    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     */
    017    package org.apache.commons.lang3.concurrent;
    018
    019    import java.util.Collections;
    020    import java.util.HashMap;
    021    import java.util.Map;
    022    import java.util.NoSuchElementException;
    023    import java.util.Set;
    024    import java.util.concurrent.ExecutorService;
    025
    026    /**
    027     * <p>
    028     * A specialized {@link BackgroundInitializer} implementation that can deal with
    029     * multiple background initialization tasks.
    030     * </p>
    031     * <p>
    032     * This class has a similar purpose as {@link BackgroundInitializer}. However,
    033     * it is not limited to a single background initialization task. Rather it
    034     * manages an arbitrary number of {@code BackgroundInitializer} objects,
    035     * executes them, and waits until they are completely initialized. This is
    036     * useful for applications that have to perform multiple initialization tasks
    037     * that can run in parallel (i.e. that do not depend on each other). This class
    038     * takes care about the management of an {@code ExecutorService} and shares it
    039     * with the {@code BackgroundInitializer} objects it is responsible for; so the
    040     * using application need not bother with these details.
    041     * </p>
    042     * <p>
    043     * The typical usage scenario for {@code MultiBackgroundInitializer} is as
    044     * follows:
    045     * <ul>
    046     * <li>Create a new instance of the class. Optionally pass in a pre-configured
    047     * {@code ExecutorService}. Alternatively {@code MultiBackgroundInitializer} can
    048     * create a temporary {@code ExecutorService} and delete it after initialization
    049     * is complete.</li>
    050     * <li>Create specialized {@link BackgroundInitializer} objects for the
    051     * initialization tasks to be performed and add them to the {@code
    052     * MultiBackgroundInitializer} using the
    053     * {@link #addInitializer(String, BackgroundInitializer)} method.</li>
    054     * <li>After all initializers have been added, call the {@link #start()} method.
    055     * </li>
    056     * <li>When access to the result objects produced by the {@code
    057     * BackgroundInitializer} objects is needed call the {@link #get()} method. The
    058     * object returned here provides access to all result objects created during
    059     * initialization. It also stores information about exceptions that have
    060     * occurred.</li>
    061     * </ul>
    062     * </p>
    063     * <p>
    064     * {@code MultiBackgroundInitializer} starts a special controller task that
    065     * starts all {@code BackgroundInitializer} objects added to the instance.
    066     * Before the an initializer is started it is checked whether this initializer
    067     * already has an {@code ExecutorService} set. If this is the case, this {@code
    068     * ExecutorService} is used for running the background task. Otherwise the
    069     * current {@code ExecutorService} of this {@code MultiBackgroundInitializer} is
    070     * shared with the initializer.
    071     * </p>
    072     * <p>
    073     * The easiest way of using this class is to let it deal with the management of
    074     * an {@code ExecutorService} itself: If no external {@code ExecutorService} is
    075     * provided, the class creates a temporary {@code ExecutorService} (that is
    076     * capable of executing all background tasks in parallel) and destroys it at the
    077     * end of background processing.
    078     * </p>
    079     * <p>
    080     * Alternatively an external {@code ExecutorService} can be provided - either at
    081     * construction time or later by calling the
    082     * {@link #setExternalExecutor(ExecutorService)} method. In this case all
    083     * background tasks are scheduled at this external {@code ExecutorService}.
    084     * <strong>Important note:</strong> When using an external {@code
    085     * ExecutorService} be sure that the number of threads managed by the service is
    086     * large enough. Otherwise a deadlock can happen! This is the case in the
    087     * following scenario: {@code MultiBackgroundInitializer} starts a task that
    088     * starts all registered {@code BackgroundInitializer} objects and waits for
    089     * their completion. If for instance a single threaded {@code ExecutorService}
    090     * is used, none of the background tasks can be executed, and the task created
    091     * by {@code MultiBackgroundInitializer} waits forever.
    092     * </p>
    093     *
    094     * @since 3.0
    095     * @version $Id: MultiBackgroundInitializer.java 1082301 2011-03-16 21:02:15Z oheger $
    096     */
    097    public class MultiBackgroundInitializer
    098            extends
    099            BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults> {
    100        /** A map with the child initializers. */
    101        private final Map<String, BackgroundInitializer<?>> childInitializers =
    102            new HashMap<String, BackgroundInitializer<?>>();
    103
    104        /**
    105         * Creates a new instance of {@code MultiBackgroundInitializer}.
