Here is the Ultimate 100-Question Master List for Spring Boot & JUnit.
This list combines Development (Spring Boot) and Quality Assurance (JUnit/Mockito). If you master these, you are technically over-prepared for any 0–3 year experience interview in 2025.
🚀 Part 1: Spring Boot Core & Architecture
| # |
Question |
Key Concept |
| 1 |
What is Spring Boot? |
Opinionated view of Spring; "Convention over Configuration". |
| 2 |
Difference: Spring Framework vs. Spring Boot? |
Spring = Engine; Boot = Car (Pre-configured, Embedded Server). |
| 3 |
What are the main advantages of Spring Boot? |
Rapid dev, No XML, Embedded Server, Production ready. |
| 4 |
What is Auto-Configuration? |
Boot scans jars on classpath & configures beans automatically. |
| 5 |
What does @SpringBootApplication do? |
Combines @Configuration, @EnableAutoConfiguration, @ComponentScan. |
| 6 |
What is a "Starter" dependency? |
A bundle of dependencies (e.g., starter-web has Tomcat + MVC). |
| 7 |
How does dependency management work? |
managed by spring-boot-dependencies (BOM) to match versions. |
| 8 |
What is the default embedded server? |
Apache Tomcat. |
| 9 |
Can we change the embedded server? |
Yes (exclude Tomcat, add Jetty or Undertow). |
| 10 |
What is SpringApplication.run()? |
Bootstraps the app, starts context, starts Tomcat. |
| 11 |
JAR vs WAR in Spring Boot? |
JAR = Embedded server (Self-contained). WAR = External server. |
| 12 |
Why is Spring Boot good for Microservices? |
Lightweight, independent deployment, cloud-native ready. |
| 13 |
What is Spring Initializr? |
Web tool (start.spring.io) to generate project structure. |
| 14 |
What is the Spring Boot CLI? |
Command line tool to run Groovy scripts (rarely used now). |
| 15 |
What is the default Logging framework? |
Logback (via SLF4J). |
⚙️ Part 2: Configuration, DI & Beans
| # |
Question |
Key Concept |
| 16 |
What is application.properties? |
Central configuration file (Port, DB URL, etc.). |
| 17 |
application.properties vs application.yml? |
YML is hierarchical/structured; Properties is flat (key=value). |
| 18 |
What is Dependency Injection (DI)? |
Spring provides objects (beans) to your class; you don't use new. |
| 19 |
What is Inversion of Control (IoC)? |
Transferring control of object creation from programmer to Spring. |
| 20 |
Constructor vs Setter vs Field Injection? |
Constructor is best (easier for testing, ensures immutability). |
| 21 |
What does @Autowired do? |
Tells Spring to inject a bean here. |
| 22 |
@Component vs @Service vs @Repository? |
All are beans. Service = Biz Logic, Repository = DB Exception translation. |
| 23 |
Difference: @Controller vs @RestController? |
Controller = Returns View/HTML. RestController = Returns Data/JSON. |
| 24 |
What is the @Bean annotation? |
Used in @Configuration classes to manually declare a bean. |
| 25 |
What is the default Scope of a Bean? |
Singleton (One instance per application). |
| 26 |
What is Prototype Scope? |
New instance created every time it is requested. |
| 27 |
What is @Lazy initialization? |
Bean is created only when first requested, not at startup. |
| 28 |
What is @Value? |
Injects property values (strings/numbers) from config file. |
| 29 |
What is @ConfigurationProperties? |
Type-safe binding of properties to a Java class (better than @Value). |
| 30 |
What are Profiles (dev, prod)? |
Separate configs for environments (application-dev.properties). |
| 31 |
