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/*
 * 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.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

/**
 * 

* A class that allows complex initialization operations in a background task. *

*

* Applications often have to do some expensive initialization steps when they * are started, e.g. constructing a connection to a database, reading a * configuration file, etc. Doing these things in parallel can enhance * performance as the CPU load can be improved. However, when access to the * resources initialized in a background thread is actually required, * synchronization has to be performed to ensure that their initialization is * complete. *

*

* This abstract base class provides support for this use case. A concrete * subclass must implement the {@link #initialize()} method. Here an arbitrary * initialization can be implemented, and a result object can be returned. With * this method in place the basic usage of this class is as follows (where * {@code MyBackgroundInitializer} is a concrete subclass): *

* *
 * MyBackgroundInitializer initializer = new MyBackgroundInitializer();
 * initializer.start();
 * // Now do some other things. Initialization runs in a parallel thread
 * ...
 * // Wait for the end of initialization and access the result object
 * Object result = initializer.get();
 * 
* *

* After the construction of a {@code BackgroundInitializer} object its * {@link #start()} method has to be called. This starts the background * processing. The application can now continue to do other things. When it * needs access to the object produced by the {@code BackgroundInitializer} it * calls its {@link #get()} method. If initialization is already complete, * {@link #get()} returns the result object immediately. Otherwise it blocks * until the result object is fully constructed. *

*

* {@code BackgroundInitializer} is a thin wrapper around a {@code Future} * object and uses an {@code ExecutorService} for running the background * initialization task. It is possible to pass in an {@code ExecutorService} at * construction time or set one using {@code setExternalExecutor()} before * {@code start()} was called. Then this object is used to spawn the background * task. If no {@code ExecutorService} has been provided, {@code * BackgroundInitializer} creates a temporary {@code ExecutorService} and * destroys it when initialization is complete. *

*

* The methods provided by {@code BackgroundInitializer} provide for minimal * interaction with the wrapped {@code Future} object. It is also possible to * obtain the {@code Future} object directly. Then the enhanced functionality * offered by {@code Future} can be used, e.g. to check whether the background * operation is complete or to cancel the operation. *

