Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.
There are several ways to execute iDocScript from Java, but in this quick example we will use a PageMerger object. There are several ways to get ahold of a PageMerger object. If you are executing in a context where a service object is readily available you may be able to get a PageMerger object reference with code like this:
PageMerger pm = m_service.m_pageMerger;
If that doesn't work for you I would try something like:
PageMerger pm = new PageMerger(databinder, executioncontext);
Now that you have your merger object you can execute all kinds of iDocScript code, an example of which might be:
pm.evaluateScript("<$lc(\"wwMyString\")$>");