Global constants in Groovy

Question

It is often desired to declare constants at the top of a script that can be referenced anywhere else in the script. In Groovy, it seems that if you declare a constant using final then it isnot accessible in child scopes. What is the solution for this very basic and common requirement? The workaround I have right now is to create an unbound variable but this is not a constant and is not elegant.

Answer 1

Groovy doesn't really have a global scope. When you have a groovy script that doesn't declare a class, it implicitly gets stuck in a class with the name of the script. So final variables at the top-level scope are really just fields of the implicit class. For example:

// foo.groovy final MYCONSTANT = "foobar" println MYCONSTANT  class Helper {     def hello() { println MYCONSTANT }  // won't work } new Helper().hello() 

Is more or less equivalent to:

class foo {     def run() {         final MYCONSTANT = "foobar"         println MYCONSTANT         new Helper().hello()     }     static main(args) {         new foo().run()     } }  class Helper {     def hello() { println MYCONSTANT }  // won't work } 

It's easy to see why it doesn't work expanded out. An easy work around is to declare your "globals" in a dummy class called e.g. Constants, and then just do a static import on it. It even works all in a single script. Example:

import static Constants.*  class Constants {     static final MYCONSTANT = "foobar" }  println MYCONSTANT  class Helper {     def hello() { println MYCONSTANT } // works! } new Helper().hello() 

EDIT:

Also, scripts are bit of a special case. If you declare a variable without def or any modifiers such as final, (i.e. just use it) it goes into a script-wide binding. So in this case:

CONSTANT = "foobar" println "foobar" 

CONSTANT is in the script-wide binding, but in:

final CONSTANT = "foobar" println "foobar" 

CONSTANT is a local variable in the script's run() method. More information on this can be found at the archived link to some Groovy - Scoping and the Semantics of "def" page.

Answer 2

In Groovy 1.8+, you can achieve this using the @Field annotation:

import groovy.transform.Field  @Field final String MY_CONSTANT = 'constant'  def printConstant() { println MY_CONSTANT }  printConstant()