MultiThreading

Exercises from Java Multithreading course by John Purcell

Course link on Udemy: https://www.udemy.com/java-multithreading/learn/v4/overview

Notes

1. Hello World - Two ways of starting threads
   1. Extends Thread class
   2. Implement Runnable and pass it to constructor of thread class
2. Volatile Keyword - Set shared variable to volatile to avoid caching between threads and guarantees that any thread will read the most recent written values
3. Synchronized keyword - Let's only one thread inside the function at a time.Avoids: Race condition
4. Lock - This example is to showcase how in a multithreaded system, Synchronized Block is faster than Synchronized method.
5. Thread Pool - Threadpool contains collection of worker threads. Once a thread completes a task in threadpool, it is recycled to complete pending tasks
6. CountDown Latch - Java class to aid in synchronization. Multiple threads wait on the coundown to become 0.
   Example of CountDownLatch is similar to catering food where the main thread waits on completion of all individual orders.
   
7. Producer Consumer - Implementation of Producer - Consumer pattern, where one/ many threads produces data and stores in shared data structure and other threads are removing/consuming data. Thread safe datastructure used: BlockingQueue
8. Wait & Notify - Wait - calling thread gives up the lock and goes to sleep. Notify - wakes up the first thread which called wait & once the thread finishes the synchronized block , the lock is relinquished. Invoking notify doesn't relinquish the lock.
9. Low Level Sync - Implemented producer consumer pattern using low level synchronization techniques : wait & notify
10. Reenterant locks - Implemented reenterant lock with two theads and used await and signal for synchronization. Lock: an object that can only be owned by a single thread at any given time. Basic operations on a lock: acquire & release Lock provides : Mutual Exclusion
    Condition Variable: Synchronization mechanism needs more than mutual exclusion;also need a way to wait for another thread to do some action.used to wait for a particular condition to become true.
    methods: wait (condition,lock) , signal (condition,lock), broadcast(condition,lock) https://web.stanford.edu/class/cs140/cgi-bin/lecture.php?topic=locks
    Disadvantage of Reentrant lock : Deadlock scenario if one of the thread signals but never unlocks
11. Deadlocks - Situation where two or more threads are blocked forever, waiting for each other.
12. Semaphores - Difference between lock , mutex and semaphore
    1. Lock - one thread at a time enters that's locked and not shared with other processes.
    2. Mutex- same as lock but can be system wide ( shared by multiple processes)
    3. Semaphore - same as mutex but allows only x number of threads to enter (Eg: to limit CPU /IO/RAM usage) https://stackoverflow.com/questions/2332765/lock-mutex-semaphore-whats-the-difference?noredirect=1&lq=1
13. Callable - Get results from threads and also allow threads to throw exceptions as opposed to runnable which doesn't do both. Future - allows you to control your threads, checking to see if they are running or not, waiting for results and even interrupting or descheduling them
14. Interrupting Threads - Threads when interupted throws InterruptedException. Another way to check if the thread is interupted is to quiz the thread and check if current thread isInterrupted()