Dependency Injection (DI) is a mechanism that allows objects to receive their dependencies from an external source, rather than creating them internally. DI is a key component of the Inversion of Control (IoC) design pattern, and it is implemented in the Spring Framework through the use of various DI techniques.
Here’s an example to help illustrate the concept of DI:
public class MyService {
private MyDao myDao;
public MyService(MyDao myDao) {
this.myDao = myDao;
}
public void doSomething() {
myDao.saveData("Hello, world!");
}
}
public class MyDao {
public void saveData(String data) {
System.out.println("Saving data: " + data);
}
}In this example, we have two classes: MyService and MyDao. MyService has a dependency on MyDao, which is injected into it using constructor injection. This means that the MyDao object is created and provided by an external source, rather than being created within the MyService class itself.
We can create an instance of MyService by passing an instance of MyDao to its constructor, like this:
MyDao myDao = new MyDao();
MyService myService = new MyService(myDao);
myService.doSomething();When the doSomething() method of MyService is called, it uses the myDao object that was injected into it to perform a save operation.
By using DI, we can decouple the MyService and MyDao classes, making them more modular and easier to test. We can also easily replace the MyDao object with a different implementation, such as a mock object, for testing purposes.
Spring provides several DI techniques, including constructor injection, setter injection, and field injection. Constructor injection is the most common and recommended technique, as it provides a clear and explicit way to define the dependencies of a class.
Overall, DI is a powerful and flexible mechanism for managing object dependencies in a Spring-based application. By using DI, you can create more modular and testable code, and reduce coupling between components in your application.