Class notes(week 9) on multithreading

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Class Notes on Multithreading (Week - 9)
Contents:- Basicsof multithreading,mainthread,threadlifecycle,creationof multiplethreads,threadsynchronization,
interthreadcommunication,deadlocksforthreads,suspending&resumingthreads.
Multithreading in Java
Multithreadingisaprocessof executingmultiple threadssimultaneously.Threadisbasicallya lightweight subprocess, a
smallest unit of processing. Multiprocessing and multithreading, both are used to achieve multitasking. But we use
multithreading than multiprocessing because threads share a common memory area. They don't allocate separate
memory area so save memory, and context-switching between the threads takes less time than processes.
Multithreading is mostly used in games, animation etc.
Multitasking
Multitaskingisaprocessof executingmultiple taskssimultaneously.We use multitaskingtoutilize the CPU.Multitasking
can be achievedbytwoways:
 Process-basedMultitasking(Multiprocessing)
 Thread-basedMultitasking(Multithreading)
1) Process-basedMultitasking(Multiprocessing)
 Each process has itsown addressinmemoryi.e.eachprocessallocates separate memoryarea.
 Processisheavyweight.
 Cost of communicationbetweenthe processes ishigh.
 Switchingfromone process toanotherrequire some time forsavingandloadingregisters,memorymaps,
updatinglistsetc.
2) Thread-basedMultitasking(Multithreading)
 Threadsshare the same addressspace.
 Threadis lightweight.
 Cost of communicationbetweenthe threadislow.
 Note:At leastone processisrequiredforeachthread.
What is Thread?
A threadisa lightweightsubprocess,asmallestunitof processing.Itisaseparate pathof execution.Itsharesthe
memoryareaof process.
As showninthe above figure, thread(t1,t2 and t3) isexecutedinside the process. There is context-switching between
the threads. There can be multiple processes inside the OS and one process can have multiple threads.
Note:At a time onlyone thread isexecuted.

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Life cycle of a Thread (Thread States)
A thread can be in one of the five states in the thread. According to sun, there is only 4 states new, runnable, non -
runnable andterminated.There isnorunningstate.Butforbetterunderstandingthe threads,we are explainingitin the
5 states. The life cycle of the thread is controlled by JVM. The thread states are as follows:
1. New
2. Runnable
3. Running
4. Non-Runnable(Blocked)
5. Terminated
1)New
The thread isinnewstate if youcreate an instance of Threadclass butbefore the invocationof start() method.
2)Runnable
The thread isinrunnable state afterinvocationof start() method,butthe threadschedulerhasnotselectedittobe the
runningthread.
3)Running
The thread isinrunningstate if the threadschedulerhasselectedit.
4)Non-Runnable(Blocked)
Thisis the state whenthe threadisstill alive,butiscurrentlynoteligibletorun.
5)Terminated
A threadisin terminatedordeadstate whenitsrun() methodexits.
How to create thread:
There are twowaysto create a thread:
1. By extendingThreadclass
2. By implementingRunnableinterface.
Thread class:
Threadclass provide constructorsandmethodstocreate and performoperationsonathread. Threadclassextends
Objectclassand implementsRunnable interface.

