Concurrency i Java – processer, kommunikation og synkronisering.

Concurrency i Java – processer, kommunikation og

synkronisering

Michael E. Caspersen, 2001 PK2, F2001 concurrency.2

1. Oversigt

• Baggrund

• Concurrency i Java– tråde (Thread og Runnable)– Javas concurrency-model

• tilstande• operationer

– kritiske regioner (låsning)• synchronized• wait, notify, notifyAll• monitor, common class (readers/writers)

– Eksempler• konto• kommunikationskanal• Conways problem


Baggrund

• Motivation– speed-up– modellering– distribuering

• Historie– guldalder: 1965-75– flerbruger-operativsystemer– programmeringssprog


Tråde i Java (eksempel)class Traade {

static class Proces extends Thread { String name;

Proces(String name) { this.name = name; } int random(int i, int j) { ... } public void run() { for (int i=0; i!=10; ++i) { System.out.println(name); try {sleep(random(0,10)); } catch (InterruptedException e) {} } } }

public static void main(String[] args) { Proces a = new Proces("aa"); Proces b = new Proces("BBBB"); a.start(); b.start(); }}


Javas concurrency-model

Created Terminated Running Non-runnable Runnable

start

stop

yield

run

stop

stop/end

dispatch

sleep timeout

Depricated:- stop- suspend- resume

Men det skal I sådan set ikke tænke på!


Bankkundeclass Customer extends Thread { String name; Account a;

Customer(String name, Account a) { this.name = name; this.a = a; } int random(int i, int j) { ... }

public void run() { for (int i=0; i!=10; ++i) { int n = random(0,10); boolean succes = a.withdraw(n); System.out.println(name+": " + n + " sucess: "+succes); try { sleep(random(0,10)); } catch (InterruptedException e) {} } }}


Kontoclass Account { privte int amount = 0; public void insert(int i) { amount = amount + i; }

public boolean withdraw(int i) { if (amount > i) { amount = amount - i; return true; } else { return false; } }}

public static void main(String[] args) { Account ac = new Account(); ac.insert(50); Customer a = new Customer("aa",ac); Customer b = new Customer("BBBB",ac); a.start(); b.start();}


Sikker konto, synchronized

class Account { privte int amount = 0; public synchronized void insert(int i) { amount = amount + i; }

public synchronized boolean withdraw(int i) { if (amount > i) { amount = amount - i; return true; } else { return false; } }}

Udførelse af en synkroniseret metode bevirker at objektet (i dette tilfælde kontoen) låses så andre synkroniserede metoder på samme objekt ikke kan udføres samtidig!


Monitor

• Klasse med udelelige metoder

• Abstraktion der letter programmering med parallelle processer

• Ikke direkte understøttet i Java, men kan simuleres vha. synchronized

• Mange standardklasser i Java er (eller kanvha wrapperklasse instansieres som) monitorer– List data = Collections.synchronizedList(new LinkedList)


En kommunikationskanal (1)

class Channel { Object p; boolean empty = true;

synchronized public void transmit(Object p) { ??? this.p = p; empty = false; }

synchronized public Object receive() { ??? empty = true; return p; }}

Et synkroniseringsproblem...




synchronized public void transmit(Object p) { await (empty); this.p = p; empty = false; }

synchronized public Object receive() { await (!empty); empty = true; return p; }}

En ”løsning” med await (ej Java)




synchronized public void transmit(Object p) { while (!empty) { try { wait(); } catch (InterruptedException ie) {} } this.p = p; empty = false; notifyAll(); }

synchronized public Object receive() { while (empty) { try { wait(); } catch (InterruptedException ie) {} } empty = true; notifyAll(); return p; }}


