Safe language technology can be used for protection within a single address space. This protection is enforced by the language's type system, which ensures that references to objects cannot be forged. A safe language alone, however, lacks many features taken for granted in more traditional operating systems, such as rights revocation, thread protection, resource management, and support for domain termination. This paper describes the J-Kernel, a portable Java-based protection system that addresses these issues. A number of micro-benchmarks are presented to characterize the costs of language-based protection, and an extensible web server based on the J-Kernel demonstrates the use of safe language techniques in a large application.