Threads

1
Threads
2
Multiple Processes

• Sometimes we need multiple processes
• Examples
• Browser download different images
• Animate moving figures
• Application you write needs to check inventory,
  verify customer status, and confirm credit line

3
What are your options?

• Write your own code to do each task
• Likely to be sequential, not concurrent
• Have a loop that does some of each task, then
  moves on to the next
• Very tough to allocate time properly
• Have the JVM allocate time to each task for you

4
Definition thread

• A program unit that executes independently
• Not quite like a separate program, in that it can
  share variables and monitor status
• Will simplify the programmer's task by letting
  the JVM alternate tasks

5
How Threads Work

• Each task implemented as a thread
• Each thread must be started
• Each thread is given a "time slice" by the
  "scheduler"
• The scheduler selects a runnable thread that is
  not sleeping or blocked

6
Two Approaches for Creating Threads

• Create a class that extends class Thread
• Implement the Runnable interface
• Why do this? (because no multiple inheritance)

7

• public class MyThread extends Thread
• public void run()
• try
• // code here to for threads actions
• catch (InterruptedException ie)
• // code here to clean up

8

• public class MyThreadTest
• public static void main(String args)
• myThread oneThread new MyThread()
• myThread twoThread new MyThread()
• oneThread.start() //start begins a thread,
  
• twoThread.start() //and not run()

9

• public void run()
• try
• for (int i 1 ilt100 i)
• System.out.println((new Date())
  "Hello")
• sleep(1000)
• catch (InterruptedException ie)
• // code here to clean up

10
Runnable Interface

• public class MyThreadClass implements Runnable

11
Other Thread Issues

• Priorities (can set 1 thru 10)
• These are closer to recommendations than demands
• Amount of time allocated to each thread can vary
  from run to run (i.e., are computers
  unpredictable?)
• Synchronization
• Possible conflicts when two or more threads
  access a single object
• Example
• public synchronized void myMethod(int myInt)
• Deadlocks (i.e., "deadly embrace")