348 lines
14 KiB
Java
348 lines
14 KiB
Java
/*
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* Licensed to the Apache Software Foundation (ASF) under one or more
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* contributor license agreements. See the NOTICE file distributed with
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* this work for additional information regarding copyright ownership.
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* The ASF licenses this file to You under the Apache License, Version 2.0
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* (the "License"); you may not use this file except in compliance with
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* the License. You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package org.apache.commons.lang3.concurrent;
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import java.util.Collections;
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import java.util.HashMap;
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import java.util.Map;
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import java.util.NoSuchElementException;
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import java.util.Set;
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import java.util.concurrent.ExecutorService;
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import org.apache.commons.lang3.Validate;
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/**
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* <p>
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* A specialized {@link BackgroundInitializer} implementation that can deal with
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* multiple background initialization tasks.
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* </p>
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* <p>
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* This class has a similar purpose as {@link BackgroundInitializer}. However,
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* it is not limited to a single background initialization task. Rather it
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* manages an arbitrary number of {@code BackgroundInitializer} objects,
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* executes them, and waits until they are completely initialized. This is
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* useful for applications that have to perform multiple initialization tasks
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* that can run in parallel (i.e. that do not depend on each other). This class
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* takes care about the management of an {@code ExecutorService} and shares it
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* with the {@code BackgroundInitializer} objects it is responsible for; so the
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* using application need not bother with these details.
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* </p>
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* <p>
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* The typical usage scenario for {@code MultiBackgroundInitializer} is as
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* follows:
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* </p>
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* <ul>
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* <li>Create a new instance of the class. Optionally pass in a pre-configured
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* {@code ExecutorService}. Alternatively {@code MultiBackgroundInitializer} can
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* create a temporary {@code ExecutorService} and delete it after initialization
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* is complete.</li>
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* <li>Create specialized {@link BackgroundInitializer} objects for the
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* initialization tasks to be performed and add them to the {@code
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* MultiBackgroundInitializer} using the
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* {@link #addInitializer(String, BackgroundInitializer)} method.</li>
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* <li>After all initializers have been added, call the {@link #start()} method.
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* </li>
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* <li>When access to the result objects produced by the {@code
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* BackgroundInitializer} objects is needed call the {@link #get()} method. The
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* object returned here provides access to all result objects created during
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* initialization. It also stores information about exceptions that have
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* occurred.</li>
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* </ul>
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* <p>
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* {@code MultiBackgroundInitializer} starts a special controller task that
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* starts all {@code BackgroundInitializer} objects added to the instance.
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* Before the an initializer is started it is checked whether this initializer
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* already has an {@code ExecutorService} set. If this is the case, this {@code
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* ExecutorService} is used for running the background task. Otherwise the
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* current {@code ExecutorService} of this {@code MultiBackgroundInitializer} is
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* shared with the initializer.
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* </p>
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* <p>
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* The easiest way of using this class is to let it deal with the management of
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* an {@code ExecutorService} itself: If no external {@code ExecutorService} is
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* provided, the class creates a temporary {@code ExecutorService} (that is
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* capable of executing all background tasks in parallel) and destroys it at the
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* end of background processing.
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* </p>
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* <p>
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* Alternatively an external {@code ExecutorService} can be provided - either at
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* construction time or later by calling the
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* {@link #setExternalExecutor(ExecutorService)} method. In this case all
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* background tasks are scheduled at this external {@code ExecutorService}.
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* <strong>Important note:</strong> When using an external {@code
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* ExecutorService} be sure that the number of threads managed by the service is
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* large enough. Otherwise a deadlock can happen! This is the case in the
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* following scenario: {@code MultiBackgroundInitializer} starts a task that
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* starts all registered {@code BackgroundInitializer} objects and waits for
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* their completion. If for instance a single threaded {@code ExecutorService}
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* is used, none of the background tasks can be executed, and the task created
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* by {@code MultiBackgroundInitializer} waits forever.
