org.apache.lucene.codecs.PostingsFormat Maven / Gradle / Ivy
/*
* 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.
*
* 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.
*
* 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 LOADER = new NamedSPILoader<>(PostingsFormat.class);
private Holder() {}
static NamedSPILoader 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.
*
* 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 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.
*
* NOTE: Only new postings formats are added, existing ones are
* never removed or replaced.
*
*
This method is expensive and should only be called for discovery
* of new postings formats on the given classpath/classloader!
*/
public static void reloadPostingsFormats(ClassLoader classloader) {
Holder.getLoader().reload(classloader);
}
}