    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.Objects;
    024import java.util.Set;
    025import java.util.concurrent.ExecutorService;
    026
    027/**
    028 * A specialized {@link BackgroundInitializer} implementation that can deal with
    029 * multiple background initialization tasks.
    030 *
    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 {@link 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 {@link ExecutorService} and shares it
    039 * with the {@link 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 {@link MultiBackgroundInitializer} is as
    044 * follows:
    045 * </p>
    046 * <ul>
    047 * <li>Create a new instance of the class. Optionally pass in a pre-configured
    048 * {@link ExecutorService}. Alternatively {@link MultiBackgroundInitializer} can
    049 * create a temporary {@link ExecutorService} and delete it after initialization
    050 * is complete.</li>
    051 * <li>Create specialized {@link BackgroundInitializer} objects for the
    052 * initialization tasks to be performed and add them to the {@code
    053 * MultiBackgroundInitializer} using the
    054 * {@link #addInitializer(String, BackgroundInitializer)} method.</li>
    055 * <li>After all initializers have been added, call the {@link #start()} method.
    056 * </li>
    057 * <li>When access to the result objects produced by the {@code
    058 * BackgroundInitializer} objects is needed call the {@link #get()} method. The
    059 * object returned here provides access to all result objects created during
    060 * initialization. It also stores information about exceptions that have
    061 * occurred.</li>
    062 * </ul>
    063 * <p>
    064 * {@link MultiBackgroundInitializer} starts a special controller task that
    065 * starts all {@link BackgroundInitializer} objects added to the instance.
    066 * Before the an initializer is started it is checked whether this initializer
    067 * already has an {@link ExecutorService} set. If this is the case, this {@code
    068 * ExecutorService} is used for running the background task. Otherwise the
    069 * current {@link ExecutorService} of this {@link 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 {@link ExecutorService} itself: If no external {@link ExecutorService} is
    075 * provided, the class creates a temporary {@link 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 {@link 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 {@link 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: {@link MultiBackgroundInitializer} starts a task that
    088 * starts all registered {@link BackgroundInitializer} objects and waits for
    089 * their completion. If for instance a single threaded {@link ExecutorService}
    090 * is used, none of the background tasks can be executed, and the task created
    091 * by {@link MultiBackgroundInitializer} waits forever.
    092 * </p>
    093 *
    094 * @since 3.0
    095 */
    096public class MultiBackgroundInitializer
    097        extends
    098        BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults> {
    099
    100    /**
    101     * A data class for storing the results of the background initialization
    102     * performed by {@link MultiBackgroundInitializer}. Objects of this inner
    103     * class are returned by {@link MultiBackgroundInitializer#initialize()}.
    104     * They allow access to all result objects produced by the
    105     * {@link BackgroundInitializer} objects managed by the owning instance. It
    106     * is also possible to retrieve status information about single
    107     * {@link BackgroundInitializer}s, i.e. whether they completed normally or
    108     * caused an exception.
    109     */
    110    public static class MultiBackgroundInitializerResults {
    111        /** A map with the child initializers. */
    112        private final Map<String, BackgroundInitializer<?>> initializers;
    113
    114        /** A map with the result objects. */
    115        private final Map<String, Object> resultObjects;
    116
    117        /** A map with the exceptions. */
    118        private final Map<String, ConcurrentException> exceptions;
    119
    120        /**
    121         * Creates a new instance of {@link MultiBackgroundInitializerResults}
    122         * and initializes it with maps for the {@link BackgroundInitializer}
    123         * objects, their result objects and the exceptions thrown by them.
    124         *
    125         * @param inits the {@link BackgroundInitializer} objects
    126         * @param results the result objects
    127         * @param excepts the exceptions
    128         */
    129        private MultiBackgroundInitializerResults(
    130                final Map<String, BackgroundInitializer<?>> inits,
    131                final Map<String, Object> results,
    132                final Map<String, ConcurrentException> excepts) {
    133            initializers = inits;
    134            resultObjects = results;
    135            exceptions = excepts;
    136        }
    137
    138        /**
    139         * Checks whether an initializer with the given name exists. If not,
    140         * throws an exception. If it exists, the associated child initializer
    141         * is returned.