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CommonlyusedConstructorsofThread class:
 Thread()
 Thread(Stringname)
 Thread(Runnabler)
 Thread(Runnabler,Stringname)
CommonlyusedmethodsofThreadclass:
1. publicvoid run():is usedto performactionfora thread.
2. publicvoid start(): starts the executionof the thread.JVMcallsthe run() methodonthe thread.
3. publicvoid sleep(longmiliseconds): Causesthe currentlyexecutingthreadtosleep(temporarilycease
execution)forthe specifiednumberof milliseconds.
4. publicvoid join():waitsfora threadto die.
5. publicvoid join(longmiliseconds): waitsfora threadto die forthe specifiedmiliseconds.
6. publicint getPriority():returnsthe priorityof the thread.
7. publicint setPriority(intpriority): changesthe priorityof the thread.
8. publicString getName():returnsthe name of the thread.
9. publicvoid setName(Stringname): changesthe name of the thread.
10. publicThread currentThread(): returnsthe reference of currentlyexecutingthread.
11. publicint getId():returnsthe idof the thread.
12. publicThread.State getState():returnsthe state of the thread.
13. publicbooleanisAlive():testsif the threadisalive.
14. publicvoid yield():causesthe currentlyexecutingthreadobjecttotemporarilypause andallow otherthreadsto
execute.
15. publicvoid suspend():isusedtosuspendthe thread(depricated).
16. publicvoid resume():isusedtoresume the suspendedthread(depricated).
17. publicvoid stop():is usedtostop the thread(depricated).
18. publicbooleanisDaemon():testsif the threadis a daemonthread.
19. publicvoid setDaemon(booleanb):marksthe threadas daemonor userthread.
20. publicvoid interrupt():interruptsthe thread.
21. publicbooleanisInterrupted(): testsif the threadhasbeeninterrupted.
22. publicstatic booleaninterrupted(): testsif the currentthreadhasbeeninterrupted.
Runnableinterface:
The Runnable interface shouldbe implementedbyanyclasswhose instancesare intendedtobe executedbyathread.
Runnable interface have onlyone methodnamedrun().
1. publicvoid run():is usedto performactionfora thread.
Startinga thread:
start() methodof Threadclass isusedto start a newlycreatedthread.Itperformsfollowingtasks:
 A newthreadstarts(withnewcallstack).
 The thread movesfromNewstate tothe Runnable state.
 Whenthe threadgets a chance to execute,itstargetrun() methodwillrun.

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1)ByextendingThreadclass:
1. class Multi extends Thread{
2. public void run(){
3. System.out.println("thread is running...");
4. }
5. public static void main(String args[]){
6. Multi t1=new Multi();
7. t1.start();
8. }
9. }
Output:thread is running...
Who makes yourclassobjectasthread object?
Thread class constructor allocatesanewthreadobject. Whenyoucreate objectof Multi class, yourclass constructoris
invoked (providedbyCompiler) from where Thread classconstructorisinvoked (bysuper() asfirststatement). Soyour
Multi class objectisthreadobjectnow.
2)Byimplementingthe Runnableinterface:
1. class Multi3 implements Runnable{
4. }
5.
7. Multi3 m1=new Multi3();
8. Thread t1 =new Thread(m1);
9. t1.start();
10. }
11. }
Output:thread is running...
If you are notextendingthe Threadclass, yourclassobjectwouldnot be treatedasa threadobject. Soyouneedto
explicitly create Threadclassobject. We are passingthe objectof yourclass thatimplementsRunnablesothatyourclass
run() methodmayexecute.
How to perform single task by multiple threads?
If you have to performsingle taskbymanythreads,have onlyone run() method. Forexample:
1. //Program of performing single task by multiple threads
2.
5. System.out.println("task one");
6. }

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11.
12. t1.start();
13. t2.start();
14. t3.start();
15. }
16. }
Output:task one
task one
task one
1. //Program of performing single task by multiple threads by implementing
Runnable.
2.
3. class Multi3 implements Runnable{
6. }
7.
9. Thread t1 =new Thread(new Multi3());//passing annonymous obj of Multi3 class
10. Thread t2 =new Thread(new Multi3());
11.
12. t1.start();
13. t2.start();
14.
15. }
16. }
Output:task one
task one
Note: Each thread runin a separatecallstack.
Howto performmultipletasksbymultiple threads(multitasking inmultithreading)?
If you have to performmultipletasksbymultiple threads,have multiplerun() methods. Forexample:
1. // Program of performing two tasks by two threads.
2.
3. class Simple1 extends Thread{
6. }
7. }
8.

