@GetMapping for GET requests in Spring Boot - Mini Project: Build & Apply

Using @GetMapping for GET requests in Spring Boot

📖 Scenario: You are building a simple Spring Boot web application that responds to HTTP GET requests. This is like setting up a friendly receptionist who listens for specific questions and gives back answers.

🎯 Goal: Create a Spring Boot controller with a method that handles GET requests at the URL /hello and returns a greeting message.

📋 What You'll Learn

Create a Spring Boot controller class named HelloController

Add a method named sayHello that returns a String

Use the @GetMapping annotation to map the method to the URL path /hello

Return the exact string Hello, Spring Boot! from the method

💡 Why This Matters

🌍 Real World

Web applications often need to respond to user requests by sending back data or messages. Using @GetMapping in Spring Boot helps you create endpoints that listen for GET requests and respond accordingly.

💼 Career

Understanding how to handle GET requests with @GetMapping is essential for backend developers working with Spring Boot to build RESTful APIs.

1

Create the controller class

Create a public class named HelloController and annotate it with @RestController.

Spring Boot

@RestController
public class HelloController {
    // Your code here
}

Hint

Use @RestController to make the class a REST controller.

2

Add the GET mapping method

Inside HelloController, add a public method named sayHello that returns a String. Annotate it with @GetMapping("/hello").

Spring Boot

import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.bind.annotation.GetMapping;

@RestController
public class HelloController {
    // Your code here
}

Hint

Use @GetMapping("/hello") above the method to map GET requests to /hello.

3

Return the greeting message

Modify the sayHello method to return the exact string "Hello, Spring Boot!".

Spring Boot

import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.bind.annotation.GetMapping;

@RestController
public class HelloController {
    @GetMapping("/hello")
    public String sayHello() {
        // Your code here
    }
}

Hint

Return the greeting message exactly as shown.

4

Complete the controller setup

Ensure the class imports org.springframework.web.bind.annotation.GetMapping and org.springframework.web.bind.annotation.RestController and that the method and class are properly closed with braces.

Spring Boot

import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.bind.annotation.GetMapping;

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

Hint

Check your imports and braces to make sure the code is complete and correct.

Practice

1. What is the main purpose of the @GetMapping annotation in a Spring Boot application?

easy

A. To map HTTP POST requests to specific controller methods

B. To map HTTP GET requests to specific controller methods

C. To configure database connections

D. To handle exceptions globally

Solution

1. Step 1: Understand HTTP methods in Spring Boot

   Spring Boot uses annotations to map HTTP methods like GET, POST, etc., to controller methods.

2. Step 2: Identify the role of @GetMapping

   @GetMapping specifically maps HTTP GET requests to the annotated method, so when a user visits a URL with a GET request, this method runs.

3. Final Answer:

   To map HTTP GET requests to specific controller methods -> Option B

4. Quick Check:

   @GetMapping = maps GET requests [OK]

Hint: Remember: @GetMapping is for GET requests only [OK]

Common Mistakes:

• Confusing @GetMapping with @PostMapping
• Thinking @GetMapping handles POST or PUT requests
• Assuming it configures database or exceptions

2. Which of the following is the correct syntax to map a GET request to the URL /hello using @GetMapping in Spring Boot?

easy

A. @GetMapping(path = "/hello") public String greet() { return "Hi"; }

B. @GetMapping(/hello) public String greet() { return "Hi"; }

C. @GetMapping('/hello') public String greet() { return "Hi"; }

D. @GetMapping(path = hello) public String greet() { return "Hi"; }

Solution

1. Step 1: Check the correct use of @GetMapping syntax

   @GetMapping requires the path to be a string, usually passed as path = "/url" or simply "/url".

2. Step 2: Identify correct string usage and syntax

   @GetMapping(path = "/hello") public String greet() { return "Hi"; } uses @GetMapping(path = "/hello") which is correct. @GetMapping(/hello) public String greet() { return "Hi"; } misses quotes around /hello. @GetMapping('/hello') public String greet() { return "Hi"; } uses single quotes, invalid for Java string literal. @GetMapping(path = hello) public String greet() { return "Hi"; } misses quotes around hello.

