@PathVariable for URL parameters in Spring Boot - Step-by-Step Execution

Concept Flow - @PathVariable for URL parameters

Client sends HTTP request with URL

↓

Spring Boot matches URL pattern

↓

Extract @PathVariable from URL

↓

Pass variable to Controller method

↓

Controller processes variable

↓

Return response to client

The flow shows how Spring Boot takes a URL with parameters, extracts those parameters using @PathVariable, and passes them to the controller method to process.

Execution Sample

Spring Boot

  @GetMapping("/users/{id}")
  public String getUser(@PathVariable String id) {
      return "User ID: " + id;
  }

This code maps a GET request with a URL containing an 'id' parameter, extracts it using @PathVariable, and returns it in the response.

Execution Table

Step	Incoming URL	URL Pattern Matched	@PathVariable Extracted	Controller Method Called	Response Returned
1	/users/42	/users/{id}	id = "42"	getUser("42")	"User ID: 42"
2	/users/abc	/users/{id}	id = "abc"	getUser("abc")	"User ID: abc"
3	/users/	No match	N/A	No method called	404 Not Found

💡 Execution stops when URL does not match pattern or after response is returned.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
id	N/A	"42"	"abc"	N/A

Key Moments - 2 Insights

Why does the method not get called when the URL is '/users/'?

Because the URL does not match the pattern '/users/{id}', so Spring Boot does not extract any @PathVariable and does not call the method. See execution_table row 3.

Can @PathVariable extract numbers and letters both?

Yes, @PathVariable extracts the part of the URL as a string regardless of content. See execution_table rows 1 and 2 where '42' and 'abc' are extracted.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'id' at Step 2?

A "42"

B "abc"

C null

D No value

💡 Hint

Check the '@PathVariable Extracted' column in row 2.

At which step does the URL not match the pattern?

A Step 1

B Step 2

C Step 3

D None

💡 Hint

Look at the 'URL Pattern Matched' column for each step.

If the URL was '/users/123', what would the response be?

A "User ID: 123"

B "User ID: abc"

C 404 Not Found

D "User ID: /users/123"

💡 Hint

Refer to how the controller returns 'User ID: ' plus the extracted id.

Concept Snapshot

@PathVariable extracts parts of the URL as method parameters.
Use @GetMapping("/path/{variable}") to define URL patterns.
Spring Boot matches URL and passes the variable to the method.
Variable is always a string unless converted.
If URL does not match, method is not called.
Useful for dynamic URLs like user IDs.

Full Transcript

This visual execution shows how Spring Boot uses @PathVariable to get parts of the URL and pass them to controller methods. When a client sends a URL like '/users/42', Spring Boot matches it to the pattern '/users/{id}', extracts '42' as the id, and calls the method getUser with id='42'. The method returns 'User ID: 42'. If the URL does not match the pattern, like '/users/', the method is not called and a 404 error is returned. This helps create dynamic web endpoints that respond based on URL parameters.

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