The Spring IoC container is at the core of the Spring Framework. The container will create the objects, wire them together, configure them, and manage their complete life cycle from creation to destruction. The Spring container uses dependency injection (DI) to manage the components that make up an application.
1. What is Spring Container?
org.springframework.context packages are the basis for Spring Framework’s IoC container. The following two interfaces are at the heart of Spring container.
- BeanFactory – provides the configuration framework and basic functionality capable of managing any type of object.
- ApplicationContext – extends BeanFactory and adds more enterprise-specific functionality such as integration with Spring AOP, message resource handling and event publication.
ApplicationContext interface represents the Spring IoC container and is responsible for instantiating, configuring, and assembling the beans by reading configuration metadata. The configuration metadata is represented in XML (legacy), Java annotations, or Java code.
2. Spring ApplicationContext
The ApplicationContext container adds more enterprise-specific functionality such as resolving textual messages from a properties file and publishing application events to interested event listeners.
The ApplicationContext container includes all functionality of the BeanFactory container, so it is generally recommended over the BeanFactory.
BeanFactory can still be used for lightweight applications like mobile devices or applet-based applications where data volume and speed are significant.
2.1. Types of ApplicationContext
The most commonly used
ApplicationContext implementations that use XML configuration sources are:
- FileSystemXmlApplicationContext – loads the definitions of the beans from an XML file.
- ClassPathXmlApplicationContext – loads the definitions of the beans from an XML file. Additionally, we can set CLASSPATH properly because this container can look bean configuration XML file in CLASSPATH.
- WebXmlApplicationContext – loads the XML file with definitions of all beans from within a web application.
In the case of Java configuration or annotation-based configuration classes, the following container types are used:
- AnnotationConfigApplicationContext – accepts component classes as input – in particular
@Configuration-annotated classes. It also plain
@Componenttypes and JSR-330 compliant classes using
- AnnotationConfigWebApplicationContext – produces a WebApplicationContext accepting similar inputs as above. It is the equivalent of AnnotationConfigApplicationContext for a web environment.
2.2. Creating ApplicationContext
A sample code for creating the application context will look like this. Note that, in most application scenarios, explicit user code is not required to instantiate a Spring IoC container. It is handled by the framework itself during the component scanning at the application startup.
After creating the ApplicationContext, we can use
getBean() to retrieve instances of the beans. Ideally, we should not getBean() at all, rather use @Autowired annotation for bean autowiring.
ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml"); HelloWorld obj = (HelloWorld) context.getBean("helloWorld");
3. Spring BeanFactory
The BeanFactory is essentially an interface for an advanced factory capable of maintaining a registry of different beans and their dependencies. The
BeanFactory enables us to read bean definitions and access them using the bean factory.
3.1. Creating BeanFactory
When using just the
BeanFactory we can create one and read in some bean definitions in the XML format as follows:
InputStream is = new FileInputStream("beans.xml"); BeanFactory factory = new XmlBeanFactory(is); //Get bean HelloWorld obj = (HelloWorld) factory.getBean("helloWorld");
Other ways to create a bean factory are as below:
Resource resource = new FileSystemResource("beans.xml"); BeanFactory factory = new XmlBeanFactory(resource); ClassPathResource resource = new ClassPathResource("beans.xml"); BeanFactory factory = new XmlBeanFactory(resource);
Basically, that’s all there is. Using
getBean(String), we can retrieve instances of the beans; the client-side view of the
BeanFactory is surprisingly simple.
3.2. BeanFactory Methods
BeanFactory interface has only six methods for client code to call:
boolean containsBean(String): returns true if the
BeanFactorycontains a bean definition or bean instance that matches the given name.
Object getBean(String): returns an instance of the bean registered under the given name. Depending on how the bean was configured by the
BeanFactoryconfiguration, either a singleton and thus shared instance or a newly created bean will be returned. A
BeansExceptionwill be thrown when either the bean cannot be found (in which case it’ll be a
NoSuchBeanDefinitionException), or an exception occurred while instantiating and preparing the bean.
Object getBean(String, Class): returns a bean, registered under the given name. The bean returned will be cast to the given Class. If the bean cannot be cast, corresponding exceptions will be thrown (
BeanNotOfRequiredTypeException). Furthermore, all rules of the
getBean(String)method apply (see above)
Class getType(String name): returns the
Classof the bean with the given name. If no bean corresponding to the given name could be found, a
NoSuchBeanDefinitionExceptionwill be thrown
boolean isSingleton(String): determines whether or not the bean definition or bean instance registered under the given name is a singleton. If no bean corresponding to the given name could be found, a
NoSuchBeanDefinitionExceptionwill be thrown
String getAliases(String): Return the aliases for the given bean name, if any were defined in the bean definition
In this Spring tutorial, we learned the basics of the Spring IoC container, how it has been implemented using ApplicationContext interface, how it can be created programmatically and access the beans if not using autowiring.
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