3. Final Answer:

   @GetMapping(path = "/hello") public String greet() { return "Hi"; } -> Option A

4. Quick Check:

   Correct syntax needs quotes and path attribute [OK]

Hint: Always use quotes around URL paths in @GetMapping [OK]

Common Mistakes:

• Omitting quotes around the path string
• Forgetting the leading slash in URL
• Using path without quotes causing syntax errors

3. Given the following Spring Boot controller method, what will be the output when a GET request is made to /welcome?

@GetMapping("/welcome")
public String welcome() {
    return "Welcome to Spring Boot!";
}

medium

A. An empty response

B. HTTP 404 Not Found error

C. "Welcome to Spring Boot!"

D. HTTP 500 Internal Server Error

Solution

1. Step 1: Understand the method mapping and return value

   The method is mapped to GET requests at "/welcome" and returns a plain string.

2. Step 2: Determine the response content

   Since the method returns a string, Spring Boot will send "Welcome to Spring Boot!" as the response body for the GET request.

3. Final Answer:

   "Welcome to Spring Boot!" -> Option C

4. Quick Check:

   GET /welcome returns method's string [OK]

Hint: Returned string is sent as response body by default [OK]

Common Mistakes:

• Expecting a view name instead of response text
• Confusing 404 error when mapping exists
• Assuming empty or error response without cause

4. Identify the error in the following Spring Boot controller method:

@GetMapping("/user/{id}")
public String getUser(@PathVariable int userId) {
    return "User ID: " + userId;
}

medium

A. The @PathVariable parameter name does not match the path variable name

B. The method should return a ResponseEntity instead of String

C. The @GetMapping path should not contain curly braces

D. The method parameter should be of type String, not int

Solution

1. Step 1: Compare path variable name and method parameter

   The path is defined as /user/{id} but the method parameter is named userId without specifying the variable name in @PathVariable.

2. Step 2: Understand @PathVariable name matching

   By default, the parameter name must match the path variable name or be explicitly specified like @PathVariable("id") int userId.

3. Final Answer:

   The @PathVariable parameter name does not match the path variable name -> Option A

4. Quick Check:

   Path variable names must match or be explicitly linked [OK]

Hint: Match @PathVariable name with URL placeholder or specify explicitly [OK]

Common Mistakes:

• Not matching method parameter name with path variable
• Assuming type mismatch causes error here
• Thinking curly braces are invalid in @GetMapping path

5. You want to create a Spring Boot controller method that handles GET requests to /product/{category}/{id} and returns a string combining both path variables. Which of the following method signatures correctly uses @GetMapping and @PathVariable annotations?

hard

A. @GetMapping("/product/{category}/{id}") public String getProduct(String category, int id) { return category + "-" + id; }

B. @GetMapping("/product/{category}/{id}") public String getProduct(@PathVariable String cat, @PathVariable int id) { return cat + "-" + id; }

C. @GetMapping("/product/{category}/{id}") public String getProduct(@PathVariable int category, @PathVariable String id) { return category + "-" + id; }

D. @GetMapping("/product/{category}/{id}") public String getProduct(@PathVariable("category") String cat, @PathVariable("id") int id) { return cat + "-" + id; }

Solution

1. Step 1: Check @GetMapping path and method parameters

   The path has two variables: {category} and {id}. The method must accept two parameters annotated with @PathVariable matching these names or explicitly linked.

2. Step 2: Analyze each option's parameter annotations and types

   @GetMapping("/product/{category}/{id}") public String getProduct(@PathVariable String cat, @PathVariable int id) { return cat + "-" + id; } uses parameter names that do not match path variables (cat != category), so fails without explicit names. @GetMapping("/product/{category}/{id}") public String getProduct(@PathVariable("category") String cat, @PathVariable("id") int id) { return cat + "-" + id; } explicitly links parameter names to path variables, which is safest. @GetMapping("/product/{category}/{id}") public String getProduct(@PathVariable int category, @PathVariable String id) { return category + "-" + id; } swaps types incorrectly. @GetMapping("/product/{category}/{id}") public String getProduct(String category, int id) { return category + "-" + id; } lacks @PathVariable annotations, so variables won't be bound.

3. Step 3: Choose the best correct and safe option

   @GetMapping("/product/{category}/{id}") public String getProduct(@PathVariable("category") String cat, @PathVariable("id") int id) { return cat + "-" + id; } explicitly names variables, avoiding errors if parameter names differ from path variables.

4. Final Answer:

   @GetMapping("/product/{category}/{id}") public String getProduct(@PathVariable("category") String cat, @PathVariable("id") int id) { return cat + "-" + id; } -> Option D

5. Quick Check:

   Explicit @PathVariable names ensure correct binding [OK]

Hint: Use @PathVariable("name") if parameter name differs from path variable [OK]

Common Mistakes:

• Omitting @PathVariable annotation
• Mismatching parameter types with path variables
• Not explicitly naming @PathVariable when names differ