Directory interface in Lucene

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.io.EOFException;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.Closeable;
import java.util.Collection; // for javadocs

import org.apache.lucene.util.IOUtils;

/** A Directory is a flat list of files.  Files may be written once, when they
 * are created.  Once a file is created it may only be opened for read, or
 * deleted.  Random access is permitted both when reading and writing.
 *  <p> Java's i/o APIs not used directly, but rather all i/o is
 * through this API.  This permits things such as: <ul>
 * <li> implementation of RAM-based indices;
 * <li> implementation indices stored in a database, via JDBC;
 * <li> implementation of an index as a single file;
 * </ul>
 *
 * Directory locking is implemented by an instance of {@link
 * LockFactory}, and can be changed for each Directory
 * instance using {@link #setLockFactory}.
 *
 */
public abstract class Directory implements Closeable {

  volatile protected boolean isOpen = true;

  /** Holds the LockFactory instance (implements locking for
   * this Directory instance). */
  protected LockFactory lockFactory;

  /**
   * Returns an array of strings, one for each file in the directory.
   * 
   * @throws NoSuchDirectoryException if the directory is not prepared for any
   *         write operations (such as {@link #createOutput(String, IOContext)}).
   * @throws IOException in case of other IO errors
   */
  public abstract String[] listAll() throws IOException;

  /** Returns true iff a file with the given name exists. */
  public abstract boolean fileExists(String name)
       throws IOException;

  /** Removes an existing file in the directory. */
  public abstract void deleteFile(String name)
       throws IOException;

  /**
   * Returns the length of a file in the directory. This method follows the
   * following contract:
   * <ul>
   * <li>Must throw {@link FileNotFoundException} if the file does not exist
   * (not {@code java.nio.file.NoSuchFileException} of Java 7).
   * <li>Returns a value &ge;0 if the file exists, which specifies its length.
   * </ul>
   * 
   * @param name the name of the file for which to return the length.
   * @throws IOException if there was an IO error while retrieving the file's
   *         length.
   */
  public abstract long fileLength(String name) throws IOException;


  /** Creates a new, empty file in the directory with the given name.
      Returns a stream writing this file. */
  public abstract IndexOutput createOutput(String name, IOContext context)
       throws IOException;

  /**
   * Ensure that any writes to these files 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.<br/>
   * <br/>
   * NOTE: Clients may call this method for same files over
   * and over again, so some impls might optimize for that.
   * For other impls the operation can be a noop, for various
   * reasons.
   */
  public abstract void sync(Collection<String> names) throws IOException;

  /** Returns a stream reading an existing file, with the
   * specified read buffer size.  The particular Directory
   * implementation may ignore the buffer size.  Currently
   * the only Directory implementations that respect this
   * parameter are {@link FSDirectory} and {@link
   * CompoundFileDirectory}.
   * <li>Must throw {@link FileNotFoundException} if the file does not exist
   * (not {@code java.nio.file.NoSuchFileException} of Java 7).
   */
  public abstract IndexInput openInput(String name, IOContext context) throws IOException; 
  
  /** Construct a {@link Lock}.
   * @param name the name of the lock file
   */
  public Lock makeLock(String name) {
      return lockFactory.makeLock(name);
  }
  /**
   * Attempt to clear (forcefully unlock and remove) the
   * specified lock.  Only call this at a time when you are
   * certain this lock is no longer in use.
   * @param name name of the lock to be cleared.
   */
  public void clearLock(String name) throws IOException {
    if (lockFactory != null) {
      lockFactory.clearLock(name);
    }
  }

  /** Closes the store. */
  @Override
  public abstract void close()
       throws IOException;

  /**
   * Set the LockFactory that this Directory instance should
   * use for its locking implementation.  Each * instance of
   * LockFactory should only be used for one directory (ie,
   * do not share a single instance across multiple
   * Directories).
   *
   * @param lockFactory instance of {@link LockFactory}.
   */
  public void setLockFactory(LockFactory lockFactory) throws IOException {
    assert lockFactory != null;
    this.lockFactory = lockFactory;
    lockFactory.setLockPrefix(this.getLockID());
  }

  /**
   * Get the LockFactory that this Directory instance is
   * using for its locking implementation.  Note that this
   * may be null for Directory implementations that provide
   * their own locking implementation.
   */
  public LockFactory getLockFactory() {
    return this.lockFactory;
  }

  /**
   * 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();
  }

  /**
   * Copies the file <i>src</i> to {@link Directory} <i>to</i> under the new
   * file name <i>dest</i>.
   * <p>
   * If you want to copy the entire source directory to the destination one, you
   * can do so like this:
   * 
   * <pre class="prettyprint">
   * Directory to; // the directory to copy to
   * for (String file : dir.listAll()) {
   *   dir.copy(to, file, newFile, IOContext.DEFAULT); // newFile can be either file, or a new name
   * }
   * </pre>
   * <p>
   * <b>NOTE:</b> this method does not check whether <i>dest</i> exist and will
   * overwrite it if it does.
   */
  public void copy(Directory to, String src, String dest, IOContext context) throws IOException {
    IndexOutput os = null;
    IndexInput is = null;
    IOException priorException = null;
    try {
      os = to.createOutput(dest, context);
      is = openInput(src, context);
      os.copyBytes(is, is.length());
    } catch (IOException ioe) {
      priorException = ioe;
    } finally {
      boolean success = false;
      try {
        IOUtils.closeWhileHandlingException(priorException, os, is);
        success = true;
      } finally {
        if (!success) {
          try {
            to.deleteFile(dest);
          } catch (Throwable t) {
          }
        }
      }
    }
  }

