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 (final BackgroundInitializer<?> child : childInitializers.values()) {
292            try {
293                child.close();
294            } catch (final 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}