    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.Collections;
    020import java.util.HashMap;
    021import java.util.Map;
    022import java.util.NoSuchElementException;
    023import java.util.Set;
    024import java.util.concurrent.ExecutorService;
    025
    026import org.apache.commons.lang3.Validate;
    027
    028/**
    029 * <p>
    030 * A specialized {@link BackgroundInitializer} implementation that can deal with
    031 * multiple background initialization tasks.
    032 * </p>
    033 * <p>
    034 * This class has a similar purpose as {@link BackgroundInitializer}. However,
    035 * it is not limited to a single background initialization task. Rather it
    036 * manages an arbitrary number of {@code BackgroundInitializer} objects,
    037 * executes them, and waits until they are completely initialized. This is
    038 * useful for applications that have to perform multiple initialization tasks
    039 * that can run in parallel (i.e. that do not depend on each other). This class
    040 * takes care about the management of an {@code ExecutorService} and shares it
    041 * with the {@code BackgroundInitializer} objects it is responsible for; so the
    042 * using application need not bother with these details.
    043 * </p>
    044 * <p>
    045 * The typical usage scenario for {@code MultiBackgroundInitializer} is as
    046 * follows:
    047 * </p>
    048 * <ul>
    049 * <li>Create a new instance of the class. Optionally pass in a pre-configured
    050 * {@code ExecutorService}. Alternatively {@code MultiBackgroundInitializer} can
    051 * create a temporary {@code ExecutorService} and delete it after initialization
    052 * is complete.</li>
    053 * <li>Create specialized {@link BackgroundInitializer} objects for the
    054 * initialization tasks to be performed and add them to the {@code
    055 * MultiBackgroundInitializer} using the
    056 * {@link #addInitializer(String, BackgroundInitializer)} method.</li>
    057 * <li>After all initializers have been added, call the {@link #start()} method.
    058 * </li>
    059 * <li>When access to the result objects produced by the {@code
    060 * BackgroundInitializer} objects is needed call the {@link #get()} method. The
    061 * object returned here provides access to all result objects created during
    062 * initialization. It also stores information about exceptions that have
    063 * occurred.</li>
    064 * </ul>
    065 * <p>
    066 * {@code MultiBackgroundInitializer} starts a special controller task that
    067 * starts all {@code BackgroundInitializer} objects added to the instance.
    068 * Before the an initializer is started it is checked whether this initializer
    069 * already has an {@code ExecutorService} set. If this is the case, this {@code
    070 * ExecutorService} is used for running the background task. Otherwise the
    071 * current {@code ExecutorService} of this {@code MultiBackgroundInitializer} is
    072 * shared with the initializer.
    073 * </p>
    074 * <p>
    075 * The easiest way of using this class is to let it deal with the management of
    076 * an {@code ExecutorService} itself: If no external {@code ExecutorService} is
    077 * provided, the class creates a temporary {@code ExecutorService} (that is
    078 * capable of executing all background tasks in parallel) and destroys it at the
    079 * end of background processing.
    080 * </p>
    081 * <p>
    082 * Alternatively an external {@code ExecutorService} can be provided - either at
    083 * construction time or later by calling the
    084 * {@link #setExternalExecutor(ExecutorService)} method. In this case all
    085 * background tasks are scheduled at this external {@code ExecutorService}.
    086 * <strong>Important note:</strong> When using an external {@code
    087 * ExecutorService} be sure that the number of threads managed by the service is
    088 * large enough. Otherwise a deadlock can happen! This is the case in the
    089 * following scenario: {@code MultiBackgroundInitializer} starts a task that
    090 * starts all registered {@code BackgroundInitializer} objects and waits for
    091 * their completion. If for instance a single threaded {@code ExecutorService}
    092 * is used, none of the background tasks can be executed, and the task created
    093 * by {@code MultiBackgroundInitializer} waits forever.
    094 * </p>
    095 *
    096 * @since 3.0
    097 */
    098public class MultiBackgroundInitializer
    099        extends
    100        BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults> {
    101    /** A map with the child initializers. */
    102    private final Map<String, BackgroundInitializer<?>> childInitializers =
    103        new HashMap<>();
    104
    105    /**
    106     * Creates a new instance of {@code MultiBackgroundInitializer}.
    107     */
    108    public MultiBackgroundInitializer() {
    109    }
    110
    111    /**
    112     * Creates a new instance of {@code MultiBackgroundInitializer} and
    113     * initializes it with the given external {@code ExecutorService}.
