Application
Programming
Hend Alkittawi

Threads & Concurrency
Introduction to Java Threads and
Synchronization

INTRODUCTION
-

All modern operating systems support concurrency, via
processes and threads.

-

A process is an instance of a program running in a computer
-

example: if you start a java program, the OS spawns a new
process, which runs in parallel to other programs.

-

A thread is a program unit that is executed concurrently with
other parts of the program.

-

One or more threads run in the context of the process.

-

Multiple threads can collaborate and work efficiently within a
single program.

THREADS
-

Multi-threaded applications
have multiple threads within
a single process
-

each thread have its own
program counter, stack and
set of registers,

-

All threads

share common

code, data, and certain
structures such as open
files.

THREADS
-

Threads are very useful in modern programming whenever a
process has multiple tasks to perform independently of the
others.

-

This is particularly true when one of the tasks may block, and
it is desired to allow the other tasks to proceed without
blocking.
-

For example in a word processor, a background thread may check
spelling and grammar while a foreground thread processes user
input ( keystrokes ), while yet a third thread loads images
from the hard drive, and a fourth does periodic automatic
backups of the file being edited.

THREADS
-

A multi-threaded application running on a single-core chip
would have to interleave the threads

-

A multi-threaded application running on a multi-core chip, the
threads could be spread across the available cores, allowing
true parallel processing

THREADS
-

The thread scheduler gives no guarantee about the order in
which threads are executed.

-

Each thread runs for a short amount of time, called a time
slice. Then the schedule activates another thread. However,
there will always be slight variations in running times. Thus,
you should expect that the order in which each thread gains
controls is somewhat random.

-

It is important to observe that the order and the timing of
operations performed by the threads are controlled by the
runtime system, and cannot be controlled by the programmer.

JAVA THREADS
-

The JVM executes each thread for a short amount of time and
then switches to another thread.

-

In a multithreaded environment, threads can be: created,
scheduled to run, paused, resumed, and terminated.

-

In Java, we can create threads within that process two
different ways
-

Create a new class of type Thread
-

-

java.lang.Thread

Create a new class that implements the Runnable interface
-

java.lang.Runnable

-

the Runnable interface has a single method called run().

JAVA THREADS
To create threads by creating a new class of type Thread
1.

Create a class that extends the Thread class.

2.

Override the run() method by placing the task code into
the run() method of your class.

3.

Create an object of the subclass

4.

Call the start method to start the thread
public class MyThread extends Thread {
@Override
public void run() {
// your code here!
}
public static void main( String[] args ){
MyThread thread = new MyThread();
thread.start();
}
}

main

-

thread

JAVA THREADS
To create threads by creating a new class that implements the
Runnable interface:
1.

Create a class that implements the Runnable interface.

2.

Place the task code into the run() method of your class.

3.

Create an object of the subclass

4.

Construct a thread object from the Runnable object.

5.

Call the start method to start the thread
public class MyRunnable implements Runnable {
public void run() {
// your code here!
}
public static void main( String[] args ){
Runnable runnable = new MyRunnable();
Thread thread = new Thread(runnable);
thread.start();
}
}

main

-

thread

JAVA THREADS
-

Main Thread
public class MainThreadDemo {
public static void main(String[] args) {
Thread t = Thread.currentThread();
System.out.println("Current thread: " + t);
t.setName("My Thread"); // set the thread name
System.out.println("After name change: " + t);
The sleep() method
try {
puts the current
for(int i = 5; i > 0; i--) {
thread to sleep for a
System.out.println(i);
given number of
Thread.sleep(500);
milliseconds
}
}
catch(InterruptedException e) {
}
Current thread: Thread[main,5,main]
After name change: Thread[My Thread,5,main]
}
5
}
4
3
2
1

Creating a thread by extending the Thread class
public class MyThreadDemo {
public static void main(String[] args) {
MyThread nt = new MyThread();
nt.start();
try {
for(int i = 5; i > 0; i--) {
System.out.printf("%-15s: %d\n", "Main Thread", i);
Thread.sleep(1000);
}
}
catch(InterruptedException e) {
}
System.out.println("Exiting Main Thread ...");
}
}

main

public class MyThread extends Thread{
public MyThread() {
super("Demo Thread");
System.out.println("Child Thread: " + this);
}
// This is the entry point for the second thread.
@Override
public void run() {
try {
for(int i = 5; i > 0; i--) {
System.out.printf("%-15s: %d\n", "Child Thread", i);
Thread.sleep(500);
}
}
catch(InterruptedException e) {
}
System.out.println("Exiting Child Thread ...");
}
}

thread

Child Thread: Thread[Demo Thread,5,main]
Main Thread: 5
Child Thread: 5
Child Thread: 4
Main Thread: 4
Child Thread: 3
Child Thread: 2
Main Thread: 3
Child Thread: 1
Exiting Child Thread ...
Main Thread: 2
Main Thread: 1
Exiting Main Thread ...

