Handling path variables and query params together in Spring Boot - Step-by-Step Execution

Concept Flow - Handling path variables and query params together

HTTP Request Received

↓

Extract Path Variable

↓

Extract Query Parameters

↓

Call Controller Method with Both

↓

Process Data Using Both

↓

Return Response

The server receives a request, extracts path variables and query parameters, then calls the controller method using both to process and respond.

Execution Sample

Spring Boot

@GetMapping("/users/{id}")
public String getUser(@PathVariable String id, @RequestParam(required = false) String filter) {
    return "User: " + id + ", Filter: " + filter;
}

This code handles a GET request with a path variable 'id' and an optional query parameter 'filter', returning a combined string.

Execution Table

Step	Action	Path Variable 'id'	Query Param 'filter'	Method Called	Returned Output
1	Request received: GET /users/42?filter=active	42	active	getUser("42", "active")	User: 42, Filter: active
2	Request received: GET /users/7?filter=all	7	all	getUser("7", "all")	User: 7, Filter: all
3	Request received: GET /users/99?filter=	99		getUser("99", "")	User: 99, Filter:
4	Request received: GET /users/5 (no filter param)	5	null (missing)	getUser("5", null)	User: 5, Filter: null

💡 Execution stops after method returns response or error if required params missing.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4
id	null	42	7	99	5
filter	null	active	all		null

Key Moments - 2 Insights

What happens if the query parameter 'filter' is missing in the request?

If 'filter' is missing, the method may receive null or throw an error depending on if @RequestParam is required. See execution_table row 4 where missing 'filter' leads to null value passed to the method.

How does Spring Boot know which part of the URL is the path variable and which is the query parameter?

Spring Boot uses @PathVariable to extract parts from the URL path (like /users/{id}) and @RequestParam to extract query parameters after '?'. This is shown in execution_table where 'id' comes from path and 'filter' from query.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'filter' at step 3?

A "active"

B "" (empty string)

C "all"

D null

💡 Hint

Check the 'Query Param filter' column at step 3 in the execution_table.

At which step does the method get called with a missing query parameter?

A Step 4

B Step 2

C Step 3

D Step 1

💡 Hint

Look for 'null (missing)' in the 'Query Param filter' column in execution_table.

If the path variable 'id' was changed to '100' in step 2, what would be the returned output?

A "User: 7, Filter: all"

B "User: 100, Filter: active"

C "User: 100, Filter: all"

D "User: 7, Filter: active"

💡 Hint

Refer to step 2 in execution_table and replace 'id' with 100 keeping 'filter' same.

Concept Snapshot

Use @PathVariable to get parts of the URL path.
Use @RequestParam to get query parameters after '?'.
Both can be used together in one controller method.
Missing query params may cause errors unless marked optional.
Example: /users/{id}?filter=active
Method signature: getUser(@PathVariable String id, @RequestParam(required = false) String filter)

Full Transcript

This visual execution shows how Spring Boot handles requests with both path variables and query parameters. The server receives a request like GET /users/42?filter=active. It extracts the path variable 'id' as 42 and the query parameter 'filter' as 'active'. Then it calls the controller method getUser with these values. The method returns a string combining both values. If the query parameter is missing, the method may throw an error or handle a default value. This is important to know when designing APIs. The execution table traces multiple requests showing how variables change and how the method output depends on inputs. The key moments clarify common confusions about missing parameters and how Spring Boot distinguishes path variables from query parameters. The quiz tests understanding by asking about variable values at specific steps and effects of changing inputs.

Practice

1. In Spring Boot, which annotation is used to extract a value from the URL path segment in a controller method?

easy

A. @PathVariable

B. @RequestParam

C. @RequestBody

D. @RequestHeader

Solution

1. Step 1: Understand URL parts

   The URL path segment is the part of the URL that defines the resource location, like /users/{id}.

2. Step 2: Identify the correct annotation

   @PathVariable is used to bind a method parameter to a URI template variable in the path.

