Multithreading in Java Object Oriented Programming language

Multithreading in Java
Multithreading in java is a process of executing multiple threads simultaneously.
A thread is a lightweight sub-process, the smallest unit of processing. Multiprocessing and
multithreading, both are used to achieve multitasking.
However, we use multithreading than multiprocessing because threads use a shared
memory area. They don't allocate separate memory area so saves memory, and context-
switching between the threads takes less time than process.
Java Multithreading is mostly used in games, animation, etc.

Advantages of Java Multithreading
1) It doesn't block the user because threads are independent and you can perform multiple
operations at the same time.
2) You can perform many operations together, so it saves time.
3) Threads are independent, so it doesn't affect other threads if an exception occurs in a
single thread.

Multitasking
Multitasking is a process of executing multiple tasks simultaneously. We use multitasking to
utilize the CPU. Multitasking can be achieved in two ways:
Process-based Multitasking (Multiprocessing)
Thread-based Multitasking (Multithreading)
1) Process-based Multitasking (Multiprocessing)
Each process has an address in memory. In other words, each process allocates a separate
memory area.
A process is heavyweight.
Cost of communication between the process is high.
Switching from one process to another requires some time for saving and loading registers,
memory maps, updating lists, etc.
2) Thread-based Multitasking (Multithreading)
Threads share the same address space.
A thread is lightweight.
Cost of communication between the thread is low.

What is Thread in java
A thread is a lightweight subprocess, the
smallest unit of processing. It is a separate
path of execution.
Threads are independent. If there occurs
exception in one thread, it doesn't affect other
threads. It uses a shared memory area.

As shown in the below figure, a thread is executed inside the
process. There is context-switching between the threads. There can
be multiple processes inside the OS, and one process can have
multiple threads.

Java Thread class
Java provides Thread class to achieve
thread programming. Thread class provides
constructors and methods to create and
perform operations on a thread. Thread class
extends Object class and implements
Runnable interface.

Multithreading in Java Object Oriented Programming language

Life cycle of a Thread (Thread States)
A thread can be in one of the five states. According to sun, there is only 4 states
in thread life cycle in java new, runnable, non-runnable and terminated. There is
no running state.
But for better understanding the threads, we are explaining it in the 5 states.
The life cycle of the thread in java is controlled by JVM. The java thread states are
as follows:
New
Runnable
Running
Non-Runnable (Blocked)
Terminated

1) New
The thread is in new state if you create an instance of Thread class but before the invocation
of start() method.
2) Runnable
The thread is in runnable state after invocation of start() method, but the thread scheduler
has not selected it to be the running thread.
3) Running
The thread is in running state if the thread scheduler has selected it.
4) Non-Runnable (Blocked)
This is the state when the thread is still alive, but is currently not eligible to run.
5) Terminated
A thread is in terminated or dead state when its run() method exits.

How to create thread
There are two ways to create a thread:
By extending Thread class
By implementing Runnable interface.
Thread class:
Thread class provide constructors and methods to create and perform operations on a
thread.Thread class extends Object class and implements Runnable interface.Commonly
used Constructors of Thread class:
Thread()
Thread(String name)
Thread(Runnable r)
Thread(Runnable r,String name)

Commonly used methods of Thread class:
public void run(): is used to perform action for a thread.
public void start(): starts the execution of the thread.JVM calls the run() method on the thread.
public void sleep(long miliseconds): Causes the currently executing thread to sleep (temporarily cease execution)
for the specified number of milliseconds.
public void join(): waits for a thread to die.
public void join(long miliseconds): waits for a thread to die for the specified miliseconds.
public int getPriority(): returns the priority of the thread.
public int setPriority(int priority): changes the priority of the thread.
public String getName(): returns the name of the thread.
public void setName(String name): changes the name of the thread.
public Thread currentThread(): returns the reference of currently executing thread.
public int getId(): returns the id of the thread.
public Thread.State getState(): returns the state of the thread.
public boolean isAlive(): tests if the thread is alive.
public void yield(): causes the currently executing thread object to temporarily pause and allow other threads to
execute.
public void suspend(): is used to suspend the thread(depricated).
public void resume(): is used to resume the suspended thread(depricated).
public void stop(): is used to stop the thread(depricated).
public boolean isDaemon(): tests if the thread is a daemon thread.
public void setDaemon(boolean b): marks the thread as daemon or user thread.
public void interrupt(): interrupts the thread.
public boolean isInterrupted(): tests if the thread has been interrupted.
public static boolean interrupted(): tests if the current thread has been interrupted.

