Threads in Java 1

Concurrency

Synchronizing threads, thread pools, etc.

Benefits of threadsMore CPU’s

• Many modern computers has more than one CPU– Dual Core

• By dividing the program into more threads the program can utilize the CPU’s– The program will run faster

Threads in Java 2

Benefits of threadsThreads waiting

• A thread may be waiting for something to happen– File to be read– Incoming network requests– Sleep()– Wait()

• While one thread is waiting other threads should use the CPU– The program will run faster

• Figure from – http://jayford040.blogspot.com/

2009/07/process-concept-operating-system.html

Threads in Java 3

Threads in Java 4

Creating threads

Class A extends Thread { public void run() { … }}

A a = new A();a.start();

• A may not extend other classes.

Class B implements Runnable { public void run() { … }}

B b = new B();Tread t = new Thread(b);t.start();

• B may extend another class.• Decouples task submission from thread scheduling, etc.

Threads in Java 5

Joining treads

• Some methods on the class Thread– Start()

• Starts the thread

– Join() • waits for the thread to die

Runnable printerA = new Printer("Anders", 10);

Thread threadA = new Thread(printerA);

threadA.start();

// threadA.join();

System.out.println( "Main is done");

Threads in Java 6

Synchronizing threads

• Having threads in a programming language is a nice feature, but threads must to be controlled.– If you have 2 or more threads with reference

to the same object, the threads might execute methods on that object simultaneously. • This must be controlled

Threads in Java 7

Race conditions and critical sections

• Race condition– 2 or more threads are reading or writing shared data

and the final result depends on the timing of the thread scheduling.

– Race conditions are generally a bad thing!• Critical section

– Part of a program (whole method or just a part of a method) where race conditions might happen.

– To avoid race conditions• We want to make sure that at most one thread executes the

critical section at any point in time.• Threads must be synchronized.

Threads in Java 8

References

• Sun Microsystems The Java Tutorial, Threads– http://java.sun.com/docs/books/tutorial/essential/threads/index.ht

ml

• Sun Microsystems Java 2 Platform Standard Edition 5.0 API Specification: java.util.concurrent– http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/packa

ge-summary.html

Threads in Java 9

More references• Brian Goetz et al Java

Concurrency in Practice, Addison Wesley 2006 http://javaconcurrencyinpractice.com

• Doug Lea Concurrent Programming in Java 2nd edition, Addison Wesley 2000

• Oaks & Wong Java Threads, O’Reilly 2004

• Niemeyer & Knudsen Learning Java, 3rd edition, O’Reilly 2005– 9. Threads, page 249-297