Why annotations drive Spring Boot in Spring Boot - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to mark the class as a Spring Boot application.

Spring Boot

@[1]
public class MyApp {
    public static void main(String[] args) {
        SpringApplication.run(MyApp.class, args);
    }
}

Drag options to blanks, or click blank then click option'

AComponent

BSpringBootApplication

CService

DRepository

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Component instead of @SpringBootApplication

Forgetting to add any annotation on the main class

The @SpringBootApplication annotation marks the main class as the entry point for Spring Boot. It enables auto-configuration and component scanning.

2fill in blank

medium

Complete the code to inject a service bean into the controller using annotation.

Spring Boot

@RestController
public class MyController {
    private final MyService service;

    public MyController([1] MyService service) {
        this.service = service;
    }
}

Drag options to blanks, or click blank then click option'

A@Service

B@Inject

C@Component

D@Autowired

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Service instead of @Autowired on constructor parameter

Not annotating the constructor parameter at all

The @Autowired annotation tells Spring to inject the service bean into the constructor.

3fill in blank

hard

Fix the error in the code by completing the annotation to define a REST endpoint.

Spring Boot

@RestController
public class HelloController {

    @[1]("/hello")
    public String sayHello() {
        return "Hello, World!";
    }
}

Drag options to blanks, or click blank then click option'

APostMapping

BRequestMapping

CGetMapping

DService

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Service instead of a mapping annotation

Using @PostMapping for a GET endpoint

The @GetMapping annotation maps HTTP GET requests to the method, defining a REST endpoint.

4fill in blank

hard

Fill both blanks to define a service class and mark it as a Spring bean.

Spring Boot

[1]
public class UserService {

    public String getUser() {
        return "User1";
    }
}

Drag options to blanks, or click blank then click option'

A@Service

B@Repository

C@Component

D@Controller

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Controller or @Repository incorrectly for service classes

Not annotating the class at all

The @Service annotation marks the class as a service bean for Spring to manage.

5fill in blank

hard

Fill all three blanks to create a REST controller with a GET endpoint that returns a greeting.

Spring Boot

@[1]
public class GreetingController {

    @[2]("/greet")
    public String greet() {
        return [3];
    }
}

Drag options to blanks, or click blank then click option'

ARestController

BGetMapping

C"Hello from Spring Boot!"

DService

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Controller instead of @RestController

Using @PostMapping instead of @GetMapping

Returning a variable instead of a string literal

The @RestController marks the class as a REST controller. @GetMapping maps GET requests to the method. The method returns a greeting string.

Practice

1. What is the main role of annotations in Spring Boot?

easy

A. They replace the need for Java classes.

B. They tell Spring Boot how to configure and run the application.

C. They are used only for styling the user interface.

D. They slow down the application startup.

Solution

1. Step 1: Understand what annotations do in Spring Boot

   Annotations provide metadata that guides Spring Boot on how to set up and manage components automatically.

2. Step 2: Compare options with this understanding

   Only They tell Spring Boot how to configure and run the application. correctly states that annotations tell Spring Boot how to configure and run the app. Other options are incorrect or unrelated.

3. Final Answer:

   They tell Spring Boot how to configure and run the application. -> Option B

4. Quick Check:

   Annotations guide configuration = B [OK]

Hint: Annotations guide setup and running in Spring Boot [OK]

Common Mistakes:

• Thinking annotations replace Java classes
• Assuming annotations are for UI styling
• Believing annotations slow down startup

2. Which of the following is the correct way to mark a class as a Spring Boot application entry point?

easy

A. @ComponentScan

B. @EnableWebMvc

C. @SpringBootApplication

D. @RequestMapping

Solution

1. Step 1: Identify the annotation that marks the main Spring Boot app class

   The @SpringBootApplication annotation is used to mark the main class that starts the Spring Boot app.

2. Step 2: Review other options for correctness

   @EnableWebMvc enables web MVC but is not the main entry point; @ComponentScan scans components; @RequestMapping maps web requests. Only @SpringBootApplication is correct here.