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9. class Simple2 extends Thread{
11. System.out.println("task two");
12. }
13. }
14.
15. class Test{
17. Simple1 t1=new Simple1();
18. Simple2 t2=new Simple2();
19.
20. t1.start();
21. t2.start();
22. }
23. }
Output:task one
task two
Sameexampleas abovebyannonymousclassthat implementsRunnableinterface:
1. //Program of performing two tasks by two threads.
2.
3. class Test{
5. Runnable r1=new Runnable(){
8. }
9. };
10.
11. Runnable r2=new Runnable(){
13. System.out.println("task two");
14. }
15. };
16.
17. Thread t1=new Thread(r1);
18. Thread t2=new Thread(r2);
19.
20. t1.start();
21. t2.start();
22. }
23. }
Output:task one
task two
The Thread Schedular:
 The thread scheduleristhe partof the JVMthat decideswhichthreadshouldrun.
 There isno guarantee thatwhichrunnable threadwill be chosentorunby the thread scheduler.
 Onlyone threadat a time canrun in a single process.
 The thread schedulermainlyusespreemptiveortime slicingschedulingtoschedule the threads.

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What is the differencebetweenpreemptiveschedulingandtimeslicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher
prioritytaskcomesintoexistence. Under time slicing, a task executes for a predefined slice of time and then reenters
the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other
factors.
Sleeping a thread (sleep() method):
The sleep() methodof Threadclassisusedto sleepathreadfor the specifiedtime.
Syntax of sleep() method:
The Thread classprovidestwomethodsforsleepingathread:
 publicstaticvoidsleep(longmiliseconds)throwsInterruptedException
 publicstaticvoidsleep(longmiliseconds,intnanos)throwsInterruptedException
1. // Program of sleep() method
2.
5. for(int i=1;i<5;i++){
6. try{Thread.sleep(500);}catch(InterruptedException e){System.out.println(e);}
7. System.out.println(i);
8. }
9. }
13.
14. t1.start();
15. t2.start();
16. }
17. }
Output:1
1
2
2
3
3
4
4
5
5
As youknowwell thatat a time onlyone threadisexecuted.If yousleepathreadforthe specifiedtime, the thread
schedulerpicksupanotherthreadandso on.
Can we start a thread twice?
No.Afterstaringa thread,it can neverbe startedagain.If you doesso,an IllegalThreadStateExceptionisthrown.For
Example:
3. System.out.println("running...");
4. }

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7. t1.start();
8. t1.start();
9. }
10. }
Output:running
Exception in thread "main" java.lang.IllegalThreadStateException
What if we call run() method directly instead start() method?
 Each thread startsin a separate call stack.
 Invokingthe run() methodfrommainthread,the run() methodgoesontothe currentcall stackrather thanat
the beginningof anewcall stack.
4. }
7. t1.run();//fine, but does not start a separate call stack
8. }
9. }
Output:running...
1. //Problem if you direct call run() method
2.
5. for(int i=1;i<5;i++){
6. try{Thread.sleep(500);}catch(InterruptedException e)
7. {System.out.println(e);}
9. }
10. }
14.
15. t1.run();

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16. t2.run();
17. }
18. }
Output:1
2
3
4
5
1
2
3
4
5
As youcan see inthe above programthat there isno context-switchingbecausehere t1andt2 will be treatedasnormal
objectnotthreadobject.
The join() method:
The join() methodwaitsforathreadto die.Inother words,itcausesthe currentlyrunningthreadstostopexecuting
until the threaditjoinswithcompletesitstask.
Syntax:
public void join()throws InterruptedException
public void join(long miliseconds)throws InterruptedException
1. // Example of join() method.
2.
3. classMulti extends Thread{
4. publicvoidrun(){
5. for(inti=1;i<=5;i++){
6. try{
7. Thread.sleep(500);
8. }catch(Exception e){System.out.println(e);}
10. }
11. }
12. publicstaticvoidmain(Stringargs[]){
16. t1.start();
17. try{
18. t1.join();
20.
21. t2.start();
22. t3.start();

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23. }
24. }
Output:1
2
3
4
5
1
1
2
2
3
3
4
4
5
5
As youcan see inthe above example,when t1completesitstaskthent2 and t3 startsexecuting.
1. //Example of join(long miliseconds) method.
2.
5. for(int i=1;i<=5;i++){
6. try{
10. }
11. }
16. t1.start();
17. try{
18. t1.join(1500);
20.
21. t2.start();
22. t3.start();
23. }
24. }
Output:1
2
3
1
4
1
2
5

