Spring Tutorials

Dependency Injection (DI) is a design pattern that removes the dependency from the programming code so that it can be easy to manage and test the application. Dependency Injection makes our programming code loosely coupled.

Dependency injection (DI) is a process whereby objects define their dependencies, ie the other objects they work with, only through constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is constructed or returned from a factory method. The container then injects those dependencies when it creates the bean.

This process is fundamentally the inverse, hence the name Inversion of Control (IOC), of the bean itself controlling the instantiation or location of its dependencies on its own by using direct construction of classes, or the Service Locator pattern.

The Dependency Injection is a design pattern that removes the dependency of the programs. In such case we provide the information from the external source such as XML file. It makes our code loosely coupled and easier for testing. In such case we write the code as:

Example

class Employee{
Address add;
Employee(Address add)
{
this.add=add;
}
public void setAddress(Address add)
 {
this.add=add;
 }
}

In this case, instance of Address class is provided by external souce such as XML file either by constructor or setter method.

The Dependency Lookup is an approach where we get the resource after demand. There can be various ways to get the resource for example:

Employee emp = new EmployeeImpl();

Here we get the instance of Employee class directly by new keyword.

Employee emp = new Employee.getEmployee();

Here we get the resource or instance of Employee class by calling the static factory method getEmployee(). There can be various ways to get the resource define above and face some problems with Dependency Lookup.

1. Tight coupling
2. Hard testing

There are Two ways to perform Dependency Injection in Spring framework as given below:

1. By Constructor: Constructor-based DI is accomplished when the container invokes a class constructor with a number of arguments, each representing a dependency on other class.
2. By Setter method: Setter-based DI is accomplished by the container calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean.

You can mix both, Constructor-based and Setter-based DI but it is a good rule of thumb to use constructor arguments for mandatory dependencies and setters for optional dependencies. Code is cleaner with the DI principle and decoupling is more effective when objects are provided with their dependencies.

The object does not look up its dependencies, and does not know the location or class of the dependencies rather everything is take care by the Spring Framework.