What is a Thread?

• Individual and separate unit of execution that is part of a process
  • multiple threads can work together to accomplish a common goal

• Video Game example
  • one thread for graphics
  • one thread for user interaction
  • one thread for networking

Advantages of Threads

• easier to program
  • 1 thread per task

• can provide better performance
  • thread only runs when needed
  • no polling to decide what to do

• multiple threads can share resources
• utilize multiple processors if available

Disadvantage of Threads

• multiple threads can lead to deadlock
  • much more on this later

• overhead of switching between threads

Creating Threads (method 1)

• extending the Thread class
  • must implement the run() method
  • thread ends when run() method finishes
  • call .start() to get the thread ready to run

Creating Threads Example 1
class Output extends Thread {
private String toSay;
public Output(String st) {
toSay = st;
}
public void run() {
try {
for(;;) {
System.out.println(toSay);
sleep(1000);
}
} catch(InterruptedException e) {
System.out.println(e);
}
}
}
class Program {
public static void main(String [] args) {
Output thr1 = new Output(“Hello”);
Output thr2 = new Output(“There”);
thr1.start();
thr2.start();
}
}

• main thread is just another thread (happens to start first)
• main thread can end before the others do
• any thread can spawn more threads

Creating Threads (method 2)

• implementing Runnable interface
  • virtually identical to extending Thread class
  • must still define the run()method
  • setting up the threads is slightly different

Creating Threads Example 2
class Output implements Runnable {
private String toSay;
public Output(String st) {
toSay = st;
}
public void run() {
try {
for(;;) {
System.out.println(toSay);
Thread.sleep(1000);
}
} catch(InterruptedException e) {
System.out.println(e);
}
}
}
class Program {
public static void main(String [] args) {
Output out1 = new Output(“Hello”);
Output out2 = new Output(“There”);
Thread thr1 = new Thread(out1);
Thread thr2 = new Thread(out2);
thr1.start();
thr2.start();
}
}

• main is a bit more complex
• everything else identical for the most part

Advantage of Using Runnable

• remember – can only extend one class
• implementing runnable allows class to extend something else

Controlling Java Threads

• _.start(): begins a thread running
• wait() and notify(): for synchronization
• _.stop(): kills a specific thread (deprecated)
• _.suspend() and resume(): deprecated
• _.join(): wait for specific thread to finish
• _.setPriority(): 0 to 10 (MIN_PRIORITY to MAX_PRIORITY); 5 is default (NORM_PRIORITY)

Java Thread Scheduling

• highest priority thread runs
  • if more than one, arbitrary

• yield(): current thread gives up processor so another of equal priority can run
  • if none of equal priority, it runs again

• sleep(msec): stop executing for set time
  • lower priority thread can run

States of Java Threads

There are four separate states

• new: just created but not started
• runnable: created, started, and able to run
• blocked: created and started but unable to run because it is waiting for some event to occur
• dead: thread has finished or been stopped

Java Thread Example 1
class Job implements Runnable {
private static Thread [] jobs = new Thread[4];
private int threadID;
public Job(int ID) {
threadID = ID;
}
public void run() { do something }
public static void main(String [] args) {
for(int i=0; i<jobs.length; i++) {
jobs[i] = new Thread(new Job(i));
jobs[i].start();
}
try {
for(int i=0; i<jobs.length; i++) {
jobs[i].join();
}
} catch(InterruptedException e) { System.out.println(e); }
}
}
Java Thread Example 2
class Schedule implements Runnable {
private static Thread [] jobs = new Thread[4];
private int threadID;
public Schedule(int ID) {
threadID = ID;
}
public void run() { do something }
public static void  main(String [] args) {
int nextThread = 0;
setPriority(Thread.MAX_PRIORITY);
for(int i=0; i<jobs.length; i++) {
jobs[i] = new Thread(new Job(i));
jobs[i].setPriority(Thread.MIN_PRIORITY);
jobs[i].start();
}
try {
for(;;) {
jobs[nextThread].setPriority(Thread.NORM_PRIORITY);
Thread.sleep(1000);
jobs[nextThread].setPriority(Thread.MIN_PRIORITY);
nextThread = (nextThread + 1) % jobs.length;
}
} catch(InterruptedException e) { System.out.println(e); }
}
}