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2
3
3
4
4
5
5
In the above example,when t1iscompletesitstaskfor1500 milliseconds(3times) thent2andt3 starts executing.
getName(),setName(String)andgetId() method:
public String getName()
public void setName(String name)
public long getId()
1. class Multi6 extends Thread{
4. }
6. Multi6 t1=new Multi6();
8. System.out.println("Name of t1:"+t1.getName());
10. System.out.println("id of t1:"+t1.getId());
11.
12. t1.start();
13. t2.start();
14.
15. t1.setName("Sonoo Jaiswal");
16. System.out.println("After changing name of t1:"+t1.getName());
17. }
18. }
Output:Name of t1:Thread-0
Name of t2:Thread-1
id of t1:8
running...
After changling name of t1:Sonoo Jaiswal
running...
ThecurrentThread()method:
The currentThread() methodreturnsareference tothe currentlyexecutingthread object.
Syntax:
publicstaticThreadcurrentThread()

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1. //Example of currentThread() method
2.
5. System.out.println(Thread.currentThread().getName());
6. }
7. }
11.
12. t1.start();
13. t2.start();
14. }
15. }
Output:Thread-0
Thread-1
Naming a thread:
The Thread classprovidesmethodstochange andgetthe name of a thread.
1. publicString getName():isusedto returnthe name of a thread.
2. publicvoid setName(Stringname): isusedto change the name of a thread.
Example of naming a thread:
4. }
10.
11. t1.start();
12. t2.start();
13.
14. t1.setName("Sonoo Jaiswal");
15. System.out.println("After changing name of t1:"+t1.getName());
16. }
17. }
Output: Name of t1:Thread-0
Name of t2:Thread-1
id of t1:8
running...
After changling name of t1:Sonoo Jaiswal
running...

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The currentThread() method:
The currentThread() methodreturnsareference tothe currentlyexecutingthreadobject.
Syntaxof currentThread()method:
 publicstatic Thread currentThread(): returnsthe reference of currentlyrunningthread.
Exampleof currentThread()method:
3. System.out.println(Thread.currentThread().getName());
4. }
5. }
9.
10. t1.start();
11. t2.start();
12. }
13. }
Output:Thread-0
Thread-1
Priority of a Thread (Thread Priority):
Each thread hasa priority.Prioritiesare representedbyanumberbetween1and 10. In mostcases,thread scheduler
schedulesthe threadsaccordingtotheirpriority(knownaspreemptive scheduling).Butitisnot guaranteedbecause it
dependsonJVMspecifificationthatwhich schedulingitchooses.
3 constants defined in Thread class:
1. publicstaticintMIN_PRIORITY
2. publicstaticintNORM_PRIORITY
3. publicstaticintMAX_PRIORITY
Defaultpriorityof athreadis 5 (NORM_PRIORITY).The value of MIN_PRIORITYis1 and the value of MAX_PRIORITYis
10.
ExampleofpriorityofaThread:
3. System.out.println("running thread name is:"+Thread.currentThread().getName(
));
4. System.out.println("running thread priority is:"+Thread.currentThread().g
etPriority());
5.
6. }

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10. m1.setPriority(Thread.MIN_PRIORITY);
11. m2.setPriority(Thread.MAX_PRIORITY);
12. m1.start();
13. m2.start();
14.
15. }
16. }
Output:running thread name is:Thread-0
running thread priority is:10
running thread name is:Thread-1
running thread priority is:1
Daemon Thread:
There are twotypesof threadsuserthreadand daemonthread.The daemonthreadisa service providerthread.It
providesservicestothe userthread.Itslife dependsonthe userthreadsi.e.whenall the userthreadsdies,JVM
terminates thisthreadautomatically.
Points to remember for Daemon Thread:
 It providesservicestouserthreadsforbackgroundsupportingtasks.Ithas norole in life thantoserve user
threads.
 Its life dependsonuserthreads.
 It isa lowprioritythread.
Why JVM terminates the daemonthread ifthere is no userthread remaining?
The sole purpose of the daemonthreadisthat it providesservicestouserthreadforbackgroundsupportingtask.If
there isno userthread,whyshouldJVMkeeprunningthisthread.Thatiswhy JVMterminatesthe daemonthreadif
there isno userthread.
MethodsforDaemonthread:
The java.lang.Threadclassprovidestwomethodsrelatedtodaemonthread
 publicvoid setDaemon(booleanstatus):isusedto markthe currentthreadas daemonthreador userthread.
 publicbooleanisDaemon():isusedto checkthat current isdaemon.
SimpleexampleofDaemonthread:
1. class MyThread extends Thread{
3. System.out.println("Name: "+Thread.currentThread().getName());
4. System.out.println("Daemon: "+Thread.currentThread().isDaemon());
5. }
6.
7. public static void main(String[] args){

