@RequestParam for query strings in Spring Boot - Step-by-Step Execution

Concept Flow - @RequestParam for query strings

HTTP GET Request with Query String

↓

Spring Boot Controller Method

↓

@RequestParam Annotation Reads Query String

↓

Parameter Value Assigned to Method Argument

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Method Executes Using Parameter

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Response Sent Back to Client

This flow shows how Spring Boot reads query string parameters from a GET request using @RequestParam and passes them to a controller method.

Execution Sample

Spring Boot

  @GetMapping("/greet")
  public String greet(@RequestParam String name) {
      return "Hello, " + name + "!";
  }

This code reads the 'name' query parameter from the URL and returns a greeting message including that name.

Execution Table

Step	Incoming URL	Query String Parsed	Parameter 'name' Value	Method Return Value
1	/greet?name=Alice	name=Alice	Alice	Hello, Alice!
2	/greet?name=Bob	name=Bob	Bob	Hello, Bob!
3	/greet	name missing	Error (required param missing)	Error: Required String parameter 'name' is not present

💡 Execution stops because the required 'name' parameter is missing in step 3, causing an error.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
name	null	Alice	Bob	null

Key Moments - 3 Insights

What happens if the 'name' query parameter is missing in the URL?

As shown in execution_table step 3, Spring Boot throws an error because @RequestParam by default requires the parameter. The method does not execute normally.

How does @RequestParam get the value from the URL?

In each step, the query string is parsed and the 'name' value is extracted and assigned to the method argument, as seen in the 'Parameter 'name' Value' column.

Can @RequestParam handle optional parameters?

By default, parameters are required. To make them optional, you can set required=false and provide a defaultValue, which avoids errors when missing.

Visual Quiz - 3 Questions

Look at the execution_table, what is the method return value when the URL is '/greet?name=Bob'?

A "Hello, Bob!"

B "Hello, Alice!"

C Error: Required String parameter 'name' is not present

D "Hello, Guest!"

💡 Hint

Check execution_table row 2 under 'Method Return Value' column.

At which step does the execution stop due to a missing required parameter?

A Step 1

B Step 3

C Step 2

D Execution never stops

💡 Hint

Look at the 'Parameter 'name' Value' column for missing parameter indication.

If we add 'required=false' to @RequestParam, how would step 3 change?

A It would return an error as before

B The query string would be ignored

C The method would receive null or default value and execute normally

D The server would crash

💡 Hint

Refer to key_moments about optional parameters and error handling.

Concept Snapshot

@RequestParam reads query string parameters in Spring Boot.
By default, parameters are required.
Missing required params cause errors.
Use required=false for optional params.
Parameter values are passed to controller methods.
Example: @RequestParam String name

Full Transcript

This visual execution shows how Spring Boot uses @RequestParam to read query string parameters from HTTP GET requests. When a request like '/greet?name=Alice' arrives, Spring parses the query string, extracts the 'name' parameter, and passes it to the controller method. The method then returns a greeting message including the name. If the 'name' parameter is missing, Spring throws an error because @RequestParam requires it by default. To avoid errors, you can make parameters optional by setting required=false. This flow helps beginners see how query strings connect to method arguments in Spring Boot.

Practice

1. What does the @RequestParam annotation do in a Spring Boot controller?

easy

A. It maps the URL path to a controller method.

B. It extracts query string parameters from the URL and passes them to the method.

C. It defines the HTTP method type like GET or POST.

D. It handles exceptions thrown by the controller.

Solution

1. Step 1: Understand the role of @RequestParam

   @RequestParam is used to get values from the URL query string, like ?name=John.

2. Step 2: Compare with other annotations

   @RequestMapping maps URLs, @RequestParam extracts query parameters, others handle HTTP methods or errors.

3. Final Answer:

   It extracts query string parameters from the URL and passes them to the method. -> Option B

4. Quick Check:

   @RequestParam reads query parameters = A [OK]

Hint: Remember: @RequestParam reads URL query values [OK]

Common Mistakes:

• Confusing @RequestParam with @PathVariable
• Thinking it maps URLs instead of parameters
• Assuming it handles HTTP methods

2. Which of the following is the correct syntax to make a query parameter optional with a default value using @RequestParam?

easy

A. @RequestParam(value = "id", required = true, defaultValue = "0") int id

B. @RequestParam(name = "id", optional = true, default = 0) int id

C. @RequestParam(name = "id", required = false, defaultValue = "0") int id

D. @RequestParam("id", default = 0) int id

Solution

1. Step 1: Identify correct attributes for optional param

   To make a parameter optional, use required = false and provide defaultValue as a string.

