Basics of Spring Beans

What is the IoC container?

• IoC, Inversion of Control is the major component of the Spring framework.
• It manages Spring configured beans.
• The inversion of the control is also known as the dependency injection.
• It is all about objects defining their dependencies and other objects using them in the application.
• @Component, @Service, @Controller and @Repository are automatically registered as Spring beans.

Role of IoC

• Instantiating beans
• Wiring beans together
• Configuring beans
• Managing bean life-cycle

Types of IoC

In the Spring framework, it has two IoC containers.

• BeanFactory
• Responsible for Bean instantiating and wiring
• Uses LAZY loading
• ApplicationContext
• Responsible for Bean instantiating and wiring
• Automatic BeanPostProcessor registration
• Automatic BeanFactoryPostProcessor registration
• Convenient MessageSource access
• ApplicationEvent publication
• Uses EAGER loading

How to supply metadata to the Spring IOC container

• XML based
• Annotation-based
• Java-based

This should be described in a separate post. Nowadays usually we are mostly using an annotation-based method.

Spring bean life-cycle

Bean definition

• Spring beans will be defined using annotations and XML configurations.

Bean creation and instantiation

• Once a bean is created it will be loaded into the ApplicationContext and JVM memory.

Population bean properties

• Spring container will create bean id, scope, and default values based on the bean definition.

Post initialization

• Spring provides aware interfaces to access bean metadata details and callback methods.
• Aware interfaces 
• ApplicationContextAware
• BeanClassLoaderAware
• BeanNameAware
• MessageSourceAware
• Initialization callbacks
• @PostConstruct
• InitializingBean.afterPropertySet()

Ready to serve

• Now the bean is ready to serve.

Pre destroy

• Spring has some custom logic and clean-ups before destroying the bean.
• Destruction callbacks
• @PreDestroy
• DisposableBean.destroy()

Bean destroy

• The bean will be removed from the JVM

Why do we need to smash the bean life-cycle

• Assign default values for bean properties.
• Load application metadata information while creating a bean and destroying a bean.
• To make sure application dependency and the modules are up and running fine.

Spring bean scopes 

The default scope of a bean is Singleton. We can use @Scope annotation to change the bean scope.

singleton

• This is the default scope.
• The same object is returned every time.
• Better to use for all stateless beans.

prototype

• A new object is created every time.
• Better to use for all stateful beans.

request

• Here it will create a single instance and it will available in the complete life-cycle of an HTTP request.

session

• Here it will create a single instance and it will available in the complete life-cycle of an HTTP session.

application

• Here it will create a single instance and it will available in the complete life-cycle of the servlet context.

websocket

• Here it will create a single instance and it will available in the complete life-cycle of a web socket.