using setter method in spring

[adarsh datt]

Why do we have to write a setter method inside a controller class when we declare an object variable instead of directly creating an object?

[Hussam Bamatraf]

This allows the Spring container to inject the required object by reading the proper class name from beans configuration file.
This is called "Depenedency Injection" where your class is not responsible to create its associated objects. Another concept here is "Decoupling": this means you use the interface (not the actual implementation) to decalre you class fields. This is important because you may have different implementations of the same service interface, therefore, you don't couple your class with one implementation. For example, insted of using:

private ArrayList myList;

Use:

private List myList;    

Spring container allows you to inject the actual implementation using beans configuration file.