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 express

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.