Creating a thread by implementing the Runnable interface

// This is the entry point for the second thread.
@Override
public void run() {
try {
for(int i = 5; i > 0; i--) {
System.out.printf("%-15s: %d\n", "Child Thread", i);
Thread.sleep(500);
}
}
catch(InterruptedException e) {
}
System.out.println("Exiting Child Thread ...");
}
}

public class MyRunnableDemo {
public static void main(String[] args) {
MyRunnable runnable = new MyRunnable();
Thread thread = new Thread(runnable);
thread.start();
try {
for(int i = 5; i > 0; i--) {
System.out.printf("%-15s: %d\n", "Main Thread", i);
Thread.sleep(1000);
}
}
catch(InterruptedException e) {
}
System.out.println("Exiting Main Thread ...");
}
}

main

public class MyRunnable implements Runnable{

thread

Main Thread
: 5
Child Thread
: 5
Child Thread
: 4
Main Thread
: 4
Child Thread
: 3
Child Thread
: 2
Main Thread
: 3
Child Thread
: 1
Exiting Child Thread ...
Main Thread
: 2
Main Thread
: 1
Exiting Main Thread ...

THREAD SYNCHRONIZATION
-

When threads share access to a common object, they can
conflict with each other. The shared access creates a problem.
This problem is often called a race condition.

-

To solve the problem use a lock mechanism. The lock mechanism
is used to control the threads that want to manipulate a
shared object.

THREAD SYNCHRONIZATION
-

To acquire the lock the code calls a synchronized method.

-

Methods that contain threed sensitive code are tagged with the
synchronized keyword.

-

When a thread calls a synchronized method on a shared object,
it owns that object’s lock until it returns from the method
and thereby unlocks the object.

-

When an object is locked by one thread, no other thread can
enter a synchronized method for that object, the other thread
is automatically deactivated, and it needs to wait until the
first thread has unlocked the object.

THREAD SYNCHRONIZATION
-

When multiple threads need to update information stored in a
shared object, some ordering has to be enforced to avoid
unintended consequences.

-

Java provides a locking mechanism for this purpose!

-

When one thread wants to access the shared object, it has to

-

-

Lock the object

-

Complete its operations on the object

-

Unlock the object

This way, each thread will have exclusive access to the object
when the thread needs the object

Thread Synchronization - No Threads!
public class CallMe {
public void call(String message) {
System.out.print("[ ");
System.out.print(message);
System.out.print(" ]");
}
}

public class SyncDemo {
public static void main(String[] args) {
CallMe target = new CallMe();
target.call("No threads!");
}
}

[ No threads! ]

Thread Synchronization - No Synchronization
public class CallMe {
public void call(String message) {
System.out.print("[ ");
System.out.print(message);
System.out.print(" ]");
}
}

public class SyncDemo {
public static void main(String[] args) {
CallMe target = new CallMe();
Runnable callerA = new Caller(target, "Hello");
Runnable callerB = new Caller(target, "World");
Runnable callerC = new Caller(target, "Howdy Y’all");
Thread threadA = new Thread(callerA);
Thread threadB = new Thread(callerB);
Thread threadC = new Thread(callerC);
threadA.start();
threadB.start();
threadC.start();
try {
threadA.join();
threadB.join();
threadC.join();
} catch (InterruptedException e) {
}

public class Caller implements Runnable {
String msg;
CallMe target;
public Caller(CallMe targ, String msg) {
this.target = target;
this.msg = msg;
}
@Override
public void run() {
target.call(msg);
}
}

The join() method
allows one thread to
wait until another
thread completes its
execution.

}
}

[ Hello[ Howdy Y'all ][ World ] ]
[ Hello ][ Howdy Y'all ][ World ]
[ Hello[ World ][ Howdy Y'all ] ]

Thread Synchronization - Method Synchronization
public class CallMe {
public synchronized void call(String message) {
System.out.print("[ ");
System.out.print(message);
System.out.print(" ]");
}
}

public class SyncDemo {
public static void main(String[] args) {
CallMe target = new CallMe();
Runnable callerA = new Caller(target, "Hello");
Runnable callerB = new Caller(target, "World");
Runnable callerC = new Caller(target, "Howdy Y’all");
Thread threadA = new Thread(callerA);
Thread threadB = new Thread(callerB);
Thread threadC = new Thread(callerC);
threadA.start();
threadB.start();
threadC.start();
try {
threadA.join();
threadB.join();
threadC.join();
} catch (InterruptedException e) {
}

public class Caller implements Runnable {
String msg;
CallMe target;
public Caller(CallMe targ, String msg) {
this.target = target;
this.msg = msg;
}
@Override
public void run() {
target.call(msg);
}
}

}
}

[ Hello ][ Howdy Y'all ][ World ]
[ Hello ][ World ][ Howdy Y'all ]

Thread Synchronization - Block Synchronization
public class CallMe {
public void call(String message) {
System.out.print("[ ");
System.out.print(message);
System.out.print(" ]");
}
}

public class SyncDemo {
public static void main(String[] args) {
CallMe target = new CallMe();
Runnable callerA = new Caller(target, "Hello");
Runnable callerB = new Caller(target, "World");
Runnable callerC = new Caller(target, "Howdy Y’all");
Thread threadA = new Thread(callerA);
Thread threadB = new Thread(callerB);
Thread threadC = new Thread(callerC);
threadA.start();
threadB.start();
threadC.start();
try {
threadA.join();
threadB.join();
threadC.join();
} catch (InterruptedException e) {
}

public class Caller implements Runnable {
String msg;
CallMe target;
public Caller(CallMe targ, String msg) {
this.target = target;
this.msg = msg;
}
@Override
public void run() {
synchronized(target){
target.call(msg);
}
}
}

}
}

[ Hello ][ Howdy Y'all ][ World ]
[ Hello ][ World ][ Howdy Y'all ]

DO YOU HAVE ANY
QUESTIONS?

THANK
YOU!

@

hend.alkittawi@utsa.edu

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