@PathVariable

#Introduction

One of the most frequently used annotations in Spring Boot. Most developers know how to write:

@GetMapping("/users/{id}")
public User getUser(@PathVariable Long id) {
}

But interviewers ask: How does Spring extract id from the URL? What happens if the value is not a number? How does Spring convert String to Long? Can we have multiple path variables? Can UUID, enums and LocalDate be used? What's the difference between @PathVariable and @RequestParam?

#What is @PathVariable?

@PathVariable extracts values from the URL path and binds them to method parameters.

@RestController
@RequestMapping("/users")
public class UserController {

    @GetMapping("/{id}")
    public User getUser(@PathVariable Long id) {
        return service.findById(id);
    }
}

For GET /users/101, Spring extracts 101 and assigns it to Long id.

#How Does Spring Understand the URL?

Spring stores /users/{id} as a URL template. For an incoming /users/101:

Request Lifecycle

#Internal Working

Conceptually Spring does:

String value = "101";
Long id = Long.valueOf(value);
controller.getUser(id);

Of course, Spring uses its conversion infrastructure rather than literally calling Long.valueOf(), but this illustrates the idea.

#Type Conversion

For /users/25, Spring converts "25" → 25L automatically. No parsing required.

Supported Types

@PathVariable Integer id
@PathVariable Long id
@PathVariable UUID id
@PathVariable Double amount
@PathVariable Boolean active
@PathVariable Status status       // any enum
@PathVariable LocalDate date      // ISO yyyy-MM-dd, or use @DateTimeFormat

No manual parsing is needed if a suitable converter exists.

UUID Example

@GetMapping("/{id}")
public User getUser(@PathVariable UUID id) {
}

/users/550e8400-e29b-41d4-a716-446655440000 → Spring converts the string into a UUID.

Enum Example

public enum Status {
    ACTIVE,
    INACTIVE
}

@GetMapping("/{status}")
public String status(@PathVariable Status status) {
    return status.name();
}

/users/ACTIVE → Status.ACTIVE.

#Multiple Path Variables

@GetMapping("/{userId}/orders/{orderId}")
public Order getOrder(@PathVariable Long userId,
                      @PathVariable Long orderId) {
}

/users/101/orders/5001 → userId = 101, orderId = 5001.

Nested Resources

@GetMapping("/companies/{companyId}/employees/{employeeId}/projects/{projectId}")
public Project get(@PathVariable Long companyId,
                   @PathVariable Long employeeId,
                   @PathVariable Long projectId) {
}

/companies/5/employees/101/projects/20 — Spring extracts all three values automatically.

#Path Variable Name Matching

@GetMapping("/{id}")
public User get(@PathVariable Long id) {
}

Works because {id} ↔ id — the names match.

Different Variable Names

@GetMapping("/{id}")
public User get(@PathVariable("id") Long userId) {
}

Spring maps {id} → userId using the explicit annotation value.

#What If Conversion Fails?

@GetMapping("/{id}")
public User get(@PathVariable Long id) {
}

For /users/abc, Spring tries "abc" → Long and fails:

400 Bad Request   (MethodArgumentTypeMismatchException)

#Missing Path Variable

With only @GetMapping("/{id}"), a request to /users cannot match the route → 404 Not Found, because there is no handler for /users.

#Optional Path Variables?

Many developers ask whether @PathVariable(required = false) is possible. It exists, but it only makes sense when the same method is also mapped to a URL without the variable:

@GetMapping({"", "/{id}"})
public Object get(@PathVariable(required = false) Long id) {
}

Usually it's clearer to write separate mappings:

@GetMapping
public List<User> all() {
}

@GetMapping("/{id}")
public User one(@PathVariable Long id) {
}

(Optional<Long> is also supported as the parameter type.)

#@PathVariable vs @RequestParam

An extremely common interview question.

// /users/10 — value is part of the URL path
@GetMapping("/{id}")
public User get(@PathVariable Long id) {}

// /users?id=10 — value comes from the query string
@GetMapping
public User get(@RequestParam Long id) {}

REST Best Practice

✅ GET /users/10 — the URL identifies the resource. ❌ GET /getUser?id=10.

#URL Encoding

For /users/John%20Doe, the browser sends John%20Doe and Spring automatically decodes it to John Doe before passing it to the controller.

#Internal Components

• PathPatternParser (or AntPathMatcher in older configurations) matches the URL pattern.
• PathVariableMethodArgumentResolver resolves @PathVariable parameters.
• ConversionService converts String values into target Java types.

These names are excellent interview points.

#Enterprise Example

@RestController
@RequestMapping("/employees")
public class EmployeeController {

    @GetMapping("/{id}")
    public Employee get(@PathVariable Long id) {
        return service.findById(id);
    }

    @DeleteMapping("/{id}")
    public void delete(@PathVariable Long id) {
        service.delete(id);
    }
}

GET /employees/101 returns employee 101; DELETE /employees/101 deletes employee 101.

#Summary Diagram

#Key Takeaways

• @PathVariable binds values from the URL path to method parameters.
• Spring converts string values using the ConversionService.
• Multiple path variables are fully supported.
• Prefer @PathVariable for resource identifiers and @RequestParam for filters and query options.
• PathVariableMethodArgumentResolver and ConversionService are key internal components.
• Let Spring perform type conversion instead of parsing values manually.