01: Spring DI & IoC interview Q&As

Video: Spring DIP, DI, and IoC. This extends 13 Spring interview questions & answers.

Q1. What do you understand by the terms Dependency Inversion Principle (DIP), Dependency Injection (DI) and Inversion of Control (IoC) container?
A1. Dependency Inversion Principle (DIP) is one of the 6 OO design principles abbreviated as “SOLID“, and is in some ways related to the Dependency Injection (DI) pattern. The idea of DIP is that higher layers of your application should not directly depend on lower layers. Dependency Inversion Principle does not imply Dependency Injection. This principle doesn’t say anything about how higher layers know what lower layer to use. This could be done as shown below by

1) Coding to interface using a factory pattern or

2) Coding to interface and through “Dependency Injection” by using an IoC container like Spring framework, Guice, or JEE 6+.


DIP (Dependency Inversion Principle)

The Dependency Inversion Principle (DIP) states that

– High level modules should not depend upon low level modules. Both should depend upon abstractions.

– Abstractions should not depend upon details. Details should depend upon abstractions.

When this principle is applied, the higher level classes will not be working directly with the lower level classes, but with an abstract layer (i.e. an abstract class or an interface). This gives us the flexibility at the cost of increased effort. The “CircusService” depends on the interface “AnimalHandler” and not on the concrete implementation “Tigerhandler”. The implementation can be easily swapped to the “LionHandler” as long as it implements the interface “AnimalHandler”.

#1. Tightly coupled “CircusService”

Higher layers directly depend on the implementations of the lower layers. This causes tighter coupling.

Helper implementation

Handler implementation

Service implementation


If you want “CircusService” to work with a TigerHandler and a LionHelper (instead of TigerHelper) as the business logic in “TigerHelper” is deprecated, you need to

Change #1 Modify the “TigerHandler” class as it depends directly on the “TigerHelper” implementation.

Change “private TigerHelper helper;” TO: “private LionHelper helper;”

Change the constructor “public TigerHandler(TigerHelper helper) {” TO: “public TigerHandler(LionHelper helper) {”

Change #2 Modify the “CircusService” class as shown below.


As “TigerHandler” and “TigerHelper” are tightly coupled. If you apply the DIP (i.e. coding to interface), you don’t have to make any changes to the “TigerHandler” to use a “LionHelper”. In fact the “CircusService” is also tightly coupled to the “TigerHandler” as you need to make structural modifications to get it to work with a “LionHandler”.

#2. Loosely coupled “CircusService”

by “coding to interface” as shown below.

DIP - Dependency Inversion Principle

DIP – Dependency Inversion Principle

Define interfaces

Helper implementation coded to interface

Handler implementation coded to an interface

Service implementation that can swap helpers without having to modify the handler

Now to get the “TigerHandler” to work with a “LionHelper”, you only have to change just one line:

#3. Singleton factory classes to wire up dependencies

If you have multiple callee classes other than “CircusService” like “SancturyService”, “HuntingService”, “WildlifeService”, etc then all the callee classes need to be modified to get a “LionHelper” to work with a “TigerHandler”. This is tight coupling of callees with the AnimalHandler class. This can be improved with the help of a “singletonfactory class as shown below.

Revised “CircusService” class using the factory class

Now, if any dependencies change like “TigerHandler” depending on a “LionHelper”, etc, change it only in the factory, and not in all the callees like “CircusService”, “SancturyService”, “HuntingService”, “WildlifeService”, etc.

What is DI (aka Dependency Injection)?

Dependency Injection (DI) is all about injecting a class’s dependencies into it at runtime. This is based on the “Dependency Inversion Principle” by defining the dependencies as interfaces, and then injecting in a concrete class implementing that interface to the constructor (or via setter methods). This allows you to swap different implementations without having to make any structural changes.

The “Dependency Injection” pattern also promotes high cohesion via the Single Responsibility Principle (SRP) since your dependencies are individual objects, which perform discrete specialized tasks like “handling animals”, “helping animal handlers”, etc. I.e. clear separation of responsibilities.

How does a “DI pattern” differ from a “Factory pattern”?

Dependency Injection is more of an architectural pattern for loosely coupling software components. Factory pattern is one of the ways to implement “Dependency Injection”.

A typical commercial project will have 300+ classes, and you will need to create too many singleton factory classes. When you use IoC containers like Spring IoC, Guice, JEE6+ CDI (Context & Dependency Injection), etc you can wire up the whole application dependencies with a lot fewer configuration files in Java & XML.

So, DI is in many ways like a configurable Factory Pattern.

#4. Spring as the IoC container to wire up classes via DI

Spring Ioc inverts the “flow of control” by taking the task of wiring up the dependencies at runtime.

