How Dependency Injection Manually Implemented?

Question

How we can manually inject an object without using the facility of containers. I did something similar through reflection as follows.

Class actionClass = Class.forName("SampleClass");
Object actionObject = actionClass.newInstance();
Method reqMethod = actionClass.getMethod("setRequest", HttpServletRequest.class);
reqMethod.invoke(actionObject,request);

is it the right way to do DI?

My intention is to pass request object to different controller classes dynamically from a dispatcher filter,where we get request and response objects. I am fearing about the performace of reflection.Is there any replacement for doing DI?

Answer 1

Dependency injection is nothing more than providing a class with its dependencies, rather than have it find them itself (via singletons/lookups etc.). So you can do it in code trivially thus:

DatabaseConnection dc = new DatabaseConnection();
MyDAO dao = new MyDAO(dc);

(pseudocode). Here the MyDAO is being injected with a database connection. If that database connection implements an interface you can easily mock this out during testing.

Answer 2

Well, when you set one object into another object using setter method or through a constructor it also is the dependency injection. Dependency injection only means creating relationship(dependency) in objects.

Using reflection as you did is just another form of it.