[PDF] Lecture 8 Java SE – Advanced Multithreading

1.ThreadsConcurrency&ParallelismLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671ThreadPoolTocreateanExecutorobject,usethestaticmethodsintheExecutorsclass.

1.ThreadsConcurrency&ParallelismLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

1.ThreadsConcurrency&ParallelismLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671importjava.util.concurrent.*; classPrintCharimplementsRunnable { privatecharcharacter;privateintnoOfTimes;PrintChar(charch, intn) {character= ch;noOfTimes= n;}publicvoidrun() {for(inti=0; i

1.ThreadsConcurrency&Parallelism

1.ThreadsConcurrency&Parallelismhttp://www.vogella.com/tutorials/JavaConcurrency/article.html"Classic"ThreadPool,ExecutorFramework,Callable&FuturepublicclassMyRunnableimplementsRunnable {privatefinallongcountUntil;MyRunnable(longcountUntil) {this.countUntil= countUntil;}@Overridepublicvoidrun() {longsum = 0;for(longi= 1; i< countUntil; i++) {sum += i;}System.out.println(sum);}}

1.ThreadsConcurrency&Parallelismhttp://www.vogella.com/tutorials/JavaConcurrency/article.html"Classic"ThreadPool,ExecutorFramework,Callable&Futureimport java.util.ArrayList;import java.util.List;publicclassProgMain{publicstaticvoidmain(String[] args) {// We will store the threads so that we can check if they are doneList threads = newArrayList();for(inti = 0; i < 500; i++) {Runnable task = newMyRunnable(10000000L + i);Thread worker = newThread(task);worker.setName(String.valueOf(i));// Start the thread, never call method run() directworker.start(); threads.add(worker);}intrunning = 0; do{running = 0; for(Thread thread: threads) {if(thread.isAlive()) { running++; }}System.out.println("We have "+ running + " running threads. ");} while(running > 0); } //end main} //end class

1.ThreadsConcurrency&Parallelismhttp://www.vogella.com/tutorials/JavaConcurrency/article.html"Classic"ThreadPool,ExecutorFramework,Callable&Futureimportjava.util.concurrent.ExecutorService;importjava.util.concurrent.Executors;publicclassMain {privatestaticfinalintNTHREDS= 10;publicstaticvoidmain(String[] args) {ExecutorServiceexecutor = Executors.newFixedThreadPool(NTHREDS);for(inti = 0; i < 500; i++) {Runnable worker = newMyRunnable(10000000L + i);executor.execute(worker);}// This will make the executor accept no new threads// and finish all existing threads in the queueexecutor.shutdown();// Wait until all threads are finishexecutor.awaitTermination();System.out.println("Finished all threads");}}

1.ThreadsConcurrency&Parallelismhttp://www.vogella.com/tutorials/JavaConcurrency/article.html"Classic"ThreadPool,ExecutorFramework,Callable&Future//package de.vogella.concurrency.callables;importjava.util.concurrent.Callable;publicclassMyCallableimplementsCallable { @OverridepublicLong call() throwsException { longsum = 0;for(longi = 0; i <= 100; i++) { sum += i;} returnsum; }}

1.ThreadsConcurrency&Parallelismhttp://www.vogella.com/tutorials/JavaConcurrency/article.htmlimportjava.util.*;importjava.util.concurrent.*;publicclassCallableFutures{privatestaticfinalintNTHREDS= 10;publicstaticvoidmain(String[] args) {ExecutorServiceexecutor = Executors.newFixedThreadPool(NTHREDS);List> list = newArrayList>();for(inti = 0; i < 20000; i++) {Callable worker = newMyCallable();Future submit = executor.submit(worker);list.add(submit);}longsum = 0;// now retrieve the result -System.out.println(list.size());for(Future future : list) { try{sum += future.get();} catch(InterruptedExceptione) {e.printStackTrace();} catch(ExecutionExceptione) {e.printStackTrace();}}System.out.println(sum);executor.shutdown();}}"Classic"ThreadPool,ExecutorFramework,Callable&Future

1.ThreadsConcurrency-SynchronizationLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671Asharedresourcemaybecorruptedifitisaccessedsimultaneouslybymultiplethreads.Example:twounsynchronizedthreadsaccessingthesamebankaccountmaycauseconflict.Stepbalance thread[i] thread[j]

10 newBalance = bank.getBalance() + 1;

20 newBalance = bank.getBalance() + 1;

31 bank.setBalance(newBalance);

41 bank.setBalance(newBalance);

1.ThreadsConcurrency-SynchronizationLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671Example:ShowingResourceConflict-AccountWithoutSynch.java•Objective:Writeaprogramthatdemonstratestheproblemofresourceconflict.Supposethatyoucreateandlaunchonehundredthreads,eachofwhichaddsapennytoanaccount.Assumethattheaccountisinitiallyempty.