2. Step 2: Check syntax correctness

   @RequestParam(name = "id", required = false, defaultValue = "0") int id uses required = false and defaultValue = "0" correctly; others use invalid attributes or miss required parts.

3. Final Answer:

   @RequestParam(name = "id", required = false, defaultValue = "0") int id -> Option C

4. Quick Check:

   Optional param syntax = A [OK]

Hint: Use required=false and defaultValue="value" for optional params [OK]

Common Mistakes:

• Using 'optional' instead of 'required'
• Setting defaultValue as a number instead of string
• Omitting required attribute

3. Given the controller method:

@GetMapping("/greet")
public String greet(@RequestParam String name) {
    return "Hello, " + name + "!";
}

What will be the output when accessing /greet?name=Anna?

medium

A. Hello, Anna!

B. Hello, null!

C. Hello, !

D. Error: Missing required parameter

Solution

1. Step 1: Understand @RequestParam behavior

   The method expects a query parameter 'name' which is required by default.

2. Step 2: Check the URL query string

   The URL provides ?name=Anna, so the method receives "Anna" as the parameter.

3. Final Answer:

   Hello, Anna! -> Option A

4. Quick Check:

   Query param 'name' = Anna, output greeting = D [OK]

Hint: If param is required and given, output uses it directly [OK]

Common Mistakes:

• Assuming missing param causes null instead of error
• Confusing query string with path variable
• Expecting default value without setting one

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

@GetMapping("/search")
public String search(@RequestParam int page) {
    return "Page: " + page;
}

What happens if the URL /search is accessed without any query parameters?

medium

A. The method returns "Page: 0"

B. The method returns "Page: null"

C. The method returns an empty string

D. The method throws a 400 Bad Request error

Solution

1. Step 1: Check @RequestParam default behavior

   By default, @RequestParam is required. Since 'page' is primitive int, it cannot be null.

2. Step 2: Analyze missing parameter effect

   Accessing /search without 'page' causes Spring to reject the request with 400 Bad Request due to missing required param.

3. Final Answer:

   The method throws a 400 Bad Request error -> Option D

4. Quick Check:

   Missing required param causes 400 error = C [OK]

Hint: Required params missing cause 400 error, primitives can't be null [OK]

Common Mistakes:

• Assuming default 0 for missing int param
• Expecting null for primitive types
• Thinking method returns empty string on error

5. You want to create a Spring Boot controller method that accepts an optional query parameter sort with default value "asc" and a required parameter page. Which method signature is correct?

hard

A. @GetMapping("/items")
public String items(@RequestParam(required = false, defaultValue = "asc") String sort, @RequestParam int page) { ... }

B. @GetMapping("/items")
public String items(@RequestParam(required = true, defaultValue = "asc") String sort, @RequestParam int page) { ... }

C. @GetMapping("/items")
public String items(@RequestParam(required = false) String sort, @RequestParam(required = true) int page) { ... }

D. @GetMapping("/items")
public String items(@RequestParam String sort = "asc", @RequestParam int page) { ... }

Solution

1. Step 1: Set optional param with default value

   Use required = false and defaultValue = "asc" for the optional 'sort' parameter.

2. Step 2: Ensure required param is declared correctly

   The 'page' parameter is required by default, so just @RequestParam int page is enough.

3. Step 3: Evaluate each option

   @GetMapping("/items") public String items(@RequestParam(required = false, defaultValue = "asc") String sort, @RequestParam int page) { ... } correctly combines required=false and defaultValue for 'sort' and required 'page'. Incorrect options include one with required=true and defaultValue (still required on missing), one with required=false without defaultValue (null on missing), and one with invalid syntax.

4. Final Answer:

   @GetMapping("/items") public String items(@RequestParam(required = false, defaultValue = "asc") String sort, @RequestParam int page) { ... } -> Option A

5. Quick Check:

   Optional with default + required param = B [OK]

Hint: Use required=false with defaultValue for optional params [OK]

Common Mistakes:

• Omitting required=false for optional param
• Trying to assign default value in method signature
• Forgetting defaultValue causes errors