Question

I’m developing a Java enterprise application, currently doing Java EE security stuff to restrict access for particular functions to specific users. I configured the application server and everything, and now I’m using the RolesAllowed-annotation to secure the methods:

@Documented
@Retention (RUNTIME)
@Target({TYPE, METHOD})
public @interface RolesAllowed {
    String[] value();
}

When I use the annotation like this, it works fine:

@RolesAllowed("STUDENT")
public void update(User p) { ... }

But this is not what I want, as I have to use a String here, refactoring becomes hard, and typos can happen. So instead of using a String, I would like to use an Enum value as a parameter for this annotation. The Enum looks like this:

public enum RoleType {
    STUDENT("STUDENT"),
    TEACHER("TEACHER"),
    DEANERY("DEANERY");

    private final String label;

    private RoleType(String label) {
        this.label = label;
    }

    public String toString() {
        return this.label;
    }
}

So I tried to use the Enum as a parameter like this:

@RolesAllowed(RoleType.DEANERY.name())
public void update(User p) { ... }

But then I get the following compiler error, although Enum.name just returns a String (which is always constant, isn’t it?).

The value for annotation attribute RolesAllowed.value must be a constant expression`

The next thing I tried was to add an additional final String to my Enum:

public enum RoleType {
    ...
    public static final String STUDENT_ROLE = STUDENT.toString();
    ...
}

But this also doesn’t work as a parameter, resulting in the same compiler error:

// The value for annotation attribute RolesAllowed.value must be a constant expression
@RolesAllowed(RoleType.STUDENT_ROLE)

How can I achieve the behavior I want? I even implemented my own interceptor to use my own annotations, which is beautiful, but far too much lines of code for a little problem like this.

DISCLAIMER

This question was originally a Scala question. I found out that Scala is not the source of the problem, so I first try to do this in Java.

Answer 1

I don’t think your approach of using enums is going to work. I found that the compiler error went away if I changed the STUDENT_ROLE field in your final example to a constant string, as opposed to an expression:

public enum RoleType { 
  ...
  public static final String STUDENT_ROLE = "STUDENT";
  ...
}

However, this then means that the enum values wouldn’t be used anywhere, because you’d be using the string constants in annotations instead.

It seems to me that you’d be better off if your RoleType class contained nothing more than a bunch of static final String constants.

---

To see why your code wasn’t compiling, I had a look into the Java Language Specification (JLS). The JLS for annotations states that for an annotation with a parameter of type T and value V,

if T is a primitive type or String, V is a constant expression.

A constant expression includes, amongst other things,

Qualified names of the form TypeName . Identifier that refer to constant variables

and a constant variable is defined as

a variable, of primitive type or type String, that is final and initialized with a compile-time constant expression