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8. MyThread t1=new MyThread();
10. t1.setDaemon(true);
11.
12. t1.start();
13. t2.start();
14. }
15. }
Output:Name: thread-0
Daemon: true
Name: thread-1
Daemon: false
Note: If you wantto makea userthread as Daemon, it mustnot be started otherwiseit willthrow
IllegalThreadStateException.
1. class MyThread extends Thread{
3. System.out.println("Name: "+Thread.currentThread().getName());
4. System.out.println("Daemon: "+Thread.currentThread().isDaemon());
5. }
6.
7. public static void main(String[] args){
10. t1.start();
11. t1.setDaemon(true);//will throw exception here
12. t2.start();
13. }
14. }
Output:exception in thread main: java.lang.IllegalThreadStateException
Synchronization
Synchronization is the capabilility of control the access of multiple threads to any shared resource. Synchronization is
better in case we want only one thread can access the shared resource at a time.
Why useSynchronization?
The synchronizationismainlyusedto
1. To preventthreadinterference.
2. To preventconsistencyproblem.
TypesofSynchronization
There are twotypesof synchronization
1. ProcessSynchronization
2. ThreadSynchronization
Here,we will discussonlythread synchronization.

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ThreadSynchronization
There are twotypesof threadsynchronizationmutual exclusiveandinter-threadcommunication.
 Mutual Exclusive
1. Synchronizedmethod.
2. Synchronizedblock.
3. staticsynchronization.
 Cooperation(Inter-threadcommunication)
Mutual Exclusive
Mutual Exclusive helpskeepthreadsfrominterferingwithone anotherwhilesharingdata.Thiscanbe done bythree
waysin java:
1. by synchronizedmethod
2. by synchronizedblock
3. by staticsynchronization
Understandingtheconceptof Lock
Synchronizationisbuiltaroundaninternal entityknownasthe lockor monitor.Every object has an lock associated with
it. By convention, a thread that needs consistent access to an object's fields has to acquire the object's lock before
accessing them, and then release the lock when it's done with them.
From Java5 the package java.util.concurrent.lockscontainsseverallockimplementations.
UnderstandingtheproblemwithoutSynchronization
In thisexample,there isnosynchronization,sooutputisinconsistent.Let'ssee the example:
1. Class Table{
2.
3. void printTable(int n){//method not synchronized
4. for(int i=1;i<=5;i++){
5. System.out.println(n*i);
6. try{
9. }
10.
11. }
12. }
13.
14. class MyThread1 extends Thread{
15. Table t;
16. MyThread1(Table t){
17. this.t=t;
18. }
20. t.printTable(5);
21. }

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22.
23. }
25. Table t;
27. this.t=t;
28. }
31. }
32. }
33.
34. class Use{
36. Table obj = new Table();//only one object
37. MyThread1 t1=new MyThread1(obj);
39. t1.start();
40. t2.start();
41. }
42. }
Output: 5
100
10
200
15
300
20
400
25
500
Solutionbysynchronizedmethod
 If you declare anymethodassynchronized,itisknownassynchronizedmethod.
 Synchronizedmethodisusedtolockan objectforany sharedresource.
 Whena thread invokesasynchronizedmethod,itautomaticallyacquiresthe lockforthatobjectandreleasesit
whenthe methodreturns.
1. //Program of synchronized method.
2.
3. Class Table{
4.
5. synchronized void printTable(int n){//synchronized method
6. for(int i=1;i<=5;i++){
8. try{
11. }
12.
13. }
14. }

