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