As common practice is to develop your application in a test driven way, we will write an appropriate test as first code in this recipe. In case you need more information about writing and using tests in Play, you should read http://www.playframework.org/documentation/1.2/test.

This is the test that should work:

public class UuidTest extends FunctionalTest {

    @Test
    public void testThatValidUuidWorks() {
        String uuid = UUID.randomUUID().toString();
        Response response = GET("/" + uuid);
        assertIsOk(response);
        assertContentEquals(uuid + " is valid", response);
    }

    @Test
    public void testThatInvalidUuidWorksNot() {
        Response response = GET("/absolutely-No-UUID");
        assertStatus(500, response);
    }
}

So whenever a valid UUID is used in the URL, it should be returned in the body and whenever an invalid UUID has been used, it should return HTTP error 500.

Add an appropriate configuration line to your conf/routes file:

GET     /{uuid}    Application.showUuid

Create a simple @UUID annotation, practically in its own annotations or validations package:

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Constraint(checkWith = UuidCheck.class)
public @interface Uuid {
        String message() default "validation.invalid.uuid";
}

Create the appropriate controller, which uses the @Uuid annotation:

public class Application extends Controller {

    public static void showUuid(@Uuid String uuid) {
        if (validation.hasErrors()) {
            flash.error("Fishy uuid");
            error();
        }   

        renderText(uuid + " is valid");
    }   
}

Create the check, which is triggered by the validation. You might want to put it into the checks package:

public class UuidCheck extends AbstractAnnotationCheck<Uuid> {

    @Override
    public booleanisSatisfied(Object validatedObject, Object value, OValContext context, Validator validator)
    throws OValException {

        try {
            UUID.fromString(value.toString());
            return true;
        } catch (IllegalArgumentException e) {}

        return false;
    }
}

When starting your application via play test and going to http://localhost:9000/@tests you should be able to run the UuidTest without problems.

Except the UuidCheck class, most of this here is old stuff. The Uuid annotation has two specialties. First it references the UuidCheck with a constraint annotation and second you can specify a message as argument. This message is used for internationalization.

The UuidCheck class is based on an Oval class. Oval is a Java library and used by the Play framework for most of the validation tasks and can be pretty easily extended as you can see here. All you need to implement is the isSatisfied() method. In this case it has tried to convert a String to a UUID. If it fails, the runtime exception thrown by the conversion is caught and false is returned, marking the check as invalid.

The oval framework is pretty complex and the logic performed here barely scratches the surface. For more information about oval, check the main documentation at http://oval.sourceforge.net/.

@Uuid public String registrationUuid;