30+ Java Spring Boot Developer Interview Questions and Answers

Explanation of Java features, Stream API, microservices, HTTP vs HTTPS, and SQL joins.

• Lambda expressions allow for concise syntax to implement functional interfaces.

• Stream API provides a way to process collections of objects in a functional style.

• Microservices architecture breaks down applications into smaller, independent services.

• HTTP is unencrypted protocol, HTTPS is encrypted using SSL/TLS for secure communication.

• Inner join combines rows from two tables based on a relate...read more

Spring is a framework that provides a comprehensive programming and configuration model for Java applications. Spring Boot is a framework that simplifies the setup and configuration of Spring applications.

• Spring is a larger framework that provides various modules for different functionalities like dependency injection, MVC, security, etc.

• Spring Boot is an opinionated framework that auto-configures the Spring application with sensible defaults, reducing the need for manual con...read more

Answers to common Java Spring Boot Developer interview questions.

• Java8 features include lambda expressions, functional interfaces, streams, and default methods.

• String is immutable in Java to ensure thread safety and prevent unintended modifications.

• Checked exceptions are checked at compile time, while unchecked exceptions are not. Example: IOException vs NullPointerException.

• throw is used to explicitly throw an exception, while throws is used to declare that a method may thro...read more

Functional interfaces provide clarity and enforce single abstract method constraint.

• Functional interfaces provide clarity to developers by explicitly indicating that the interface is intended to be used as a functional interface.

• Using @FunctionalInterface annotation helps enforce the single abstract method constraint, preventing accidental addition of more abstract methods.

• It also allows the compiler to perform additional checks to ensure that the interface meets the requirem...read more

Overview of Controller, Dependency Injection, Bean Lifecycle, and JPA in Spring Boot.

• Controller: Handles HTTP requests and responses, annotated with @Controller or @RestController.

• Dependency Injection: A design pattern that allows Spring to manage object creation and dependencies, using @Autowired.

• Bean Lifecycle: Involves phases like instantiation, dependency injection, initialization, and destruction, managed by Spring.

• JPA: A specification for accessing data in Java, often u...read more

SOLID principles guide software design for better maintainability and scalability. Singleton ensures a single instance of a class.

• S: Single Responsibility Principle - A class should have one reason to change.

• O: Open/Closed Principle - Software entities should be open for extension but closed for modification.

• L: Liskov Substitution Principle - Subtypes must be substitutable for their base types.

• I: Interface Segregation Principle - Clients should not be forced to depend on inte...read more

The Fibonacci series is a sequence where each number is the sum of the two preceding ones, starting from 0 and 1.

• The Fibonacci series starts with 0 and 1.

• Each subsequent number is the sum of the previous two.

• Example: For n=5, the series is 0, 1, 1, 2, 3.

• A simple implementation can be done using iteration or recursion.

A singleton class is a class that allows only one instance of itself to be created and provides a global point of access to that instance.

• Singleton classes are often used in Spring Boot for managing resources such as database connections or thread pools.

• In Spring Boot, singleton beans are created by default, meaning that only one instance of a bean is created and shared across the application.

• To create a singleton bean in Spring Boot, you can use the @Component annotation or ...read more

The question covers topics like method overriding, method overloading, exceptions, Java 8 features, and string manipulation.

• Method overriding is when a subclass provides a specific implementation of a method that is already provided by its superclass.

• Method overloading is when multiple methods have the same name but different parameters.

• Exception handling is used to handle errors that occur during the execution of a program.

• Java 8 introduced features like lambda expressions, ...read more

Rest controllers are specifically used for RESTful web services, while normal controllers are used for traditional web applications.

• Rest controllers are used to build RESTful web services that can accept and return JSON or XML data.

• Normal controllers are used for traditional web applications that render HTML views.

• Rest controllers use annotations like @RestController and @RequestMapping to define endpoints.

• Normal controllers use annotations like @Controller and @GetMapping to...read more

Logging in Spring Boot is a feature that allows developers to track and record events and errors that occur during application runtime.

• Spring Boot uses the popular logging framework, Logback, for its logging capabilities.

• Developers can configure logging levels and output formats in the application.properties or application.yml file.

• Logging can be used to track application performance, debug errors, and monitor user activity.

• Spring Boot also provides a convenient way to view a...read more

Memory leak is a situation where a program uses memory inefficiently, causing memory to be allocated but not released.

• Memory leak occurs when a program allocates memory but does not release it after it is no longer needed.

• Identifying memory leaks can be done using tools like profilers to analyze memory usage over time.

• Common signs of memory leaks include increasing memory usage over time, frequent garbage collection, and application crashes due to out-of-memory errors.

Comparable is an interface in Java used for comparing objects and sorting collections.

• Comparable interface is used to define a natural ordering of objects.

• To implement Comparable, a class must implement the compareTo() method.

• Example: Implementing Comparable interface in a class to compare instances based on a specific attribute.

Abstract classes, streams, and the static keyword are key concepts in Java for structuring code and managing data flow.

