@GetMapping for GET requests in Spring Boot - Step-by-Step Execution

Concept Flow - @GetMapping for GET requests

Client sends GET request

↓

Spring Boot matches URL

↓

@GetMapping method called

↓

Method executes and returns data

↓

Spring Boot sends response back to client

This flow shows how a GET request from a client is handled by a Spring Boot controller method annotated with @GetMapping, which returns data back to the client.

Execution Sample

Spring Boot

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

This code defines a method that handles GET requests to '/hello' and returns a greeting string.

Execution Table

Step	Action	Input/Condition	Result/Output
1	Client sends GET request	GET /hello	Request received by Spring Boot
2	Spring Boot matches URL	URL = /hello	Matches @GetMapping("/hello") method
3	Call method sayHello()	No parameters	Method executes
4	Method returns	Return value = "Hello, World!"	Response body set to "Hello, World!"
5	Spring Boot sends response	Response body = "Hello, World!"	Client receives "Hello, World!"
6	End	No more actions	Request handling complete

💡 Request handled successfully and response sent back to client

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
requestUrl	null	/hello	/hello	/hello
methodReturnValue	null	null	"Hello, World!"	"Hello, World!"

Key Moments - 3 Insights

Why does the method execute only for GET requests?

Because @GetMapping specifically maps HTTP GET requests to the method, as shown in execution_table step 2 where Spring Boot matches the URL and HTTP method.

What happens if the URL does not match any @GetMapping?

Spring Boot will not call the method and will return a 404 Not Found error, since no matching handler is found.

Can the method return other types besides String?

Yes, the method can return objects, which Spring Boot converts to JSON automatically if annotated properly.

Visual Quiz - 3 Questions

Look at the execution table, what is the methodReturnValue after step 4?

A "Hello, World!"

B null

C /hello

D GET /hello

💡 Hint

Check the variable_tracker row for methodReturnValue at After Step 4

At which step does Spring Boot match the URL to the @GetMapping method?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the execution_table Action column for URL matching

If the client sends a POST request instead of GET, what happens?

A The method returns "Hello, World!"

B The @GetMapping method is called anyway

C Spring Boot returns 404 Not Found

D The server crashes

💡 Hint

Refer to key_moments about URL and method matching

Concept Snapshot

@GetMapping maps HTTP GET requests to controller methods.
Use @GetMapping("/path") above a method to handle GET requests to that path.
The method runs when a matching GET request arrives.
Return values become the response body.
If no match, Spring Boot returns 404.
Works for simple strings or objects (auto JSON).

Full Transcript

When a client sends a GET request to a Spring Boot server, the framework looks for a method annotated with @GetMapping that matches the request URL. If found, it calls that method. The method runs and returns a value, such as a string. Spring Boot then sends this value back as the response body to the client. If no matching method is found, the server returns a 404 error. This process ensures that GET requests are handled cleanly and simply by methods marked with @GetMapping.

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