lucene/core/src/java/org/apache/lucene/codecs/PostingsFormat.java

/*
 * 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.
 */
package org.apache.lucene.codecs;

import java.io.IOException;
import java.util.ServiceLoader;
import java.util.Set;
import org.apache.lucene.codecs.perfield.PerFieldPostingsFormat; // javadocs
import org.apache.lucene.index.SegmentReadState;
import org.apache.lucene.index.SegmentWriteState;
import org.apache.lucene.util.NamedSPILoader;

/**
 * Encodes/decodes terms, postings, and proximity data.
 *
 * <p>Note, when extending this class, the name ({@link #getName}) may written into the index in
 * certain configurations. In order for the segment to be read, the name must resolve to your
 * implementation via {@link #forName(String)}. This method uses Java's {@link ServiceLoader Service
 * Provider Interface} (SPI) to resolve format names.
 *
 * <p>If you implement your own format, make sure that it has a no-arg constructor so SPI can load
 * it.
 *
 * @see ServiceLoader
 * @lucene.experimental
 */
public abstract class PostingsFormat implements NamedSPILoader.NamedSPI {

  /**
   * This static holder class prevents classloading deadlock by delaying init of postings formats
   * until needed.
   */
  private static final class Holder {
    private static final NamedSPILoader<PostingsFormat> LOADER =
        new NamedSPILoader<>(PostingsFormat.class);

    private Holder() {}

    static NamedSPILoader<PostingsFormat> getLoader() {
      if (LOADER == null) {
        throw new IllegalStateException(
            "You tried to lookup a PostingsFormat by name before all formats could be initialized. "
                + "This likely happens if you call PostingsFormat#forName from a PostingsFormat's ctor.");
      }
      return LOADER;
    }
  }

  /** Zero-length {@code PostingsFormat} array. */
  public static final PostingsFormat[] EMPTY = new PostingsFormat[0];

  /** Unique name that's used to retrieve this format when reading the index. */
  private final String name;

  /**
   * Creates a new postings format.
   *
   * <p>The provided name will be written into the index segment in some configurations (such as
   * when using {@link PerFieldPostingsFormat}): in such configurations, for the segment to be read
   * this class should be registered with Java's SPI mechanism (registered in META-INF/ of your jar
   * file, etc).
   *
   * @param name must be all ascii alphanumeric, and less than 128 characters in length.
   */
  protected PostingsFormat(String name) {
    // TODO: can we somehow detect name conflicts here?  Two different classes trying to claim the
    // same name?  Otherwise you see confusing errors...
    NamedSPILoader.checkServiceName(name);
    this.name = name;
  }

  /** Returns this posting format's name */
  @Override
  public final String getName() {
    return name;
  }

  /** Writes a new segment */
  public abstract FieldsConsumer fieldsConsumer(SegmentWriteState state) throws IOException;

  /**
   * Reads a segment. NOTE: by the time this call returns, it must hold open any files it will need
   * to use; else, those files may be deleted. Additionally, required files may be deleted during
   * the execution of this call before there is a chance to open them. Under these circumstances an
   * IOException should be thrown by the implementation. IOExceptions are expected and will
   * automatically cause a retry of the segment opening logic with the newly revised segments.
   */
  public abstract FieldsProducer fieldsProducer(SegmentReadState state) throws IOException;

  @Override
  public String toString() {
    return "PostingsFormat(name=" + name + ")";
  }

  /** looks up a format by name */
  public static PostingsFormat forName(String name) {
    return Holder.getLoader().lookup(name);
  }

  /** returns a list of all available format names */
  public static Set<String> availablePostingsFormats() {
    return Holder.getLoader().availableServices();
  }

  /**
   * Reloads the postings format list from the given {@link ClassLoader}. Changes to the postings
   * formats are visible after the method ends, all iterators ({@link
   * #availablePostingsFormats()},...) stay consistent.
   *
   * <p><b>NOTE:</b> Only new postings formats are added, existing ones are never removed or
   * replaced.
   *
   * <p><em>This method is expensive and should only be called for discovery of new postings formats
   * on the given classpath/classloader!</em>
   */
  public static void reloadPostingsFormats(ClassLoader classloader) {
    Holder.getLoader().reload(classloader);
  }
}