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I was looking for a way to add a simple configurable extension-point to an application. The target was to be able to write a few lines of code or configure a few properties to be able to setup custom-filters. Luckily the task was not to be executed by a user of the system, but by a programmer. The open source product of choice here was BeanShell.

Here is an example of the code used to execute a simple script and give it access to part of the service-layer of the application:

Interpreter interpreter = new Interpreter();
interpreter.set("log", log);
interpreter.set("xxxxService", xxxxService);
interpreter.set("yyyyService", yyyyService);
interpreter.set("inputObject", someObject);
interpreter.eval(script);

After which it is possible to write scripts in the BeanShell language (which is very much like Java) to modify the object inputted, or as in my case validate some business-rules by making a few service-calls.