Java code won't compile due to attribute must be a constant expression error

Question

I can't figure out why the following won't compile. The error the IDE gives me is "The value for annotation attribute RequestParam.defaultValue must be a constant expression".

My project involves Spring and Maven, and it goes the following:

private static final String MAX_LONG_AS_STRING = Long.toString(Long.MAX_VALUE);

@RequestMapping(method=RequestMethod.GET)
public List<Spittle> spittles(
        @RequestParam(value="max",
                    defaultValue=MAX_LONG_AS_STRING) long max,
        @RequestParam(value="count", defaultValue="20") int count) {
    return spittleRepository.findSpittles(max, count);
}

I'm thinking the error comes from the conversion of Long to String, but I do not know how to fix it. I will appreciate any help, I am new to annotations and Spring.

Answer 1

The Java rules say that when you have an annotation, and it has a parameter that expects a primitive type (such as an int) or a String, the value must be a constant expression. [This has nothing to do with Spring.] Roughly speaking, a constant expression is one whose value the compiler can figure out at compile time. However, there are rules for what constitutes a constant expression. These rules are in JLS 15.28. Only certain types of operations can be used in a constant expression. A method call, such as Long.toString(), isn't one of those. So using that makes your expression not a constant expression, even though it looks like it should be. (It looks like it to you, because you know what Long.toString does. However, the compiler doesn't keep a catalog of all methods to know which ones are "constant" methods whose values can be figured out at compile time.)

However, the example at the link shows that the + operator can be used, even when one of the arguments is not a string and therefore a toString() method is implicitly called. This suggests that you might be able to make things work like this:

private static final String MAX_LONG_AS_STRING = "" + Long.MAX_VALUE;

I haven't tried it, though.