code: public Set<String> restrictedClasses() {
          if (null == _restrictedClasses) {
              String s = get(SANDBOX_RESTRICTED_CLASS);
              s += ";org.rythmengine.Rythm;RythmEngine;RythmSecurityManager,java.io;java.nio;java.security;java.rmi;java.net;java.awt;java.applet";
              _restrictedClasses = new HashSet<String>();
              for (String cls : Arrays.asList(s.split(";"))) {
                  cls = cls.trim();
                  if ("".equals(cls)) {
                      continue;
                  }
                  _restrictedClasses.add(cls);
              }
          }
          return new HashSet<String>(_restrictedClasses);
      }
docs: Return {@link RythmConfigurationKey#SANDBOX_RESTRICTED_CLASS} without lookup
  <p/>
  <p>Note, the return value also contains rythm's built-in restricted classes</p>
  
  @return a set of restricted classes