com.hazelcast.jet.impl.util.ConcurrentMemoizingSupplier Maven / Gradle / Ivy
/*
* Copyright (c) 2008-2018, Hazelcast, Inc. All Rights Reserved.
*
* Licensed 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 com.hazelcast.jet.impl.util;
import java.util.function.Supplier;
/**
* Wraps a {@code Supplier} and returns a thread-safe memoizing supplier
* which calls it only on the first invocation of {@code get()} and
* afterwards returns the remembered instance.
*
* The provided {@code Supplier} must return a non-null value.
*/
final class ConcurrentMemoizingSupplier implements Supplier {
private final Supplier onceSupplier;
private volatile T remembered;
ConcurrentMemoizingSupplier(Supplier onceSupplier) {
this.onceSupplier = onceSupplier;
}
@Override
public T get() {
// The common path will use a single volatile load
T loadResult = remembered;
if (loadResult != null) {
return loadResult;
}
synchronized (this) {
// The uncommon path can use simpler code with multiple volatile loads
if (remembered != null) {
return remembered;
}
remembered = onceSupplier.get();
if (remembered == null) {
throw new NullPointerException("Supplier returned null.");
}
return remembered;
}
}
}