Why we are using interface in spring MVC pattern?

Question

I am new to spring MVC , I have downloaded a small spring MVC project . The project is executing fine but it this project interfaces and classes are being used . like

public interface EmployeeService {

    public void addEmployee(Employee employee);

    public List listEmployeess();

    public Employee getEmployee(int empid);

    public void deleteEmployee(Employee employee);
}

And

public class EmployeeServiceImpl implements EmployeeService {

    @Autowired
    private EmployeeDao employeeDao;

    @Transactional(propagation = Propagation.REQUIRED, readOnly = false)
    public void addEmployee(Employee employee) {
        employeeDao.addEmployee(employee);
    }

    public List listEmployeess() {
        return employeeDao.listEmployeess();
    }

    public Employee getEmployee(int empid) {
        return employeeDao.getEmployee(empid);
    }

    public void deleteEmployee(Employee employee) {
        employeeDao.deleteEmployee(employee);
    }

}

My doubt is if we are using EmployeeServiceImpl what is the need of implementing EmployeeService ? same thing is there in EmployeeDao and EmployeeDaoImpl.

Answer 1

Interfaces are always a good practice for decoupling, but also, when speaking about Spring, there are several features you can use having interfaces rather than concrete classes.

A big advantage is proxying - Spring AOP.

You can find more information here: http://docs.spring.io/spring/docs/3.0.x/spring-framework-reference/html/aop.html

There are other advantages, like post processing and things like that, but I think you will have an interesting reading on Spring AOP.

Answer 2

Weather spring mvc or not, one should always code to interface. Interface gives me better readability when i just want to see what the class does instead of worrying about how it does it, kind of API exposed to outer world.

Another benefit is there could be multiple implementations of 'how to do' it and spring helps to switch easily between multiple implementations. For e.g. you could have one more implementation of EmployeeService say FullTimeEmployeeServiceImpl, RemoteEmployeeServiceImpl.

Now if you have client class which uses EmployeeService:

class EmployeeManager{
    private EmployeeService service;
}

you can inject any of bean here

<bean id="employeeManager" class="com.abc.EmployeeManager">
  <property name="service" ref="fullTimeEmployee | remoteEmployee" >
</bean>

<bean id="fullTimeEmployee" class="com.abc.FullTimeEmployeeServiceImpl" />
<bean id="remoteEmployee" class="com.abc.RemoteEmployeeServiceImpl" />