@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

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Extract @PathVariable from URL

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Pass variable to Controller method

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Controller processes variable

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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.