code: public void setupDecorateMethods(ClassLoader cl) {
          synchronized (STAR_IMPORTS) {
              if (DECORATED) return;
  
              GroovyShell shell = new GroovyShell(cl, new Binding(), getCompilerConfiguration());
              try {
                  shell.run(
                          new InputStreamReader(this.getClass().getResourceAsStream("extend.groovy")),
                          "dumpling-metaclass-setup",
                          Collections.emptyList()
                  );
              } catch (Exception ex) {
                  AssertionError err = new AssertionError("Unable to decorate object model");
                  err.initCause(ex);
                  throw err; // Java 6
              }
  
              DECORATED = true;
          }
      }
docs: Decorate Dumpling API with groovy extensions.