/*
* 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.Set;
import java.util.concurrent.ExecutorService;
/**
* <p>
* A specialized {@link BackgroundInitializer} implementation that can deal with
* multiple background initialization tasks.
* </p>
* <p>
* 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 {@code 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 {@code ExecutorService} and shares it
* with the {@code BackgroundInitializer} objects it is responsible for; so the
* using application need not bother with these details.
* </p>
* <p>
* The typical usage scenario for {@code MultiBackgroundInitializer} is as
* follows:
* <ul>
* <li>Create a new instance of the class. Optionally pass in a pre-configured
* {@code ExecutorService}. Alternatively {@code MultiBackgroundInitializer} can
* create a temporary {@code ExecutorService} and delete it after initialization
* is complete.</li>
* <li>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.</li>
* <li>After all initializers have been added, call the {@link #start()} method.
* </li>
* <li>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.</li>
* </ul>
* </p>
* <p>
* {@code MultiBackgroundInitializer} starts a special controller task that
* starts all {@code BackgroundInitializer} objects added to the instance.
* Before the an initializer is started it is checked whether this initializer
* already has an {@code ExecutorService} set. If this is the case, this {@code
* ExecutorService} is used for running the background task. Otherwise the
* current {@code ExecutorService} of this {@code MultiBackgroundInitializer} is
* shared with the initializer.
* </p>
* <p>
* The easiest way of using this class is to let it deal with the management of
* an {@code ExecutorService} itself: If no external {@code ExecutorService} is
* provided, the class creates a temporary {@code ExecutorService} (that is
* capable of executing all background tasks in parallel) and destroys it at the
* end of background processing.
* </p>
* <p>
* Alternatively an external {@code 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 {@code ExecutorService}.
* <strong>Important note:</strong> 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: {@code MultiBackgroundInitializer} starts a task that
* starts all registered {@code BackgroundInitializer} objects and waits for
* their completion. If for instance a single threaded {@code ExecutorService}
* is used, none of the background tasks can be executed, and the task created
* by {@code MultiBackgroundInitializer} waits forever.
* </p>
*
* @since 3.0
* @version $Id: MultiBackgroundInitializer.java 1082301 2011-03-16 21:02:15Z oheger $
*/
public class MultiBackgroundInitializer
extends
BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults> {
/** A map with the child initializers. */
private final Map<String, BackgroundInitializer<?>> childInitializers =
new HashMap<String, BackgroundInitializer<?>>();
/**
* Creates a new instance of {@code MultiBackgroundInitializer}.
*/
public MultiBackgroundInitializer() {
super();
}
/**
* Creates a new instance of {@code MultiBackgroundInitializer} and
* initializes it with the given external {@code ExecutorService}.
*
* @param exec the {@code ExecutorService} for executing the background
* tasks
*/
public MultiBackgroundInitializer(ExecutorService exec) {
super(exec);
}
/**
* Adds a new {@code BackgroundInitializer} to this object. When this
* {@code 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 <b>null</b>)
* @param init the {@code BackgroundInitializer} to add (must not be
* <b>null</b>)
* @throws IllegalArgumentException if a required parameter is missing
* @throws IllegalStateException if {@code start()} has already been called
*/
public void addInitializer(String name, BackgroundInitializer<?> init) {
if (name == null) {
throw new IllegalArgumentException(
"Name of child initializer must not be null!");
}
if (init == null) {
throw new IllegalArgumentException(
"Child initializer must not be null!");
}
synchronized (this) {
if (isStarted()) {
throw new IllegalStateException(
"addInitializer() must not be called after start()!");
}
childInitializers.put(name, init);
}
}
/**
* 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 {@code 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() {
int result = 1;
for (BackgroundInitializer<?> bi : childInitializers.values()) {