Java Thread Example by extending Thread class
class Multi extends Thread{
public void run(){
System.out.println("thread is running...");
}
public static void main(String args[]){
Multi t1=new Multi();
t1.start();
}
}

Java Thread Example by implementing Runnable interface
class Multi3 implements Runnable{
public void run(){
System.out.println("thread is running...");
}
Multi3 m1=new Multi3();
Thread t1 =new Thread(m1);
t1.start();
}
}
If you are not extending the Thread class, your class object would not be treated as a thread
object.So you need to explicitely create Thread class object. We are passing the object of
your class that implements Runnable so that your class run() method may execute.

Using the Thread Class: Thread(String Name)
We can directly use the Thread class to spawn new threads using the constructors defined
above.
1.public class MyThread1
2.{
3.// Main method
4.public static void main(String argvs[])
5.{
6.// creating an object of the Thread class using the constructor Thread(String name)
7.Thread t= new Thread("My first thread");
8.
9.// the start() method moves the thread to the active state
10.t.start();
11.// getting the thread name by invoking the getName() method
12.String str = t.getName();
13.System.out.println(str);
14.}
15.}

Using the Thread Class: Thread(Runnable r, String name)
1.public class MyThread2 implements Runnable
2.{
3.public void run()
4.{
5.System.out.println("Now the thread is running ...");
6.}
7. // main method
9.{
10.// creating an object of the class MyThread2
11.Runnable r1 = new MyThread2();
12.
13.// creating an object of the class Thread using Thread(Runnable r, String name)
14.Thread th1 = new Thread(r1, "My new thread");
15. // the start() method moves the thread to the active state
16.th1.start();
17. // getting the thread name by invoking the getName() method
18.String str = th1.getName();
19.System.out.println(str);
20.}
21.}

Thread Scheduler in Java
Thread scheduler in java is the part of the JVM that decides which thread should run.
There is no guarantee that which runnable thread will be chosen to run by the thread
scheduler.
Only one thread at a time can run in a single process.
The thread scheduler mainly uses preemptive or time slicing scheduling to schedule the
threads.
Difference between preemptive scheduling and time slicing
Under preemptive scheduling, the highest priority task executes until it enters the waiting or
dead states or a higher priority task comes into existence. 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.

Sleep method in java
The sleep() method of Thread class is used to sleep a thread for the specified amount of
time.
Syntax of sleep() method in java
The Thread class provides two methods for sleeping a thread:
public static void sleep(long miliseconds)throws InterruptedException
public static void sleep(long miliseconds, int nanos)throws InterruptedException

Example of sleep method in java
class TestSleepMethod1 extends Thread{
public void run(){
for(int i=1;i<5;i++){
try{Thread.sleep(500);}catch(InterruptedException e){System.out.println(e);}
System.out.println(i);
}
}
TestSleepMethod1 t1=new TestSleepMethod1();
TestSleepMethod1 t2=new TestSleepMethod1();
t1.start();
t2.start();
}
}
As you know well that at a time only one thread is executed. If you sleep a thread for the
specified time,the thread shedular picks up another thread and so on.

Example of the sleep() Method in Java : on the main thread
1.// important import statements
2.import java.lang.Thread;
3.import java.io.*;
4.public class TestSleepMethod2
5.{
6. // main method
8.{
9. try {
10.for (int j = 0; j < 5; j++)
11.{
12.// The main thread sleeps for the 1000 milliseconds, which is 1 sec
13.// whenever the loop runs
14.Thread.sleep(1000);
15. // displaying the value of the variable
16.System.out.println(j);
17.} }
18.catch (Exception expn)
19.{
20.// catching the exception
21.System.out.println(expn);
22.}
23.}

Example of the sleep() Method in Java: When the sleeping time is -ive
The following example throws the exception IllegalArguementException when the time for
sleeping is negative.
1./ important import statements
2.import java.lang.Thread;
3.import java.io.*;
4. public class TestSleepMethod3
5.{ // main method
6.public static void main(String argvs[]) {
7.// we can also use throws keyword followed by // exception name for throwing the excep
tion
8.try {
9.for (int j = 0; j < 5; j++) {
10.// it throws the exception IllegalArgumentException // as the time is -ive which is -100
11.Thread.sleep(-100);
12.// displaying the variable's value
13.System.out.println(j); } }
14.catch (Exception expn) { // the exception iscaught here
15.System.out.println(expn);
16.}
17.}
18.}

