Abstract: Though Java is expected to have a wide range of embedded applications, there exist some problems in applying Java to real-time systems. The hybrid architecture, in which the Java runtime environment is implemented on a real-time OS, parts of the application requiring real-time properties are implemented on the real-time OS, and parts of the application requiring GUI features, extension of features, and replacing of programs should be implemented on the Java runtime environment, is practical. In this paper, we present interface specifications between Java threads and real-time tasks based the hybrid architecture we build, called JTRON specification, and experiences based on application development using its implementation. First, cooperative computation models between Java threads and real- time tasks in the architecture are described. Based on these models, we categorize three types of communication interface between Java threads and real-time tasks: real-time OS attach class, shared object with explicit locking mechanism, and stream-based communication. By standardizing the interface, not only the Java program, but also the real-time program becomes portable and reusable. Two of the three types of the interface can be applied to other real-time OSes also .