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When multiple threads are based on the same object of a class that has implemented Runnable interface or extended Thread class, a synchronized run of these multiple threads can be achieved just by marking the implemented run() method with synchronized keyword.

In a synchronized run of multiple threads, when a thread enters a synchronized run() method, it is not replaced by any other thread wishing to enter the run() method at the same time, not until the thread in execution ends up completing the execution of synchronized run() method.

Two important conditions for a synchronized run of multiple threads -:



• Multiple threads based on a same object of the class, which has implemented Runnable interface.
• Implemented method run() is marked synchronized.

Running multiple threads based on one object, synchronized run

• We are going to create three threads based on a same object of a class, which has implemented Runnable interface and method start() on each thread will be called one by one, next to each other.
• Each thread that enters the run() method will end up completing its execution of run() method, without being replaced by any other thread, right in the middle of its execution of run() method.

// class A is implementing Runnable interface and its run() method

 class A implements Runnable
{

synchronized public void run() //run() is synchronized, i.e. synchronized run of multiple threads,
{			       // A thread executing run() is not replaced by another thread
for(int i=0;i<3;i++)
System.out.println(Thread.currentThread().getName());
}


public static void main(String... ar)
{
A ob= new A();

Thread t1= new Thread(ob,"ThreadA");
Thread t2= new Thread(ob,"ThreadB");
Thread t3= new Thread(ob,"ThreadC");	 //Calling Thread's constructor & passing the object of
					 //A class that  implemented  Runnable interface &
					 //the name of new thread.

t1.start();
t2.start();
t3.start();
}
}

ThreadB
ThreadB
ThreadB
ThreadC
ThreadC
ThreadC
ThreadA
ThreadA
ThreadA





Program Analysis



• We have created 3 threads based on the same object of A class. Each thread is given a different name.
• Method start() is called on each thread, which automatically calls its run() method to start its execution
• In run() method, each thread cycles through for-loop three times, printing its own name .
• As you may see the output, it was a synchronized run of threads, and no thread was replaced by another thread(wishing to enter run()) in the middle of its execution of synchronized run() method.





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Running multiple threads with a delay in each thread's run(synchronized run)


We are creating three threads based on a different object of the class that has implemented Runnable interface. The first method to enter the run() method, goes to sleep for 1 second but it is not replaced by any other thread i.e. it wakes up after 1 second and completes its execution of run() method .

 class A implements Runnable
{

synchronized public void run() //run method is marked synchronized, causing synchronized run of muliple threads
{
for(int i=0;i<3;i++)
{
try
{
Thread.sleep(1000);
System.out.println(Thread.currentThread().getName());
}
catch(Exception e)
{
System.out.println(e);
}

}
}


public static void main(String... ar)
{
A ob= new A();

Thread t1= new Thread(ob,"ThreadA");
Thread t2= new Thread(ob,"ThreadB");
Thread t3= new Thread(ob,"ThreadC");

t1.start();
t2.start();
t3.start();
}
}





Output is -:




ThreadA	
ThreadA
ThreadA
ThreadC
ThreadC
ThreadC
ThreadB
ThreadB
ThreadB





Program Analysis



• We have created three threads, with each thread based on a different object of A class.
• Method start() is called on each thread, which automatically calls its run() method to start its execution
• In run() method, each thread cycles in for-loop, three times. In each cycle, there is a delay of 1 second and the name of executing thread is printed.
• You may see in the output, it's been a synchronized run of threads. Each thread is allowed to complete its execution of synchronized run() & even when a thread sleeps for 1 second, no thread replaces it.





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