• Abstract Class: A class that cannot be instantiated and may contain abstract methods (without implementation). Example: `abstract class Animal { abstract void sound(); }`

• Streams: A sequence of data elements supporting sequential and parallel aggregate operations. Example: `List<String> names = Arrays.asList('John', 'Jane'); names.stream().filter(name -> name....read more

Stateless API does not store client state on the server, while stateful API does.

• Stateless API: Each request from client to server must contain all necessary information for the server to fulfill the request. No client state is stored on the server. Example: RESTful APIs.

• Stateful API: The server stores client state, which can be accessed by subsequent requests. Example: HTTP sessions.

Run Length Encoding compresses consecutive characters into counts followed by the character.

• Iterate through the string while counting consecutive characters.

• When a different character is encountered, append the count and character to the result.

• Example: For 'AABBBCCCC', the output is '2A3B4C'.

• Edge case: For an empty string, the output should also be an empty string.

This code demonstrates how to convert a string into a character array in Java.

• Use the `toCharArray()` method: `char[] chars = str.toCharArray();`

• Example: `String str = "Hello"; char[] chars = str.toCharArray();`

• Iterate through the array: `for (char c : chars) { System.out.println(c); }`

• You can also use `String.split()` for separating by delimiters, but it returns a String array.

String is immutable, String Buffer is mutable.

• String is immutable, meaning its value cannot be changed once it is created.

• String Buffer is mutable, meaning its value can be changed after it is created.

• String is faster and more memory efficient for simple string operations.

• String Buffer is slower but more efficient for operations that require frequent modifications.

Spring Actuator is a tool that provides endpoints to monitor and manage Spring Boot applications.

• Actuator provides health, metrics, info, and other endpoints

• It can be used to monitor and manage the application's health and performance

• Actuator endpoints can be customized and secured using Spring Security

• Examples of endpoints include /health, /metrics, /info, /env, and /shutdown

Java 8 provides a concise way to reverse a string using streams and collectors.

• Use the StringBuilder class: StringBuilder sb = new StringBuilder(originalString); sb.reverse();

• Utilize Java 8 Streams: String reversed = originalString.chars().mapToObj(c -> (char) c).reduce("", (s, c) -> c + s);

• Another approach is to convert the string to a char array, reverse it, and create a new string: char[] charArray = originalString.toCharArray();

HTTP status 204 means no content, while 404 means not found.

• HTTP status 204 indicates a successful request, but with no content to return.

• HTTP status 404 indicates that the requested resource could not be found on the server.

• HTTP status 204 is often used for successful DELETE requests where no content needs to be returned.

• HTTP status 404 is commonly seen when a user tries to access a non-existent URL.

Spring Security is a powerful and highly customizable authentication and access-control framework.

• Provides authentication and authorization support for web applications

• Offers a wide range of security features such as password hashing, CSRF protection, and session management

• Can be easily integrated with other Spring frameworks like Spring MVC and Spring Boot

• Supports multiple authentication mechanisms such as form-based, basic, and OAuth2

• Allows for custom security configuration...read more

Yes, Java is a statically typed language, meaning variable types are known at compile time.

• In Java, you must declare the type of a variable when you create it. Example: 'int number = 5;'

• Static typing helps catch type-related errors at compile time rather than at runtime.

• Java's type system includes primitive types (int, char, etc.) and reference types (objects).

• You cannot assign a value of one type to a variable of another type without explicit casting. Example: 'double d = 5;...read more

Spring is a framework for Java development, while Spring Boot simplifies Spring application setup and configuration.

• Spring requires extensive configuration, while Spring Boot uses convention over configuration.

• Spring Boot includes embedded servers (like Tomcat), eliminating the need for external server setup.

• Spring Boot provides starters, which are pre-configured dependencies for common tasks (e.g., spring-boot-starter-web).

• Spring applications can be complex and verbose, wher...read more

Dependency injection is a design pattern where the dependencies of a class are provided externally rather than being created within the class itself.

• Dependency injection helps in achieving loose coupling between classes.

• It allows for easier testing and maintenance of code.

• There are three types of dependency injection: constructor injection, setter injection, and interface injection.

• Example: In Spring Boot, dependencies are injected using annotations like @Autowired.

Spring is a framework for building Java applications, while Spring Boot is a tool for easily creating stand-alone, production-grade Spring-based applications.

• Spring is a framework that provides comprehensive infrastructure support for developing Java applications.

• Spring Boot is a tool that simplifies the process of creating and deploying Spring-based applications.

• Spring Boot includes embedded servers, which makes it easy to run applications as standalone JAR files.

• Spring Boot...read more

@Qualifier is used in Spring to resolve ambiguity when multiple beans of the same type exist in the application context.

• @Qualifier annotation helps in injecting specific beans when multiple candidates are available.

• It can be used with @Autowired to specify which bean to inject.

• Example: If you have two beans of type 'DataSource', use @Qualifier("myDataSource") to specify which one to use.

• It can also be used with constructor injection, setter injection, and method parameters.