    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 *      https://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.Objects;
    020import java.util.concurrent.Callable;
    021import java.util.concurrent.ExecutorService;
    022
    023/**
    024 * A specialized {@link BackgroundInitializer} implementation that wraps a
    025 * {@link Callable} object.
    026 *
    027 * <p>
    028 * An instance of this class is initialized with a {@link Callable} object when
    029 * it is constructed. The implementation of the {@link #initialize()} method
    030 * defined in the super class delegates to this {@link Callable} so that the
    031 * {@link Callable} is executed in the background thread.
    032 * </p>
    033 * <p>
    034 * The {@link java.util.concurrent.Callable} interface is a standard mechanism
    035 * of the JDK to define tasks to be executed by another thread. The {@code
    036 * CallableBackgroundInitializer} class allows combining this standard interface
    037 * with the background initializer API.
    038 * </p>
    039 * <p>
    040 * Usage of this class is very similar to the default usage pattern of the
    041 * {@link BackgroundInitializer} class: Just create an instance and provide the
    042 * {@link Callable} object to be executed, then call the initializer's
    043 * {@link #start()} method. This causes the {@link Callable} to be executed in
    044 * another thread. When the results of the {@link Callable} are needed the
    045 * initializer's {@link #get()} method can be called (which may block until
    046 * background execution is complete). The following code fragment shows a
    047 * typical usage example:
    048 * </p>
    049 *
    050 * <pre>{@code
    051 * // a Callable that performs a complex computation
    052 * Callable<Integer> computationCallable = new MyComputationCallable();
    053 * // setup the background initializer
    054 * CallableBackgroundInitializer<Integer> initializer =
    055 *     new CallableBackgroundInitializer(computationCallable);
    056 * initializer.start();
    057 * // Now do some other things. Initialization runs in a parallel thread
    058 * ...
    059 * // Wait for the end of initialization and access the result
    060 * Integer result = initializer.get();
    061 * }
    062 * </pre>
    063 *
    064 * @param <T> the type of the object managed by this initializer class
    065 * @since 3.0
    066 */
    067public class CallableBackgroundInitializer<T> extends BackgroundInitializer<T> {
    068    /** The Callable to be executed. */
    069    private final Callable<T> callable;
    070
    071    /**
    072     * Creates a new instance of {@link CallableBackgroundInitializer} and sets
    073     * the {@link Callable} to be executed in a background thread.
    074     *
    075     * @param call the {@link Callable} (must not be <strong>null</strong>).
    076     * @throws IllegalArgumentException if the {@link Callable} is <strong>null</strong>.
    077     */
    078    public CallableBackgroundInitializer(final Callable<T> call) {
    079        checkCallable(call);
    080        callable = call;
    081    }
    082
    083    /**
    084     * Creates a new instance of {@link CallableBackgroundInitializer} and initializes it with the {@link Callable} to be executed in a background thread and
    085     * the {@link ExecutorService} for managing the background execution.
    086     *
    087     * @param call the {@link Callable} (must not be <strong>null</strong>).
    088     * @param exec an external {@link ExecutorService} to be used for task execution.
    089     * @throws IllegalArgumentException if the {@link Callable} is <strong>null</strong>.
    090     */
    091    public CallableBackgroundInitializer(final Callable<T> call, final ExecutorService exec) {
    092        super(exec);
    093        checkCallable(call);
    094        callable = call;
    095    }
    096
    097    /**
    098     * Tests the passed in {@link Callable} and throws an exception if it is undefined.
    099     *
    100     * @param callable the object to check.
    101     * @throws IllegalArgumentException if the {@link Callable} is <strong>null</strong>.
    102     */
    103    private void checkCallable(final Callable<T> callable) {
    104        Objects.requireNonNull(callable, "callable");
    105    }
    106
    107    /**
    108     * {@inheritDoc}
    109     */
    110    @Override
    111    protected Exception getTypedException(final Exception e) {
    112        //This Exception object will be used for type comparison in AbstractConcurrentInitializer.initialize but not thrown
    113        return new Exception(e);
    114    }
    115
    116    /**
    117     * Performs initialization in a background thread. This implementation delegates to the {@link Callable} passed at construction time of this object.
    118     *
    119     * @return the result of the initialization.
    120     * @throws Exception if an error occurs.
    121     */
    122    @Override
    123    protected T initialize() throws Exception {
    124        return callable.call();
    125    }
    126}