Spring REST - Extract URL Path Variables

REST APIs have resource names and resource id present in the URL path unlike a traditional web API. This article focuses on how to extract that information and use it.

Take a typical REST URL:

GET https://api.bookstore.com/authors/twain-mark/books/roughing-it

Here, the name of the author and the name of the book are inside the book and not part of the request body or query parameters. This is a standard practice while developing REST APIs. To extract the path variables in Spring, we use the annotation @PathVariable:

@RequestMapping("/authors/{author}/books/{book}")
public ResponseEntity<String> greeting(@PathVariable String author, @PathVariable String book) {
    System.out.println("Requested Author: " + author);
    System.out.println("Requested Book: " + book);
    return new ResponseEntity<String>("ok", HttpStatus.OK);
}

Similar to the @RequestParam annotation,

if a variable is not present, a 400 Bad Request error is thrown
if a variable can be optional, the it can be marked like that using required = false. (there’s a catch which we will see in a bit)
There is no default value for the PathVariable

Marking URL Variables as Optional

Request variables can be marked as optional using required = false. But unlike RequestParam, an alternate URL has to be specified in the path mapping. Example:

@RequestMapping(path = { "/authors/{author}/books/{book}", "authors/{author}/books" })
public ResponseEntity<String> greeting(@PathVariable(name = "author") String author,
        @PathVariable(name = "book", required = false) String book, @RequestParam() int p) {
    System.out.println("Requested Author: " + author);
    System.out.println("Requested Book: " + book);
    return new ResponseEntity<String>("ok", HttpStatus.OK);
}

Here the book field is marked as optional but there are 2 paths in the RequestMapping.

Get all Path Variables in a Map

All the path variables can be extracted in a Map similar to @RequestParam and @RequestHeader in Spring. Simply specify a Map and you are done.

@RequestMapping(path = { "/authors/{author}/books/{book}", "authors/{author}/books" })
public ResponseEntity<String> greeting(@PathVariable Map<String, String> pathParams) {
    System.out.println("Requested Author: " + pathParams.get("author"));
    System.out.println("Requested Book: " + pathParams.get("book"));
    return new ResponseEntity<String>("ok", HttpStatus.OK);
}

Note: Do not forget to give alternate URLs in case of optional path variables other wise Spring will throw a 404 Not Found error.

Conclusion

Path Variable can be extracted by specifying the variables in {field} in the RequestMapping
They can be extracted using PathVariable annotation either one at a time or all together using a Map
If they are optional, do not forget to add additional path mapping in the RequestMapping

[How to extract Request Headers]({% post_url java/2019-11-23-spring-boot-request-headers %})
[Introduction to Java Streams]({% post_url java/2019-11-18-introduction-to-java-stream %})

Marking URL Variables as Optional

Get all Path Variables in a Map

Conclusion

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