Luna: A Flexible Java Protection System

Extensible Java systems face a difficult trade-off between sharing and protection. On one hand, Java's ability to run different protection domains in a single virtual machine enables domains to share data easily and communicate without address space switches. On the other hand, unrestricted sharing blurs the boundaries between protection domains, making it difficult to terminate domains and enforce restrictions on resource usage. Existing solutions to these problems restrict sharing in an ad-hoc fashion, ruling out many desirable programming styles.

This paper presents an extension to Java's type system that systematically addresses the issues of data sharing, revocation, thread control, and resource control. Multiple tasks running in a single virtual machines share data using special remote pointers, which have different types from local pointers. The distinction between local and remote pointers allows the Java run-time system to mediate the communication between tasks without slowing down operations on ordinary pointers. The extensions to Java are implemented by a system called Luna, based on the Guavac and Marmot compilers, extended with special optimizations to support both fast inter-task communication and dynamic access control. The paper describes two applications written in Luna: a simple extensible web server, and an extension of the Squid web cache to support dynamic content generation.