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[LANG-496] Added LazyInitializer class plus test class. 

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/lang/trunk@819141 13f79535-47bb-0310-9956-ffa450edef68
This commit is contained in:
Oliver Heger 2009-09-26 14:27:32 +00:00
parent 31a812e4eb
commit 8958fdcc73
2 changed files with 222 additions and 0 deletions
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113
src/java/org/apache/commons/lang/concurrent/LazyInitializer.java Normal file
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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.lang.concurrent;
/**
* <p>
* This class provides a generic implementation of the lazy initialization
* pattern.
* </p>
* <p>
* Sometimes an application has to deal with an object only under certain
* circumstances, e.g. when the user selects a specific menu item or if a
* special event is received. If the creation of the object is costly or the
* consumption of memory or other system resources is significant, it may make
* sense to defer the creation of this object until it is really needed. This is
* a use case for the lazy initialization pattern.
* </p>
* <p>
* This abstract base class provides an implementation of the double-check idiom
* for an instance field as discussed in Joshua Bloch's "Effective Java", 2nd
* edition, item 71. The class already implements all necessary synchronization.
* A concrete subclass has to implement the {@code initialize()} method, which
* actually creates the wrapped data object.
* </p>
* <p>
* As an usage example consider that we have a class {@code ComplexObject} whose
* instantiation is a complex operation. In order to apply lazy initialization
* to this class, a subclass of {@code LazyInitializer} has to be created:
*
* <pre>
* public class ComplexObjectInitializer extends LazyInitializer&lt;ComplexObject&gt; {
* &#064;Override
* protected ComplexObject initialize() {
* return new ComplexObject();
* }
* }
* </pre>
*
* Access to the data object is provided through the {@code get()} method. So,
* code that wants to obtain the {@code ComplexObject} instance would simply
* look like this:
*
* <pre>
* // Create an instance of the lazy initializer
* ComplexObjectInitializer initializer = new ComplexObjectInitializer();
* ...
* // When the object is actually needed:
* ComplexObject cobj = initializer.get();
* </pre>
*
* </p>
* <p>
* If multiple threads call the {@code get()} method when the object has not yet
* been created, they are blocked until initialization completes. The algorithm
* guarantees that only a single instance of the wrapped object class is
* created, which is passed to all callers. Once initialized, calls to the
* {@code get()} method are pretty fast because no synchronization is needed
* (only an access to a <b>volatile</b> member field).
* </p>
*
* @version $Id$
* @param <T> the type of the object managed by this initializer class
*/
public abstract class LazyInitializer<T> {
/** Stores the managed object. */
private volatile T object;
/**
* Returns the object wrapped by this instance. On first access the object
* is created. After that it is cached and can be accessed pretty fast.
*
* @return the object initialized by this {@code LazyInitializer}
*/
public T get() {
T result = object;
if (result == null) {
synchronized (this) {
result = object;
if (result == null) {
object = result = initialize();
}
}
}
return result;
}
/**
* Creates and initializes the object managed by this {@code
* LazyInitializer}. This method is called by {@link #get()} when the object
* is accessed for the first time. An implementation can focus on the
* creation of the object. No synchronization is needed, as this is already
* handled by {@code get()}.
*
* @return the managed data object
*/
protected abstract T initialize();
}

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src/test/org/apache/commons/lang/concurrent/LazyInitializerTest.java Normal file
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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.lang.concurrent;
import java.util.concurrent.CountDownLatch;
import junit.framework.TestCase;
/**
* Test class for {@code LazyInitializer}.
*
* @version $Id$
*/
public class LazyInitializerTest extends TestCase {
/** The initializer to be tested. */
private LazyInitializerTestImpl initializer;
@Override
protected void setUp() throws Exception {
super.setUp();
initializer = new LazyInitializerTestImpl();
}
/**
* Tests obtaining the managed object.
*/
public void testGet() {
assertNotNull("No managed object", initializer.get());
}
/**
* Tests whether sequential get() invocations always return the same
* instance.
*/
public void testGetMultipleTimes() {
Object obj = initializer.get();
for (int i = 0; i < 10; i++) {
assertEquals("Got different object at " + i, obj, initializer.get());
}
}
/**
* Tests invoking get() from multiple threads concurrently.
*/
public void testGetConcurrent() throws InterruptedException {
final int threadCount = 20;
final CountDownLatch startLatch = new CountDownLatch(1);
class GetThread extends Thread {
Object object;
@Override
public void run() {
try {
// wait until all threads are ready for maximum parallelism
startLatch.await();
// access the initializer
object = initializer.get();
} catch (InterruptedException iex) {
// ignore
}
}
}
GetThread[] threads = new GetThread[threadCount];
for (int i = 0; i < threadCount; i++) {
threads[i] = new GetThread();
threads[i].start();
}
// fire all threads and wait until they are ready
startLatch.countDown();
for (Thread t : threads) {
t.join();
}
// check results
Object managedObject = initializer.get();
for (GetThread t : threads) {
assertEquals("Wrong object", managedObject, t.object);
}
}
/**
* A test implementation of LazyInitializer. This class creates a plain
* Object. As Object does not provide a specific equals() method, it is easy
* to check whether multiple instances were created.
*/
private static class LazyInitializerTestImpl extends
LazyInitializer<Object> {
@Override
protected Object initialize() {
return new Object();
}
}
}