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Creating basic deadlocked system

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A deadlock occurs when two competing actions wait for the other to finish, and thus neither ever does. In java there is one lock associated with each object. To avoid concurrent modification done by multiple threads on single object we can use synchronized keyword, but everything comes at a cost. Using synchronized keyword wrongly can lead to stuck systems called as deadlocked system.

Consider there are 2 threads working on 1 instance, Lets call threads as First and Second, and lets say we have 2 resources R1 and R2. First acquires R1 and also needs R2 for its completion while Second acquires R2 and needs R1 for completion.

so say at time t=0,

First has R1 and Second has R2. now First is waiting for R2 while Second is waiting for R1. this wait is indefinite and this leads to deadlock. 

public class Example2 {
    
    public static void main(String[] args) throws InterruptedException {
        final DeadLock dl = new DeadLock();
        Thread t1 = new Thread(new Runnable() {
    
            @Override
            public void run() {
                // TODO Auto-generated method stub
                dl.methodA();
            }
        });
   
        Thread t2 = new Thread(new Runnable() {
    
            @Override
            public void run() {
                // TODO Auto-generated method stub
                try {
                    dl.method2();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        });
        t1.setName("First");
        t2.setName("Second");
        t1.start();
        t2.start();
    }
}

class DeadLock {
    
    Object mLock1 = new Object();
    Object mLock2 = new Object();
    

    public void methodA() {
        System.out.println("methodA wait for mLock1  " + Thread.currentThread().getName());
        synchronized (mLock1) {
            System.out.println("methodA mLock1 acquired   " + Thread.currentThread().getName());
            try {
                Thread.sleep(100);
                method2();
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
    public void method2() throws InterruptedException {
        System.out.println("method2 wait for mLock2  " + Thread.currentThread().getName());
        synchronized (mLock2) {
            System.out.println("method2  mLock2 acquired   " + Thread.currentThread().getName());
            Thread.sleep(100);
            method3();
        }
    }
    public void method3() throws InterruptedException {
        System.out.println("method3  mLock1  "+ Thread.currentThread().getName());
        synchronized (mLock1) {
            System.out.println("method3   mLock1 acquired  " + Thread.currentThread().getName());
        }
    }
}

Output of this program: 

methodA wait for mLock1  First
method2 wait for mLock2  Second
method2  mLock2 acquired   Second
methodA mLock1 acquired   First
method3  mLock1  Second
method2 wait for mLock2  First
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Concurrent Programming Threads:
* Concurrent Programming Threads
* Callable and Future
* CountDownLatch
* Basic Multithreading
* Locks as Synchronisation aids
* Semaphore
* Runnable Object
* Creating basic deadlocked system
* Synchronization
* Creating a java.lang.Thread instance
* Atomic operations
* Exclusive write Concurrent read access
* Visualizing readwrite barriers while using synchronized volatile
* Get status of all threads started by your program excluding system threads
* Producer-Consumer
* Multiple producerconsumer example with shared global queue
* Using ThreadLocal
* Add two int arrays using a Threadpool
* Pausing Execution
* Thread Interruption Stopping Threads
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