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}