How to activate a Profile? |
spring.profiles.active=dev. |
| 32 |
What is @PostConstruct? |
Runs logic immediately after bean creation/DI is done. |
| 33 |
What is @PreDestroy? |
Runs logic just before the bean is removed (cleanup). |
| 34 |
What is a Circular Dependency? |
A needs B, B needs A. Fix using @Lazy. |
| 35 |
What is Spring Boot DevTools? |
Enables hot-swap/live-reload during development. |
🌐 Part 3: Web (REST API) & Exception Handling
| # |
Question |
Key Concept |
| 36 |
Dependency for REST API? |
spring-boot-starter-web. |
| 37 |
What does @RequestMapping do? |
Maps HTTP requests (URL) to Handler methods. |
| 38 |
Methods: Get, Post, Put, Delete? |
Get=Read, Post=Create, Put=Update, Delete=Remove. |
| 39 |
@RequestParam vs @PathVariable? |
PathVar = /user/1 (Clean URL). ReqParam = /user?id=1 (Filter/Sort). |
| 40 |
What is @RequestBody? |
Maps JSON body to Java Object (Deserialization). |
| 41 |
What is ResponseEntity? |
Wrapper to customize HTTP Status, Headers, and Body. |
| 42 |
Return specific Status Code (e.g. 201)? |
return new ResponseEntity<>(HttpStatus.CREATED);. |
| 43 |
What is a DTO? Why use it? |
Data Transfer Object. Decouples DB Entity from API response. |
| 44 |
How to handle Exceptions Globally? |
Use @ControllerAdvice + @ExceptionHandler. |
| 45 |
@ExceptionHandler vs @ControllerAdvice? |
ExceptionHandler = Local (one controller). Advice = Global. |
| 46 |
What is Swagger/OpenAPI? |
Library (springdoc-openapi) to auto-generate API docs & UI. |
| 47 |
Difference: put vs patch? |
Put = Replace entire object. Patch = Partial update. |
| 48 |
What is Serialization? |
Converting Java Object -> JSON (Jackson library does this). |
| 49 |
How to validate input? |
Use @Valid and Javax annotations (@NotNull, @Size). |
| 50 |
How to consume external APIs? |
RestTemplate (Legacy) or WebClient (Modern). |
💾 Part 4: Data (JPA, DB & Transactions)
| # |
Question |
Key Concept |
| 51 |
What is Spring Data JPA? |
Layer on top of Hibernate to reduce boilerplate code. |
| 52 |
What is JpaRepository? |
Interface providing CRUD methods (save, findAll) out of the box. |
| 53 |
What is the H2 Database? |
In-memory DB. Data is lost when app stops. Good for testing. |
| 54 |
spring.jpa.hibernate.ddl-auto? |
create, update, validate, none (Schema management). |
| 55 |
What are Derived Query Methods? |
findByName(String name) -> Auto-generates SQL. |
| 56 |
What is @Query? |
Used to write custom SQL or JPQL queries. |
| 57 |
What does @Transactional do? |
Ensures atomicity. If error occurs, rolls back all DB changes. |
| 58 |
What is Lazy Loading in JPA? |
Associated data (e.g., Orders of a User) loads only when accessed. |
| 59 |
N+1 Problem in Hibernate? |
1 Query for Parent, N queries for Children. Fix using JOIN FETCH. |
| 60 |
How to configure DB password? |
In application.properties (spring.datasource.password). |
🛡️ Part 5: Advanced (Security, Ops, AOP)
| # |
Question |
Key Concept |
| 61 |
What is Spring Boot Actuator? |
Production monitoring endpoints (/health, /metrics, /info). |
| 62 |
How to enable Actuator? |
Add spring-boot-starter-actuator. |
| 63 |
Basic Security setup? |
Add starter-security. Default: Basic Auth (User + Generated Pwd). |
| 64 |
Authentication vs Authorization? |
Auth = Who are you? (Login). AuthZ = What can you do? (Roles). |
| 65 |
What is AOP? |
Aspect-Oriented Programming. Separates cross-cutting concerns (Logs). |
| 66 |
AOP: Advice vs Pointcut? |
