Real Time meets Java

1
Real Time meets Java

• A presentation on Real Time Java
• The specification and its implementations

2
What is Java?

• Object-Oriented
• Garbage Collected
• Compiled/Interpreted (Virtual Machine)
• In all Something very far from real time

3
The Java Technology Stack
4
Adding Real Time to Java

• An API covers the necessary framework
• JSR-000001
• Just skeletal no implementation
• An output from the Java Community Process (JCP)

5
The RTJ Specification

• Defines 7 aspects of Real Time
• Scheduling
• Memory Management
• Synchronisation
• Asynchronous event handling
• Asynchronous control transfer
• Asynchronous thread termination
• Access to physical memory

6
The RTJ spec, contd.

• Scheduling encompasses various priority levels
  (28)
• Memory management in 3 forms
• Immortal (persistent throughout execution)
• Scoped (associated with thread memory space)
• Heap (traditional object heap)
• Note only the last one is garbage collected

7
The RTJ spec, contd.

• Synchronisation enforces use of the synchronized
  keyword, and uses a FIFO model.
• Asynchrony in events allows for hooks into code
  to be executed immediately

8
The RTJ spec, contd.

• Asynchronous control and termination allows
  threads to be paused in order to handle events,
  or even be terminated
• Access to physical memory
• Required for real time
• Not portable, against the ideals of Java
• Security issues?

9
Implementations

• Various implementations of the specification
  exist
• JTime TimeSys
• JamaicaVM
• jRate

10
JTime

• Reference Implementation
• Supports all of the RTJ specification
• Limited platform support
• Requires a vendor-modified Linux kernel
• Limited architecture support
• x86 and PowerPC
• Ahead of Time (AoT) Compilation

11
JamaicaVM

• Everything is real time e.g. threads
• Makes memory access semantics clearer
• Very small footprint (128KB)
• Runs on multiple real time OSes
• VxWorks, QNX, RT/Linux, etc.
• Runs on multiple architectures
• x86, SPARC, StrongARM, etc.
• Not fully compliant with RTJ spec

12
JRate

• Educational Research
• Aims to fully implement RTJ spec
• Add-in to existing GNU GCJ implementation
• Not reliable yet work in progress

13
Conclusion

• Safe at 35,000 feet?
• Probably better in environments that are not
  safety-critical.
• The first implementation released mid 2003.
• A final reminder

14

• The user acknowledges that Java is not
  designed or intended for use in the design,
  construction, operation or maintenance of any
  nuclear facility.
•  
• You acknowledge that Software may contain errors
  and is not designed or intended for use in
  on-line control of aircraft, air traffic,
  aircraft navigation or aircraft communications
  or in the design, construction, operation or
  maintenance of any nuclear facility ("High Risk
  Activities").
• The Java license agreements (J2SE, J2ME J2EE).