Use groovy categories to add dynamic properties - groovy

Question

Expanding on this blog post, I am trying to use a category to create a simple DSL for use with the javax.measure (JSR-275) classes (similar to TimeCategory for time intervals)
However, I do not want to add boilerplate code for each of the possible available methods (getMeter, getMilliMeter, getKelvin, getSecond etc.). I thought overriding the getProperty(String) method would work, but alas, it looks like the getProperty method defined in the category is not used when accessing the property directly.
Here is some simplified code to demonstrate:
import javax.measure.quantity.Length;
import javax.measure.unit.Unit;
import javax.measure.Measure;
#Category(Number)
class LengthCategory {
public Measure<BigDecimal, Length> getProperty(String unit){
return Measure.valueOf(this,Unit.valueOf(unit));
}
}
use(LengthCategory){
println 3.getProperty("m") // this works
println 3.m // this reports a non-exisiting property
prinlln 3.'m' // as does this
}
Assuming other methods of dynamically adding properties to a runtime object (e.g. Expando, subclassing GroovyInterceptible, mixins and other metaclass manipulations) is not viable and I would really rather not have to manually code getters for every possible unit and SI prefix combination. There are obviously other ways to go about creating a DSL for measurements, but I would still like to understand why this method would not work.
Could someone explain why the getProperty method of the category does not override .propertyName usage? I am obviously missing something important about the resolution of property names using the metaclass during runtime.

Answer 1

I don't know why getProperty doesn't work on categories. But you can define a get method on them that does basically the same (i think). This works:
#Category(Number)
class LengthCategory {
def get(String unit) {
"$this $unit"
}
}
use (LengthCategory) {
println 3.m // 3 m
println 3.'m' // 3 m
}

Answer 2

As far as I can tell, you can't actually extend Integers with full (i.e., readable and writable) properties using Category -- only with methods.
You can extend an Integer using read-only properties by using the method version of the property. You can even make it writable by including a set method. However, there doesn't seem to be a way to store the value passed in other than in a static variable and that ends up affecting all Integers.
Example:
$ cat catprop
#!/usr/local/bin/groovy
#Category(Integer)
class CatInteger {
private static String str = "default"
public static String setN(Integer i, String _str) { str = _str }
public static String getN(Integer i) { return str }
}
use (CatInteger) {
3.n = "333a"
println "3.n is " + 3.n
3.n = "333b"
println "3.n is " + 3.n
4.n = "444"
println "4.n is " + 4.n
println "3.n is " + 3.n
}
$ catprop
3.n is 333a
3.n is 333b
4.n is 444
3.n is 444
$
Note that in the last line 3.n return "444" because the stored field is static. I suppose that one could use a private HashMap and store a value for every Integer accessed, but that's too ugly to contemplate.
Another possibility would be to use the MetaClass Interface's getProperty() and setProperty(). However, I haven't looked into that so I don't know if it would work or not (just a thought).

Answer 3

Nice answer, but not sure, if you's still want to use JSR-275 now that JSR-363 is final?;-)