Step 1: Use Spring framework. The pom.xml file will bring in the Spring dependency jar files.

Step 2: Create an xml “applicationContext.xml“, which wires up the dependencies. This is the replacement for the factory class “AnimalHandlerFactory”.

The helper is injected into handler via constructor injection in Spring.

Step 3: The revised “CircusService” class using the Spring IoC container for DI instead of the factory class “AnimalHandlerFactory”.

Q. Why DI frameworks are better than factory classes?

Flexibility & easier maintenance as you don’t have to create too many factory classes, and also when using a DI framework, you are outsourcing the responsibility of wiring up the classes to an external framework like Spring, which is separate from your code.

The DI Frameworks give you flexibility of how to register your abstractions against your concrete types with Java Config, XML and Annotations.

Q. What is an IoC container?

The Inversion of Control Container (IoC) is a container that supports Dependency Injection. An IoC container defines what concrete classes should be used for what dependencies throughout your application. This brings in an added flexibility through looser coupling, and makes it much easier to change what dependencies are used.

The basic concept of the Inversion of Control pattern is that you do not create your objects but describe how they should be created. You don’t directly connect your components and services together in the code, but describe which services are needed by which components via configuration files in XML/Java and annotations. The IoC container is then responsible for hooking it all up. The objects are given their dependencies at creation time by some external entity that coordinates each object in the system.

Q. What are the benefits of IoC containers like Spring or JEE CDI?

The real power of DI and IoC is realized in its ability to replace the compile time binding of the relationships between classes with binding those relationships at runtime. For example, in Seam framework, you can have a real and mock implementation of an interface, and at runtime decide which one to use based on a property, presence of another file, or some precedence values. This is incredibly useful if you think you may need to modify the way your application behaves in different scenarios/environments.

Another real benefit of DI and IoC is that it makes your code easier to unit test. You can inject mock implementations into your unit test classes. Junit with Mockito tutorial to fully mock the DAO layer

There are other benefits like promoting looser coupling without any proliferation of factory and singleton design patterns, follows a consistent approach for lesser experienced developers to follow, etc. These benefits can come in at the cost of the added complexity to your application and has to be carefully manged by using them only at the right places where the real benefits are realized, and not just using them because many others are using them.

Note: The CDI (Contexts and Dependency Injection) is standard on Dependency Injection. CDI is a part of the Java EE 6 stack, meaning an application running in a Java EE 6 compatible container can leverage CDI out-of-the-box. Weld is the reference implementation of CDI.

Q2. In your experience, why would you use Spring framework?
A2. Spring framework has been very popular and been filling the gap in the Java EE stack for a number of years now. Java EE has finally caught up with DI, AOP, batch jobs, Managed Beans, etc promoting POJO driven development with JEE 6 and 7, hence for new Java EE projects JEE 6 or 7 might be the way to go. If you are after enhancing the current projects developed in Spring, or you are in more favor of Spring then this framewok has been popular for a number of reasons compared to pre JEE 6.

1) A key design principle in Spring in general is the “Open for extension, closed for modification” principle. So, some of the methods in the core classes are marked “final”.

2) Spring is an IoC container that supports both constructor injection (supplying arguments to constructors) and setter-based injection (calling setters on a call) to get the benefits of Dependency Injection (DI) like loose coupling, easier to test, etc.

3) It is modular with spring core, spring batch, spring mvc, spring orm, etc. When Spring was out, it was only a small core with IoC container and it was fast and easy to use. Now, I cannot even count how many Spring Components are available today. Spring Boot is a new framework to address this complexity by simplifying the bootstrapping and development of a new Spring application.

4) It is very matured and has been used in many large Java/JEE applications.

5) It supports AOP to implement cross cutting concerns.

6) It supports transaction management.

7) It promotes uniformity to handle exceptions (e.g. checked vs unchecked) and integration with JMS, JDBC, JNDI, remoting, etc.

8) Closes the JDBC/JMS connection resources automatically without having to you write verbose try/catch/finally blocks with logic to close the resources.

Q3. In your experience, what do you don’t like about Spring? Are there any pitfalls?

1) Spring has become very huge and bulky. So, don’t over do it by using all its features because of the hype that Spring is good. Look at what parts of Spring really provides some benefits for your project and use those parts. In most cases, it is much better to use proven frameworks like “Spring Boot” than create your own equivalent solution from a maintenance and applying the best practices perspective.

2) Spring MVC is probably not the best Web framework. There are JavaScript based alternatives like angular js, react js, etc. You will be using Spring with “Spring Boot” for building the microservices. Evaluate & benchmark other alternative frameworks like Dropwizard, Vertx, etc.

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