Limit object's access with SecurityManager?

I'm trying to solve how to code a system where an Java application loads
external classes with class loader. The problem is that those externally
loaded classes can be malicious so i must somehow limit their actions.

Is it valid solution to create a new SecurityManager that "denies
everything" only in the case that there is a class loaded with a certain
classloader in the stack?

Example source from
http://forum.java.sun.com/thread.js...4&thread=457210

public class MySecurityManager {

public MySecurityManager() {}

public void checkRead(String file) {
Class[] c = getClassContext(); // current execution stack

for (int i = 0; i < c.length; i++) {
ClassLoader cl = c[ i ].getClassLoader();
if (cl != null) {
if (cl.getClass().equals(MyClassLoader.class))
throw new SecurityException();
}
}
}
}


Michael Maier