org.apache.parquet.hadoop.metadata.Canonicalizer 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.parquet.hadoop.metadata;
import java.util.concurrent.ConcurrentHashMap;
/**
* returns canonical representation of objects (similar to String.intern()) to save memory
* if a.equals(b) then canonicalize(a) == canonicalize(b)
* this class is thread safe
*
* @param the type of values canonicalized by subclasses
*/
public class Canonicalizer {
private ConcurrentHashMap canonicals = new ConcurrentHashMap();
/**
* @param value the value to canonicalize
* @return the corresponding canonical value
*/
final public T canonicalize(T value) {
T canonical = canonicals.get(value);
if (canonical == null) {
value = toCanonical(value);
T existing = canonicals.putIfAbsent(value, value);
// putIfAbsent is atomic, making sure we always return the same canonical representation of the value
if (existing == null) {
canonical = value;
} else {
canonical = existing;
}
}
return canonical;
}
/**
* @param value the value to canonicalize if needed
* @return the canonicalized value
*/
protected T toCanonical(T value) {
return value;
}
}