The high-level and portable nature of the Java platform allows applications to be written once and executed on all the supported systems. However, such a feature comes at the cost of hardware abstraction, making it more difficult or even impossible to access several low-level functionalities. Overseer is a Java framework that makes it possible on Linux systems by simplifying access to real-time measurement of low-level data such as Hardware Performance Counters (HPCs), IPMI sensors, and Java VM internal events. Overseer supports functionalities such as HPC-management, process/thread affinity settings, hardware topology identification, as well as power-consumption and temperature monitoring. In this paper we describe Overseer and how to use it to extend Java applications with functionalities not provided by the default runtime. A public release of Overseer is available.