Account

-balance: int +getBalance(): int +deposit(amount: int): void 100

AccountWithoutSync

-bank: Account -thread: Thread[] +main(args: String[]): void

AddAPennyTask

+run(): void java.lang.Runnable -char token +getToken +setToken +paintComponet +mouseClicked 1 1 1

1.ThreadsConcurrency-SynchronizationLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671RaceConditionWhat,then,causedtheerrorintheexample?Hereisapossiblescenario:•Effect: Task 1 did nothing (in Step 4 Task 2 overrides the result)•Problem: Task 1and Task 2are accessing a common resource in a way that causes conflict. •Known as a race conditionin multithreaded programs. •A thread-safeclass does not cause a race condition in the presence of multiple threads. •The Accountclass is not thread-safe.

Step balance Task 1 Task 2

1 0 newBalance = balance + 1;

2 0 newBalance = balance + 1;

3 1 balance = newBalance;

4 1 balance = newBalance;

synchronized•Problem:raceconditions•Solution:giveexclusiveaccesstoonethreadatatimetocodethatmanipulatesasharedobject.•Synchronizationkeepsotherthreadswaitinguntiltheobjectisavailable.•Thesynchronizedkeywordsynchronizesthemethodsothatonlyonethreadcanaccessthemethodatatime.•ThecriticalregionintheAccountWithoutSynch.javaistheentiredepositmethod.•Onewaytocorrecttheprobleminsourcecode:makeAccountthread-safebyaddingthesynchronizedkeywordindeposit:publicsynchronizedvoiddeposit(doubleamount)1.ThreadsConcurrency-SynchronizationLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

SynchronizingInstanceMethodsandStaticMethods•Asynchronizedmethodacquiresalockbeforeitexecutes.•Instancemethod:thelockisontheobjectforwhichitwasinvoked.•Staticmethod:thelockisontheclass.•Ifonethreadinvokesasynchronizedinstancemethod(respectively,staticmethod)onanobject,thelockofthatobject(respectively,class)isacquired,thenthemethodisexecuted,andfinallythelockisreleased.•Anotherthreadinvokingthesamemethodofthatobject(respectively,class)isblockeduntilthelockisreleased.1.ThreadsConcurrency-SynchronizationLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

Acquire a lock on the object account

-char token +getToken +setToken +paintComponet +mouseClicked

Execute the deposit method

-char token +getToken +setToken +paintComponet +mouseClicked

Release the lock

-char token +getToken +setToken +paintComponet +mouseClicked

Task 1

-char token +getToken +setToken +paintComponet +mouseClicked

Acqurie a lock on the object account

-char token +getToken +setToken +paintComponet +mouseClicked

Execute the deposit method

-char token +getToken +setToken +paintComponet

Release the lock

Task 2

-char token +getToken +setToken +paintComponet +mouseClicked

Wait to acquire the lock

-char token +getToken +setToken +paintComponet +mouseClicked

1.ThreadsConcurrency-SynchronizationLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

SynchronizingStatements•Invokingasynchronizedinstancemethodofanobjectacquiresalockontheobject.•Invokingasynchronizedstaticmethodofaclassacquiresalockontheclass.•Asynchronizedblockcanbeusedtoacquirealockonanyobject,notjustthisobject,whenexecutingablockofcode.synchronized(expr){statements;}•exprmustevaluatetoanobjectreference.•Iftheobjectisalreadylockedbyanotherthread,thethreadisblockeduntilthelockisreleased.•Whenalockisobtainedontheobject,thestatementsinthesynchronizedblockareexecuted,andthenthelockisreleased.1.ThreadsConcurrency-SynchronizationLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

SynchronizingStatementsvs.MethodsAnysynchronizedinstancemethodcanbeconvertedintoasynchronizedstatement.Supposethatthefollowingisasynchronizedinstancemethod:publicsynchronizedvoidxMethod(){//methodbody}ThismethodisequivalenttopublicvoidxMethod(){synchronized(this){//methodbody}}1.ThreadsConcurrency-SynchronizationLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

Same as ReentrantLock(false).

Creates a lock with the given fairness policy. When the fairness is true, the longest-waiting thread will get the lock. Otherwise, there is no particular access order. "interface» java.util.concurrent.locks.Lock +lock(): void +unlock(): void +newCondition(): Condition

Acquires the lock.

Releases the lock.

