Handling path variables and query params together in Spring Boot - Mini Project: Build & Apply

Handling path variables and query params together in Spring Boot

📖 Scenario: You are building a simple Spring Boot web service for a bookstore. You want to create an endpoint that takes a book category as a path variable and an optional filter as a query parameter.This will help users get books from a specific category and optionally filter them by author name.

🎯 Goal: Build a Spring Boot controller with a method that handles a path variable category and an optional query parameter author. The method should return a string message showing the received category and author filter.

📋 What You'll Learn

Create a Spring Boot controller class named BookController

Add a GET mapping method named getBooksByCategory

Use @PathVariable to get the category from the URL path

Use @RequestParam to get the optional author query parameter with default value "all"

Return a string message showing the category and author filter values

💡 Why This Matters

🌍 Real World

Web services often need to get data from both the URL path and query parameters to filter or customize responses.

💼 Career

Understanding how to handle path variables and query parameters is essential for backend developers building REST APIs with Spring Boot.

1

Create the controller class and method signature

Create a public class named BookController annotated with @RestController. Inside it, create a public method named getBooksByCategory that returns a String. Add a @GetMapping annotation with the path "/books/{category}".

Spring Boot

@RestController
public class BookController {
    
    // Create getBooksByCategory method with @GetMapping("/books/{category}")
    // Your code here
}

Hint

Use @RestController on the class. Use @GetMapping("/books/{category}") on the method. The method should return a String.

2

Add the path variable parameter

Modify the getBooksByCategory method to accept a String parameter named category annotated with @PathVariable.

Spring Boot

@RestController
public class BookController {
    
    @GetMapping("/books/{category}")
    public String getBooksByCategory(@PathVariable String category) {
        // Your code here
        return "";
    }
}

Hint

Add @PathVariable String category as a parameter to the method.

3

Add the optional query parameter

Add a second parameter to getBooksByCategory named author of type String. Annotate it with @RequestParam with required = false and defaultValue = "all".

Spring Boot

@RestController
public class BookController {
    
    @GetMapping("/books/{category}")
    public String getBooksByCategory(@PathVariable String category, @RequestParam(required = false, defaultValue = "all") String author) {
        // Your code here
        return "";
    }
}

Hint

Use @RequestParam(required = false, defaultValue = "all") String author as the second parameter.

4

Return a message showing category and author

In the getBooksByCategory method, return a string formatted as "Category: {category}, Author filter: {author}" using string concatenation or String.format.

Spring Boot

@RestController
public class BookController {
    
    @GetMapping("/books/{category}")
    public String getBooksByCategory(@PathVariable String category, @RequestParam(required = false, defaultValue = "all") String author) {
        // Your code here
        return "";
    }
}

Hint

Use return String.format("Category: %s, Author filter: %s", category, author); to create the message.

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