Monkey patching a Java class?

[Marc Downie]

Does adding new methods to Java.extend based subclasses actually work?
Currently I have (staying close to the example code):

var ArrayList = Java.type("java.util.ArrayList")
var ArrayListExtender = Java.extend(ArrayList)
var printSizeInvokedArrayList = new ArrayListExtender() {
    size: function() { print("size invoked!"); },
  banana: function() { print("Banana"); } // this doesn't override anything
in ArrayList
}

printSizeInvokedArrayList.size() // WORKS
printSizeInvokedArrayList.banana() // TYPE ERROR ([] has no such function
"banana")

// and even:

printSizeInvokedArrayList.peach = function(){print("Peach");} // no error,
but....
printSizeInvokedArrayList.peach() // TYPE ERROR ([] has no such function
"peach")

(1.9.0-ea-b23 and 1.8.0_20-ea-b23)

Marc.



On Thu, May 1, 2014 at 11:23 PM, A. Sundararajan <
sundararajan.athijegannathan at oracle.com> wrote:

> No, you can't add/remove a method (or public field) of a Java class to use
> within the script. You could subclass and expose that subclass as
> "java.io.File" by
>
>     var oldFile = java.io.File;
>     java.io.File = Java.extend(oldFile, ...)
>
> But, I'd not recommend it - besides user can still get original
> java.io.File via Java.type (unless you do similar hack on Java.type as
> well!!)
>
> Cleaner approach is to expose a script API wrapping java.io.File.
>
> -Sundar
>
>
> On Thursday 17 April 2014 12:03 AM, HRJet wrote:
>
>> Is it possible to monkey patch a Java class for use within Javascript?
>>
>> For example, I want to add a convenience method to java.io.File class, say
>> "readAsString()".
>>
>> Then, in javascript I want to call file.readAsString()  where file is an
>> instance of java.io.File. Note that the file instance may be created by
>> some third-party code, over which I have no control.
>>
>> In Java land, this seems to be usually done with CGLib or AspectJ, etc.
>>
>> I was wondering if nashorn had some trick up its sleeve for doing this in
>> script land, since this sort of thing is common in Javascript.
>>
>> thanks,
>> HRJ
>>
>
>