java - When should AccessController.doPrivileged() be used?

Question

If I understand AccessController.doPrivileged correctly, it is saying that untrusted code should be able to invoke methods requiring permissions (such as System.getProperty()) through an intermediate method that does have permissions.

That brings up the question: when should AccessController.doPrivileged() be used? When should untrusted code be allowed to invoke privileged code through intermediate methods? When should it fail?

Following your reasoning, please explain why ClassLoader creation should always be allowed: http://findbugs.sourceforge.net/bugDescriptions.html#DP_CREATE_CLASSLOADER_INSIDE_DO_PRIVILEGED

Answer 1

Agree with Suraj's answer, but thought I'd add a specific example where I've required the use of a privileged block.

Imagine you've built an application that provides a number of services to pluggable modules. So your app and its services are trusted code. The pluggable modules, however, are not necessarily trusted and are loaded in their own class loaders (and have their own protection domains).

When a pluggable module invokes a service, you are implementing custom security checks ("does pluggable module X have permission to use this service"). But the service itself might require some core Java permission (read a system property, write to a file, etc). The code that requires these permissions is wrapped in a doPrivileged() so that the insufficient permissions from the untrusted pluggable modules are effectively ignored - only the privileges of your trusted services module apply.