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