Transcript public class PlatformAccess
START
Translation of process algebras to Java
Paul Bilokon Samuel Lau Andrew Roberts
Introduction
Modelling Finite State Processes (FSP) Threads and monitors Mission statement
Modelling
Simplicity: abstraction from irrelevant details Determination of relevant factors Predict long term behaviour “What if” scenarios
Finite State Processes (FSP)
“A process calculus is a small language that allows us to give precise descriptions of the essential properties of concurrent and communicating programs.”
FSP is a
process calculus
.
FSP is
finite
, so we can do proofs, safety checks, etc.
Example:
SUPREMA_GROUP = (enter -> present -> leave -> SUPREMA_GROUP).
AUDIENCE = (yawn -> sleep -> wakeUp -> AUDIENCE).
||FUN = (SUPREMA_GROUP || AUDIENCE).
Wishful thinking: yawn sleep enter wakeUp present leave …
Mission statement
Investigate
the relationship between FSP and Java
Restrict
the subset of FSP under consideration Consider
examples
, develop the
theory
Discover interesting patterns and methods
Formalize
it, if possible, using mathematical notation
Evaluate
the results. Refine the model, if necessary
Could we use these results to build an automated FSP to Java translator? Would this be a useful tool?
Translation
The action prefix and simple processes Guarded actions Choice Variables Simple parallel composition Parallel composition of dependent processes Other FSP constructs
Quick FSP Recap
Actions
P1 = (a1 -> a2 -> … -> an -> STOP).
Guarded Actions
P = (when(B) a -> STOP).
Choice
DRINKS_MACHINE = (red -> coffee -> DRINKS_MACHINE |blue -> tea -> DRINKS_MACHINE).
FSP Processes Java classes FSP Actions Java Methods Guards While-wait loop Choice External factor allows choice
Variables
range T = 0..5
STORE = STORE[0], STORE[i:T]=(put[i:T]->STORE[i]).
FSP Constructor Java constructor
public class Store { private int i; public Store() { this.i = 0; } synchronized public int put(int i) { this.i = i; } } //END class
STORE has a state variable, represented in Java as a field variable ‘put’ has an action variable, represented as a parameter for the put method
Simple parallel composition
||COMPOSITION = (a1 || a2 || … || an), No shared actions!
public class Composition { protected A1 _a1; protected A2 _a2; … protected An _an; } public Composition() { new Thread(_a1 = new A1()); new Thread(_a2 = new A2()); … new Thread(_an = new An()); }
Instantiate and start threads Non-simple composition uses similar composite class
Composition: caller/callee pattern
Two shared actions: designate one the caller and the other callee Caller is part of a thread Calls callee method, which is part of a monitor Only works with 2 shared actions!
public class Caller implements Runnable { public class Callee { public run() { … Callee.a(); … } } } synchronized public void a() { //do something }
Composition: semaphores
Uses semaphores – not intuitive Java Works for many shared actions – can become complex!
BILL = (play -> meet -> eat -> STOP).
BEN = (work -> meet -> sleep -> STOP).
public class Bill implements Runnable { public void run() { play(); release(A); acquire(B); eat(); } } //END class public class Ben implements Runnable { public void run() { work(); acquire(A); release(B); sleep(); } } //END class
General composition of processes
P1 = (… -> a -> … -> STOP).
P2 = (… -> a -> … -> STOP).
… P(n – 1) = (… -> a -> … -> STOP).
Pn = (… -> a -> … -> STOP).
Method 1.
Extend the semaphore method . For n shared methods, (2n-2) semaphores required.
Initialize to an ‘acquired’ state.
Method 2.
Synchronization object.
Uses Java’s inbuilt synchronization.
This object is a monitor and counts in the shared actions. Once they have all ‘reported in’ it will let all the threads continue.
Case study: Roller Coaster
Apology Monitors and threads revisited Caller/callee pattern Problem: parameters or return values?
Problem: action order Well-formed FSP
Caller/callee pattern
Only one type of process interaction – monitor/thread.
In general, this is the most common form of process communication.
‘Directionality’ is an important criterion for preferring this design pattern. E.g. the thread
Passengers
tells the monitor
Controller
that a
newPassenger
has arrived.
Problem: parameters vs return
COASTERCAR
has action
getPassenger[i:1..MCar]
(cf. Prolog).
CONTROLLER
has action
getPassenger[carSize]
. The variable carSize is bound.
A method of
Coastercar Controller
(monitor)?
with a ‘free’ variable (thread) to be called from Use return values instead:
synchronized public int getPassenger()
in Controller.
Problem: action order
PLATFORMACCESS = (arrive -> depart -> PLATFORMACCESS).
PLATFORMACCESS = ({arrive -> depart} -> PLATFORMACCESS).
public class PlatformAccess { public boolean arrive_done = false; synchronized public void arrive() { while (arrive_done) wait(); arrive_done = true; } } synchronized public void depart() { while (! arrive_done) wait(); arrive_done = false; }
Well-formed FSP
Is this translation ‘natural’?
Does Java ‘match’ FSP?
Is it easy for an automated FSP2Java program to spot this?
Could ask the user to re-write the FSP to comply with a well-formed FSP standard.
For example…
Well-formed FSP II
PLATFORMACCESS = (arrive -> depart -> PLATFORMACCESS).
PLATFORMACCESS = PLATFORMACCESS[0], PLATFORMACCESS[i:0..1] = (when(i=0) arrive->PLATFORMACCESS[1] |when(i=1) arrive->PLATFORMACCESS[0] ).
Conclusions
What is a good translation?
A subset of FSP Limitation Automatic Translation Further work FSP modification Validation of translation Rest of FSP
What is a good translation?
The factors are: Complexity Readability We would like to read and understand the Java code easily. We have found out that the Caller/Callee pattern is the most intuitive. However, this pattern cannot be used for more than 2 shared actions.
Limitation
Caller/Callee pattern: Monitor? Thread?
Data flow between processes can be difficult to translate. One solution is to rewrite the FSP and merge the actions.
A FSP can send more than 1 item of data but Java cannot.
Automatic translation
Given more time, we may write a program that automatically translates FSP to Java.
Closer relation between FSP and Java code Humans have benefit of context, which will effect implementation Is this feasible?
FSP modification
We may rewrite the FSP in some circumstances.
Aim: Make the translation more effective and concise.
Validation of translation
We are able to translate a large subset of FSP.
But how can we prove that the Java code actually corresponds to the FSP given?
Very difficult: Logic reasoning?
I Want More!
You are dazzled, fascinated, intrigued…
where do you want to go today?
Prof. Magee’s home page: http://www.doc.ic.ac.uk/~jnm/ Our project home page: http://www.doc.ic.ac.uk/~pb401/ Suprema/ Web articles and project report.
Test yourself – try Q&A!