@PathVariable for URL parameters in Spring Boot - Mini Project: Build & Apply

Using @PathVariable for URL Parameters in Spring Boot

📖 Scenario: You are building a simple Spring Boot web service that returns greetings for users based on their names provided in the URL.

🎯 Goal: Create a Spring Boot controller that uses @PathVariable to extract a user's name from the URL and return a greeting message.

📋 What You'll Learn

Create a Spring Boot controller class named GreetingController.

Add a method that handles GET requests to /greet/{name}.

Use @PathVariable to capture the name parameter from the URL.

Return a greeting message that includes the captured name.

💡 Why This Matters

🌍 Real World

Web services often need to get data from the URL path to customize responses, like user profiles or product details.

💼 Career

Understanding @PathVariable is essential for backend developers working with Spring Boot to build REST APIs.

1

Create the GreetingController class

Create a public class called GreetingController annotated with @RestController.

Spring Boot

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

Hint

Use @RestController to make the class a REST controller in Spring Boot.

2

Add a GET mapping method with @PathVariable

Inside GreetingController, add a public method called greetUser that handles GET requests to /greet/{name} using @GetMapping. The method should take a String name parameter annotated with @PathVariable.

Spring Boot

@RestController
public class GreetingController {
    // Add a method with @GetMapping("/greet/{name}") and @PathVariable String name
}

Hint

Use @GetMapping("/greet/{name}") and @PathVariable String name in the method signature.

3

Implement the greeting message

In the greetUser method, return a greeting string that says "Hello, " followed by the name parameter.

Spring Boot

@RestController
public class GreetingController {
    @GetMapping("/greet/{name}")
    public String greetUser(@PathVariable String name) {
        // Return greeting message
    }
}

Hint

Use string concatenation to combine "Hello, " and the name variable.

4

Add class-level request mapping

Add @RequestMapping("/api") annotation to the GreetingController class to prefix all routes with /api.

Spring Boot

// Add @RequestMapping("/api") above the class declaration
@RestController
public class GreetingController {
    @GetMapping("/greet/{name}")
    public String greetUser(@PathVariable String name) {
        return "Hello, " + name;
    }
}

Hint

Use @RequestMapping("/api") above the class to add a common URL prefix.

Practice

1. What is the main purpose of the @PathVariable annotation in Spring Boot?

easy

A. To bind form data to a Java object

B. To handle exceptions in controller methods

C. To define a fixed URL mapping for a controller method

D. To extract values from the URL path and pass them as method parameters

Solution

1. Step 1: Understand the role of @PathVariable

   @PathVariable is used to get dynamic parts of the URL and pass them to method parameters.

2. Step 2: Compare with other annotations

   Other options like binding form data or exception handling are done by different annotations, not @PathVariable.

3. Final Answer:

   To extract values from the URL path and pass them as method parameters -> Option D

4. Quick Check:

   @PathVariable = extract URL values [OK]

Hint: Remember: @PathVariable grabs values directly from URL path [OK]

Common Mistakes:

• Confusing @PathVariable with @RequestParam
• Thinking it binds form data
• Assuming it handles exceptions

2. Which of the following is the correct syntax to use @PathVariable in a Spring Boot controller method to get an id from the URL /user/{id}?

easy

A. @GetMapping("/user/{id}") public String getUser(@PathVariable int id) { ... }

B. @GetMapping("/user") public String getUser(@PathVariable int id) { ... }

C. @GetMapping("/user/{id}") public String getUser(@RequestParam int id) { ... }

D. @GetMapping("/user/{id}") public String getUser(@PathVariable String name) { ... }

Solution

1. Step 1: Match URL pattern with method parameter

   The URL has {id}, so the method must have @PathVariable int id to match the placeholder.

2. Step 2: Check annotation and parameter type

   @GetMapping("/user/{id}") public String getUser(@PathVariable int id) { ... } correctly uses @PathVariable with int id and matches the URL pattern.

