可能有些讀者會疑惑爲何本身在Demo中，建立完索引，close後還有write.lock文件存在，由於如今lucene的默認實現是NativeFSLockFactory，也是上文說起的使用nio調用本地方法去實現的lock。