org.apache.lucene.codecs.Codec.java Source code

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Here is the source code for org.apache.lucene.codecs.Codec.java

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/*
 * 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;
        }

        // TODO: should we use this, or maybe a system property is better?
        static Codec defaultCodec = LOADER.lookup("Lucene80");
    }

    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();

    /** 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;
    }
}