The document discusses the importance of dependency injection in achieving a scalable modular architecture using Java, led by Jerry Kurian from Learnym Technologies. It explains how Java annotations can simplify code by managing dependencies at runtime, allowing for components to work independently while still functioning effectively. The document details the process by which the Spring framework handles dependency injection, ensuring that relevant instances are created and injected appropriately.

1. Dependency Injection via Annotations
HOW SPRING DOES IT – SORT OF 

2. About Presenter
•I am Jerry Kurian. Founder and Maker in Chief at Learnym Technologies
(www.Learnym.com)
•I have worked with Java for almost 20 years.
• Developed distributed systems at Huawei Technologies
• Lead developer on EJB container for our in-house app server
•Post Huawei, worked on multiple products using Java and non-Java techs
•Have kept in touch with the evolving eco-system and now code a lot with
Java, Scala, Angular 4 and anything that seems like the right tool for the job
•At Learnym, helping businesses define their products clearly and then help
develop a scalable solution. We also do workshops on interesting topics
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3. Why do we need dependency
Injection
•Modular architecture is scalable
•Modularization has been the aim since long and various technologies
and architectures have come and gone
• EJB as a Component
• JNDI for Lookup
•Most of the componentization solutions suffer from being overly
complex and too hard to maintain !!
•Injection methods make code much more simpler, leaving the
frameworks to do the heavy lifting
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4. What’s the challenge?
•Modularization requires that two components be as independent of each
other as possible
•Module A should access Module B functionalities through the interface B,
the contract
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Module A Module B
Interface B

5. Approaches
•There are multiple approaches like lookup via factory, JNDI
•Adds dependency on the factory or makes code and packaging complex !!
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Module A Module B
Interface B
Factory
Register
Lookup

6. A better approach
•The problem with the lookup and such approaches is that there is a
dependency with the framework, which makes the code complex
•A better approach is to make the code unspoilt with external concerns
•A meta-data based approach provides a solution
•Java Annotations is the way to define meta-data in Java code
•Annotations are just place holder for the compiler or the run-time
system to do something meaningful with it
•At runtime, Annotations in collaboration with Reflection can do a lot of
magic
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7. How does it work?
public class ModuleA{
@Autowired
private ModuleB moduleB;
}
•The ModuleA “indicates” its dependence on the ModuleB contract
•At runtime, it expects an appropriate implementation of the ModuleB
interface to be available to it.
•Spring does it by injecting an instance of ModuleBImpl into the private
member variable
•How does this happen?
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8. The process
•The application classes
• Define the module classes and contract
• In the dependent module, add a member variable with an Annotation
• The type of the member variable should be that of the dependency
•The Framework
• The framework should find out the classes that are dependent on other
modules by reflecting over them
• Create instances of the module classes via reflection
• On the dependent instance, set the member value to the instance of the
dependency
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9. Breaking It Down
The framework does the following
1. Create an instance of the module that is desired
2. Identify the dependencies of this module by finding out fields that
are annotated as dependents
3. For each dependent field, identify the type and look through the
packages to find a class that implements the type
4. Find out if the implementation makes itself injectable via an
annotation and create an instance
5. Inject transitive dependencies
6. Set the private member variable of the target module “accessible”
7. Set the dependency instance to the private variable
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10. Questions?
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11. Thank You
•Do get in touch for any workshops on advanced Java and Scala.
•Look forward to hearing about any challenges you face that could be
covered in a workshop
•Feel free to connect with me on linkedIn @
https://www.linkedin.com/in/jerryk/
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