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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.Objects;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
/**
* A specialized {@link BackgroundInitializer} implementation that wraps a
* {@link Callable} object.
*
*
* An instance of this class is initialized with a {@link Callable} object when
* it is constructed. The implementation of the {@link #initialize()} method
* defined in the super class delegates to this {@link Callable} so that the
* {@link Callable} is executed in the background thread.
*
*
* The {@code java.util.concurrent.Callable} interface is a standard mechanism
* of the JDK to define tasks to be executed by another thread. The {@code
* CallableBackgroundInitializer} class allows combining this standard interface
* with the background initializer API.
*
*
* Usage of this class is very similar to the default usage pattern of the
* {@link BackgroundInitializer} class: Just create an instance and provide the
* {@link Callable} object to be executed, then call the initializer's
* {@link #start()} method. This causes the {@link Callable} to be executed in
* another thread. When the results of the {@link Callable} are needed the
* initializer's {@link #get()} method can be called (which may block until
* background execution is complete). The following code fragment shows a
* typical usage example:
*
*
*
* // a Callable that performs a complex computation
* Callable<Integer> computationCallable = new MyComputationCallable();
* // setup the background initializer
* CallableBackgroundInitializer<Integer> initializer =
* new CallableBackgroundInitializer(computationCallable);
* initializer.start();
* // Now do some other things. Initialization runs in a parallel thread
* ...
* // Wait for the end of initialization and access the result
* Integer result = initializer.get();
*
*
* @since 3.0
* @param the type of the object managed by this initializer class
*/
public class CallableBackgroundInitializer extends BackgroundInitializer {
/** The Callable to be executed. */
private final Callable callable;
/**
* Creates a new instance of {@link CallableBackgroundInitializer} and sets
* the {@link Callable} to be executed in a background thread.
*
* @param call the {@link Callable} (must not be null)
* @throws IllegalArgumentException if the {@link Callable} is null
*/
public CallableBackgroundInitializer(final Callable call) {
checkCallable(call);
callable = call;
}
/**
* Creates a new instance of {@link CallableBackgroundInitializer} and
* initializes it with the {@link Callable} to be executed in a background
* thread and the {@link ExecutorService} for managing the background
* execution.
*
* @param call the {@link Callable} (must not be null)
* @param exec an external {@link ExecutorService} to be used for task
* execution
* @throws IllegalArgumentException if the {@link Callable} is null
*/
public CallableBackgroundInitializer(final Callable call, final ExecutorService exec) {
super(exec);
checkCallable(call);
callable = call;
}
/**
* Tests the passed in {@link Callable} and throws an exception if it is
* undefined.
*
* @param callable the object to check
* @throws IllegalArgumentException if the {@link Callable} is null
*/
private void checkCallable(final Callable callable) {
Objects.requireNonNull(callable, "callable");
}
/**
* {@inheritDoc}
*/
@Override
protected Exception getTypedException(Exception e) {
//This Exception object will be used for type comparison in AbstractConcurrentInitializer.initialize but not thrown
return new Exception(e);
}
/**
* Performs initialization in a background thread. This implementation
* delegates to the {@link Callable} passed at construction time of this
* object.
*
* @return the result of the initialization
* @throws Exception if an error occurs
*/
@Override
protected T initialize() throws Exception {
return callable.call();
}
}