Advice = What to do. Pointcut = Where to apply it. |
| 67 |
How to schedule tasks? |
@EnableScheduling + @Scheduled(cron = "..."). |
| 68 |
How to run Async methods? |
@EnableAsync + @Async (Runs in separate thread). |
| 69 |
What is Project Lombok? |
Auto-generates Getters/Setters/Constructors to reduce code. |
| 70 |
CORS in Spring Boot? |
Cross-Origin Resource Sharing. Configured via @CrossOrigin. |
🧪 Part 6: JUnit 5 (Testing Core)
| # |
Question |
Key Concept |
| 71 |
What is JUnit 5 (Jupiter)? |
The standard testing framework for Java. |
| 72 |
Why Unit Test? |
Catch bugs early, ensure code quality, safe refactoring. |
| 73 |
@Test annotation? |
Marks a method as a test case. |
| 74 |
@BeforeEach vs @BeforeAll? |
Each = Before every test method. All = Once per class (Static). |
| 75 |
@AfterEach vs @AfterAll? |
Cleanup methods (after every test vs once at end). |
| 76 |
How to ignore a test? |
@Disabled. |
| 77 |
What is @DisplayName? |
Custom name for test in reports (e.g., "Should return 404"). |
| 78 |
assertEquals vs assertSame? |
Equals = Checks value (.equals()). Same = Checks object reference. |
| 79 |
How to test Exceptions? |
assertThrows(Exception.class, () -> service.method()). |
| 80 |
What is @ParameterizedTest? |
Run same test multiple times with different inputs (@ValueSource). |
| 81 |
What is assertTimeout? |
Fails if test takes longer than X seconds. |
| 82 |
Difference: Failure vs Error? |
Failure = Assertion failed. Error = Exception/Crash in code. |
| 83 |
What is the AAA Pattern? |
Arrange (Setup), Act (Call method), Assert (Verify result). |
| 84 |
What is Code Coverage? |
Percentage of code lines executed by tests (Aim for >80%). |
| 85 |
TDD (Test Driven Development)? |
Write Test first (Fail) -> Write Code (Pass) -> Refactor. |
🦜 Part 7: Mockito & Integration Testing (The Tricky Stuff)
| # |
Question |
Key Concept |
| 86 |
Why do we Mock? |
To isolate the unit. Don't hit real DB or API in Unit Tests. |
| 87 |
@Mock vs @InjectMocks? |
Mock = Creates fake dependency. InjectMocks = Target class accepting mocks. |
| 88 |
What is when(...).thenReturn(...)? |
Stubs behavior: "When repo.findById(1) is called, return User A". |
| 89 |
How to verify a call? |
verify(repo, times(1)).save(user). |
| 90 |
Mocking void methods? |
Cannot use when. Use doNothing().when(mock).method(). |
| 91 |
What is @Spy? |
Partial mock. Real object is called unless specific method is stubbed. |
| 92 |
@SpringBootTest vs @WebMvcTest? |
SpringBootTest = Full Context (Slow). WebMvcTest = Only Controllers (Fast). |
| 93 |
@Mock (Mockito) vs @MockBean (Spring)? |
Use @Mock in Unit tests. Use @MockBean to replace bean in Spring Context. |
| 94 |
What is MockMvc? |
Tool to simulate HTTP requests in Controller tests without starting Server. |
| 95 |
How to test Repository Layer? |
@DataJpaTest (Uses in-memory H2 DB automatically). |
| 96 |
What is ArgumentCaptor? |
Captures arguments passed to a mock to assert their values later. |
| 97 |
How to test private methods? |
Don't. Test the public method calling it. Or use Reflection (bad practice). |
| 98 |
Mocking Static Methods? |
Use MockedStatic (Available in Mockito 3.4+). |
| 99 |
What are Testcontainers? |
Docker-based testing. Spins up real DB (Postgres) for integration tests. |
| 100 |
Testing JSON responses? |
Use JsonPath ($.id) to verify specific fields in JSON string. |
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