3. Final Answer:

   @GetMapping("/user/{id}") public String getUser(@PathVariable int id) { ... } -> Option A

4. Quick Check:

   URL {id} matches @PathVariable int id [OK]

Hint: URL placeholder and @PathVariable name must match [OK]

Common Mistakes:

• Missing {id} in URL path
• Using @RequestParam instead of @PathVariable
• Mismatching parameter name and URL placeholder

3. Given the following Spring Boot controller method:

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

What will be the output when a client requests /product/electronics/42?

medium

A. electronics-42

B. 42-electronics

C. category-id

D. electronics42

Solution

1. Step 1: Identify parameter values from URL

   URL /product/electronics/42 means category = "electronics", id = 42.

2. Step 2: Understand return statement

   The method returns category + "-" + id, so it returns "electronics-42".

3. Final Answer:

   electronics-42 -> Option A

4. Quick Check:

   Return = category + "-" + id = electronics-42 [OK]

Hint: Match URL parts to parameters in order [OK]

Common Mistakes:

• Swapping parameter order
• Ignoring the dash '-' in return
• Confusing @PathVariable with @RequestParam

4. Consider this Spring Boot controller method:

@GetMapping("/order/{orderId}")
public String getOrder(@PathVariable int id) {
    return "Order: " + id;
}

What will happen when a client requests /order/100?

medium

A. Returns "Order: 0"

B. Returns "Order: 100"

C. Throws an error because parameter name does not match path variable

D. Returns null

Solution

1. Step 1: Check URL placeholder and method parameter names

   URL has {orderId}, but method parameter is named id without specifying name in @PathVariable.

2. Step 2: Understand default matching behavior

   Without specifying the name in @PathVariable, Spring tries to match parameter name to path variable name. They differ, causing an error.

3. Final Answer:

   Throws an error because parameter name does not match path variable -> Option C

4. Quick Check:

   Parameter name must match path variable or specify name [OK]

Hint: Parameter name must match path variable or use @PathVariable("name") [OK]

Common Mistakes:

• Assuming names can differ without annotation
• Ignoring the need to specify variable name in @PathVariable
• Expecting default value assignment

5. You want to create a Spring Boot controller method that handles URLs like /blog/{year}/{month}/{slug} where year and month are integers and slug is a string. Which of the following method signatures correctly uses @PathVariable to extract these values?

hard

A. @GetMapping("/blog/{year}/{month}/{slug}")
public String getPost(@PathVariable int yr, @PathVariable int mo, @PathVariable String sl) { ... }

B. @GetMapping("/blog/{year}/{month}/{slug}")
public String getPost(@PathVariable("year") int y, @PathVariable("month") int m, @PathVariable("slug") String s) { ... }

C. @GetMapping("/blog/{year}/{month}/{slug}")
public String getPost(@PathVariable String year, @PathVariable String month, @PathVariable String slug) { ... }

D. @GetMapping("/blog/{year}/{month}/{slug}")
public String getPost(@PathVariable int y, @PathVariable int m, @PathVariable String s) { ... }

Solution

1. Step 1: Match URL placeholders with method parameters

   The URL has {year}, {month}, {slug}. Method parameters must match these names or specify them explicitly.

2. Step 2: Check parameter names and @PathVariable usage

   @GetMapping("/blog/{year}/{month}/{slug}") public String getPost(@PathVariable("year") int y, @PathVariable("month") int m, @PathVariable("slug") String s) { ... } uses different parameter names (y, m, s) but explicitly maps them to URL placeholders using @PathVariable("name"), which is correct.

3. Final Answer:

   @GetMapping("/blog/{year}/{month}/{slug}") public String getPost(@PathVariable("year") int y, @PathVariable("month") int m, @PathVariable("slug") String s) { ... } -> Option B

4. Quick Check:

   Explicit name mapping allows different parameter names [OK]

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

Common Mistakes:

• Using mismatched parameter names without specifying name
• Using wrong data types for year/month
• Assuming parameter names must always match URL placeholders