lucene/core/src/java/org/apache/lucene/codecs/Codec.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.util.Objects;
import java.util.ServiceLoader; // javadocs
import java.util.Set;
import org.apache.lucene.index.IndexWriterConfig; // javadocs
import org.apache.lucene.util.NamedSPILoader;

/**
 * Encodes/decodes an inverted index segment.
 *
 * <p>Note, when extending this class, the name ({@link #getName}) is written into the index. 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 codec names.
 *
 * <p>If you implement your own codec, make sure that it has a no-arg constructor so SPI can load
 * it.
 *
 * @see ServiceLoader
 */
public abstract class Codec implements NamedSPILoader.NamedSPI {

  /**
   * This static holder class prevents classloading deadlock by delaying init of default codecs and
   * available codecs until needed.
   */
  private static final class Holder {
    private static final NamedSPILoader<Codec> LOADER = new NamedSPILoader<>(Codec.class);

    private Holder() {}

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

    static Codec defaultCodec = LOADER.lookup("Lucene90");
  }

  private final String name;

  /**
   * Creates a new codec.
   *
   * <p>The provided name will be written into the index segment: in order to 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 Codec(String name) {
    NamedSPILoader.checkServiceName(name);
    this.name = name;
  }

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

  /** Encodes/decodes postings */
  public abstract PostingsFormat postingsFormat();

  /** Encodes/decodes docvalues */
  public abstract DocValuesFormat docValuesFormat();

  /** Encodes/decodes stored fields */
  public abstract StoredFieldsFormat storedFieldsFormat();

  /** Encodes/decodes term vectors */
  public abstract TermVectorsFormat termVectorsFormat();

  /** Encodes/decodes field infos file */
  public abstract FieldInfosFormat fieldInfosFormat();

  /** Encodes/decodes segment info file */
  public abstract SegmentInfoFormat segmentInfoFormat();

  /** Encodes/decodes document normalization values */
  public abstract NormsFormat normsFormat();

  /** Encodes/decodes live docs */
  public abstract LiveDocsFormat liveDocsFormat();

  /** Encodes/decodes compound files */
  public abstract CompoundFormat compoundFormat();

  /** Encodes/decodes points index */
  public abstract PointsFormat pointsFormat();

  /** Encodes/decodes numeric vector fields */
  public abstract VectorFormat vectorFormat();

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

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

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

  /** expert: returns the default codec used for newly created {@link IndexWriterConfig}s. */
  public static Codec getDefault() {
    if (Holder.defaultCodec == null) {
      throw new IllegalStateException(
          "You tried to lookup the default Codec before all Codecs could be initialized. "
              + "This likely happens if you try to get it from a Codec's ctor.");
    }
    return Holder.defaultCodec;
  }

  /** expert: sets the default codec used for newly created {@link IndexWriterConfig}s. */
  public static void setDefault(Codec codec) {
    Holder.defaultCodec = Objects.requireNonNull(codec);
  }

  /**
   * returns the codec's name. Subclasses can override to provide more detail (such as parameters).
   */
  @Override
  public String toString() {
    return name;
  }
}