    142         *
    143         * @param name the name to check
    144         * @return the initializer with this name
    145         * @throws NoSuchElementException if the name is unknown
    146         */
    147        private BackgroundInitializer<?> checkName(final String name) {
    148            final BackgroundInitializer<?> init = initializers.get(name);
    149            if (init == null) {
    150                throw new NoSuchElementException(
    151                        "No child initializer with name " + name);
    152            }
    153
    154            return init;
    155        }
    156
    157        /**
    158         * Returns the {@link ConcurrentException} object that was thrown by the
    159         * {@link BackgroundInitializer} with the given name. If this
    160         * initializer did not throw an exception, the return value is
    161         * <b>null</b>. If the name cannot be resolved, an exception is thrown.
    162         *
    163         * @param name the name of the {@link BackgroundInitializer}
    164         * @return the exception thrown by this initializer
    165         * @throws NoSuchElementException if the name cannot be resolved
    166         */
    167        public ConcurrentException getException(final String name) {
    168            checkName(name);
    169            return exceptions.get(name);
    170        }
    171
    172        /**
    173         * Returns the {@link BackgroundInitializer} with the given name. If the
    174         * name cannot be resolved, an exception is thrown.
    175         *
    176         * @param name the name of the {@link BackgroundInitializer}
    177         * @return the {@link BackgroundInitializer} with this name
    178         * @throws NoSuchElementException if the name cannot be resolved
    179         */
    180        public BackgroundInitializer<?> getInitializer(final String name) {
    181            return checkName(name);
    182        }
    183
    184        /**
    185         * Returns the result object produced by the {@code
    186         * BackgroundInitializer} with the given name. This is the object
    187         * returned by the initializer's {@code initialize()} method. If this
    188         * {@link BackgroundInitializer} caused an exception, <b>null</b> is
    189         * returned. If the name cannot be resolved, an exception is thrown.
    190         *
    191         * @param name the name of the {@link BackgroundInitializer}
    192         * @return the result object produced by this {@code
    193         * BackgroundInitializer}
    194         * @throws NoSuchElementException if the name cannot be resolved
    195         */
    196        public Object getResultObject(final String name) {
    197            checkName(name);
    198            return resultObjects.get(name);
    199        }
    200
    201        /**
    202         * Returns a set with the names of all {@link BackgroundInitializer}
    203         * objects managed by the {@link MultiBackgroundInitializer}.
    204         *
    205         * @return an (unmodifiable) set with the names of the managed {@code
    206         * BackgroundInitializer} objects
    207         */
    208        public Set<String> initializerNames() {
    209            return Collections.unmodifiableSet(initializers.keySet());
    210        }
    211
    212        /**
    213         * Returns a flag whether the {@link BackgroundInitializer} with the
    214         * given name caused an exception.
    215         *
    216         * @param name the name of the {@link BackgroundInitializer}
    217         * @return a flag whether this initializer caused an exception
    218         * @throws NoSuchElementException if the name cannot be resolved
    219         */
    220        public boolean isException(final String name) {
    221            checkName(name);
    222            return exceptions.containsKey(name);
    223        }
    224
    225        /**
    226         * Returns a flag whether the whole initialization was successful. This
    227         * is the case if no child initializer has thrown an exception.
    228         *
    229         * @return a flag whether the initialization was successful
    230         */
    231        public boolean isSuccessful() {
    232            return exceptions.isEmpty();
    233        }
    234    }
    235
    236    /** A map with the child initializers. */
    237    private final Map<String, BackgroundInitializer<?>> childInitializers = new HashMap<>();
    238
    239    /**
    240     * Creates a new instance of {@link MultiBackgroundInitializer}.
    241     */
    242    public MultiBackgroundInitializer() {
    243    }
    244
    245    /**
    246     * Creates a new instance of {@link MultiBackgroundInitializer} and
    247     * initializes it with the given external {@link ExecutorService}.
    248     *
    249     * @param exec the {@link ExecutorService} for executing the background
    250     * tasks
    251     */
    252    public MultiBackgroundInitializer(final ExecutorService exec) {
    253        super(exec);
    254    }
    255
    256    /**
    257     * Adds a new {@link BackgroundInitializer} to this object. When this
    258     * {@link MultiBackgroundInitializer} is started, the given initializer will
    259     * be processed. This method must not be called after {@link #start()} has
    260     * been invoked.
