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