* * @since 3.0 * @param the type of the object managed by this initializer class */ public abstract class BackgroundInitializer implements ConcurrentInitializer { /** The external executor service for executing tasks. */ private ExecutorService externalExecutor; // @GuardedBy("this") /** A reference to the executor service that is actually used. */ private ExecutorService executor; // @GuardedBy("this") /** Stores the handle to the background task. */ private Future future; // @GuardedBy("this") /** * Creates a new instance of {@code BackgroundInitializer}. No external * {@code ExecutorService} is used. */ protected BackgroundInitializer() { this(null); } /** * Creates a new instance of {@code BackgroundInitializer} and initializes * it with the given {@code ExecutorService}. If the {@code ExecutorService} * is not null, the background task for initializing this object will be * scheduled at this service. Otherwise a new temporary {@code * ExecutorService} is created. * * @param exec an external {@code ExecutorService} to be used for task * execution */ protected BackgroundInitializer(final ExecutorService exec) { setExternalExecutor(exec); } /** * Returns the external {@code ExecutorService} to be used by this class. * * @return the {@code ExecutorService} */ public final synchronized ExecutorService getExternalExecutor() { return externalExecutor; } /** * Returns a flag whether this {@code BackgroundInitializer} has already * been started. * * @return a flag whether the {@link #start()} method has already been * called */ public synchronized boolean isStarted() { return future != null; } /** * Sets an {@code ExecutorService} to be used by this class. The {@code * ExecutorService} passed to this method is used for executing the * background task. Thus it is possible to re-use an already existing * {@code ExecutorService} or to use a specially configured one. If no * {@code ExecutorService} is set, this instance creates a temporary one and * destroys it after background initialization is complete. Note that this * method must be called before {@link #start()}; otherwise an exception is * thrown. * * @param externalExecutor the {@code ExecutorService} to be used * @throws IllegalStateException if this initializer has already been * started */ public final synchronized void setExternalExecutor( final ExecutorService externalExecutor) { if (isStarted()) { throw new IllegalStateException( "Cannot set ExecutorService after start()!"); } this.externalExecutor = externalExecutor; } /** * Starts the background initialization. With this method the initializer * becomes active and invokes the {@link #initialize()} method in a * background task. A {@code BackgroundInitializer} can be started exactly * once. The return value of this method determines whether the start was * successful: only the first invocation of this method returns true, * following invocations will return false. * * @return a flag whether the initializer could be started successfully */ public synchronized boolean start() { // Not yet started? if (!isStarted()) { // Determine the executor to use and whether a temporary one has to // be created ExecutorService tempExec; executor = getExternalExecutor(); if (executor == null) { executor = tempExec = createExecutor(); } else { tempExec = null; } future = executor.submit(createTask(tempExec)); return true; } return false; } /** * Returns the result of the background initialization. This method blocks * until initialization is complete. If the background processing caused a * runtime exception, it is directly thrown by this method. Checked * exceptions, including {@code InterruptedException} are wrapped in a * {@link ConcurrentException}. Calling this method before {@link #start()} * was called causes an {@code IllegalStateException} exception to be * thrown. * * @return the object produced by this initializer * @throws ConcurrentException if a checked exception occurred during * background processing * @throws IllegalStateException if {@link #start()} has not been called */ @Override public T get() throws ConcurrentException { try { return getFuture().get(); } catch (final ExecutionException execex) { ConcurrentUtils.handleCause(execex); return null; // should not be reached } catch (final InterruptedException iex) { // reset interrupted state Thread.currentThread().interrupt(); throw new ConcurrentException(iex); } } /** * Returns the {@code Future} object that was created when {@link #start()} * was called. Therefore this method can only be called after {@code * start()}. * * @return the {@code Future} object wrapped by this initializer * @throws IllegalStateException if {@link #start()} has not been called */ public synchronized Future getFuture() { if (future == null) { throw new IllegalStateException("start() must be called first!"); } return future; } /** * Returns the {@code ExecutorService} that is actually used for executing * the background task. This method can be called after {@link #start()} * (before {@code start()} it returns null). If an external executor * was set, this is also the active executor. Otherwise this method returns * the temporary executor that was created by this object. * * @return the {@code ExecutorService} for executing the background task */ protected final synchronized ExecutorService getActiveExecutor() { return executor; } /** * Returns the number of background tasks to be created for this * initializer. This information is evaluated when a temporary {@code * ExecutorService} is created. This base implementation returns 1. Derived * classes that do more complex background processing can override it. This * method is called from a synchronized block by the {@link #start()} * method. Therefore overriding methods should be careful with obtaining * other locks and return as fast as possible. * * @return the number of background tasks required by this initializer */ protected int getTaskCount() { return 1; } /** * Performs the initialization. This method is called in a background task * when this {@code BackgroundInitializer} is started. It must be * implemented by a concrete subclass. An implementation is free to perform * arbitrary initialization. The object returned by this method can be * queried using the {@link #get()} method. * * @return a result object * @throws Exception if an error occurs */ protected abstract T initialize() throws Exception; /** * Creates a task for the background initialization. The {@code Callable} * object returned by this method is passed to the {@code ExecutorService}. * This implementation returns a task that invokes the {@link #initialize()} * method. If a temporary {@code ExecutorService} is used, it is destroyed * at the end of the task. * * @param execDestroy the {@code ExecutorService} to be destroyed by the * task * @return a task for the background initialization */ private Callable createTask(final ExecutorService execDestroy) { return new InitializationTask(execDestroy); } /** * Creates the {@code ExecutorService} to be used. This method is called if * no {@code ExecutorService} was provided at construction time. * * @return the {@code ExecutorService} to be used */ private ExecutorService createExecutor() { return Executors.newFixedThreadPool(getTaskCount()); } private class InitializationTask implements Callable { /** Stores the executor service to be destroyed at the end. */ private final ExecutorService execFinally; /** * Creates a new instance of {@code InitializationTask} and initializes * it with the {@code ExecutorService} to be destroyed at the end. * * @param exec the {@code ExecutorService} */ InitializationTask(final ExecutorService exec) { execFinally = exec; } /** * Initiates initialization and returns the result. * * @return the result object * @throws Exception if an error occurs */ @Override public T call() throws Exception { try { return initialize(); } finally { if (execFinally != null) { execFinally.shutdown(); } } } } }




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