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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();
}
}
}
}
}