// Java code for thread creation by extending
// the Thread class
class MultithreadingDemo extends Thread
{ public void run()
{ try
{ // Displaying the thread that is running
System.out.println ("Thread " + Thread.currentThread().getId() +
" is running"); }
catch (Exception e) {
// Throwing an exception
System.out.println ("Exception is caught"); }
} }
// Main Class
public class Multithread{
public static void main(String[] args) {
int n = 8; // Number of threads
for (int i=0; i<8; i++) {
MultithreadingDemo object = new MultithreadingDemo();
object.start();
}}}

Can we start a thread twice
No. After starting a thread, it can never be started again. If you does so,
an IllegalThreadStateException is thrown. In such case, thread will run once but for
second time, it will throw exception.
Let's understand it by the example given below:
public class TestThreadTwice1 extends Thread{
public void run(){
System.out.println("running...");
}
TestThreadTwice1 t1=new TestThreadTwice1();
t1.start();
t1.start();
}
}

Naming Thread and Current Thread
class TestMultiNaming1 extends Thread{
public void run(){
System.out.println("running...");
}
TestMultiNaming1 t1=new TestMultiNaming1();
System.out.println("Name of t1:"+t1.getName());
System.out.println("Name of t2:"+t2.getName());
t1.start();
t2.start();
t1.setName("Sonoo Jaiswal");
System.out.println("After changing name of t1:"+t1.getName());
}
}

Example of currentThread() method
class TestMultiNaming2 extends Thread{
public void run(){
System.out.println(Thread.currentThread().getName());
}
t1.start();
t2.start();
}
}

Priority of a Thread (Thread Priority):
Each thread have a priority. Priorities are represented by a number between 1
and 10. In most cases, thread schedular schedules the threads according to
their priority (known as preemptive scheduling). But it is not guaranteed
because it depends on JVM specification that which scheduling it chooses.
3 constants defined in Thread class:
1. public static int MIN_PRIORITY
2.public static int NORM_PRIORITY
3.public static int MAX_PRIORITY
Default priority of a thread is 5 (NORM_PRIORITY). The value of MIN_PRIORITY
is 1 and the value of MAX_PRIORITY is 10.

Example of priority of a Thread:
class TestMultiPriority1 extends Thread{
public void run(){
System.out.println("running thread name is:"+Thread.currentThread().getNa
me());
System.out.println("running thread priority is:"+Thread.currentThread().getPri
ority());
}
TestMultiPriority1 m1=new TestMultiPriority1();
TestMultiPriority1 m2=new TestMultiPriority1();
m1.setPriority(Thread.MIN_PRIORITY);
m2.setPriority(Thread.MAX_PRIORITY);
m1.start();
m2.start();
}
}

Daemon Thread in Java
Daemon thread in java is a service provider thread that provides services to
the user thread. Its life depend on the mercy of user threads i.e. when all the
user threads dies, JVM terminates this thread automatically.
There are many java daemon threads running automatically e.g. gc, finalizer
etc.
You can see all the detail by typing the jconsole in the command prompt. The
jconsole tool provides information about the loaded classes, memory usage,
running threads etc.
Points to remember for Daemon Thread in Java
• It provides services to user threads for background supporting tasks. It has
no role in life than to serve user threads.
• Its life depends on user threads.
• It is a low priority thread.

Why JVM terminates the daemon thread if there is no user thread?
The sole purpose of the daemon thread is that it provides services to user
thread for background supporting task. If there is no user thread, why should
JVM keep running this thread. That is why JVM terminates the daemon thread if
there is no user thread.
Methods for Java Daemon thread by Thread class
The java.lang.Thread class provides two methods for java daemon thread.
No. Method Description
1) public void
setDaemon(boolean
status)
is used to mark the
current thread as
daemon thread or user
thread.
2) public boolean
isDaemon()
is used to check that
current is daemon.

