package org.apache.lucene.index;/** * 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.util.List;import java.io.IOException;/** * <p>Expert: policy for deletion of stale {@link IndexCommit index commits}.  *  * <p>Implement this interface, and pass it to one * of the {@link IndexWriter} or {@link IndexReader} * constructors, to customize when older * {@link IndexCommit point-in-time commits} * are deleted from the index directory.  The default deletion policy * is {@link KeepOnlyLastCommitDeletionPolicy}, which always * removes old commits as soon as a new commit is done (this * matches the behavior before 2.2).</p> * * <p>One expected use case for this (and the reason why it * was first created) is to work around problems with an * index directory accessed via filesystems like NFS because * NFS does not provide the "delete on last close" semantics * that Lucene's "point in time" search normally relies on. * By implementing a custom deletion policy, such as "a * commit is only removed once it has been stale for more * than X minutes", you can give your readers time to * refresh to the new commit before {@link IndexWriter} * removes the old commits.  Note that doing so will * increase the storage requirements of the index.  See <a * target="top" * href="http://issues.apache.org/jira/browse/LUCENE-710">LUCENE-710</a> * for details.</p> */public interface IndexDeletionPolicy {  /**   * <p>This is called once when a writer is first   * instantiated to give the policy a chance to remove old   * commit points.</p>   *    * <p>The writer locates all index commits present in the    * index directory and calls this method.  The policy may    * choose to delete some of the commit points, doing so by   * calling method {@link IndexCommit#delete delete()}    * of {@link IndexCommit}.</p>   *    * <p><u>Note:</u> the last CommitPoint is the most recent one,   * i.e. the "front index state". Be careful not to delete it,   * unless you know for sure what you are doing, and unless    * you can afford to lose the index content while doing that.    *   * @param commits List of current    * {@link IndexCommit point-in-time commits},   *  sorted by age (the 0th one is the oldest commit).   */  public void onInit(List commits) throws IOException;  /**   * <p>This is called each time the writer completed a commit.   * This gives the policy a chance to remove old commit points   * with each commit.</p>   *   * <p>The policy may now choose to delete old commit points    * by calling method {@link IndexCommit#delete delete()}    * of {@link IndexCommit}.</p>   *    * <p>If writer has <code>autoCommit = true</code> then   * this method will in general be called many times during   * one instance of {@link IndexWriter}.  If   * <code>autoCommit = false</code> then this method is   * only called once when {@link IndexWriter#close} is   * called, or not at all if the {@link IndexWriter#abort}   * is called.    *   * <p><u>Note:</u> the last CommitPoint is the most recent one,   * i.e. the "front index state". Be careful not to delete it,   * unless you know for sure what you are doing, and unless    * you can afford to lose the index content while doing that.   *     * @param commits List of {@link IndexCommit},   *  sorted by age (the 0th one is the oldest commit).   */  public void onCommit(List commits) throws IOException;}