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* </p>
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*
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* @since 3.0
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*/
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public class MultiBackgroundInitializer
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extends
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BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults> {
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/** A map with the child initializers. */
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private final Map<String, BackgroundInitializer<?>> childInitializers =
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new HashMap<>();
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/**
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* Creates a new instance of {@code MultiBackgroundInitializer}.
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*/
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public MultiBackgroundInitializer() {
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}
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/**
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* Creates a new instance of {@code MultiBackgroundInitializer} and
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* initializes it with the given external {@code ExecutorService}.
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*
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* @param exec the {@code ExecutorService} for executing the background
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* tasks
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*/
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public MultiBackgroundInitializer(final ExecutorService exec) {
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super(exec);
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}
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/**
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* Adds a new {@code BackgroundInitializer} to this object. When this
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* {@code MultiBackgroundInitializer} is started, the given initializer will
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* be processed. This method must not be called after {@link #start()} has
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* been invoked.
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*
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* @param name the name of the initializer (must not be <b>null</b>)
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* @param backgroundInitializer the {@code BackgroundInitializer} to add (must not be
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* <b>null</b>)
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* @throws IllegalArgumentException if a required parameter is missing
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* @throws IllegalStateException if {@code start()} has already been called
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*/
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public void addInitializer(final String name, final BackgroundInitializer<?> backgroundInitializer) {
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Validate.notNull(name, "name");
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Validate.notNull(backgroundInitializer, "backgroundInitializer");
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synchronized (this) {
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if (isStarted()) {
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throw new IllegalStateException("addInitializer() must not be called after start()!");
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}
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childInitializers.put(name, backgroundInitializer);
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}
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}
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/**
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* Returns the number of tasks needed for executing all child {@code
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* BackgroundInitializer} objects in parallel. This implementation sums up
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* the required tasks for all child initializers (which is necessary if one
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* of the child initializers is itself a {@code MultiBackgroundInitializer}
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* ). Then it adds 1 for the control task that waits for the completion of
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* the children.
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*
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* @return the number of tasks required for background processing
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*/
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@Override
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protected int getTaskCount() {
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int result = 1;
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synchronized (this) {
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for (final BackgroundInitializer<?> bi : childInitializers.values()) {
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result += bi.getTaskCount();
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}
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}
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return result;
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}
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/**
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* Creates the results object. This implementation starts all child {@code
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* BackgroundInitializer} objects. Then it collects their results and
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* creates a {@code MultiBackgroundInitializerResults} object with this
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* data. If a child initializer throws a checked exceptions, it is added to
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* the results object. Unchecked exceptions are propagated.
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*
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* @return the results object
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* @throws Exception if an error occurs
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*/
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@Override
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protected MultiBackgroundInitializerResults initialize() throws Exception {
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final Map<String, BackgroundInitializer<?>> inits;
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synchronized (this) {
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// create a snapshot to operate on
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inits = new HashMap<>(
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childInitializers);
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}
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// start the child initializers
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final ExecutorService exec = getActiveExecutor();
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for (final BackgroundInitializer<?> bi : inits.values()) {
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if (bi.getExternalExecutor() == null) {
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// share the executor service if necessary
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bi.setExternalExecutor(exec);
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}
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bi.start();
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}
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// collect the results
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final Map<String, Object> results = new HashMap<>();
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final Map<String, ConcurrentException> excepts = new HashMap<>();
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for (final Map.Entry<String, BackgroundInitializer<?>> e : inits.entrySet()) {
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try {
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results.put(e.getKey(), e.getValue().get());
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} catch (final ConcurrentException cex) {
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excepts.put(e.getKey(), cex);
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}
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}
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return new MultiBackgroundInitializerResults(inits, results, excepts);
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}
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/**
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* A data class for storing the results of the background initialization
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* performed by {@code MultiBackgroundInitializer}. Objects of this inner
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* class are returned by {@link MultiBackgroundInitializer#initialize()}.
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* They allow access to all result objects produced by the
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* {@link BackgroundInitializer} objects managed by the owning instance. It
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* is also possible to retrieve status information about single
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* {@link BackgroundInitializer}s, i.e. whether they completed normally or
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* caused an exception.