Simple example of Daemon thread in java
public class TestDaemonThread1 extends Thread{
public void run(){
if(Thread.currentThread().isDaemon()){//checking for daemon thread
System.out.println("daemon thread work");
}
else{
System.out.println("user thread work");
}
}
public static void main(String[] args){
TestDaemonThread1 t1=new TestDaemonThread1();//creating thread
TestDaemonThread1 t2=new TestDaemonThread1();
TestDaemonThread1 t3=new TestDaemonThread1();
t1.setDaemon(true);//now t1 is daemon thread
t1.start();//starting threads
t2.start();
t3.start();
}
}

Note: If you want to make a user thread as Daemon, it must not be started otherwise it will
throw IllegalThreadStateException.
1.class TestDaemonThread2 extends Thread{
2. public void run(){
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){
8. TestDaemonThread2 t1=new TestDaemonThread2();
9. TestDaemonThread2 t2=new TestDaemonThread2();
10. t1.start();
11. t1.setDaemon(true);//will throw exception here
12. t2.start();
13. }
14.}
Output: Output:exception in thread main: java.lang.IllegalThreadStateException

join() method
• The join() method in Java is provided by the
java.lang.
• Thread class that permits one thread to wait
until the other thread to finish its execution.
• Suppose th be the object the class Thread
whose thread is doing its execution currently,
then the th.join(); statement ensures that th is
finished before the program does the execution
of the next statement.
• When there are more than one thread invoking
the join() method, then it leads to overloading on
the join() method that permits the developer or
programmer to mention the waiting period.

1.class TestJoinMethod1 extends Thread{
2. public void run(){
3. for(int i=1;i<=5;i++){
4. try{
5. Thread.sleep(500);
6. }catch(Exception e){System.out.println(e);}
7. System.out.println(i);
8. } }
9.public static void main(String args[]){
10. TestJoinMethod1 t1=new TestJoinMethod1();
13. t1.start();
14. try{
15. t1.join();
16. }catch(Exception e){System.out.println(e);}
17. t2.start();
18. t3.start();
19. }
20.}

Java Thread Synchronization
In multithreading, there is the asynchronous behavior of the programs. If one thread is
writing some data and another thread which is reading data at the same time, might create
inconsistency in the application.
When there is a need to access the shared resources by two or more threads, then
synchronization approach is utilized.
Java has provided synchronized methods to implement synchronized behavior.
In this approach, once the thread reaches inside the synchronized block, then no other
thread can call that method on the same object. All threads have to wait till that thread
finishes the synchronized block and comes out of that.
In this way, the synchronization helps in a multithreaded application. One thread has to wait
till other thread finishes its execution only then the other threads are allowed for execution.
It can be written in the following form:
Synchronized(object) { //Block of statements to be synchronized }

Why use Synchronization
The synchronization is mainly used to
To prevent thread interference.
To prevent consistency problem.
Types of Synchronization
There are two types of synchronization
Process Synchronization
Thread Synchronization
Thread Synchronization
There are two types of thread synchronization mutual exclusive and inter-thread
communication.
Mutual Exclusive
Synchronized method.
Synchronized block.
static synchronization.
Cooperation (Inter-thread communication in java)

Mutual Exclusive
Mutual Exclusive helps keep threads from interfering with one another while sharing data.
This can be done by three ways in java:
by synchronized method
by synchronized block
by static synchronization

Understanding the problem without Synchronization
class Table{
void printTable(int n){//method not synchronized
for(int i=1;i<=5;i++){
System.out.println(n*i);
try{
Thread.sleep(400);
}catch(Exception e){System.out.println(e);} }}}
class MyThread1 extends Thread{
Table t;
MyThread1(Table t){
this.t=t;}
public void run(){
t.printTable(5); } }
Table t;
MyThread2(Table t){
this.t=t; }
public void run(){
t.printTable(100); }}
class TestSynchronization1{
Table obj = new Table();//only one object
MyThread1 t1=new MyThread1(obj);
t1.start();
t2.start(); } }

Output: 5 100 10 200 15 300 20 400 25 500

Java synchronized method
//example of java synchronized method
class Table{
synchronized void printTable(int n){//synchronized method
for(int i=1;i<=5;i++){
System.out.println(n*i);
try{
Thread.sleep(400);
}catch(Exception e){System.out.println(e);} } } }
Table t;
MyThread1(Table t){
this.t=t; }
public void run(){
Table t;
MyThread2(Table t){
this.t=t; }
public void run(){
public class TestSynchronization2{
Table obj = new Table();//only one object
t1.start();
t2.start(); } }

Output: 5 10 15 20 25 100 200 300 400 500

References
https://www.javatpoint.com/java-tutorial
https://www.tutorialspoint.com/java/index.htm
https://www.oreilly.com/library/view/java-the-complete/9781259589348/

Multithreading in Java Object Oriented Programming language

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