    261     *
    262     * @param name the name of the initializer (must not be <b>null</b>)
    263     * @param backgroundInitializer the {@link BackgroundInitializer} to add (must not be
    264     * <b>null</b>)
    265     * @throws NullPointerException if either {@code name} or {@code backgroundInitializer}
    266     *         is {@code null}
    267     * @throws IllegalStateException if {@code start()} has already been called
    268     */
    269    public void addInitializer(final String name, final BackgroundInitializer<?> backgroundInitializer) {
    270        Objects.requireNonNull(name, "name");
    271        Objects.requireNonNull(backgroundInitializer, "backgroundInitializer");
    272
    273        synchronized (this) {
    274            if (isStarted()) {
    275                throw new IllegalStateException("addInitializer() must not be called after start()!");
    276            }
    277            childInitializers.put(name, backgroundInitializer);
    278        }
    279    }
    280
    281    /**
    282     * Calls the closer of all child {@code BackgroundInitializer} objects
    283     *
    284     * @throws ConcurrentException throws an ConcurrentException that will have all other exceptions as suppressed exceptions. ConcurrentException thrown by children will be unwrapped.
    285     * @since 3.14.0
    286     */
    287    @Override
    288    public void close() throws ConcurrentException {
    289        ConcurrentException exception = null;
    290
    291        for (BackgroundInitializer<?> child : childInitializers.values()) {
    292            try {
    293                child.close();
    294            } catch (Exception e) {
    295                if (exception == null) {
    296                    exception = new ConcurrentException();
    297                }
    298
    299                if (e instanceof ConcurrentException) {
    300                    // Because ConcurrentException is only created by classes in this package
    301                    // we can safely unwrap it.
    302                    exception.addSuppressed(e.getCause());
    303                } else {
    304                    exception.addSuppressed(e);
    305                }
    306            }
    307        }
    308
    309        if (exception != null) {
    310            throw exception;
    311        }
    312    }
    313
    314    /**
    315     * Returns the number of tasks needed for executing all child {@code
    316     * BackgroundInitializer} objects in parallel. This implementation sums up
    317     * the required tasks for all child initializers (which is necessary if one
    318     * of the child initializers is itself a {@link MultiBackgroundInitializer}
    319     * ). Then it adds 1 for the control task that waits for the completion of
    320     * the children.
    321     *
    322     * @return the number of tasks required for background processing
    323     */
    324    @Override
    325    protected int getTaskCount() {
    326        return 1 + childInitializers.values().stream().mapToInt(BackgroundInitializer::getTaskCount).sum();
    327    }
    328
    329    /**
    330     * Creates the results object. This implementation starts all child {@code
    331     * BackgroundInitializer} objects. Then it collects their results and
    332     * creates a {@link MultiBackgroundInitializerResults} object with this
    333     * data. If a child initializer throws a checked exceptions, it is added to
    334     * the results object. Unchecked exceptions are propagated.
    335     *
    336     * @return the results object
    337     * @throws Exception if an error occurs
    338     */
    339    @Override
    340    protected MultiBackgroundInitializerResults initialize() throws Exception {
    341        final Map<String, BackgroundInitializer<?>> inits;
    342        synchronized (this) {
    343            // create a snapshot to operate on
    344            inits = new HashMap<>(childInitializers);
    345        }
    346
    347        // start the child initializers
    348        final ExecutorService exec = getActiveExecutor();
    349        inits.values().forEach(bi -> {
    350            if (bi.getExternalExecutor() == null) {
    351                // share the executor service if necessary
    352                bi.setExternalExecutor(exec);
    353            }
    354            bi.start();
    355        });
    356
    357        // collect the results
    358        final Map<String, Object> results = new HashMap<>();
    359        final Map<String, ConcurrentException> excepts = new HashMap<>();
    360        inits.forEach((k, v) -> {
    361            try {
    362                results.put(k, v.get());
    363            } catch (final ConcurrentException cex) {
    364                excepts.put(k, cex);
    365            }
    366        });
    367
    368        return new MultiBackgroundInitializerResults(inits, results, excepts);
    369    }
    370
    371    /**
    372     * Tests whether this all child {@code BackgroundInitializer} objects are initialized.
    373     * Once initialized, always returns true.
    374     *
    375     * @return whether all child {@code BackgroundInitializer} objects instance are initialized. Once initialized, always returns true. If there are no child {@code BackgroundInitializer} objects return false.
    376     * @since 3.14.0
    377     */
    378    @Override
    379    public boolean isInitialized() {
    380        if (childInitializers.isEmpty()) {
    381            return false;
    382        }
    383
    384        return childInitializers.values().stream().allMatch(BackgroundInitializer::isInitialized);
    385    }
    386}