Question

In our application, users are able to define Java expressions that are executed by our main engine (those expressions are method calls only: e.g., Math.abs(42) ). They’re executed via reflection.

What are the different solutions to prevent those expressions calling for example System.exit (but as well File access and others…) either directly or via a method call that will internally call eventually System.exit ?

Note that several different expressions can be executed in different threads. Preventing for example File access with a SecurityManager does not work as the main engine must still be able to access the File system while the expressions are executed.

Answer 1

You are wrong to say you can’t use the SecurityManager — this is precisely what it’s for: hosting untrusted code, as in an Applet container or RMI server. The modern SecurityManager is configured with policy files that grant specific, fine-grained rights, including limited access to the file system. You need to use the SecurityManager, but you need to become an expert in it.

This is an enormous topic; the best thing to do would be to Google “Java security policy files” and just read everything you can.