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Lucene example source code file (PersistentSnapshotDeletionPolicy.java)

This example Lucene source code file (PersistentSnapshotDeletionPolicy.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Lucene tags/keywords

corruptindexexception, document, document, entry, field, field, illegalstateexception, indexwriter, io, ioexception, ioexception, override, persistentsnapshotdeletionpolicy, string, string, util

The Lucene PersistentSnapshotDeletionPolicy.java source code

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.io.IOException;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;
import org.apache.lucene.document.Fieldable;
import org.apache.lucene.document.Field.Index;
import org.apache.lucene.document.Field.Store;
import org.apache.lucene.index.IndexWriterConfig.OpenMode;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.LockObtainFailedException;
import org.apache.lucene.util.Version;

 * A {@link SnapshotDeletionPolicy} which adds a persistence layer so that
 * snapshots can be maintained across the life of an application. The snapshots
 * are persisted in a {@link Directory} and are committed as soon as
 * {@link #snapshot(String)} or {@link #release(String)} is called.
 * <p>
 * <b>NOTE: this class receives a {@link Directory} to persist the data into
 * a Lucene index. It is highly recommended to use a dedicated directory (and on
 * stable storage as well) for persisting the snapshots' information, and not
 * reuse the content index directory, or otherwise conflicts and index
 * corruptions will occur.
 * <p>
 * <b>NOTE: you should call {@link #close()} when you're done using this
 * class for safetyness (it will close the {@link IndexWriter} instance used).
public class PersistentSnapshotDeletionPolicy extends SnapshotDeletionPolicy {

  // Used to validate that the given directory includes just one document w/ the
  // given ID field. Otherwise, it's not a valid Directory for snapshotting.
  private static final String SNAPSHOTS_ID = "$SNAPSHOTS_DOC$";

  // The index writer which maintains the snapshots metadata
  private final IndexWriter writer;

   * Reads the snapshots information from the given {@link Directory}. This
   * method can be used if the snapshots information is needed, however you
   * cannot instantiate the deletion policy (because e.g., some other process
   * keeps a lock on the snapshots directory).
  public static Map<String, String> readSnapshotsInfo(Directory dir) throws IOException {
    IndexReader r = IndexReader.open(dir, true);
    Map<String, String> snapshots = new HashMap();
    try {
      int numDocs = r.numDocs();
      // index is allowed to have exactly one document or 0.
      if (numDocs == 1) {
        Document doc = r.document(r.maxDoc() - 1);
        Field sid = doc.getField(SNAPSHOTS_ID);
        if (sid == null) {
          throw new IllegalStateException("directory is not a valid snapshots store!");
        for (Fieldable f : doc.getFields()) {
          snapshots.put(f.name(), f.stringValue());
      } else if (numDocs != 0) {
        throw new IllegalStateException(
            "should be at most 1 document in the snapshots directory: " + numDocs);
    } finally {
    return snapshots;
   * {@link PersistentSnapshotDeletionPolicy} wraps another
   * {@link IndexDeletionPolicy} to enable flexible snapshotting.
   * @param primary
   *          the {@link IndexDeletionPolicy} that is used on non-snapshotted
   *          commits. Snapshotted commits, by definition, are not deleted until
   *          explicitly released via {@link #release(String)}.
   * @param dir
   *          the {@link Directory} which will be used to persist the snapshots
   *          information.
   * @param mode
   *          specifies whether a new index should be created, deleting all
   *          existing snapshots information (immediately), or open an existing
   *          index, initializing the class with the snapshots information.
   * @param matchVersion
   *          specifies the {@link Version} that should be used when opening the
   *          IndexWriter.
  public PersistentSnapshotDeletionPolicy(IndexDeletionPolicy primary,
      Directory dir, OpenMode mode, Version matchVersion)
      throws CorruptIndexException, LockObtainFailedException, IOException {
    super(primary, null);

    // Initialize the index writer over the snapshot directory.
    writer = new IndexWriter(dir, new IndexWriterConfig(matchVersion, null).setOpenMode(mode));
    if (mode != OpenMode.APPEND) {
      // IndexWriter no longer creates a first commit on an empty Directory. So
      // if we were asked to CREATE*, call commit() just to be sure. If the
      // index contains information and mode is CREATE_OR_APPEND, it's a no-op.

    try {
      // Initializes the snapshots information. This code should basically run
      // only if mode != CREATE, but if it is, it's no harm as we only open the
      // reader once and immediately close it.
      for (Entry<String, String> e : readSnapshotsInfo(dir).entrySet()) {
        registerSnapshotInfo(e.getKey(), e.getValue(), null);
    } catch (RuntimeException e) {
      writer.close(); // don't leave any open file handles
      throw e;
    } catch (IOException e) {
      writer.close(); // don't leave any open file handles
      throw e;

  public synchronized void onInit(List<? extends IndexCommit> commits)
  throws IOException {
    // super.onInit() needs to be called first to ensure that initialization
    // behaves as expected. The superclass, SnapshotDeletionPolicy, ensures
    // that any snapshot IDs with empty IndexCommits are released. Since this 
    // happens, this class needs to persist these changes.
    persistSnapshotInfos(null, null);

   * Snapshots the last commit using the given ID. Once this method returns, the
   * snapshot information is persisted in the directory.
   * @see SnapshotDeletionPolicy#snapshot(String)
  public synchronized IndexCommit snapshot(String id) throws IOException {
    if (SNAPSHOTS_ID.equals(id)) {
      throw new IllegalArgumentException(id + " is reserved and cannot be used as a snapshot id");
    persistSnapshotInfos(id, lastCommit.getSegmentsFileName());
    return super.snapshot(id);

   * Deletes a snapshotted commit by ID. Once this method returns, the snapshot
   * information is committed to the directory.
   * @see SnapshotDeletionPolicy#release(String)
  public synchronized void release(String id) throws IOException {
    persistSnapshotInfos(null, null);

  /** Closes the index which writes the snapshots to the directory. */
  public void close() throws CorruptIndexException, IOException {

   * Persists all snapshots information. If the given id and segment are not
   * null, it persists their information as well.
  private void persistSnapshotInfos(String id, String segment) throws IOException {
    Document d = new Document();
    d.add(new Field(SNAPSHOTS_ID, "", Store.YES, Index.NO));
    for (Entry<String, String> e : super.getSnapshots().entrySet()) {
      d.add(new Field(e.getKey(), e.getValue(), Store.YES, Index.NO));
    if (id != null) {
      d.add(new Field(id, segment, Store.YES, Index.NO));


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