Et modelleringseksempel

Edit Find

Adjust Collect

Produktion af avisspalter

SkrivemaskineIndlæser teksten en linie ad gangen

TokenizerOpdeler teksten i ord

BundtlerBundtler ord til linie

PrinterFordeler ekstra blanktegn mellem ordene på linien

Channel ofString

Channel ofString

Channel ofBunch


Hjælpeklasser

class Channel { synchronized public void transmit(Object p) { ... } synchronized public Object receive() { ... }}

class Bunch { int blanks; String[] words;

Bunch(int blanks, String[] words) { this.blanks = blanks; this.words = words; }

// i eksemplet tilgås blanks og words direkte!!! // men her kunne være get-metoder ...}


Hovedprogram

class Spalte {

public static void main(String[] args) { Channel ll = new Channel(); Channel ww = new Channel(); Channel bb = new Channel(); Edit er = new Edit("demo.txt",ll); Find fi = new Find(ll,ww); Collect co = new Collect(ww,bb,38); Adjust ad = new Adjust(bb); ad.start(); co.start(); fi.start(); er.start(); }}


Editclass Edit extends Thread { BufferedReader in; String inname; Channel out;

Edit(String inname, Channel out) { this.inname = inname; this.out = out; }

public void run() { try { in = new BufferedReader(new FileReader(inname)); while(in.ready()) { String s = in.readLine(); out.transmit(s); } in.close(); } catch (IOException e) {} }}


Find

class Find extends Thread {

Channel in; Channel out;

Find(Channel in, Channel out) { this.in = in; this.out = out; }

public void run() { while (true) { StringTokenizer s = new StringTokenizer((String) in.receive()," "); while (s.hasMoreTokens()) { out.transmit(s.nextToken()); } } }}


Collectclass Collect extends Thread { Channel in; Channel out; int col_width;

Collect(Channel in, Channel out, int col_width) { this.in = in; this.out = out; this.col_width = col_width; }

public void run() { String[] words = new String[col_width]; int no_chars = 0; int no_words = 0; while (true) { String s = (String) in.receive(); if (no_chars + no_words + s.length() > col_width) { String[] l = new String[no_words]; for (int i=0; i!=no_words; ++i) l[i] = words[i]; out.transmit(new Bunch(col_width-no_chars-no_words+1,l)); no_words = 0; no_chars = 0; } words[no_words] = s; ++no_words; no_chars = no_chars+s.length(); } }}


Adjust (1)

class Adjust extends Thread { Channel in;

Adjust(Channel in) { this.in = in; } int random(int i, int j) { ... }

public void run() { while (true) { Bunch b = (Bunch) in.receive(); String[] blanks = distribute(b); for (int i=0; i!= b.words.length; ++i) { System.out.print(b.words[i]); System.out.print(blanks[i]); } System.out.println(); } }}


Adjust (2)

String[] distribute(Bunch b) { String[] blanks = new String[b.words.length]; for (int i=0; i!=blanks.length-1; ++i) blanks[i] = " "; blanks[blanks.length-1] = ""; while (b.blanks>0) { int i = random(0,blanks.length-1); blanks[i] = blanks[i]+" "; --b.blanks; } return blanks; }


Kritik af monitorer

Monitor

Writer

Readers

. . .

En monitor sikrer gensidig udelukkelse på samtlige operationer, men det er ofte unødvendig forsigtigt.


Alternativ: Common class (1)

Monitor

Writer

Readers

. . .


Alternativ: Common class (2)

Monitor

Writer

Readers

. . .


Common Class

RWBuffer = common class var state nr, nw: integer; data item: T;

procedure entry write(e: T); begin entry: [when nw + nr = 0 do nw:= nw + 1] body: item:= e exit: [nw:= nw - 1] end;

procedure entry read(var e: T); begin entry: [when nw = 0 do nr:= nr + 1] body: e:= item exit: [nr:= nr - 1] end;

begin nr:= 0; nw:= 0; item:= ... end

entry- og exit-statements udføres udeleligt og med gensidig udelukkel-se i forhold til alle andre entry- og exit-statements i klassen.

Concurrency i Java – processer, kommunikation og synkronisering.

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