3. Final Answer:

   @SpringBootApplication -> Option C

4. Quick Check:

   Main app class annotation = A [OK]

Hint: Main app class uses @SpringBootApplication [OK]

Common Mistakes:

• Confusing @EnableWebMvc as main app annotation
• Using @RequestMapping on main class
• Thinking @ComponentScan alone starts the app

3. Given this code snippet:

@RestController
public class HelloController {
  @GetMapping("/hello")
  public String sayHello() {
    return "Hello, Spring!";
  }
}

What will be the output when accessing /hello in a browser?

medium

A. The plain text 'Hello, Spring!'

B. A 404 Not Found error

C. A JSON object with message 'Hello, Spring!'

D. An HTML page with 'Hello, Spring!' inside <h1> tags

Solution

1. Step 1: Understand @RestController and @GetMapping behavior

   @RestController returns data directly as the response body, not a view. @GetMapping maps the URL '/hello' to the method.

2. Step 2: Determine the response content

   The method returns a String 'Hello, Spring!'. This will be sent as plain text in the HTTP response body.

3. Final Answer:

   The plain text 'Hello, Spring!' -> Option A

4. Quick Check:

   @RestController returns plain text = C [OK]

Hint: @RestController returns plain text or JSON, not HTML views [OK]

Common Mistakes:

• Expecting JSON output without @ResponseBody
• Assuming HTML page is returned
• Thinking URL mapping is missing

4. Consider this Spring Boot controller code:

@RestController
public class MyController {
  @GetMapping("/data")
  public String getData() {
    return null;
  }
}

What problem will occur when accessing /data?

medium

A. The server returns HTTP 204 No Content with empty body.

B. The server returns the string 'null' as plain text.

C. The server returns HTTP 404 Not Found because mapping is missing.

D. The server returns HTTP 500 Internal Server Error due to null response.

Solution

1. Step 1: Analyze the method returning null in @RestController

   Returning null from a controller method that expects a response body results in Spring returning HTTP 204 No Content with an empty body.

2. Step 2: Understand the effect on HTTP response

   Spring treats null return as no content, so it responds with HTTP 204 No Content instead of an error.

3. Final Answer:

   The server returns HTTP 204 No Content with empty body. -> Option A

4. Quick Check:

   Null return in @RestController causes 204 No Content = B [OK]

Hint: Returning null in @RestController results in 204 No Content [OK]

Common Mistakes:

• Expecting HTTP 204 No Content automatically
• Thinking null returns 'null' string
• Assuming 404 error due to missing mapping

5. You want to create a Spring Boot service that automatically scans and registers all components in the package com.example.app. Which annotation combination should you use on your main application class to achieve this?

hard

A. @SpringBootApplication
@EnableWebMvc(scanBasePackages = "com.example.app")

B. @SpringBootApplication
@ComponentScan("com.example.app")

C. @EnableAutoConfiguration
@ComponentScan("com.example.app")

D. @SpringBootApplication(scanBasePackages = "com.example.app")

Solution

1. Step 1: Understand how to specify component scanning in Spring Boot

   @SpringBootApplication supports a property scanBasePackages to define packages to scan for components automatically.

2. Step 2: Compare options for correct usage

   @SpringBootApplication(scanBasePackages = "com.example.app") uses @SpringBootApplication with scanBasePackages attribute correctly. @SpringBootApplication @ComponentScan("com.example.app") redundantly uses @ComponentScan which is unnecessary if scanBasePackages is set. @EnableAutoConfiguration @ComponentScan("com.example.app") misses @SpringBootApplication. @SpringBootApplication @EnableWebMvc(scanBasePackages = "com.example.app") misuses @EnableWebMvc which does not have scanBasePackages.

3. Final Answer:

   @SpringBootApplication(scanBasePackages = "com.example.app") -> Option D

4. Quick Check:

   Use scanBasePackages in @SpringBootApplication = A [OK]

Hint: Use scanBasePackages in @SpringBootApplication for scanning [OK]

Common Mistakes:

• Using @ComponentScan separately when scanBasePackages suffices
• Confusing @EnableWebMvc with scanning packages
• Omitting @SpringBootApplication annotation