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);
  }
}