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
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 * </p>
046 * <ul>
047 * <li>Create a new instance of the class. Optionally pass in a pre-configured
048 * {@code ExecutorService}. Alternatively {@code MultiBackgroundInitializer} can
049 * create a temporary {@code 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 * {@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 1583482 2014-03-31 22:54:57Z niallp $
096 */
097public 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(final 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(final String name, final 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 (final 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        final ExecutorService exec = getActiveExecutor();
195        for (final 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        final Map<String, Object> results = new HashMap<String, Object>();
205        final Map<String, ConcurrentException> excepts = new HashMap<String, ConcurrentException>();
206        for (final Map.Entry<String, BackgroundInitializer<?>> e : inits.entrySet()) {
207            try {
208                results.put(e.getKey(), e.getValue().get());
209            } catch (final 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                final Map<String, BackgroundInitializer<?>> inits,
248                final Map<String, Object> results,
249                final 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(final 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(final 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(final 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(final 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(final String name) {
343            final 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}