What you should know about Spring before passing an Interview (Basics)

Introduction

So you are about to pass an interview as Backend Spring developer and you are wondering what are
the things that I should know about. Then you are in the right place. This will be a good start for you.
This will be a work in-progress, in which I will try to add as many questions and answers as possible.
Interested. Let's dive in!

The Spring Ecosystem

As a rule of thumb, Spring is too big to cover, that's why, I believe that you didn't include in your resume that you master Spring EcoSystem without specifying what frameworks and modules you mean. If you do so, you are screwed! and don't blame anyone for not picking up you, though that is another topic. That's why we will try to cover the most used frameworks and modules.

Speaking about a modern java-based enterprise application, you will mostly need:

Spring Core: Ioc (Inversion of Control) container and Spring AOP (Aspect oriented programming)
Spring WEB: Spring MVC, RestFul Web Services and Spring WebSocket (it depends)
Spring Test: with JUnit and Mockito
Spring Boot: less configuration, more focus on business logic
Spring Security: Who doesn't need that! a Powerful framework to manage authentication and access-control
Spring Data JPA: an easy way to implement JPA based repositories
Spring Social: Connect your application with known social providers, Google, Facebook and Twitter and Others.

First things first: Basics

What's JEE:

JEE or Java Enterprise Edition is a collection of specifications, APIs and technologies maintained by JCP(Java Community Process) designed to support Enterprise Applications which usually are large-scale, distributed, difficult to maintain and mission-critical.

The so-called application servers, like WildFly (formely known as JBoss), GlassFish, etc. are implementations of these specifications.

What's Spring?

Spring means different things in different contexts. In the old days, people refer to Spring as a framework to handle Dependency Injection. Over time, Spring has been much more improved to become an Ecosystem that provides everything you need to embrace the java in an enterprise environment. Nowadays, Spring refer to the whole family of Spring Projects and modules.
You should know that Spring maintain and develop it's framework around what's called Projects. Inside any Project, you will find different sub-projects maintained either by the community or the Spring developers. Every Project depends on different modules. refer to this article for a more thorough explanation about they relate to each other.

Let's have a look inside spring social project, the project contains different sub-projects:

Spring Social Facebook
Spring Social Twitter
Spring Social Linkden

as well as different community projects, they are by tens:

Spring Social DropBox
Spring Social Google

Spring and JEE:

Now you might wonder, how does JEE relate to Spring? in both definitions, I didn't mention one another!

That's quite true! because Spring isn't an application server nor an implementation of JEE.
In fact, JEE and Spring are competing for enterprise java. Spring doesn't necessary require JEE to run.
As an example, Tomcat, which is an implementation of Servlet and JSP and also provide support for different view technologies including and not limited to Thymeleaf, is more than sufficent to run All Spring Ecosystem.

Dependency Injection: Spring IOC vs CDN
AOP: Spring AOP vs interceptor
Presentation Framework: Spring MVC vs JSF
Security: Spring Security vs JAAS/JASPIC
Testing: Spring Testing vs Arquilian
Spring Beans vs EJB

What's Dependency of Injection

So far, the simplest definition that I've found for dependency injection is the following:
Basically, instead of having your objects creating a dependency or asking a factory object to make one for them, you pass the needed dependencies in to the object externally, and you make it somebody else's problem. This "someone" is either an object further up the dependency graph, or a dependency injector (framework) that builds the dependency graph. A dependency as I'm using it here is any other object the current object needs to hold a reference to.

In Spring terms, instead of creating your objects with the new operator, let The IoC Container handle them for you. All you need to do, is to @Autowired your object and define the referenced object with an xml file, with annotations or with java configuration.

What's the IoC Container

The Spring IoC container is responsible for instantiating, configuring and assembling objects known as beans, as well as managing their lifecycle. It is represented by the interface ApplicationContext.

How to instantiate Spring Ioc Container?

As a general rule of thumb, use ApplicationContext unless otherwise you have a good reason not to do so. Why? because ApplicationContext is built on top of BeanFactory and it offers what BeanFactory provides and other more enterprise-specific functionality.

Define a bean:

Every Object created and maintained by Spring IoC is called a bean.

Define an Inner Bean:

Inner beans are beans that are defined within the scope of another bean. Thus, a <bean/> element inside the <property/> or <constructor-arg/> elements is called inner bean.

Outside an Xml configuration, I didn't see how to configure inner beans

What are Beans' scopes?

They are 6 predefined and supported beans:

Singleton: (Default) Scopes a single bean definition to a single object instance per Spring IoC container.
ProtoType: Scopes a single bean definition to any number of object instances.
Request: Scopes a single bean definition to the lifecycle of a single HTTP request
Session: Scopes a single bean definition to the lifecycle of an HTTP Session.
Application: Scopes a single bean definition to the lifecycle of a ServletContext.
WebSocket: Scopes a single bean definition to the lifecycle of a WebSocket.

