How to use Spring @Component, @Repository, @Service and @Controller Annotations?

In spring
autowiring concepts, we learned about

@Autowired

annotation
that it handles only wiring. You still have to define the beans themselves so the container is aware of them and can inject them for you. But with

@Component

,

@Repository

,

@Service

and

@Controller

annotations
in place and after enabling automatic component scanning, spring will automatically import the beans into the container so you don’t have to define them explicitly with XML. These annotations are called Stereotype
annotations as well.

Before jumping to example use of these annotations, let’s learn quick facts about these annotations which will help you in making a better decision about when to use which annotation.

@Component, @Repository, @Service and @Controller annotations

1) The

@Component

annotation
marks a java class as a bean so the component-scanning mechanism of spring can pick it up and pull it into the application context. To use this annotation, apply it over class as below:

2) Although above use of

@Component

is
good enough but you can use more suitable annotation that provides additional benefits specifically for DAOs i.e.

@Repository

annotation.
The

@Repository

annotation
is a specialization of the

@Component

annotation
with similar use and functionality. In addition to importing the DAOs into the DI container, it also makes the
unchecked exceptions (thrown from DAO methods) eligible for translation into Spring

DataAccessException

.

3) The

@Service

annotation
is also a specialization of the component annotation. It doesn’t currently provide any additional behavior over the

@Component

annotation,
but it’s a good idea to use

@Service

over

@Component

in
service-layer classes because it specifies intent better. Additionally, tool support and additional behavior
might rely on it in the future.

4)

@Controller

annotation
marks a class as a Spring Web MVC controller. It too is a

@Component

specialization,
so beans marked with it are automatically imported into the DI container. When you add the

@Controller

annotation
to a class, you can use another annotation i.e.

@RequestMapping

;
to map URLs to instance methods of a class.

In real life, you will face very rare situations where you will need to use

@Component

annotation.
Most of the time, you will using

@Repository

,

@Service

and

@Controller

annotations.

@Component

should
be used when your class does not fall into either of three categories i.e. controller, manager and dao.

If you want to define name of the bean with which they will be registered in DI container, you can pass the name in annotation itself
e.g. @Service (“employeeManager”).

How to enable component scanning

Above four annotation will be scanned and configured only when they are scanned by DI container of spring framework. To enable this scanning, you will need to use “context:component-scan”
tag in your

applicationContext.xml

file.
e.g.

The context:component-scan element requires a base-package attribute, which, as its name suggests, specifies
a starting point for a recursive component search. You may not want to give your top package for scanning to spring, so you should declare three component-scan elements,
each with a base-package attribute pointing to a different package.

When component-scan is declared, you no longer need to declare context:annotation-config, because autowiring
is implicitly enabled when component scanning is enabled.

How to use @Component, @Repository, @Service and @Controller Annotations

As I already said that you use

@Repository

,

@Service

and

@Controller

annotations
over DAO, manager and controller classes. But in real life, at DAO and manager layer we often have separate classes and interfaces. Interface for defining the contract, and classes for defining the implementations of contracts. Where to use these annotations?
Let’s find out.

Always use these annotations over concrete classes; not over interfaces.

Once you have these stereotype annotations on beans, you can directly use bean references defined inside concrete classes. Note the references are of type interfaces. Spring DI container is smart enough to inject the correct instance in this case.

EmployeeDAO.java and EmployeeDAOImpl.java

EmployeeManager.java and EmployeeManagerImpl.java

EmployeeController.java

EmployeeDTO.java

Let’s test the above configuration and annotations:

TestSpringContext.java

Drop me a comment/query if something needs more explanation.

Happy Learning !!