result += bi.getTaskCount();
}
return result;
}
/**
* Creates the results object. This implementation starts all child {@code
* BackgroundInitializer} objects. Then it collects their results and
* creates a {@code 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 {
Map<String, BackgroundInitializer<?>> inits;
synchronized (this) {
// create a snapshot to operate on
inits = new HashMap<String, BackgroundInitializer<?>>(
childInitializers);
}
// start the child initializers
ExecutorService exec = getActiveExecutor();
for (BackgroundInitializer<?> bi : inits.values()) {
if (bi.getExternalExecutor() == null) {
// share the executor service if necessary
bi.setExternalExecutor(exec);
}
bi.start();
}
// collect the results
Map<String, Object> results = new HashMap<String, Object>();
Map<String, ConcurrentException> excepts = new HashMap<String, ConcurrentException>();
for (Map.Entry<String, BackgroundInitializer<?>> e : inits.entrySet()) {
try {
results.put(e.getKey(), e.getValue().get());
} catch (ConcurrentException cex) {
excepts.put(e.getKey(), cex);
}
}
return new MultiBackgroundInitializerResults(inits, results, excepts);
}
/**
* A data class for storing the results of the background initialization
* performed by {@code 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<String, BackgroundInitializer<?>> initializers;
/** A map with the result objects. */
private final Map<String, Object> resultObjects;
/** A map with the exceptions. */
private final Map<String, ConcurrentException> exceptions;
/**
* Creates a new instance of {@code MultiBackgroundInitializerResults}
* and initializes it with maps for the {@code BackgroundInitializer}
* objects, their result objects and the exceptions thrown by them.
*
* @param inits the {@code BackgroundInitializer} objects
* @param results the result objects
* @param excepts the exceptions
*/
private MultiBackgroundInitializerResults(
Map<String, BackgroundInitializer<?>> inits,
Map<String, Object> results,
Map<String, ConcurrentException> excepts) {
initializers = inits;
resultObjects = results;
exceptions = excepts;
}
/**
* Returns the {@code BackgroundInitializer} with the given name. If the
* name cannot be resolved, an exception is thrown.
*
* @param name the name of the {@code BackgroundInitializer}
* @return the {@code BackgroundInitializer} with this name
* @throws NoSuchElementException if the name cannot be resolved
*/
public BackgroundInitializer<?> getInitializer(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
* {@code BackgroundInitializer} caused an exception, <b>null</b> is
* returned. If the name cannot be resolved, an exception is thrown.
*
* @param name the name of the {@code BackgroundInitializer}
* @return the result object produced by this {@code
* BackgroundInitializer}
* @throws NoSuchElementException if the name cannot be resolved
*/
public Object getResultObject(String name) {
checkName(name);
return resultObjects.get(name);
}
/**
* Returns a flag whether the {@code BackgroundInitializer} with the
* given name caused an exception.
*
* @param name the name of the {@code BackgroundInitializer}
* @return a flag whether this initializer caused an exception
* @throws NoSuchElementException if the name cannot be resolved
*/
public boolean isException(String name) {
checkName(name);
return exceptions.containsKey(name);
}
/**
* Returns the {@code ConcurrentException} object that was thrown by the
* {@code BackgroundInitializer} with the given name. If this
* initializer did not throw an exception, the return value is
* <b>null</b>. If the name cannot be resolved, an exception is thrown.
*
* @param name the name of the {@code BackgroundInitializer}
* @return the exception thrown by this initializer
* @throws NoSuchElementException if the name cannot be resolved
*/
public ConcurrentException getException(String name) {
checkName(name);
return exceptions.get(name);
}
/**
* Returns a set with the names of all {@code BackgroundInitializer}
* objects managed by the {@code MultiBackgroundInitializer}.
*
* @return an (unmodifiable) set with the names of the managed {@code
* BackgroundInitializer} objects
*/
public Set<String> 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(String name) {
BackgroundInitializer<?> init = initializers.get(name);
if (init == null) {
throw new NoSuchElementException(
"No child initializer with name " + name);
}
return init;
}
}
}