registerSingleton

1. Overview

In this article, we will learn the usage of the registerSingleton method defined by the ConfigurableBeanFactory class. The ConfigurableBeanFactory is a configuration interface to be implemented by most bean factories.

You must not directly use the ConfigurableBeanFactory in your normal application code instead, use the sub-interfaces or implementations. Here, we will use the most common implementation DefaultListableBeanFactory to explain the registerSingleton method.

2. Bean Definitions

A BeanFactory configuration comprises, at its most basic level, definitions of one or more beans that the BeanFactory must manage.

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"        
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
    <bean id="sampleBean" class="com.tedblob.beanfactory.models.SampleBean">
        <constructor-arg value="1000" />
    </bean>
</beans>

3. Spring DefaultListableBeanFactory Instantiation

You can instantiate the DefaultListableBeanFactory explicitly by referring to the previously written bean definition:

The FileSystemResource return the absolute resource path and look for the resource in the file system such as C:/.../Core/IOC/beanfactory/src/main/resources/package/config. xml.

public class BeanfactoryApplication {
	public static void main(String[] args) {
		DefaultListableBeanFactory beanDefinitionRegistry = new DefaultListableBeanFactory();
		XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(beanDefinitionRegistry);
		reader.loadBeanDefinitions(new FileSystemResource("//Core//IOC//beanfactory//src//main//resources//config.xml"));
		SampleBean sampleBean = beanDefinitionRegistry.getBean(SampleBean.class);
		System.out.println(sampleBean.getValue() + "");
	}
}

The ClassPathResource looks for the resource on the classpath files inside /WEB-INF/classes.

public class BeanfactoryApplication {
	public static void main(String[] args) {
		DefaultListableBeanFactory beanDefinitionRegistry = new DefaultListableBeanFactory();
		XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(beanDefinitionRegistry);
		reader.loadBeanDefinitions(new ClassPathResource("config.xml"));
		SampleBean sampleBean = beanDefinitionRegistry.getBean(SampleBean.class);
		System.out.println(sampleBean.getValue() + "");
	}
}

Note that we have used the getBean method of the BeanFactory implementation to access the beans. The BeanFactory uses lazy initialization whereas ApplicationContext uses eager initialization. You can refer to this article to see the differences between ApplicationContext vs BeanFactory.

The BeanFactory creates the beans when you invoke or use the beans such as getBeans() method. You can refer to this article to understand more about BeanFactory lazy loading.

3.1. registerSingleton method

The bean definitions provide information on how to create a specific bean for the BeanFactory. However, you can also allow registering existing bean objects that are created outside the factory (by custom code).

DefaultListableBeanFactory supports this through the registerSingleton method, as defined by the org.springframework.beans.factory.config.ConfigurableBeanFactory interface.

For example, the following code registers the CustomBean object using the registerSingleton method.

public class BeanfactoryApplication {
	public static void main(String[] args) {
		DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
		XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(beanFactory);
		reader.loadBeanDefinitions(new ClassPathResource("config.xml"));
		beanFactory.registerSingleton("customBean", new CustomBean("custom bean"));
		CustomBean customBean = beanFactory.getBean(CustomBean.class);
		System.out.println(customBean.getValue() + "");
           }
}

Note that this customBean singleton is not available in BeanDefinition as we haven’t registered this through bean definition. You can verify this by calling the getBeanDefinitionNames method on BeanFactory.

System.out.println(Arrays.asList(beanFactory.getBeanDefinitionNames()));
// prints only [sampleBean]
// customBean 

3.2. Destroy singleton

You can destroy the singleton bean by calling the destroySingleton method on the BeanFactory.

//Destroy a singleton bean from context
beanFactory.destroySingleton(customBean);

4. Conclusion

To sum up, we have learned the Spring registerSingleton method. You can find code samples of this article in our GitHub repository.