org.apache.jackrabbit.oak.core.LazyValue 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.jackrabbit.oak.core;
/**
* An instances of this class represents a lazy value of type {@code T}.
* {@code LazyValue} implements an evaluate by need semantics:
* {@link #createValue()} is called exactly once when {@link #get()}
* is called for the first time.
*
* {@code LazyValue} instances are thread safe.
*/
abstract class LazyValue {
private volatile T value;
/**
* Factory method called to create the value on an as need basis.
* @return a new instance for {@code T}.
*/
protected abstract T createValue();
/**
* @return {@code true} iff {@link #get()} has been called at least once.
*/
public boolean hasValue() {
return value != null;
}
/**
* Get value. Calls {@link #createValue()} if called for the first time.
* @return the value
*/
public T get () {
// Double checked locking is fine since Java 5 as long as value is volatile.
// See http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
if (value == null) {
synchronized (this) {
if (value == null) {
value = createValue();
}
}
}
return value;
}
}