MemoizingSupplier.java

package io.blobkeeper.common.util;

/*
 * Copyright (C) 2015 by Denis M. Gabaydulin
 *
 * 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.
 */

import java.util.function.Supplier;

public class MemoizingSupplier<T> implements Supplier<T> {
    final Supplier<T> delegate;
    transient volatile boolean initialized;
    // "value" does not need to be volatile; visibility piggy-backs
    // on volatile read of "initialized".
    transient T value;

    public MemoizingSupplier(Supplier<T> delegate) {
        this.delegate = delegate;
    }

    @Override
    public T get() {
        // A 2-field variant of Double Checked Locking.
        if (!initialized) {
            synchronized (this) {
                if (!initialized) {
                    T t = delegate.get();
                    value = t;
                    initialized = true;
                    return t;
                }
            }
        }
        return value;
    }

    public boolean isInit() {
        return initialized;
    }
}