/*
 * 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.ExecutorService;

import org.apache.commons.lang3.Validate;

 A specialized BackgroundInitializer implementation that wraps a Callable object. 
 An instance of this class is initialized with a Callable object when it is constructed. The implementation of the initialize() method defined in the super class delegates to this Callable so that the Callable is executed in the background thread. 
 The java.util.concurrent.Callable interface is a standard mechanism of the JDK to define tasks to be executed by another thread. The 
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 BackgroundInitializer class: Just create an instance and provide the Callable object to be executed, then call the initializer's BackgroundInitializer.start() method. This causes the Callable to be executed in another thread. When the results of the Callable are needed the initializer's BackgroundInitializer.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();

Type parameters:
<T> – the type of the object managed by this initializer class
Since:
3.0/**
 * <p>
 * A specialized {@link BackgroundInitializer} implementation that wraps a
 * {@code Callable} object.
 * </p>
 * <p>
 * An instance of this class is initialized with a {@code Callable} object when
 * it is constructed. The implementation of the {@link #initialize()} method
 * defined in the super class delegates to this {@code Callable} so that the
 * {@code Callable} is executed in the background thread.
 * </p>
 * <p>
 * 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.
 * </p>
 * <p>
 * Usage of this class is very similar to the default usage pattern of the
 * {@link BackgroundInitializer} class: Just create an instance and provide the
 * {@code Callable} object to be executed, then call the initializer's
 * {@link #start()} method. This causes the {@code Callable} to be executed in
 * another thread. When the results of the {@code 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:
 * </p>
 *
 * <pre>
 * // a Callable that performs a complex computation
 * Callable&lt;Integer&gt; computationCallable = new MyComputationCallable();
 * // setup the background initializer
 * CallableBackgroundInitializer&lt;Integer&gt; 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();
 * </pre>
 *
 *
 * @since 3.0
 * @param <T> the type of the object managed by this initializer class
 */
public class CallableBackgroundInitializer<T> extends BackgroundInitializer<T> {
    
The Callable to be executed. /** The Callable to be executed. */
    private final Callable<T> callable;

    
Creates a new instance of CallableBackgroundInitializer and sets the Callable to be executed in a background thread. 
Params:
call – the Callable (must not be null)
Throws:
IllegalArgumentException – if the Callable is null/**
     * Creates a new instance of {@code CallableBackgroundInitializer} and sets
     * the {@code Callable} to be executed in a background thread.
     *
     * @param call the {@code Callable} (must not be <b>null</b>)
     * @throws IllegalArgumentException if the {@code Callable} is <b>null</b>
     */
    public CallableBackgroundInitializer(final Callable<T> call) {
        checkCallable(call);
        callable = call;
    }

    
Creates a new instance of CallableBackgroundInitializer and initializes it with the Callable to be executed in a background thread and the ExecutorService for managing the background execution. 
Params:
call – the Callable (must not be null)
exec – an external ExecutorService to be used for task execution
Throws:
IllegalArgumentException – if the Callable is null/**
     * Creates a new instance of {@code CallableBackgroundInitializer} and
     * initializes it with the {@code Callable} to be executed in a background
     * thread and the {@code ExecutorService} for managing the background
     * execution.
     *
     * @param call the {@code Callable} (must not be <b>null</b>)
     * @param exec an external {@code ExecutorService} to be used for task
     * execution
     * @throws IllegalArgumentException if the {@code Callable} is <b>null</b>
     */
    public CallableBackgroundInitializer(final Callable<T> call, final ExecutorService exec) {
        super(exec);
        checkCallable(call);
        callable = call;
    }

    
Performs initialization in a background thread. This implementation delegates to the Callable passed at construction time of this object. 
Throws:
Exception – if an error occurs
Returns:
the result of the initialization/**
     * Performs initialization in a background thread. This implementation
     * delegates to the {@code 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();
    }

    
Tests the passed in Callable and throws an exception if it is undefined. 
Params:
call – the object to check
Throws:
IllegalArgumentException – if the Callable is null/**
     * Tests the passed in {@code Callable} and throws an exception if it is
     * undefined.
     *
     * @param call the object to check
     * @throws IllegalArgumentException if the {@code Callable} is <b>null</b>
     */
    private void checkCallable(final Callable<T> call) {
        Validate.isTrue(call != null, "Callable must not be null!");
    }
}