    106         */
    107        public MultiBackgroundInitializer() {
    108            super();
    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(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 init 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(String name, BackgroundInitializer<?> init) {
    135            if (name == null) {
    136                throw new IllegalArgumentException(
    137                        "Name of child initializer must not be null!");
    138            }
    139            if (init == null) {
    140                throw new IllegalArgumentException(
    141                        "Child initializer must not be null!");
    142            }
    143
    144            synchronized (this) {
    145                if (isStarted()) {
    146                    throw new IllegalStateException(
    147                            "addInitializer() must not be called after start()!");
    148                }
    149                childInitializers.put(name, init);
    150            }
    151        }
    152
    153        /**
    154         * Returns the number of tasks needed for executing all child {@code
    155         * BackgroundInitializer} objects in parallel. This implementation sums up
    156         * the required tasks for all child initializers (which is necessary if one
    157         * of the child initializers is itself a {@code MultiBackgroundInitializer}
    158         * ). Then it adds 1 for the control task that waits for the completion of
    159         * the children.
    160         *
    161         * @return the number of tasks required for background processing
    162         */
    163        @Override
    164        protected int getTaskCount() {
    165            int result = 1;
    166
    167            for (BackgroundInitializer<?> bi : childInitializers.values()) {
    168                result += bi.getTaskCount();
    169            }
    170
    171            return result;
    172        }
    173
    174        /**
    175         * Creates the results object. This implementation starts all child {@code
    176         * BackgroundInitializer} objects. Then it collects their results and
    177         * creates a {@code MultiBackgroundInitializerResults} object with this
    178         * data. If a child initializer throws a checked exceptions, it is added to
    179         * the results object. Unchecked exceptions are propagated.
    180         *
    181         * @return the results object
    182         * @throws Exception if an error occurs
    183         */
    184        @Override
    185        protected MultiBackgroundInitializerResults initialize() throws Exception {
    186            Map<String, BackgroundInitializer<?>> inits;
    187            synchronized (this) {
    188                // create a snapshot to operate on
    189                inits = new HashMap<String, BackgroundInitializer<?>>(
    190                        childInitializers);
    191            }
    192
    193            // start the child initializers
    194            ExecutorService exec = getActiveExecutor();
    195            for (BackgroundInitializer<?> bi : inits.values()) {
    196                if (bi.getExternalExecutor() == null) {
    197                    // share the executor service if necessary
    198                    bi.setExternalExecutor(exec);
    199                }
    200                bi.start();
    201            }
    202
    203            // collect the results
    204            Map<String, Object> results = new HashMap<String, Object>();
    205            Map<String, ConcurrentException> excepts = new HashMap<String, ConcurrentException>();
    206            for (Map.Entry<String, BackgroundInitializer<?>> e : inits.entrySet()) {
    207                try {
    208                    results.put(e.getKey(), e.getValue().get());
    209                } catch (ConcurrentException cex) {
    210                    excepts.put(e.getKey(), cex);
    211                }
    212            }
    213
    214            return new MultiBackgroundInitializerResults(inits, results, excepts);
    215        }
    216
    217        /**
    218         * A data class for storing the results of the background initialization
    219         * performed by {@code MultiBackgroundInitializer}. Objects of this inner
    220         * class are returned by {@link MultiBackgroundInitializer#initialize()}.
    221         * They allow access to all result objects produced by the
    222         * {@link BackgroundInitializer} objects managed by the owning instance. It
    223         * is also possible to retrieve status information about single
    224         * {@link BackgroundInitializer}s, i.e. whether they completed normally or
    225         * caused an exception.
    226         */
    227        public static class MultiBackgroundInitializerResults {
    228            /** A map with the child initializers. */
    229            private final Map<String, BackgroundInitializer<?>> initializers;
    230
    231            /** A map with the result objects. */
    232            private final Map<String, Object> resultObjects;
    233
    234            /** A map with the exceptions. */
    235            private final Map<String, ConcurrentException> exceptions;
    236
    237            /**
    238             * Creates a new instance of {@code MultiBackgroundInitializerResults}
    239             * and initializes it with maps for the {@code BackgroundInitializer}
    240             * objects, their result objects and the exceptions thrown by them.