Explain the bean lifecycle:

Instantiation: The Spring IoC will look for a bean definition inside a configuration xml file or annotation. After it will find it, it will create an instance of the bean.
Properties injection: After creation of that instance, dependency will be injected.
setBeanName(): This method of BeanNameAware interface will be called if a bean has implemented it. It sets the bean name.
setBeanClassLoader(): This method of BeanClassLoaderAware interface will be called if a bean has implemented it.
setBeanFactory(): This method of BeanFactoryAware interface will be called if a bean has implemented it. It provides the owning factory.
postProcessBeforeInitialization(): This method of BeanPostProcessor interface will be called if a bean has implemented it
@PostConstruct: The method annotated with this annotation will be called in this phase. Note that Spring recommends to use this annotation over implementing InitializingBean interface.
afterPropertiesSet(): This method of InitializingBean interface will be called if a bean has implemented it.
init-method: in case you have used an xml configuration, custom init method defined via this attribute will be called. (no need to define it, if you have used @PostConstruct or java based configuration @Bean(initMethod = "") ).
postProcessAfterInitialization(): This method of BeanPostProcessor interface will be called if a bean has implemented it.

At this stage, the bean is fully ready to be used.

@PreDestroy: The method annotated with this annotation will be called in this phase. Note that Spring recommends to use this annotation over implementing DisposableBean interface.
destroy(): This method of DisposableBean interface will be called if a bean has implemented it
destroy-method: in case you have used an xml configuration, custom destroy method defined via this attribute will be called. (no need to define it, if you have used @PreDestroy or java based configuration @Bean(destroyMethod = "") ).

List the different ways to Inject Dependencies in Spring:

From the Spring documentation, DI exists in two major variants: Constructor-based and Setter-based but field injection is also used when you apply @Autowired annotation on a field.

This is all done using Java Reflection. But we will not dig there, we will just follow Spring convention:

We focus on the plumbing so you can focus on the business logic.

How configuration metadata is passed to the Spring Container?

Basically, they are three ways to configre a Spring Application:

The old way: Xml-based
The new way: Annotation-based: still we need an Xml file with the following tag <context:annotation-config\>
The newest way: Java-based: no longer an Xml file is used.

What are Autowiring Modes?

In general they are four different autowiring modes:

Default: by reference. What I mean is that you need to define inside the Xml file the ref attribute.
byName: Spring looks for a bean with the same name as the property that needs to be autowired
byType: Allows a property to be autowired if exactly one bean of the property type exists in the container.
constructor: For example, if a bean definition is set to autowire by constructor in configuration file, and it has a constructor with one of the arguments of Student type, Spring looks for a bean definition named Student, and uses it to set the constructor's argument

Nowadays, most developers use Annotation based configuration. In a sense, we only use @Autowired annotation.

So bear in mind that

@Autowired uses byType autowiring by default. when you have multiple beans definitions and you need one and not the other use @Qualifier("Bean Name").
Most of the time, we are working with interfaces as best practice, and later on we implement them. So, let's suppose we have the following code:

public Interface Student { String getName(); }

@Component public class StudentImp implements Student{ @Override String getName() { return "Ilias"; }

public class SchoolService { @Autowired private Student studentImp; }

In this schenario, by defining the bean property name, studentImp, Spring matches that to the StudentImp implementation and injects that specific implementation.

What are the differences between `@Controller`, `@Component`, `@Repository`, `@Configuration`, `@Bean` and `@Service`

@Repository: is a marker for any class that fulfills the role or stereotype of a repository (also known as Data Access Object or DAO)
@Component: is a generic stereotype for any Spring-managed component. As a general rule of thumb, use this component if other annotations are not suited for your needs, since all of them serve a specific use case.
@Service: hold business logic and call method in repository layer.
@Controller: indicates that a particular class serves the role of a controller.
@Configuration: indicates that its primary purpose is as a source of bean definitions
@Bean: is used to indicate that a method instantiates, configures and initializes a new object to be managed by the Spring IoC container

What's the use of `@Import` and `@Required` annotations?

@Import simplifies container instantiation, as only one class needs to be dealt with, rather than requiring the developer to remember a potentially large number of @Configuration classes during construction
@Required simply indicates that the property must be populated, otherwise the container throws an exception.

What's AOP?

Aspect-Oriented Programming (AOP) complements Object-Oriented Programming (OOP) by providing another way of thinking about program structure. The key unit of modularity in OOP is the class, whereas in AOP the unit of modularity is the aspect.

What's an Aspect, a JoinPoint, a PointCut, Weaving and an Advice respectively?

An aspect is a common feature that's typically scattered across methods, classes, object hierarchies, or even entire object models. It is behavior that looks and smells like it should have structure, but you can't find a way to express this structure in code with traditional object-oriented techniques.

In Spring AOP, it is simply a class annotated with @Aspect.

A joinpoint is any point in your java program. The call of a method. The execution of a constructor the access of a field. All these are joinpoints. You could also think of a joinpoint as a particular Java event. Where an event is a method call, constructor call, field access etc.

In Spring AOP, a join point always represents a method execution.

A Pointcut are AOP's Regular expressions matching a particual join point. execution(public * *(..)) is an example of a point cut that matches the execution of all public methods.

Weaving is linking aspects with other application types or objects to create an advised object.

In Spring AOP, the weaving is performed at Run Time. Other frameworks might performed it at compile time.

An Advice is an action taken by an aspect at a particular join point. Think of it as an Event Handler. Different types of advice exist in Spring AOP including @Before, @After, etc.

Review this article for a complete explanation about AOP concepts