Multithreading Concepts and Programs

 

1. Producer Consumer Problem using wait and notify.

2. Producer consumer problem using Blocking Queue.

3. Producer consumer using semaphore

4. CountDownLatch Example

5. When should we use CountDownLatch in Java :

Use CountDownLatch when one of Thread like main thread, require to wait for one or more thread to complete, before its start doing processing. Classical example of using CountDownLatch in Java  is any server side core Java application which uses services architecture,  where multiple services is provided by multiple threads and application cannot start processing  until all services have started successfully as shown in our CountDownLatch example.

CountDownLatch in Java – Things to remember

Few points about Java CountDownLatch which is worth remembering:

1) You cannot reuse CountDownLatch once count is reaches to zero, this is the main difference between CountDownLatch and CyclicBarrier, which is frequently asked in core Java interviews and multi-threading  interviews.

2) Main Thread wait on Latch by calling CountDownLatch.await() method while other thread calls CountDownLatch.countDown() to inform that they have completed.

6. CyclicBarrier Example

7. Dining Philosopher Problem

The diagram above represents the problem. There are five silent philosophers (P1 – P5) sitting around a circular table, spending their lives eating and thinking.

There are five forks for them to share (1 – 5) and to be able to eat, a philosopher needs to have forks in both his hands. After eating, he puts both of them down and then they can be picked by another philosopher who repeats the same cycle.

An initial solution would be to make each of the philosophers follow the following protocol:

As the above pseudo code describes, each philosopher is initially thinking. After a certain amount of time, the philosopher gets hungry and wishes to eat.

At this point, he reaches for the forks on his either side and once he’s got both of them, proceeds to eat. Once the eating is done, the philosopher then puts the forks down, so that they’re available for his neighbor.

 

8. What is Thread Pool in Java and why we need it?
   
   As I said Thread pool is a pool of already created worker thread ready to do the job. The thread pool is one of essential facility any multi-threaded server side Java application requires. One example of using thread pool is creating a web server, which processes client request. If you are familiar with socket programming then you know that ServerSocket.accept() is blocking method and blocks until a socket connection made. 
   
   If only one thread is used to process client request, than it subsequently limit how many client can access server concurrently. In order to support large number of clients, you may decide to use one thread per request paradigm, in which each request is processed by separate Thread, but this require Thread to be created, when request arrived.  Since creation of Thread is time consuming process, it delays request processing. 
   
   It also limits number of clients based upon how many thread per JVM is allowed, which is obviously a limited number. Thread pool solves this problem for you, It creates Thread and manage them. Instead of creating Thread and discarding them once task is done, thread-pool reuses threads in form of worker thread. 
   
   Since Thread are usually created and pooled when application starts, your server can immediately start request processing, which can further improve server’s response time. Apart from this, there are several other benefits of using Thread pool in Java applications, which we will see in subsequent section. 
   
   In short, we need thread pools to better manage threads and decoupling task submission from execution. Thread pool and Executor framework introduced in Java 5 is an excellent thread pool provided by library.

9. Java Thread Pool - Executor Framework in Java 5

Java 5 introduced several useful features like Enum, Generics, Variable arguments and several concurrency collections and utilities like ConcurrentHashMap and BlockingQueue etc, It also introduced a full feature built-in Thread Pool framework commonly known as Executor framework. 

The core of this thread pool framework is Executor interface which defines an abstraction of task execution with method execute(Runnable task) and ExecutorService which extends Executor to add various life-cycle and thread pool management facilities like shutting down thread pool. 

Executor framework also provides a static utility class called Executors ( similar to Collections) which provides several static factory method to create various type of Thread Pool implementation in Java e.g. fixed size thread pool, cached thread pool and scheduled thread pool. Runnable and Callable interface are used to represent task executed by worker thread managed in these Thread pools. 

Interesting point about Executor framework is that, it is based on Producer consumer design pattern, where application thread produces task and worker thread consumers or execute those task, So it also suffers with limitation of Producer consumer task like if production speed is substantially higher than consumption than you may run OutOfMemory because of queued task, of course only if your queue is unbounded.

10. How to create fixed size thread pool using Executor framework in Java?

Creating fixed size thread pool using Java 5 Executor framework is pretty easy because of static factory methods provided by Executors class. All you need to do is define your task which you want to execute concurrently and then submit that task to ExecutorService. from them Thread pool will take care of how to execute that task, it can be executed by any free worker thread and if you are interested in result you can query Future object returned by submit() method. Executor framework also provides different kind of Thread Pool e.g. SingleThreadExecutor which creates just one worker thread or CachedThreadPool which creates worker threads as and when necessary. You can also check Java documentation of Executor Framework for complete details of services provided by this API. Java concurrency in Practice also has a couple of chapters dedicated to the effective use of Java 5 Executor framework, which is worth reading for any senior Java developer.

11. Benefits of Thread Pool in Java

Thread Pool offers several benefit to Java application, biggest of them is separating submission of a task to execution of the task, which results if more loose coupled and flexible design than tightly coupled create and execute pattern. Here are some more benefits of using Thread pool in Java:

1) Use of Thread Pool reduces response time by avoiding thread creation during request or task processing.

2) Use of Thread Pool allows you to change your execution policy as you need. you can go from single thread to multiple threads by just replacing ExecutorService implementation.

3) Thread Pool in Java application increases the stability of the system by creating a configured number of threads decided based on system load and available resource.

4) Thread Pool frees application developer from thread management stuff and allows to focus on business logic.

That's all on Thread pool in Java 5. we have seen what is thread pool in Java, what is executor framework in java 5, how to create thread pool in Java and some benefits of using thread pool in Java application. no doubt knowledge of thread pool is essential for a server side core Java developer and I suggest reading Java Threads and Concurrency Practice in Java to learn more about concurrency and thread pool.

12. Example of Thread Pool in java