3. Final Answer:

   @PathVariable -> Option A

4. Quick Check:

   Path segment value = @PathVariable [OK]

Hint: Use @PathVariable for URL path parts, not query strings [OK]

Common Mistakes:

• Confusing @RequestParam with @PathVariable
• Using @RequestBody for path variables
• Trying to get path variables from headers

2. Which of the following method signatures correctly handles a path variable id and a query parameter sort in Spring Boot?

easy

A. public String getUser(@PathVariable String id, @RequestParam String sort)

B. public String getUser(@RequestParam String id, @PathVariable String sort)

C. public String getUser(@RequestParam String id, @RequestParam String sort)

D. public String getUser(@PathVariable String id, @PathVariable String sort)

Solution

1. Step 1: Match annotations to parameter types

   Path variables come from the URL path, so use @PathVariable for id. Query parameters come after ?, so use @RequestParam for sort.

2. Step 2: Check method signature correctness

   public String getUser(@PathVariable String id, @RequestParam String sort) correctly uses @PathVariable for id and @RequestParam for sort.

3. Final Answer:

   public String getUser(@PathVariable String id, @RequestParam String sort) -> Option A

4. Quick Check:

   Path variable = @PathVariable, Query param = @RequestParam [OK]

Hint: Path variables use @PathVariable, query params use @RequestParam [OK]

Common Mistakes:

• Swapping @PathVariable and @RequestParam
• Using @PathVariable for query parameters
• Missing one of the annotations

3. Given the controller method:

@GetMapping("/items/{itemId}")
public String getItem(@PathVariable String itemId, @RequestParam(defaultValue = "asc") String order) {
    return itemId + "-" + order;
}

What will be the output when a client requests /items/42?order=desc?

medium

A. "42-asc"

B. "42-desc"

C. "desc-42"

D. Runtime error due to missing parameter

Solution

1. Step 1: Identify values from URL

   Path variable itemId is "42" from /items/42. Query parameter order is "desc" from ?order=desc.

2. Step 2: Understand method return

   The method returns itemId + "-" + order, so "42-desc".

3. Final Answer:

   "42-desc" -> Option B

4. Quick Check:

   Path variable + query param = "42-desc" [OK]

Hint: Path variable fills {itemId}, query param fills order value [OK]

Common Mistakes:

• Ignoring query parameter and using default
• Mixing order of concatenation
• Expecting error when query param is present

4. Consider this controller method:

@GetMapping("/books/{bookId}")
public String getBook(@PathVariable int bookId, @RequestParam int page) {
    return "Book: " + bookId + ", Page: " + page;
}

What error will occur if a client calls /books/abc?page=2?

medium

A. 404 Not Found because path variable missing

B. 500 Internal Server Error due to null page

C. 400 Bad Request due to type mismatch

D. No error, returns "Book: abc, Page: 2"

Solution

1. Step 1: Check path variable type

   The method expects bookId as int, but URL provides "abc" which is not a number.

2. Step 2: Understand Spring Boot behavior on type mismatch

   Spring Boot returns 400 Bad Request when it cannot convert path variable to required type.

3. Final Answer:

   400 Bad Request due to type mismatch -> Option C

4. Quick Check:

   Type mismatch in @PathVariable causes 400 error [OK]

Hint: Non-numeric path variable for int causes 400 error [OK]

Common Mistakes:

• Expecting 404 Not Found instead of 400
• Assuming null page causes error
• Thinking method returns string with invalid input

5. You want to create a Spring Boot endpoint that handles a user ID as a path variable and optionally accepts a query parameter filter. If filter is missing, it should default to "all". Which method signature correctly implements this behavior?

hard

A. public String getUser(@PathVariable String userId, @RequestParam Optional<String> filter)

B. public String getUser(@RequestParam String userId, @RequestParam String filter)

C. public String getUser(@PathVariable String userId, @RequestParam(required = true) String filter)

D. public String getUser(@PathVariable String userId, @RequestParam(defaultValue = "all") String filter)

Solution

1. Step 1: Identify path variable usage

   User ID is part of the URL path, so use @PathVariable for userId.

2. Step 2: Handle optional query parameter with default

   Use @RequestParam with defaultValue = "all" to provide a default when filter is missing.

3. Final Answer:

   public String getUser(@PathVariable String userId, @RequestParam(defaultValue = "all") String filter) -> Option D

4. Quick Check:

   Default query param value = @RequestParam(defaultValue) [OK]

Hint: Use @RequestParam(defaultValue) for optional query params [OK]

Common Mistakes:

• Using @RequestParam(required = true) for optional params
• Using @RequestParam without defaultValue causing errors
• Confusing @RequestParam and @PathVariable roles