Real-Time Java and ATC

7/27/2019 Real-Time Java and ATC

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Real-time Java&

Asynchronous Transfer of Control

Jihua Zhong

Marc E Loy

Maung Han

Seminor in Real-time SystemsInstructor: Professor Insup Lee


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Different Flavours of Java

(JavaFamily)

Java

(J2SE,J2EE)

Personal

Java(J2ME)

Embedded

Java

ROM Size 4 MB


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Real-time Java

(A brief intro)


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What is Real-time Java (RTJ)?

Standard Java is not enough to handle real-time constraints.

Java (and JVM) lacks semantic for standardreal-time programming techniques.

Embedded Java Specification was there, butmerely a subset of standard Java API.

There is a gap for a language real-timecapable and equipped with all Javas powerfuladvantages.


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Real-time Spec. for Java (RTSJ)

IBM, Sun and other partners formedReal-time for Java Expert Group

sponsored by NIST in 1998.It came up with RTSJ to fill this gap for

real-time systems.

RTSJ proposed seven areas ofenhancements to the standard Java.


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RTSJ 7 Areas of Enhancement

1. Thread scheduling and dispatching.

2. Memory management.

3. Synchronization and Resource sharing.4. Asynchronous Event Handling.

5. Asynchronous Transfer of Control.

6. Asynchronous Thread Termination.7. Physical Memory Access.


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RTSJ Implementations

RTSJ is still under evaluation.

Sun has no practical implementation.

Some implementations by others:- VisualAge by IBM

- Simple RTJ by RTJ Computing

Theres another spec by J-consortium backedby Microsoft, HP.

Other just sit on the fence.


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Asynchrony


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Asynchrony

Asynchronous event handling andtransfer of control in execution.

Not all real-life events are predictablein time and frequency.

Ability of real-time systems to react to

them correctly requires reliableasynchrony techniques.


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Asynchrony (contd.)

Conventional ways of achieving it inJava is by interrupts and exceptions.

They are deadlock prone.Can cause data structure corruption.


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Asynchrony (contd.)

Leaving exiting techniques intact forconventional codes,

RTSJ offers two extended approachesfor real-time threads-

1. Asynchronous Events (AEv).

2. Asynchronously InterruptedExceptions (AIE).


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I. Asynchronous EventsSome internal or

external event thathappens.

System need torespond to thechangingenvironment.

Not known inadvance about timeor frequency.


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Class Hierarchy

AsyncEvent

Schedulable (I)

AsyncEventHandler

BoundAsyncEventHandler

AsyncEventHandler behavelike Thread. It has run()method.

BoundAsynchronousHandler

can be used for addedtimeliness by binding eachhandler to a dedicated thread.

Object

Runnable (I)


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Asynchronous Event (contd.)

Event1Event2

Event3

Handler AHandler B

One event can be handled by more than one handler.

One handler can handle more than one event.

Many to many relationship.


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AsyncEvent (how it works)

A Handler can choose to process itsfire-counts individually or collectively.

AsyncEvent

bindTo()Realworldevent

abstract class AsyncEventHandler:

run() // final

handleAsyncEvent()

getAndDecrementPendingFireCount()

addHandler()

Handler implementsSchedulable and Runnableinterfaces.


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Sample code for AsyncEvent handling

public static void main(String args){

AsyncEventHandler hdlrA = new AsyncEventHandler(){

public void handleAsyncEvent(){

do{

System.out.print(Handler A executed.);

} while(getAndDecrementPendingFireCount()>0);

}}


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AsyncEvent Sample Code

AsyncEvent event1 = new AsyncEvent();

event1.addHandler(hdlrA);

System.out.println(AsyncEvent Test.\n);

event1.fire();

System.out.println(Event fired.\n);

} // main()


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Use of AEv Handling

In real-time systems, there can be hundreds

(or even thousands) of possible events.

But usually, only few happens at one time.

Impractical to create and assign one threadfor each possible event.

AEv Handling offers low cost alternative to

threads.Not like methods, AEv handlers can be

scheduled and executed asynchronously.


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II. ATC

(Asynchronous Transfer of Control)

Some change inthe systemenvironment needs

immediateattention.

Abandon currentexecution or take

appropriate action.


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AIE(AsynchronouslyInterruptedException)

Interruptible (I)

Exception

Object

Throwable

InterruptedException

AsynchronouslyInterruptedException

Timed

Parameter forAIE.doInterruptible()


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AIE (contd.)

In RT Java, ATC is achieved by throwing aspecial exception AIE.

RT threads and methods can choose toreceive (or not to) AIE by including it inthrows clause.

e.g. void run() throwsasynchronouslyInterruptedException {}

Existing Non-RT implementations are notaffected.


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AIE (contd.)

AIE can be thrown to cut short a threadsexecution and take appropriate action.

It can be thrown explicitly by firing inprogram codes, or implicitly by interruptinga real-time thread.

e.g. aie.fire() or thread.interrupt()

If AIE is fired in a methods it did not declareAIE in throws clause, AIE is put to pendingstate until control reaches AIE-enabledmethod.


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Some rules on AIE

Only one AIE can become active at anyone time, no nested AIEs.

If newly generated AIE is less in depththan the pending AIE, it is replaced bynew one.

If newly generated AIE is deeper indepth, it is discarded.


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How AIE works

AIE Handler should invoke theAIE.happened() method to clear it.

Otherwise, it will continue to propagateoutward.

AIE method

Non-AIE method

AIE method (catch)

AIE fired

Deferred untilAIE method

AIE caughtand cleared


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When AIE is deferred

In AIE methods, it is deferred if controlis in synchronized block.

