    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.lang3.concurrent;
    018
    019import java.util.concurrent.CancellationException;
    020import java.util.concurrent.ConcurrentHashMap;
    021import java.util.concurrent.ConcurrentMap;
    022import java.util.concurrent.ExecutionException;
    023import java.util.concurrent.Future;
    024import java.util.function.Function;
    025
    026import org.apache.commons.lang3.exception.ExceptionUtils;
    027
    028/**
    029 * Definition of an interface for a wrapper around a calculation that takes a single parameter and returns a result. The
    030 * results for the calculation will be cached for future requests.
    031 *
    032 * <p>
    033 * This is not a fully functional cache, there is no way of limiting or removing results once they have been generated.
    034 * However, it is possible to get the implementation to regenerate the result for a given parameter, if an error was
    035 * thrown during the previous calculation, by setting the option during the construction of the class. If this is not
    036 * set the class will return the cached exception.
    037 * </p>
    038 * <p>
    039 * Thanks go to Brian Goetz, Tim Peierls and the members of JCP JSR-166 Expert Group for coming up with the
    040 * original implementation of the class. It was also published within Java Concurrency in Practice as a sample.
    041 * </p>
    042 *
    043 * @param <I> the type of the input to the calculation
    044 * @param <O> the type of the output of the calculation
    045 *
    046 * @since 3.6
    047 */
    048public class Memoizer<I, O> implements Computable<I, O> {
    049
    050    private final ConcurrentMap<I, Future<O>> cache = new ConcurrentHashMap<>();
    051    private final Function<? super I, ? extends Future<O>> mappingFunction;
    052    private final boolean recalculate;
    053
    054    /**
    055     * Constructs a Memoizer for the provided Computable calculation.
    056     *
    057     * <p>
    058     * If a calculation throws an exception for any reason, this exception will be cached and returned for all future
    059     * calls with the provided parameter.
    060     * </p>
    061     *
    062     * @param computable the computation whose results should be memorized
    063     */
    064    public Memoizer(final Computable<I, O> computable) {
    065        this(computable, false);
    066    }
    067
    068    /**
    069     * Constructs a Memoizer for the provided Computable calculation, with the option of whether a Computation that
    070     * experiences an error should recalculate on subsequent calls or return the same cached exception.
    071     *
    072     * @param computable the computation whose results should be memorized
    073     * @param recalculate determines whether the computation should be recalculated on subsequent calls if the previous call
    074     *        failed
    075     */
    076    public Memoizer(final Computable<I, O> computable, final boolean recalculate) {
    077        this.recalculate = recalculate;
    078        this.mappingFunction = k -> FutureTasks.run(() -> computable.compute(k));
    079    }
    080
    081    /**
    082     * Constructs a Memoizer for the provided Function calculation.
    083     *
    084     * <p>
    085     * If a calculation throws an exception for any reason, this exception will be cached and returned for all future
    086     * calls with the provided parameter.
    087     * </p>
    088     *
    089     * @param function the function whose results should be memorized
    090     * @since 2.13.0
    091     */
    092    public Memoizer(final Function<I, O> function) {
    093        this(function, false);
    094    }
    095
    096    /**
    097     * Constructs a Memoizer for the provided Function calculation, with the option of whether a Function that
    098     * experiences an error should recalculate on subsequent calls or return the same cached exception.
    099     *
    100     * @param function the computation whose results should be memorized
    101     * @param recalculate determines whether the computation should be recalculated on subsequent calls if the previous call
    102     *        failed
    103     * @since 2.13.0
    104     */
    105     public Memoizer(final Function<I, O> function, final boolean recalculate) {
    106        this.recalculate = recalculate;
    107        this.mappingFunction = k -> FutureTasks.run(() -> function.apply(k));
    108    }
    109
    110    /**
    111     * This method will return the result of the calculation and cache it, if it has not previously been calculated.
    112     *
    113     * <p>
    114     * This cache will also cache exceptions that occur during the computation if the {@code recalculate} parameter in the
    115     * constructor was set to {@code false}, or not set. Otherwise, if an exception happened on the previous calculation,
    116     * the method will attempt again to generate a value.
    117     * </p>
    118     *
    119     * @param arg the argument for the calculation
    120     * @return the result of the calculation
    121     * @throws InterruptedException thrown if the calculation is interrupted
    122     */
    123    @Override
    124    public O compute(final I arg) throws InterruptedException {
    125        while (true) {
    126            final Future<O> future = cache.computeIfAbsent(arg, mappingFunction);
    127            try {
    128                return future.get();
    129            } catch (final CancellationException e) {
    130                cache.remove(arg, future);
    131            } catch (final ExecutionException e) {
    132                if (recalculate) {
    133                    cache.remove(arg, future);
    134                }
    135                throw launderException(e.getCause());
    136            }
    137        }
    138    }
    139
    140    /**
    141     * This method launders a Throwable to either a RuntimeException, Error or any other Exception wrapped in an
    142     * IllegalStateException.
    143     *
    144     * @param throwable the throwable to laundered
    145     * @return a RuntimeException, Error or an IllegalStateException
    146     */
    147    private RuntimeException launderException(final Throwable throwable) {
    148        throw new IllegalStateException("Unchecked exception", ExceptionUtils.throwUnchecked(throwable));
    149    }
    150}