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15.
17. Table t;
19. this.t=t;
20. }
23. }
24.
25. }
27. Table t;
29. this.t=t;
30. }
33. }
34. }
35.
36. class Use{
41. t1.start();
42. t2.start();
43. }
44. }
Output: 5
10
15
20
25
100
200
300
400
500
SameExampleofsynchronizedmethodbyusingannonymousclass
In thisprogram,we have createdthe two threadsby anonymous class,solesscodingisrequired.
1. //Program of synchronized method by using annonymous class.
2.
3. Class Table{
4.
5. synchronized void printTable(int n){//synchronized method
6. for(int i=1;i<=5;i++){
8. try{
11. }
12.

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13. }
14. }
15.
16. class Use{
18. final Table obj = new Table();//only one object
19.
20. MyThread1 t1=new MyThread1(){
22. obj.printTable(5);
23. }
24. };
25. MyThread1 t2=new MyThread1(){
28. }
29. };
30.
31. t1.start();
32. t2.start();
33. }
34. }
Output: 5
10
15
20
25
100
200
300
400
500
Synchronized block
Synchronized block can be used to perform synchronization on any specific resource of the method.
Suppose youhave 50 linesof code inyourmethod, but you want to synchronize only 5 lines, you can use synchronized
block.
If you put all the codes of the method in the synchronized block, it will work same as the synchronized method.
Pointsto rememberforSynchronizedblock
 Synchronizedblockisusedtolockan objectforany sharedresource.
 Scope of synchronizedblockissmallerthanthe method.
Syntax to use synchronizedblock
1. synchronized (object reference expression) {
2. //code block
3. }

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Exampleofsynchronizedblock
Let's see the simple exampleof synchronizedblock.
1. //Program of synchronized block.
2.
3. class Table{
4.
5. void printTable(int n){
6. synchronized(this){//synchronized block
7. for(int i=1;i<=5;i++){
9. try{
12. }
13. }
14. }//end of the method
15. }
16.
18. Table t;
20. this.t=t;
21. }
24. }
25.
26. }
28. Table t;
30. this.t=t;
31. }
34. }
35. }
36.
37. class Use{
42. t1.start();
43. t2.start();
44. }
45. }
Output:5
10
15
20
25
100
200

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300
400
500
SameExampleofsynchronizedblockbyusingannonymousclass:
1. //Program of synchronized block by using annonymous class.
2.
3. class Table{
4.
5. void printTable(int n){
6. synchronized(this){//synchronized block
7. for(int i=1;i<=5;i++){
9. try{
12. }
13. }
14. }//end of the method
15. }
16.
17. class Use{
19. final Table obj = new Table();//only one object
20.
21. Thread t1=new Thread(){
24. }
25. };
29. }
30. };
31.
32. t1.start();
33. t2.start();
34. }
35. }
Output:5
10
15
20
25
100
200
300
400
500

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Static synchronization
If you make any staticmethodas synchronized,the lockwill be onthe classnoton object.
Problemwithoutstatic synchronization
Suppose there are twoobjectsof a shared class(e.g.Table) namedobject1andobject2.In case of synchronized method
and synchronizedblock there cannot be interference between t1 and t2 or t3 and t4 because t1 and t2 both refers to a
commonobjectthat have a single lock.Butthere can be interference betweent1and t3 or t2 and t4 because t1acquires
anotherlockand t3 acquiresanother lock.I want no interference between t1 and t3 or t2 and t4.Static synchronization
solves this problem.
Exampleofstatic synchronization
In thisexample we are applyingsynchronizedkeywordonthe staticmethodtoperformstatissynchrnization.
class Table{
synchronized static void printTable(int n){
for(int i=1;i<=10;i++){
System.out.println(n*i);
try{
Thread.sleep(400);
}catch(Exception e){}
}
}
}
class MyThread1 extends Thread{
public void run(){
Table.printTable(1);
}
}
public void run(){
}
}
public void run(){
}
}