In non-AIE methods, deferred untilcontrol reaches in AIE method.

If both AIE and other exceptions

occurred when control enters AIEmethod, AIE overrides others.


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When AIEs overlap

Another AIE can get generated whileone is in execution.

If AIE of outer block is fired, executionstops and control transfers to outerblock.


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Asynchronous Thread Termination

AEv and AIE in RTJ provides ways tomanage execution loci in real-time

systems asynchronously.Using these two together can achieve

asynchronous termination of threads.


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Sample Implementation

Single ElevatorControl system.

Separate classes for: Carriage (floor pos,

door open?, moving?)

MotorControl (up,

down) PanelControl (buttons

pressed,next dest)


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Elevator (contd.)

Events;

A floor select button is pressed.

Elevator request button is pressed. Fire alarm sounded.

Exceptions;

Hazard (fire, blackout, etc.).


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AsyncEvent classes

class ButtonEvent extends AsyncEvent{

public ButtonEvent(String bindstring){

bindTo(bindstring);

}}

class FireAlarm extends AsyncEvent{

public FireAlarm(){

super();bindTo("FireSignal");

}

}


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Event Handlers (inner classes)

class ReqBtnHandlerextends AsyncEventHandler{

int flr;

ReqBtnHandler(int f){flr=f;}

publicvoid handleAsyncEvent(){floorReq[flr]=true;}}

class SelBtnHandlerextends AsyncEventHandler{

int flr;

SelBtnHandler(int f){flr=f;}publicvoid handleAsyncEvent(){floorSel[flr]=true;}

}

Code


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Hazard Handler(in Carriage class)

class HzHandlerextends AsyncEventHandler{

publicvoid handleAsyncEvent(){

aie.fire(); // fire AIE to interrupt control thread

// open the door

if(curFloor==0 && !moving) openDoor();

}

}

Code


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Linking Events to Handlers

// in PanelControl class

ButtonEvent sb0=new ButtonEvent("selbutton0"); // 1,2,

ButtonEvent rb0=new ButtonEvent("reqbutton0"); // 1,2,

sb0.addHandler(newSelBtnHandler(0)); // 1,2,

rb0.addHandler(newReqBtnHandler(0)); // 1,2,

// in Carriage class

HzHandler hh=new HzHandler();

e.addHandler(hh); // fire alarm is stored in attribute e of Carriage.

// similarly in PanelControl, share the same event.Need to link every single event and handler, cannot differentiatebetween different events of the same class. Better if we canpass instance specific data along with event. But RTSJ does notallow that.

Code

Code


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AIE exception

class HazardAie extends

synchronouslyInterruptedException{

public HazardAie(){super();}

}

HazardAie myAie= new HazardAie();

Carriage car=new Carriage(fa,myAie);

Somebody has to fire it. So pass to Carriage that

receive the fire event


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AIE handling

publicclass MotorControl implements Interruptible{

.

// action when AIE is fired.

publicvoidinterruptAction(AsynchronouslyInterruptedException e){

if(carriage.getFloor()>0){

carriage.closeDoor();

godown();}

}


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MotorControl (contd.)

// action when interruptible code is run

publicvoid run(AsynchronouslyInterruptedException e){

while(true){

try{myrun(e);}catch(AsynchronouslyInterruptedException

e1){}

}// while

}void myrun(AsynchronouslyInterruptedException e)

throws AsynchronouslyInterruptedException {

// normal control sequence

}


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Main body of program

publicclass ElevatorSim {

publicstaticvoid main(java.lang.String[] args) {

FireAlarm fa = new FireAlarm(); // create fire alarm

eventHazardAie myAie = new HazardAie();

Carriage car = new Carriage(fa,myAie);

PanelControl pc = new PanelControl(5,fa);

MotorControl mc = new MotorControl(car, pc, 4, 0);

myAie.doInterruptible(mc); // run interruptible code

}

}


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Memory Management


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Purpose

To allow memory areas of differentbehavior to be accessed by real-time

tasks.Specify memory consumption behavior

of RT tasks.

RT tasks can enter and exit memoryareas and allocate objects as needed.


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Memory Class Types

MemoryArea Heap Memory (normal Java Heap)

Scoped Memory (limited lifetime.)

Immortal Memory (app lifetime) ImmortalPhysicalMemory (app lifetime, physical)

PhysicalMemoryFactory (Supply physicalmem to other mem objects, e.g. DMA, Byte

swapping)RawMemoryAccess (Drivers, Memory

mapped I/O)


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Memory Class Hierarchy

Object

MemoryArea

HeapMemory

ImmortalPhysicalMemory

ScopedMemoryLTMemory

ImmortalMemory

ScopedPhysicalMemory

VTMemory

RawMemoryAcess

PhysicalMemoryFactory

RawMemoryFloatAcess


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Scoped Memory Rules

Reference to an object in ScopeMemory;

Can never be stored in an object in Java Heap.

Can never be stored in an object inImmortalMemory.

Can be stored in an object in the same scope orinner scope memory.

An object in Immortal or Heap may storereferences in ScopeMemory.


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ScopeMemory activation

An area of ScopeMemory can become activefor a thread by;

Calling enter() method of ScopeMemory object.

Creating instances of objects in ScopeMemory.

ScopeMemory areas can be activated innested manner.

Objects in an inner scope can refer to outerscopes, but not the other way.


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Conclusion

Java is trying to become real-time capable.

Most systems today are either non real-time

or implement only a subset of RTSJ.We expect to see more complete and robust

implementations soon with increasinglypopular embedded and real-time systems.

Documents

Real-Time Java and ATC