    114     *
    115     * @param exec the {@code ExecutorService} for executing the background
    116     * tasks
    117     */
    118    public MultiBackgroundInitializer(final ExecutorService exec) {
    119        super(exec);
    120    }
    121
    122    /**
    123     * Adds a new {@code BackgroundInitializer} to this object. When this
    124     * {@code MultiBackgroundInitializer} is started, the given initializer will
    125     * be processed. This method must not be called after {@link #start()} has
    126     * been invoked.
    127     *
    128     * @param name the name of the initializer (must not be <b>null</b>)
    129     * @param backgroundInitializer the {@code BackgroundInitializer} to add (must not be
    130     * <b>null</b>)
    131     * @throws IllegalArgumentException if a required parameter is missing
    132     * @throws IllegalStateException if {@code start()} has already been called
    133     */
    134    public void addInitializer(final String name, final BackgroundInitializer<?> backgroundInitializer) {
    135        Validate.notNull(name, "name");
    136        Validate.notNull(backgroundInitializer, "backgroundInitializer");
    137
    138        synchronized (this) {
    139            if (isStarted()) {
    140                throw new IllegalStateException("addInitializer() must not be called after start()!");
    141            }
    142            childInitializers.put(name, backgroundInitializer);
    143        }
    144    }
    145
    146    /**
    147     * Returns the number of tasks needed for executing all child {@code
    148     * BackgroundInitializer} objects in parallel. This implementation sums up
    149     * the required tasks for all child initializers (which is necessary if one
    150     * of the child initializers is itself a {@code MultiBackgroundInitializer}
    151     * ). Then it adds 1 for the control task that waits for the completion of
    152     * the children.
    153     *
    154     * @return the number of tasks required for background processing
    155     */
    156    @Override
    157    protected int getTaskCount() {
    158        int result = 1;
    159
    160        for (final BackgroundInitializer<?> bi : childInitializers.values()) {
    161            result += bi.getTaskCount();
    162        }
    163
    164        return result;
    165    }
    166
    167    /**
    168     * Creates the results object. This implementation starts all child {@code
    169     * BackgroundInitializer} objects. Then it collects their results and
    170     * creates a {@code MultiBackgroundInitializerResults} object with this
    171     * data. If a child initializer throws a checked exceptions, it is added to
    172     * the results object. Unchecked exceptions are propagated.
    173     *
    174     * @return the results object
    175     * @throws Exception if an error occurs
    176     */
    177    @Override
    178    protected MultiBackgroundInitializerResults initialize() throws Exception {
    179        final Map<String, BackgroundInitializer<?>> inits;
    180        synchronized (this) {
    181            // create a snapshot to operate on
    182            inits = new HashMap<>(
    183                    childInitializers);
    184        }
    185
    186        // start the child initializers
    187        final ExecutorService exec = getActiveExecutor();
    188        for (final BackgroundInitializer<?> bi : inits.values()) {
    189            if (bi.getExternalExecutor() == null) {
    190                // share the executor service if necessary
    191                bi.setExternalExecutor(exec);
    192            }
    193            bi.start();
    194        }
    195
    196        // collect the results
    197        final Map<String, Object> results = new HashMap<>();
    198        final Map<String, ConcurrentException> excepts = new HashMap<>();
    199        for (final Map.Entry<String, BackgroundInitializer<?>> e : inits.entrySet()) {
    200            try {
    201                results.put(e.getKey(), e.getValue().get());
    202            } catch (final ConcurrentException cex) {
    203                excepts.put(e.getKey(), cex);
    204            }
    205        }
    206
    207        return new MultiBackgroundInitializerResults(inits, results, excepts);
    208    }
    209
    210    /**
    211     * A data class for storing the results of the background initialization
    212     * performed by {@code MultiBackgroundInitializer}. Objects of this inner
    213     * class are returned by {@link MultiBackgroundInitializer#initialize()}.
    214     * They allow access to all result objects produced by the
    215     * {@link BackgroundInitializer} objects managed by the owning instance. It
    216     * is also possible to retrieve status information about single
    217     * {@link BackgroundInitializer}s, i.e. whether they completed normally or
    218     * caused an exception.
