Java Threads, I

Mt Hebron CS Clubhttp://mthcompsci.wordpress.com/

Nov 2013

Fundamental Idea• Processes– Large, serialized, and have their own

data storage– Have you ever asked yourself :– How can we run two things at once in

Java?• (This is a fair bit complex!)• Threads can run simultaneously and

share identical data from a heap/stack

A static example – pausing Code

• Thread.sleep (long ms);• ^This method can cause a thread to

pause for the specified amount of time, in milliseconds

• Danger – interruption (threads could be interrupted by another command)

Implementation in Java• Thread Object is built-in to Java• Runnable – interface used when specifying

the class given to a thread to run (“allocating”)

• In this class, which you’re giving a thread to run, you’ll need to override the run() and start() methods to say what you want this thread to run.

• Reserved word synchronized• ^This is for more advanced threaded

programming; we may get to this topic later.

Source: Oracle Documentation: http://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html

These “Runnable” parameters can be the classes which you define!(as long as you ‘implement Runnable’ in your class)

Examples – Starting Simple• Make a program to print:

“Hello from a threaded Object!”• …using a Thread Constructor and a

second class.• Remember! JVM invokes the class’s

run() method when you invoke [threadName].start()public class Driver

{ public static void main (String[] args) { // Use a Constructor of thread // invoke: [thread’s identifier].start(); /* Then wait for a second, or use join() method */ }}

public class PrintHello implements Runnable{

}

public PrintHello (){}public void run (){ // What are you trying to do?}

A More Complex Example

An Example Program, cont.

An Example Program, cont.

The Serial Program (for Comparison)

Some Notes• Each Thread has been

given a separate class to run (this is why each class was called ‘Runnable’)

• The threaded program waits about 200 seconds for both threads to complete before finally terminating

• The Serial program has the same code as the Threaded one, but it never invokes any Thread methods.• The Serial program terminates the moment it prints the output

More Important Points• Key point: how many threads does this

program have?• Answer: 3• There is a main thread that begins, and two

others are created manually in this program.• You may be wondering:• This is all interesting, but how can I (or, say,

some company) use this feature as a benefit?• I ran some tests to determine execution time…

Results of Comparison

• Threaded programming can be 20-30% quicker in my example (perhaps more efficient in other examples!)

• Execution time – very important, in competitions and in real programming jobs

Implementation in Java, II• If two different threads try to access/act on

the same data,– Memory consistency error could occur.

• synchronized methods try to reduce this problem

• ‘Liveness’ – efficiency• Issues possible with liveness

w/threads:– deadlock (≥2 threads waiting for each other to act),– livelock (≥2 threads both continuously replying to each

other),– Starvation (one thread has absolute access to data for

long amounts of time)

Sources / For more Info:• Main source: http

://docs.oracle.com/javase/tutorial/essential/concurrency/index.html

• http://www.seas.gwu.edu/~simhaweb/java/lectures/module12/module12.html

• http://www.cs.uic.edu/~jbell/CourseNotes/OperatingSystems/4_Threads.html

To Be Continued…• Thanks for Attending!• If you have questions, feel free to ask• Next meeting: next Thursday afternoon