This document provides an introduction to Java concurrency and the ExecutorService API. It discusses parallelism and threads in Java, shows how to create and manage thread pools using ExecutorService, and emphasizes that ExecutorService is now preferred over directly starting and managing threads. The goal is to give a simple overview of Java concurrency basics and how ExecutorService can help manage threads.

1. Java concurrency API
By Franck Benault
Created 06/12/2015
Last update 22/11/2015

2. Java concurrency introduction
● Parallelism can speed up you program
– But is Java the best language for concurrency ?

3. Java concurrency links
● My examples in Github
– https://github.com/franck-benault/test-concurrence

4. Java concurrency plan

5. Goal of this presentation
● Goal
– Very simple presentation
● Focus on ExecutorService

6. Java concurrency : Basics threads
● Principle
– Class Worker implemented Runnable interface
– Worker w = new Worker(...) ;
– Thread t = new Thread(w) ;
– t.start() ;
– t.join() ; //wait until the end of the thread
● Problems
– Run() method does not return any information (return value, exception)
– How to manage a pool of threads ?
– Do not use this way anymore !

7. Java concurrency : Sleep
● Old way
– Thread.sleep(100);
● Since Java5
– TimeUnit.MILLISECONDS.sleep(100);

8. Java concurrency : show threads
with jvisualVM

9. Java concurrency : Basics
isSynchronized
● Key word
– On a bloc (with a parameter)
– On a method
● Thread safe

10. Java concurrency :
ExcecutorService
● Management of a pool of threads
● Accept Callable and Runnable
ExecutorService executor;
executor = Executors.newFixedThreadPool(threadPoolSize);
WorkerThread workerThread = new WorkerThread(...);
executor.submit(workerThread);
...
executor.shutdown();
while (!executor.isTerminated()) Thread.sleep(10);

11. Conclusion
● Let's start with concurrency in Java