  /**
   * Creates an {@link IndexInputSlicer} for the given file name.
   * IndexInputSlicer allows other {@link Directory} implementations to
   * efficiently open one or more sliced {@link IndexInput} instances from a
   * single file handle. The underlying file handle is kept open until the
   * {@link IndexInputSlicer} is closed.
   * <li>Must throw {@link FileNotFoundException} if the file does not exist
   * (not {@code java.nio.file.NoSuchFileException} of Java 7).
   *
   * @throws IOException
   *           if an {@link IOException} occurs
   * @lucene.internal
   * @lucene.experimental
   */
  public IndexInputSlicer createSlicer(final String name, final IOContext context) throws IOException {
    ensureOpen();
    return new IndexInputSlicer() {
      private final IndexInput base = Directory.this.openInput(name, context);
      @Override
      public IndexInput openSlice(String sliceDescription, long offset, long length) {
        return new SlicedIndexInput("SlicedIndexInput(" + sliceDescription + " in " + base + ")", base, offset, length);
      }
      @Override
      public void close() throws IOException {
        base.close();
      }
      @Override
      public IndexInput openFullSlice() {
        return base.clone();
      }
    };
  }

  /**
   * @throws AlreadyClosedException if this Directory is closed
   */
  protected final void ensureOpen() throws AlreadyClosedException {
    if (!isOpen)
      throw new AlreadyClosedException("this Directory is closed");
  }
  
  /**
   * Allows to create one or more sliced {@link IndexInput} instances from a single 
   * file handle. Some {@link Directory} implementations may be able to efficiently map slices of a file
   * into memory when only certain parts of a file are required.   
   * @lucene.internal
   * @lucene.experimental
   */
  public abstract class IndexInputSlicer implements Closeable {
    /**
     * Returns an {@link IndexInput} slice starting at the given offset with the given length.
     */
    public abstract IndexInput openSlice(String sliceDescription, long offset, long length) throws IOException;

    /**
     * Returns an {@link IndexInput} slice starting at offset <i>0</i> with a
     * length equal to the length of the underlying file
     * @deprecated Only for reading CFS files from 3.x indexes.
     */
    @Deprecated
    // can we remove this somehow?
    public abstract IndexInput openFullSlice() throws IOException;
  }
  
  /** Implementation of an IndexInput that reads from a portion of
   *  a file.
   */
  private static final class SlicedIndexInput extends BufferedIndexInput {
    IndexInput base;
    long fileOffset;
    long length;
    
    SlicedIndexInput(final String sliceDescription, final IndexInput base, final long fileOffset, final long length) {
      this(sliceDescription, base, fileOffset, length, BufferedIndexInput.BUFFER_SIZE);
    }
    
    SlicedIndexInput(final String sliceDescription, final IndexInput base, final long fileOffset, final long length, int readBufferSize) {
      super("SlicedIndexInput(" + sliceDescription + " in " + base + " slice=" + fileOffset + ":" + (fileOffset+length) + ")", readBufferSize);
      this.base = base.clone();
      this.fileOffset = fileOffset;
      this.length = length;
    }
    
    @Override
    public SlicedIndexInput clone() {
      SlicedIndexInput clone = (SlicedIndexInput)super.clone();
      clone.base = base.clone();
      clone.fileOffset = fileOffset;
      clone.length = length;
      return clone;
    }
    
    /** Expert: implements buffer refill.  Reads bytes from the current
     *  position in the input.
     * @param b the array to read bytes into
     * @param offset the offset in the array to start storing bytes
     * @param len the number of bytes to read
     */
    @Override
    protected void readInternal(byte[] b, int offset, int len) throws IOException {
      long start = getFilePointer();
      if(start + len > length)
        throw new EOFException("read past EOF: " + this);
      base.seek(fileOffset + start);
      base.readBytes(b, offset, len, false);
    }
    
    /** Expert: implements seek.  Sets current position in this file, where
     *  the next {@link #readInternal(byte[],int,int)} will occur.
     * @see #readInternal(byte[],int,int)
     */
    @Override
    protected void seekInternal(long pos) {}
    
    /** Closes the stream to further operations. */
    @Override
    public void close() throws IOException {
      base.close();
    }
    
    @Override
    public long length() {
      return length;
    }
  }
}

Directory在讀寫時能夠隨機訪問，其中的文件只用於讀和寫，不能更新，更新index是經過讀和寫複合來實現的。