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Deadlock

Deadlock describes a situation where two or more threads are blocked forever, waiting for each other. Deadlock occurs when multiple threads need the same locks but obtain them in different order. A Java multithreaded program may suffer from the deadlock condition because the synchronized keyword causes the executing thread to block while waiting for the lock, or monitor, associated with the specified object.

Important points:

• If threads are waiting for each other to finish, then the condition is known as Deadlock.
• Deadlock condition is a complex condition which occurs only in case of multiple threads.
• Deadlock condition can break our code at run time and can destroy business logic.
• We should avoid this condition as much as we can.

Example:

Alphonse and Gaston are friends, and great believers in courtesy. A strict rule of courtesy is that when you bow to a friend, you must remain bowed until your friend has a chance to return the bow. Unfortunately, this rule does not account for the possibility that two friends might bow to each other at the same time. This example application, Deadlock, models this possibility:

public class Deadlock {
    static class Friend {
        private final String name;
        public Friend(String name) {
            this.name = name;
        }
        public String getName() {
            return this.name;
        }
        public synchronized void bow(Friend bower) {
            System.out.format("%s: %s"
                + "  has bowed to me!%n", 
                this.name, bower.getName());
            bower.bowBack(this);
        }
        public synchronized void bowBack(Friend bower) {
            System.out.format("%s: %s"
                + " has bowed back to me!%n",
                this.name, bower.getName());
        }
    }

    public static void main(String[] args) {
        final Friend alphonse =
            new Friend("Alphonse");
        final Friend gaston =
            new Friend("Gaston");
        new Thread(new Runnable() {
            public void run() { alphonse.bow(gaston); }
        }).start();
        new Thread(new Runnable() {
            public void run() { gaston.bow(alphonse); }
        }).start();
    }
}

When Deadlock runs, it’s extremely likely that both threads will block when they attempt to invoke bowBack. Neither block will ever end, because each thread is waiting for the other to exit bow.

Conclusion

For avoid deadlock consider this:

• Avoid Nested Locks: This is the main reason for dead lock. Dead Lock mainly happens when we give locks to multiple threads. Avoid giving lock to multiple threads if we already have given to one.
• Avoid Unnecessary Locks: We should have lock only those members which are required. Having lock on unnecessarily can lead to dead lock.
• Using thread join: Dead lock condition appears when one thread is waiting other to finish. If this condition occurs we can use Thread.join with maximum time you think the execution will take.

Links

https://www.javatpoint.com/deadlock-in-java
https://www.geeksforgeeks.org/deadlock-in-java-multithreading/
https://www.tutorialspoint.com/java/java_thread_deadlock.htm

credit goes to @swayangjit