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public void run(){
}
}
class Use{
public static void main(String t[]){
MyThread1 t1=new MyThread1();
t1.start();
t2.start();
t3.start();
t4.start();
}
}
Output: 1
2
3
4
5
6
7
8
9
10
10
20
30
40
50
60
70
80
90
100
100
200
300
400
500
600
700
800
900
1000
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000

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Sameexampleof static synchronizationbyannonymousclass
In thisexample,we are usingannonymousclasstocreate the threads.
1. class Table{
2.
3. synchronized static void printTable(int n){
4. for(int i=1;i<=10;i++){
6. try{
8. }catch(Exception e){}
9. }
10. }
11. }
12.
13. public class Test {
14. public static void main(String[] args) {
15.
18. Table.printTable(1);
19. }
20. };
21.
25. }
26. };
27.
31. }
32. };
33.
37. }
38. };
39. t1.start();
40. t2.start();
41. t3.start();
42. t4.start();
43.
44. }
45. }
Output: 1
2
3
4
5

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6
7
8
9
10
10
20
30
40
50
60
70
80
90
100
100
200
300
400
500
600
700
800
900
1000
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Synchronizedblockonaclasslock:
The block synchronizes on the lock of the object denoted by the reference .class. A static synchronized method
printTable(int n) in class Table is equivalent to the following declaration:
1. static void printTable(int n) {
2. synchronized (Table.class) { // Synchronized block on class A
3. // ...
4. }
5. }
Deadlock:
Deadlock can occur in a situation when a thread is waiting for an object lock, that is acquired by another thread and
secondthreadiswaitingforan objectlockthat isacquiredbyfirstthread.Since,boththreadsare waiting for each other
to release the lock, the condition is called deadlock.

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ExampleofDeadlockinjava:
1. public class DeadlockExample {
3. final String resource1 = "ratan jaiswal";
4. final String resource2 = "vimal jaiswal";
5. // t1 tries to lock resource1 then resource2
6. Thread t1 = new Thread() {
7. public void run() {
8. synchronized (resource1) {
9. System.out.println("Thread 1: locked resource 1");
10.
11. try { Thread.sleep(100);} catch (Exception e) {}
12.
15. }
16. }
17. }
18. };
19.
20. // t2 tries to lock resource2 then resource1
21. Thread t2 = new Thread() {
22. public void run() {
25.
26. try { Thread.sleep(100);} catch (Exception e) {}
27.
30. }
31. }
32. }
33. };
34.
35.
36. t1.start();
37. t2.start();
38. }
39. }
40.
Output: Thread 1: locked resource 1
Thread 2: locked resource 2

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Inter-thread communication (Cooperation):
Cooperation (Inter-thread communication) is all about making synchronized threads communicate with each other.
Cooperation(Inter-threadcommunication) isamechanisminwhichathreadis paused running in its critical section and
anotherthreadisallowed to enter (or lock) in the same critical section to be executed.It is implemented by following
methods of Object class:
 wait()
 notify()
 notifyAll()
1)wait()method:
Causescurrentthreadto release the lockandwaituntil eitheranotherthreadinvokesthe notify() methodorthe
notifyAll()methodforthisobject,oraspecifiedamountof time haselapsed.The currentthreadmustownthisobject's
monitor.Syntax:
 public final void wait()throws InterruptedException
 public final void wait(long timeout)throws InterruptedException
2)notify()method:
Wakesup a single threadthatiswaitingonthisobject'smonitor.If anythreadsare waitingonthisobject,one of them is
chosento be awakened.The choice isarbitraryandoccurs at the discretionof the implementation.Syntax:
 public final void notify()
3)notifyAll()method:
Wakesup all threadsthat are waitingonthisobject'smonitor.
 public final void notifyAll()
ExampleofInter threadCommunication:
1. public class Customer {
2. int amount=0;
3. int flag=0;
4. public synchronized int withdraw(int amount){
5. System.out.println(Thread.currentThread().getName()+" is going to withdr
aw");
6.
7. if(flag==0){
8. try{
9. System.out.println("waiting....");
10. wait();
11. }catch(Exception e){}
12. }
13. this.amount-=amount;
14. System.out.println("withdraw completed");
15. return amount;

