Java SecurityManager @Override public void checkPermission(Permission perm)

Question

I'm building a SWING application and also need to write a custom SecurityManager. If I write an empty class which extends SecurityManager like this

public class Sandbox extends SecurityManager {}

it works fine, meaning that the GUI is rendered correctly and all privileges like I/O are revoked. However I need to customize the checkPermission method and whenever I override it nothing works anymore... Why even something like this shouldn't work??

public class Sandbox extends SecurityManager {
  @Overide
  public void checkPermission(Permission perm) {
    super.checkPermission(perm);
  }
}

Update: a very basic example that shows the problem is this

public static void main(String[] args) {

    System.setSecurityManager(new SecurityManager() {
        @Override
        public void checkPermission(Permission p) {
            if (some_condition_here) {
              // Do something here
            } else {
              // Resort to default implementation
              super.checkPermission(p);
            }
        }
    });

    new JFrame().setVisible(true);

}

Removing the "checkPermission" method the application works correctly, but I really can't get my head around this.

Answer 1

The permissions are granted based on all the code on the stack. All callers must have the required permission. If you override the method and call the superclass method, your code is on the stack as well which implies that your codebase (where your custom SecurityManager belongs to) must have the permission you (your callers) ask for.

That’s the difference between overriding or not. If you don’t override that method only the (possibly privileged) caller’s code is on the stack and it will get the requested permission. If you override that method your code is also on the stack and must have the permission as well.

So if you want to implement a custom SecurityManager which invokes the inherited check method you must configure the inherited (policy based) logic to give your SecurityManager all permissions it should be able to grant. It’s recommended to separate the SecurityManager from the rest of the application into a different codebase so only the SecurityManager and nothing else gets the generous permissions.