MemorizedCallable.java

/*
 * Copyright (c) 2001-2017, Zoltan Farkas All Rights Reserved.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 *
 * Additionally licensed with:
 *
 * 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 org.spf4j.base;

import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
import java.util.concurrent.Callable;

/**
 * use Callables.memorize to create a instance.
 * @author zoly
 */
@SuppressFBWarnings("NOS_NON_OWNED_SYNCHRONIZATION")
public final class MemorizedCallable<V> implements Callable<V> {

    private static final Object NOTHING = new Object();

    private volatile V value;

    private final Callable<V> callable;

    MemorizedCallable(final Callable<V> callable) {
        this.callable = callable;
        value = (V) NOTHING;
    }

    @Override
    public V call() throws Exception {
        V result = value;
        if (result == NOTHING) {
            synchronized (this) {
                result = value;
                if (result == NOTHING) {
                    result = callable.call();
                    value = result;
                }
            }
        }
        return result;
    }

    public V clear() {
      V result = value;
      value = (V) NOTHING;
      return result;
    }

    public NullAbleOptional<V> getIfMemorized() {
      V rv = value;
      if (rv == NOTHING) {
        return NullAbleOptional.EMPTY;
      } else {
        return NullAbleOptional.of(rv);
      }
    }

    public void set(final V v) {
      this.value = v;
    }

    @Override
    public String toString() {
        return "MemorizedCallable{" + "value=" + value + '}';
    }

}