Many modern mobile devices, such as mobile phones or Personal Digital Assistants (PDAs), are able to run Java applications, such as games, Internet browsers, chat tools and so on. These applications perform some operations on the mobile device, that are critical from the security point of view, such as connecting to the Internet, sending and receiving SMS messages, connecting to other devices through the Bluetooth interface, browsing the user's contact list, and so on. Hence, an adequate security support is required to protect the device from malicious applications. This paper proposes an enhanced security support for next generation Java Micro Edition platform. This support performs a runtime monitoring of the operations performed by the Java applications, and enforces a security policy that defines which operations applications are allowed to perform. Two possible design approaches for the security support are presented and compared.