Java Security Model

Attributes

Focused on Protecting Users from Hostile Programs Downloaded from Untrusted Sources Across a Network

Accomplished via a Customizable "Sandbox" in which Java Programs Run

Sandbox for Untrusted Applets, for example, Prohibits Many Activities incl

Reading from or writing to local disk
Making a networ connection to any host, except the one from which the applet came
Creating a new process
Loading a new dynamic library or directly calling a native method

Java Cryptology

This is different from Java "Security" as we are discussing here. It is used to encrypt data that is used in applications and certainly provides "security" for such data and applications. It is provided by an entirel separate API of its own.