Returns a new Condition instance that is bound to this

Lock instance.

java.util.concurrent.locks.ReentrantLock +ReentrantLock() +ReentrantLock(fair: boolean)

1.ThreadsConcurrency-SynchronizationusingLocksLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

FairnessPolicy•ReentrantLock:concreteimplementationofLockforcreatingmutuallyexclusivelocks.•Createalockwiththespecifiedfairnesspolicy.•Truefairnesspoliciesguaranteethelongest-waitthreadtoobtainthelockfirst.•Falsefairnesspoliciesgrantalocktoawaitingthreadwithoutanyaccessorder.•Programsusingfairlocksaccessedbymanythreadsmayhavepooroverallperformancethanthoseusingthedefaultsetting,buthavesmallervariancesintimestoobtainlocksandguaranteelackofstarvation.1.ThreadsConcurrency-SynchronizationLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

Example:UsingLocksThisexamplerevisesAccountWithoutSync.javainAccountWithoutSyncUsingLock.javatosynchronizetheaccountmodificationusingexplicitlocks.1.ThreadsConcurrency-SynchronizationLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

"interface» java.util.concurrent.Condition +await(): void +signal(): void +signalAll(): Condition Causes the current thread to wait until the condition is signaled.

Wakes up one waiting thread.

Wakes up all waiting threads.

1.ThreadsConcurrency-SynchronizationLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

while (balance < withdrawAmount) newDeposit.await();

Withdraw Task

-char token +getToken +setToken +paintComponet +mouseClicked balance -= withdrawAmount -char token +getToken +setToken lock.unlock();

Deposit Task

-char token +getToken +setToken +paintComponet +mouseClicked lock.lock(); -char token +getToken +setToken +paintComponet +mouseClicked newDeposit.signalAll(); balance += depositAmount -char token +getToken +setToken +paintComponet +mouseClicked lock.unlock(); -char token lock.lock(); -char token +getToken +setToken +paintComponet +mouseClicked

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

Example:ThreadCooperationWriteaprogramthatdemonstratesthreadcooperation(ThreadCooperation.java).Supposethatyoucreateandlaunchtwothreads,onedepositstoanaccount,andtheotherwithdrawsfromthesameaccount.Thesecondthreadhastowaitiftheamounttobewithdrawnismorethanthecurrentbalanceintheaccount.Whenevernewfundisdepositedtotheaccount,thefirstthreadnotifiesthesecondthreadtoresume.Iftheamountisstillnotenoughforawithdrawal,thesecondthreadhastocontinuetowaitformorefundintheaccount.Assumetheinitialbalanceis0andtheamounttodepositandtowithdrawisrandomlygenerated.Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 01360126711.ThreadsCooperation

Java'sBuilt-inMonitors•LocksandconditionsarenewstartingwithJava5.•PriortoJava5,threadcommunicationswereprogrammedusingobject'sbuilt-inmonitors.•Locksandconditionsaremorepowerfulandflexiblethanthebuilt-inmonitor.•Amonitorisanobjectwithmutualexclusionandsynchronizationcapabilities.•Onlyonethreadcanexecuteamethodatatimeinthemonitor.•Athreadentersthemonitorbyacquiringalock(synchronizedkeywordonmethod/block)onthemonitorandexitsbyreleasingthelock.•Athreadcanwaitinamonitoriftheconditionisnotrightforittocontinueexecutinginthemonitor.•Anyobjectcanbeamonitor.Anobjectbecomesamonitoronceathreadlocksit.Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 01360126711.ThreadsSynchronization-Monitors

wait(),notify(),andnotifyAll()Usethewait(),notify(),andnotifyAll()methodstofacilitatecommunicationamongthreads.Thewait(),notify(),andnotifyAll()methodsmustbecalledinasynchronizedmethodorasynchronizedblockonthecallingobjectofthesemethods.Otherwise,anIllegalMonitorStateExceptionwouldoccur.Thewait()methodletsthethreadwaituntilsomeconditionoccurs.Whenitoccurs,youcanusethenotify()ornotifyAll()methodstonotifythewaitingthreadstoresumenormalexecution.ThenotifyAll()methodwakesupallwaitingthreads,whilenotify()picksuponlyonethreadfromawaitingqueue.Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 01360126711.ThreadsSynchronization-Monitors

synchronized (anObject) { try { // Wait for the condition to become true while (!condition) anObject.wait(); // Do something when condition is true catch (InterruptedException ex) { ex.printStackTrace();

Task 1

synchronized (anObject) { // When condition becomes true anObject.notify(); or anObject.notifyAll();

Task 2

resume

1.ThreadsSynchronization-MonitorsLiang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671

while (count == CAPACITY) notFull.await(); -char token +getToken +setToken +paintComponet +mouseClicked

Task for adding an int

-char token +getToken +setToken +paintComponet +mouseClicked

Add an int to the buffer

-char tokenquotesdbs_dbs11.pdfusesText_17