    241             *
    242             * @param inits the {@code BackgroundInitializer} objects
    243             * @param results the result objects
    244             * @param excepts the exceptions
    245             */
    246            private MultiBackgroundInitializerResults(
    247                    Map<String, BackgroundInitializer<?>> inits,
    248                    Map<String, Object> results,
    249                    Map<String, ConcurrentException> excepts) {
    250                initializers = inits;
    251                resultObjects = results;
    252                exceptions = excepts;
    253            }
    254
    255            /**
    256             * Returns the {@code BackgroundInitializer} with the given name. If the
    257             * name cannot be resolved, an exception is thrown.
    258             *
    259             * @param name the name of the {@code BackgroundInitializer}
    260             * @return the {@code BackgroundInitializer} with this name
    261             * @throws NoSuchElementException if the name cannot be resolved
    262             */
    263            public BackgroundInitializer<?> getInitializer(String name) {
    264                return checkName(name);
    265            }
    266
    267            /**
    268             * Returns the result object produced by the {@code
    269             * BackgroundInitializer} with the given name. This is the object
    270             * returned by the initializer's {@code initialize()} method. If this
    271             * {@code BackgroundInitializer} caused an exception, <b>null</b> is
    272             * returned. If the name cannot be resolved, an exception is thrown.
    273             *
    274             * @param name the name of the {@code BackgroundInitializer}
    275             * @return the result object produced by this {@code
    276             * BackgroundInitializer}
    277             * @throws NoSuchElementException if the name cannot be resolved
    278             */
    279            public Object getResultObject(String name) {
    280                checkName(name);
    281                return resultObjects.get(name);
    282            }
    283
    284            /**
    285             * Returns a flag whether the {@code BackgroundInitializer} with the
    286             * given name caused an exception.
    287             *
    288             * @param name the name of the {@code BackgroundInitializer}
    289             * @return a flag whether this initializer caused an exception
    290             * @throws NoSuchElementException if the name cannot be resolved
    291             */
    292            public boolean isException(String name) {
    293                checkName(name);
    294                return exceptions.containsKey(name);
    295            }
    296
    297            /**
    298             * Returns the {@code ConcurrentException} object that was thrown by the
    299             * {@code BackgroundInitializer} with the given name. If this
    300             * initializer did not throw an exception, the return value is
    301             * <b>null</b>. If the name cannot be resolved, an exception is thrown.
    302             *
    303             * @param name the name of the {@code BackgroundInitializer}
    304             * @return the exception thrown by this initializer
    305             * @throws NoSuchElementException if the name cannot be resolved
    306             */
    307            public ConcurrentException getException(String name) {
    308                checkName(name);
    309                return exceptions.get(name);
    310            }
    311
    312            /**
    313             * Returns a set with the names of all {@code BackgroundInitializer}
    314             * objects managed by the {@code MultiBackgroundInitializer}.
    315             *
    316             * @return an (unmodifiable) set with the names of the managed {@code
    317             * BackgroundInitializer} objects
    318             */
    319            public Set<String> initializerNames() {
    320                return Collections.unmodifiableSet(initializers.keySet());
    321            }
    322
    323            /**
    324             * Returns a flag whether the whole initialization was successful. This
    325             * is the case if no child initializer has thrown an exception.
    326             *
    327             * @return a flag whether the initialization was successful
    328             */
    329            public boolean isSuccessful() {
    330                return exceptions.isEmpty();
    331            }
    332
    333            /**
    334             * Checks whether an initializer with the given name exists. If not,
    335             * throws an exception. If it exists, the associated child initializer
    336             * is returned.
    337             *
    338             * @param name the name to check
    339             * @return the initializer with this name
    340             * @throws NoSuchElementException if the name is unknown
    341             */
    342            private BackgroundInitializer<?> checkName(String name) {
    343                BackgroundInitializer<?> init = initializers.get(name);
    344                if (init == null) {
    345                    throw new NoSuchElementException(
    346                            "No child initializer with name " + name);
    347                }
    348
    349                return init;
    350            }
    351        }
    352    }