【Lucene】store包FSDirectory

源码中涉及以下知识点：

1.java.security.MessageDigest

2.org.apache.lucene.store.LockFactory

org.apache.lucene.store.FSLockFactory

见FSDirectory构造

3.32bit与64bit操作系统

4.sync中RandomAccessFile、FileDescriptor

5.getLockId中位运算

 

相比父类Directory

新增方法1：static open()

因为SUN的JRE长期在WIN平台上存在问题，所以使用NIOFSDirectory性能会较差，这样需要根据不同平台来选择合理的

FSDirectory，静态方法open提供了一种简化的方法来创建平台合适的FSDirectory实例，免除了开发者自己进行平台判断。

 

public static FSDirectory open(File path) throws IOException {
    return open(path, null);
  }

public static FSDirectory open(File path, LockFactory lockFactory) throws IOException {
    if (Constants.WINDOWS) {
      return new SimpleFSDirectory(path, lockFactory);
    } else {
      return new NIOFSDirectory(path, lockFactory);
    }
  }

 

覆盖方法1：sync()

在父类abstract Directory声明如下：

 

/** Ensure that any writes to this file are moved to
   *  stable storage.  Lucene uses this to properly commit
   *  changes to the index, to prevent a machine/OS crash
   *  from corrupting the index. */
  public void sync(String name) throws IOException {}

 子类abstract FSDirectory进行了override

 

@Override
  public void sync(String name) throws IOException {
    ensureOpen();
    File fullFile = new File(directory, name);
    boolean success = false;
    int retryCount = 0;
    IOException exc = null;
    while(!success && retryCount < 5) {
      retryCount++;
      RandomAccessFile file = null;
      try {
        try {
          file = new RandomAccessFile(fullFile, "rw");
          file.getFD().sync();
          success = true;
        } finally {
          if (file != null)
            file.close();
        }
      } catch (IOException ioe) {
        if (exc == null)
          exc = ioe;
        try {
          // Pause 5 msec
          Thread.sleep(5);
        } catch (InterruptedException ie) {
          throw new ThreadInterruptedException(ie);
        }
      }
    }
    if (!success)
      // Throw original exception
      throw exc;
  }

 

覆盖方法2：getLockID()

在父类abstract Directory声明如下：

 

/**
   * Return a string identifier that uniquely differentiates
   * this Directory instance from other Directory instances.
   * This ID should be the same if two Directory instances
   * (even in different JVMs and/or on different machines)
   * are considered "the same index".  This is how locking
   * "scopes" to the right index.
   */
  public String getLockID() {
      return this.toString();
  }

  @Override
  public String toString() {
    return super.toString() + " lockFactory=" + getLockFactory();
  }

  子类abstract FSDirectory进行了override

 

@Override
  public String getLockID() {
    ensureOpen();
    String dirName;                               // name to be hashed
    try {
      dirName = directory.getCanonicalPath();
    } catch (IOException e) {
      throw new RuntimeException(e.toString(), e);
    }

    byte digest[];
    synchronized (DIGESTER) {
      digest = DIGESTER.digest(dirName.getBytes());
    }
    StringBuilder buf = new StringBuilder();
    buf.append("lucene-");
    for (int i = 0; i < digest.length; i++) {
      int b = digest[i];
      buf.append(HEX_DIGITS[(b >> 4) & 0xf]);
      buf.append(HEX_DIGITS[b & 0xf]);
    }

    return buf.toString();
  }