    219     */
    220    public static class MultiBackgroundInitializerResults {
    221        /** A map with the child initializers. */
    222        private final Map<String, BackgroundInitializer<?>> initializers;
    223
    224        /** A map with the result objects. */
    225        private final Map<String, Object> resultObjects;
    226
    227        /** A map with the exceptions. */
    228        private final Map<String, ConcurrentException> exceptions;
    229
    230        /**
    231         * Creates a new instance of {@code MultiBackgroundInitializerResults}
    232         * and initializes it with maps for the {@code BackgroundInitializer}
    233         * objects, their result objects and the exceptions thrown by them.
    234         *
    235         * @param inits the {@code BackgroundInitializer} objects
    236         * @param results the result objects
    237         * @param excepts the exceptions
    238         */
    239        private MultiBackgroundInitializerResults(
    240                final Map<String, BackgroundInitializer<?>> inits,
    241                final Map<String, Object> results,
    242                final Map<String, ConcurrentException> excepts) {
    243            initializers = inits;
    244            resultObjects = results;
    245            exceptions = excepts;
    246        }
    247
    248        /**
    249         * Returns the {@code BackgroundInitializer} with the given name. If the
    250         * name cannot be resolved, an exception is thrown.
    251         *
    252         * @param name the name of the {@code BackgroundInitializer}
    253         * @return the {@code BackgroundInitializer} with this name
    254         * @throws NoSuchElementException if the name cannot be resolved
    255         */
    256        public BackgroundInitializer<?> getInitializer(final String name) {
    257            return checkName(name);
    258        }
    259
    260        /**
    261         * Returns the result object produced by the {@code
    262         * BackgroundInitializer} with the given name. This is the object
    263         * returned by the initializer's {@code initialize()} method. If this
    264         * {@code BackgroundInitializer} caused an exception, <b>null</b> is
    265         * returned. If the name cannot be resolved, an exception is thrown.
    266         *
    267         * @param name the name of the {@code BackgroundInitializer}
    268         * @return the result object produced by this {@code
    269         * BackgroundInitializer}
    270         * @throws NoSuchElementException if the name cannot be resolved
    271         */
    272        public Object getResultObject(final String name) {
    273            checkName(name);
    274            return resultObjects.get(name);
    275        }
    276
    277        /**
    278         * Returns a flag whether the {@code BackgroundInitializer} with the
    279         * given name caused an exception.
    280         *
    281         * @param name the name of the {@code BackgroundInitializer}
    282         * @return a flag whether this initializer caused an exception
    283         * @throws NoSuchElementException if the name cannot be resolved
    284         */
    285        public boolean isException(final String name) {
    286            checkName(name);
    287            return exceptions.containsKey(name);
    288        }
    289
    290        /**
    291         * Returns the {@code ConcurrentException} object that was thrown by the
    292         * {@code BackgroundInitializer} with the given name. If this
    293         * initializer did not throw an exception, the return value is
    294         * <b>null</b>. If the name cannot be resolved, an exception is thrown.
    295         *
    296         * @param name the name of the {@code BackgroundInitializer}
    297         * @return the exception thrown by this initializer
    298         * @throws NoSuchElementException if the name cannot be resolved
    299         */
    300        public ConcurrentException getException(final String name) {
    301            checkName(name);
    302            return exceptions.get(name);
    303        }
    304
    305        /**
    306         * Returns a set with the names of all {@code BackgroundInitializer}
    307         * objects managed by the {@code MultiBackgroundInitializer}.
    308         *
    309         * @return an (unmodifiable) set with the names of the managed {@code
    310         * BackgroundInitializer} objects
    311         */
    312        public Set<String> initializerNames() {
    313            return Collections.unmodifiableSet(initializers.keySet());
    314        }
    315
    316        /**
    317         * Returns a flag whether the whole initialization was successful. This
    318         * is the case if no child initializer has thrown an exception.
    319         *
    320         * @return a flag whether the initialization was successful
    321         */
    322        public boolean isSuccessful() {
    323            return exceptions.isEmpty();
    324        }
    325
    326        /**
    327         * Checks whether an initializer with the given name exists. If not,
    328         * throws an exception. If it exists, the associated child initializer
    329         * is returned.
    330         *
    331         * @param name the name to check
    332         * @return the initializer with this name
    333         * @throws NoSuchElementException if the name is unknown
    334         */
    335        private BackgroundInitializer<?> checkName(final String name) {
    336            final BackgroundInitializer<?> init = initializers.get(name);
    337            if (init == null) {
    338                throw new NoSuchElementException(
    339                        "No child initializer with name " + name);
    340            }
    341
    342            return init;
    343        }
    344    }
    345}