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    }