1 G53SRP: Introduction to Real Time Specification for Java (RTSJ) Chris Greenhalgh School of Computer Science Including material © Andy Wellings from his site supporting the course text

2 Contents BackgroundBackground –NIST requirements –Principles OverviewOverview RTSJ Time-lineRTSJ Time-line Further readingFurther reading Book: Wellings 7 (part)Book: Wellings 7 (part)

Background and NIST Requirements In the late 1990s: US National Institute of Standards and Technology (NIST) guiding principles and a set of requirements for real- time extensions to the Java platformIn the late 1990s: US National Institute of Standards and Technology (NIST) guiding principles and a set of requirements for real- time extensions to the Java platform Including: Real-Time Java (RTJ) shouldIncluding: Real-Time Java (RTJ) should –take into account current real-time practices and –facilitate advances in the state of the art of real- time systems implementation technology

Support for Current State of Practice Fixed priority and round robin schedulingFixed priority and round robin scheduling Mutual exclusion locking (avoiding priority inversion)Mutual exclusion locking (avoiding priority inversion) Inter-thread communication (e.g. semaphores)Inter-thread communication (e.g. semaphores) User-defined interrupt handlers and device drivers — including the ability to manage interrupts (e.g., enabling and disabling)User-defined interrupt handlers and device drivers — including the ability to manage interrupts (e.g., enabling and disabling) Timeouts and aborts on running threadsTimeouts and aborts on running threads The NIST group recognized that profiles of RTJ were necessary in order to cope with the wide variety of potential applications, these included: safety critical, no dynamic loading, and distributed real-time profiles

Implementations must provide A framework for finding available profilesA framework for finding available profiles Bounded pre-emption latency on any garbage collectionBounded pre-emption latency on any garbage collection A well-defined model for real-time Java threadsA well-defined model for real-time Java threads Communication and synchronization between real-time and non real- time threadsCommunication and synchronization between real-time and non real- time threads Mechanisms for handling internal and external asynchronous eventsMechanisms for handling internal and external asynchronous events Asynchronous thread terminationAsynchronous thread termination Mutual exclusion without blockingMutual exclusion without blocking The ability to determine whether the running thread is real-time or non real-timeThe ability to determine whether the running thread is real-time or non real-time A well-defined relationship between real-time and non real-time threadsA well-defined relationship between real-time and non real-time threads

RTSJ Guiding Principles Be backward compatible with non real-time Java programsBe backward compatible with non real-time Java programs Support the principle of “Write Once, Run Anywhere” but not at the expense of predictabilitySupport the principle of “Write Once, Run Anywhere” but not at the expense of predictability Address the current real-time system practice and allow future implementations to include advanced featuresAddress the current real-time system practice and allow future implementations to include advanced features Give priority to predictable execution in all design trade- offsGive priority to predictable execution in all design trade- offs Require no syntactic extensions to the Java languageRequire no syntactic extensions to the Java language Allow implementers flexibilityAllow implementers flexibility NB Real Time Java functionality requires Real Time OS Support!

Overview of Enhancements The RTSJ enhances Java in the following areas:The RTSJ enhances Java in the following areas: –memory management –time values and clocks –schedulable objects and scheduling –real-time threads –asynchronous event handling and timers –asynchronous transfer of control –synchronization and resource sharing –physical memory access

Warning To date, RTSJ only really addresses the execution of real-time Java programs on a single processor systems. It attempts not to preclude execution on a shared-memory multiprocessor systems but it has no facilities directly to control, say, allocation of threads to processors.

Real Time Specification for Java (RTSJ) time-line Version 1.0 – JSR-1 – Version 1.0 – JSR-1 – –Version – 2005 –Version – 2006 Version 1.1 – JSR-282 – in progressVersion 1.1 – JSR-282 – in progress –Mainly incremental fixes and enhancements but starting to include multi-processor supportbut starting to include multi-processor support N.B. RTSJ = SpecificationN.B. RTSJ = Specification –RI = Reference Implementation (but not only implementation) 9

Further Reading The NIST requirements: Carnahan, L., and Ruark, M. (Eds) (1999), “Requirements for Real-time Extensions for the Java Platform”, NIST Publication , NIST requirements: Carnahan, L., and Ruark, M. (Eds) (1999), “Requirements for Real-time Extensions for the Java Platform”, NIST Publication , The RTSJ Homepage RTSJ Homepage “JSR-282 Status Report”, “JSR-282 Status Report”,