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*/
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public static class MultiBackgroundInitializerResults {
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/** A map with the child initializers. */
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private final Map<String, BackgroundInitializer<?>> initializers;
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/** A map with the result objects. */
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private final Map<String, Object> resultObjects;
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/** A map with the exceptions. */
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private final Map<String, ConcurrentException> exceptions;
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/**
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* Creates a new instance of {@code MultiBackgroundInitializerResults}
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* and initializes it with maps for the {@code BackgroundInitializer}
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* objects, their result objects and the exceptions thrown by them.
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*
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* @param inits the {@code BackgroundInitializer} objects
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* @param results the result objects
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* @param excepts the exceptions
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*/
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private MultiBackgroundInitializerResults(
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final Map<String, BackgroundInitializer<?>> inits,
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final Map<String, Object> results,
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final Map<String, ConcurrentException> excepts) {
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initializers = inits;
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resultObjects = results;
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exceptions = excepts;
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}
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/**
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* Returns the {@code BackgroundInitializer} with the given name. If the
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* name cannot be resolved, an exception is thrown.
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*
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* @param name the name of the {@code BackgroundInitializer}
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* @return the {@code BackgroundInitializer} with this name
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* @throws NoSuchElementException if the name cannot be resolved
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*/
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public BackgroundInitializer<?> getInitializer(final String name) {
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return checkName(name);
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}
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/**
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* Returns the result object produced by the {@code
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* BackgroundInitializer} with the given name. This is the object
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* returned by the initializer's {@code initialize()} method. If this
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* {@code BackgroundInitializer} caused an exception, <b>null</b> is
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* returned. If the name cannot be resolved, an exception is thrown.
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*
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* @param name the name of the {@code BackgroundInitializer}
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* @return the result object produced by this {@code
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* BackgroundInitializer}
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* @throws NoSuchElementException if the name cannot be resolved
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*/
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public Object getResultObject(final String name) {
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checkName(name);
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return resultObjects.get(name);
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}
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/**
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* Returns a flag whether the {@code BackgroundInitializer} with the
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* given name caused an exception.
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*
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* @param name the name of the {@code BackgroundInitializer}
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* @return a flag whether this initializer caused an exception
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* @throws NoSuchElementException if the name cannot be resolved
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*/
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public boolean isException(final String name) {
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checkName(name);
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return exceptions.containsKey(name);
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}
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/**
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* Returns the {@code ConcurrentException} object that was thrown by the
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* {@code BackgroundInitializer} with the given name. If this
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* initializer did not throw an exception, the return value is
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* <b>null</b>. If the name cannot be resolved, an exception is thrown.
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*
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* @param name the name of the {@code BackgroundInitializer}
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* @return the exception thrown by this initializer
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* @throws NoSuchElementException if the name cannot be resolved
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*/
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public ConcurrentException getException(final String name) {
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checkName(name);
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return exceptions.get(name);
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}
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/**
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* Returns a set with the names of all {@code BackgroundInitializer}
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* objects managed by the {@code MultiBackgroundInitializer}.
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*
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* @return an (unmodifiable) set with the names of the managed {@code
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* BackgroundInitializer} objects
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*/
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public Set<String> initializerNames() {
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return Collections.unmodifiableSet(initializers.keySet());
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}
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/**
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* Returns a flag whether the whole initialization was successful. This
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* is the case if no child initializer has thrown an exception.
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*
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* @return a flag whether the initialization was successful
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*/
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public boolean isSuccessful() {
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return exceptions.isEmpty();
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}
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/**
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* Checks whether an initializer with the given name exists. If not,
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* throws an exception. If it exists, the associated child initializer
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* is returned.
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*
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* @param name the name to check
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* @return the initializer with this name
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* @throws NoSuchElementException if the name is unknown
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*/
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private BackgroundInitializer<?> checkName(final String name) {
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final BackgroundInitializer<?> init = initializers.get(name);
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if (init == null) {
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throw new NoSuchElementException(
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"No child initializer with name " + name);
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}
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return init;
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}
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}
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}
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