The Top Java Web Developer Interview Questions You Need to Know

Java web development is an in-demand skill for backends websites, web apps and more. As a Java web developer, you need to master both Java and web development basics.

This means being ready for common Java interview questions on topics like:

• Java language features
• OOP concepts
• JVM, JRE, JDK
• Multithreading
• Garbage collection
• Exception handling

You also need to know web development topics like

• HTTP
• REST APIs
• JSON/XML
• Web frameworks like Spring and Hibernate
• Web servers like Tomcat
• Testing web apps

This article will cover the most frequently asked Java web developer interview questions so you can ace your next interview

Java Interview Questions

Let’s start with some common Java interview questions:

What is Java?

Java is an object-oriented programming language developed by Sun Microsystems (now Oracle) in the 1990s. It is a compiled language that runs on the Java Virtual Machine (JVM). Java is known for being platform-independent, secured, robust and multi-threaded.

Why is Java platform-independent?

Java is platform-independent because the Java code is compiled into an intermediate bytecode format that is then interpreted by the JVM on any platform. This allows Java code to run on any device with a JVM, like Windows, Linux, macOS, smartphones and more.

What are the differences between C++ and Java?

Some key differences between C++ and Java:

• C++ is compiled to native machine code, Java is compiled to bytecode that runs on the JVM.
• Java does not support pointers, structures, unions or operator overloading like C++.
• Java fully supports OOP with no global variables or functions. C++ supports OOP but still has global functions.
• Java exception handling is easier with try/catch blocks. C++ requires header files and helper functions.
• Java has automatic memory management with garbage collection. C++ requires manual memory management.

Why is Java not a pure object-oriented language?

Java has 8 primitive data types – char, boolean, byte, short, int, long, float and double. These are not objects, so Java is not a pure object-oriented language. But other than the primitives, everything else in Java is an object.

What are some key features of Java?

• Simple: Easy to learn syntax with a C/C++ like syntax.
• Object-oriented: Fully supports OOP with classes, inheritance, polymorphism, abstraction and encapsulation.
• Platform independent: Java bytecode runs on any platform with a JVM.
• Secured: Java programs run inside a virtual machine sandbox and cannot access system resources directly.
• Robust: Java uses strong memory management with no pointer arithmetic.
• Multi-threaded: Java supports concurrent programming with threading built into the language.

What do you get in the Java JDK download?

The Java Development Kit (JDK) includes:

• JRE (Java Runtime Environment): Libraries and JVM needed to run Java programs.
• Development tools: Compiler (javac), debugger (jdb), JAR, documentation generator etc.
• API docs: Java API documentation for all built-in classes.

Explain JVM, JRE and JDK

• JVM: Java Virtual Machine that interprets and runs Java bytecode.
• JRE: Java Runtime Environment provides core Java libraries and JVM needed to run Java code.
• JDK: Java Development Kit contains JRE + development tools like compiler, debugger etc.

These cover the basics of the Java language itself. Now let’s move on to web development topics.

Web Development Interview Questions

Here are common interview questions on web development with Java:

Explain HTTP request methods GET vs POST.

• GET requests data from the server. Parameters are sent in the URL.
• POST submits data to the server for creation/update. Parameters are sent in request body.
• GET is idempotent, can be cached, bookmarked, pierced. POST is non-idempotent.

How does HTTP work?

The HTTP protocol works on a request-response model:

1. Client opens a connection and sends an HTTP request to the server.
2. Server processes the request and sends back an HTTP response.
3. The connection is closed once the response is received by the client.

HTTP is stateless, so each request-response is executed independently.

What is REST and RESTful web services?

REST (Representational State Transfer) is an architectural style for building web APIs that uses HTTP methods explicitly and follows REST constraints:

• Client–server – Separate client and server concerns
• Stateless – No client context on server between requests
• Cacheable – Responses must be cacheable
• Uniform interface – Resources accessed in same manner
• Layered system – Client doesn’t know if connected directly to end server
• Code on demand (optional) – Transfer executable code when required

RESTful web services follow the REST architectural constraints for building web APIs.

Compare REST and SOAP web services

What are some advantages of using REST?

• Lightweight, more scalable
• Faster performance
• Easy to build and consume
• Works well for public APIs over the internet
• Easier integration with JavaScript/frontends

Explain stateless and stateful web services.

• Stateless: No client state is stored on the server. Each request is independent. REST APIs are stateless.
• Stateful: Client state persists on the server across requests. Useful for workflows or transactions. SOAP web services can be stateful.

Stateless services are easier to scale and work better for internet-based services. Stateful services allow transactions and workflows spanning multiple requests.

How do you secure REST APIs?

Some ways to secure REST APIs:

• Require API key authentication
• Use HTTPS instead of HTTP
• OAuth2 for authorization and access control
• JSON Web Tokens (JWT) to securely transmit user identity
• Rate limiting to prevent abuse/DDoS attacks

What are some popular Java web frameworks?

Some widely used Java web frameworks include:

• Spring MVC – Model-View-Controller framework for building web apps
• JSF – JavaServer Faces framework to build server-side UIs
• Struts – MVC framework for web apps based on Servlets/JSP
• Play – For scalable and lightweight web apps
• Blade – Lightweight and high performance web framework
• Vaadin – Framework for modern web UIs and apps
• Grails – Groovy-based web framework similar to Ruby on Rails
• Jersey – JAX-RS reference implementation for RESTful web services

Explain the architecture of a typical Java web application

A typical Java web app architecture has:

• Client-side: HTML/CSS/JavaScript runs on browser
• Web server: Apache Tomcat, Jetty etc. listen for requests
• Servlet container: Forwards requests to Servlets/JSPs
• Controller: Servlet/Filter handles requests and routing
• Model: POJOs represent data and business logic
• Persistence: Database like MySQL via JDBC, JPA
• View: JSPs generate HTML response sent back to client

What is dependency injection? When is it used?

Dependency injection (DI) passes required dependencies to a class instead of having it create them. This decouples classes from their dependencies and makes code more testable and maintainable.

For example, instead of a ShoppingCart class directly instantiating a DiscountCalculator class, the calculator can be passed via constructor injection:

public ShoppingCart(DiscountCalculator calculator) {  this.calculator = calculator; }

Now ShoppingCart is not tightly coupled to DiscountCalculator and unit testing is easier using mock objects.

How do you test web applications and APIs?

Some ways to test web apps and APIs:

• Unit testing components in isolation
• Integration testing assembling components
• Functional/acceptance testing against requirements
• Automated UI testing for web pages
• Load and performance testing for scalability
• Security testing like SQL injection, XSS, CSRF etc.
• Mocking dependencies for fast and reliable unit testing

What are some tools used for API testing?

Some popular API testing tools:

• Postman – Very popular GUI-based API testing
• JMeter – Load testing and performance testing web apps
• Cucumber – BDD testing framework, works well with Selenium
• Karate – API test automation framework
• REST Assured – Java DSL for API testing
• SoapUI – Functional testing of SOAP and REST APIs