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Preinstantiating Singleton Beans
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By default, the BeanFactory will not preinstantiate singletons, but will instead create each singleton bean when it s first requested. In contrast, the ApplicationContext, discussed a bit later in this chapter, will preinstantiate singleton beans. In practice, you will almost always use an ApplicationContext, so you will rarely need to deal with this issue. When you do use the BeanFactory, it makes sense to preinstantiate singletons. For one, you will discover configuration errors that become apparent only when beans are created early on. Let s see how to preinstantiate singletons in the BeanFactory using the InstantiationCounterClass class, which counts the number of instantiations via a static variable, as shown in Listing 2-25. Listing 2-25. The InstantiationCounterClass Class, Which Tracks the Number of Instantiations package com.apress.springbook.chapter02; import java.util.concurrent.atomic.AtomicInteger;
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CHAPTER 2 THE CORE CONTAINER
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public class InstantiationCounterClass { private static AtomicInteger instantiationCounter = new AtomicInteger(0); public static int getNumberOfInstantiations() { /* returns the internal int value */ return instantiationCounter.get(); } public InstantiationCounterClass() { /* increment the internal int value with one */ instantiationCounter.incrementAndGet(); } } We configure the InstantiationCounterClass class in a configuration file, as shown in Listing 2-26. Listing 2-26. Configuring the InstantiationCounterClass in a Bean Definition < xml version="1.0" encoding="UTF-8" > <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd"> <beans> <bean id="instantiationCounterClass" class="com.apress.springbook.chapter02.InstantiationCounterClass"/> </beans>
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The InstantiationCounterClass uses the java.util.concurrent.atomic.AtomicInteger class introduced in Java 5 to ensure that updates to the static instantiationCounter variable happen atomically, and therefore in a thread-safe fashion. Although this class is used purely for demonstration purposes, we don t want to show you code that updates a shared variable without a second thought. We advise you to take a similar approach when you increment counter variables in your code, even if it s just for testing purposes. We know from experience that you cannot tell who will look at your code after you write it.
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Listing 2-27 demonstrates, in the BeanFactoryPreInstantiateSingletonsIntegrationTests test case, how to preinstantiate singletons when using ConfigurableListableBeanFactory. We need to use ConfigurableListableBeanFactory because we are working with the container s configuration. Before we preinstantiate the singletons, we will count the number of instantiations on InstantiationCounterClass. Listing 2-27. Testing the Preinstantiation of Singletons on BeanFactory package com.apress.springbook.chapter02; import junit.framework.TestCase; import org.springframework.core.io.ClassPathResource; import org.springframework.beans.factory.config.ConfigurableListableBeanFactory; import org.springframework.beans.factory.xml.XmlBeanFactory; public class BeanFactoryPreInstantiateSingletonsIntegrationTests extends TestCase { public void testPreInstantiateSingletons() { ConfigurableListableBeanFactory beanFactory =
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new XmlBeanFactory( new ClassPathResource( "com/apress/springbook/chapter02/preInstantiate-singletons.xml" ) ); assertEquals(0, InstantiationCounterClass.getNumberOfInstantiations()); beanFactory.preInstantiateSingletons(); assertEquals(1, InstantiationCounterClass.getNumberOfInstantiations()); } } Prototype beans are not preinstantiated by the BeanFactory or ApplicationContext container.
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Bean Initialization
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The container can optionally initialize beans, either via an interface that is implemented by beans or via a custom initialization method. This initialization step can be used to check the internal state of beans. If you use setter injection, it s possible to have forgotten to configure one or more properties. You can use the initialization step to let beans check if all the required collaborating objects are present.
Using the InitializingBean Interface
The container will initialize any bean that implements the org.springframework.beans.factory. InitializingBean interface, as shown in Listing 2-28. Listing 2-28. The InitializingBeanNumbersClass Class, Which Implements the InitializingBean Interface package com.apress.springbook.chapter02; import org.springframework.beans.factory.InitializingBean; public class InitializingBeanNameClass implements InitializingBean { private String firstName; public String getFirstName() { return this.firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } public void afterPropertiesSet() throws Exception { if (this.firstName == null) { throw new RequiredPropertyNotSetException("firstName property is required."); } }
CHAPTER 2 THE CORE CONTAINER
private class RequiredPropertyNotSetException extends Exception { private RequiredPropertyNotSetException(String msg) { super(msg); } } } The afterPropertiesSet() method is invoked by the container and can throw an exception, which will interrupt the life cycle of the bean. In this case, it is a custom RequiredPropertyNotSet Exception that warns about missing properties. The InitializingBeanNameClass does not require any special configuration, as shown in Listing 2-29. Listing 2-29. Configuring InitializingBeanNumberClass in the Container < xml version="1.0" encoding="UTF-8" > <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd"> <beans> <bean id="numberClass" class="com.apress.springbook.chapter02.InitializingBeanNumberClass"/> </beans> The container calls the afterPropertiesSet() method, as shown in the test case in Listing 2-30. Listing 2-30. The InitializingBeanNumberClassIntegrationTests Test Case package com.apress.springbook.chapter02; import junit.framework.TestCase; import org.springframework.core.io.ClassPathResource; import org.springframework.beans.factory.BeanFactory; import org.springframework.beans.factory.xml.XmlBeanFactory; public class InitializingBeanNameClassIntegrationTests extends TestCase { public void testPreInstantiateSingletons() { BeanFactory beanFactory = new XmlBeanFactory( new ClassPathResource( "com/apress/springbook/chapter02/initializing-bean.xml" ) ); InitializingBeanNameClass nameClass = (InitializingBeanNameClass)beanFactory.getBean("nameClass"); assertEquals("Roger", nameClass.getFirstName()); } } The test case uses BeanFactory so singleton beans are not preinstantiated. This means that the nameClass bean is created when the getBean() method is called. and any exception thrown by the afterProperties() method is rethrown by the getBean() method. If you try to run this test, you will receive a RequiredPropertyNotSetException before the test executes, because the container won t instantiate the bean without a firstName property.
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