CODEWITHSUNDEEP
javascalaconstants
Alan Ho
Alan Ho

Reputation: 181

Best practice for use constants in scala annotations

I use tapestry 5 as my choice of web framework. Tapestry allows me to define symbols in the configure class and inject symbols into other components.

for example, 

public interface SymbolConstants {
  static String DEFAULT_TIMEOUT_KEY = "default.timeout"; 
}

public class AppModule {
   void contributeApplicationDefault(Configuration conf) {
       conf.add(SymbolConstants.DEFAULT_TIMEOUT_KEY, "10");
   }
}

public class MyComponent {
  @Symbol(SymbolConstants.DEFAULT_VALUE_KEY)
  private long timeout;
}

The ability to define static constants and use them as annotation values gives me compile time check.

I am wondering how to define constants and use them as values of scala annotations. If not, what is the best practice to define/limit the value that we can assign to annotations in scala.

Upvotes: 18

Views: 9217

Answers (2)

F. P. Freely
F. P. Freely

Reputation: 1134

The 'final' keyword is required to make the compiler emit it as you would do it in Java. E.g.,

object Foo
{
   final val MY_SYMBOLIC_CONSTANT="whatever"
}

It seems that, otherwise, you only get an accessor method under the hood which is not statically calculable.

Upvotes: 23

Jay Taylor
Jay Taylor

Reputation: 13562

It doesn't seem possible w/ scala versions 2.8.1.final, 2.8.2.final, or 2.9.1.final (the result was the same with all):

object Constant { val UNCHECKED = "unchecked" }

class Test {                                       
    @SuppressWarnings(Array(Constant.UNCHECKED))   
    def test: Unit = println("testing.. 1, 2... 3")
}

.

<console>:7: error: annotation argument needs to be a constant; found: Constant.UNCHECKED
           @SuppressWarnings(Array(Constant.UNCHECKED))

Upvotes: 1

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