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16. }
17.
18. public synchronized void deposit(int amount){
19. System.out.println(Thread.currentThread().getName()+"is going to depos
it");
20. this.amount+=amount;
21.
22. notifyAll();
23. System.out.println("deposit completed");
24. flag=1;
25. }
26.
27.
28. }
29.
30. public class SynMethod {
32. final Customer c=new Customer();
33.
36. c.withdraw(5000);
37. System.out.println("After withdraw amount is"+c.amount);
38. }
39. };
40.
43. c.deposit(9000);
44. System.out.println("After deposit amount is "+c.amount);
45. }
46. };
47.
48.
49. t1.start();
50. t2.start();
51.
52.
53. }
54. }
Output: Thread-0 is going to withdraw
waiting....
Thread-1 is going to deposit
deposit completed
After deposit amount is 9000
withdraw completed
After withdraw amount is 4000
Interrupting a Thread:
If any threadis insleepingorwaitingstate (i.e.sleep() orwait() isinvoked),callingthe interrupt() methodonthe thre ad,
breaksout the sleepingorwaitingstate throwingInterruptedException.If the threadisnotinthe sleepingorwaiting
state,callingthe interrupt()methodperformsnormal behaviouranddoesn'tinterruptthe threadbutsetsthe interrupt

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flagto true.Let's firstsee the methodsprovidedbythe Threadclassfor threadinterruption.
The 3 methods provided by the Thread class for interrupting a thread
 publicvoid interrupt()
 publicstatic booleaninterrupted()
 publicbooleanisInterrupted()
Example of interrupting a thread that stops working
In thisexample,afterinterruptingthe thread,we are propagatingit,soit will stopworking.If we don'twantto stopthe
thread,we can handle itwhere sleep() orwait() methodisinvoked.Let'sfirstsee the example where we are propagating
the exception.
1. class A extends Thread{
3. try{
5. System.out.println("task");
6. }catch(InterruptedException e){
7. throw new RuntimeException("Thread interrupted..."+e);
8. }
9.
10. }
11.
13. A t1=new A();
14. t1.start();
15. try{
16. t1.interrupt();
17. }catch(Exception e){System.out.println("Exception handled "+e);}
18.
19. }
20. }
Output:Exception in thread-0
java.lang.RuntimeException: Thread interrupted...
java.lang.InterruptedException: sleep interrupted
at A.run(A.java:7)
Example of interrupting a thread that doesn't stop working
In thisexample,afterinterrupting the thread,we handle the exception,soitwill breakoutthe sleepingbutwill notstop
working.
3. try{
5. System.out.println("task");

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6. }catch(InterruptedException e){
7. System.out.println("Exception handled "+e);
8. }
10. }
11.
13. A t1=new A();
14. t1.start();
15.
16. t1.interrupt();
17.
18. }
19. }
Output:Exception handled
java.lang.InterruptedException: sleep interrupted
thread is running...
Example of interrupting thread that behaves normally
If thread isnot insleepingorwaitingstate,callingthe interrupt() methodsetsthe interruptedflagtotrue that can be
usedto stopthe thread bythe java programmerlater.
2.
4. for(int i=1;i<=5;i++)
6. }
7.
9. A t1=new A();
10. t1.start();
11.
12. t1.interrupt();
13.
14. }
15. }
Output:1
2
3
4
5
What about isInterrupted and interrupted method?
The isInterrupted() methodreturnsthe interruptedflageithertrue orfalse.The staticinterrupted()methodreturnsthe
interruptedflagafterthatitsetsthe flagtofalse if itistrue.

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1. class InterruptedDemo extends Thread{
2.
4. for(int i=1;i<=2;i++){
5. if(Thread.interrupted()){
6. System.out.println("code for interrupted thread");
7. }
8. else{
9. System.out.println("code for normal thread");
10. }
11.
12. }//end of for loop
13. }
14.
16.
17. InterruptedDemo t1=new InterruptedDemo();
18. InterruptedDemo t2=new InterruptedDemo();
19.
20. t1.start();
21. t1.interrupt();
22.
23. t2.start();
24.
25. }
26. }
Output:Code for interrupted thread
code for normal thread

Class notes(week 9) on multithreading

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