CS4273: Distributed System Technologies and Programming I

Lecture 4: Java Threads and Multithread Programming

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Multi-threading

Threads

• A thread is a thread of control flow of executing a piece program, similar to a process.

• Multi-threading is a technique of concurrent programming, which runs multiple threads on one piece of program.

• Threads are running in a pseudo-parallel fashion.

• Servers and animation programs often use multiple threads.

PC

PC

PC

Memory

Code Segment

Thread1

Thread2

Thread3

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States of a Thread Life Cycle

Blocked

NewBorn Dead

Runnable Running

start()

yield()

wait()notify()

sleep()

• run queue: priority queue

head = Thread.currentThread();

• wait queue: activated by notify()

• sleep queue:

Head

Tail

Head Tail

Head Tail

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Methods in Thread Class

run() method

• run() method is the program body of a thread. A thread executes run method when it starts. You need to override this method to let the thread do your work!

Methods called by threads:

• sleep(ms): make the caller sleep for N milli-seconds.

• yield(): make the caller non-selfish thread. The caller doesn’t mind to give out the CPU to other threads (of the same priority).

• wait() : the caller blocks itself until being waken up.

• notify(): wake up a thread in wait-queue (by moving the thread from wait-queue to run-queue). The caller still holds CPU after the call.

• notifyAll(): wake up all threads in wait-queue.

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Methods in Thread Class (Cont.)

Methods called by control programs (or threads)

• start(): start the thread. It places the thread to the tail of run-queue.

• setPriority(Thread.MAX_PRIORITY - 2).

• isAlive(): check if a thread is still executing run method.

• currentThread(): return the current running thread.

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Example of a thread program

public class mythread extends JApplet implements Runnable {……….

public void start() { if (runner == null) { runner = new Thread(this); runner.start(); } } public void stop() {

// N.B. thread.stop method was deprecated! runner = null; } public void run() { Thread thisThread = Thread.currentThread();

// N.B. if “runner’ is null, it won’t do anythingwhile (runner == thisThread) {

thisThread.sleep(t);repaint();………

}

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Thread Creation

There are two ways to create threads:

1. extend the Thread class to define your thread-class and then construct objects out of your thread-class.

2. use the Runnable interface and implement the run method of the interface.

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Example of extending Thread class

class BellThread extends Thread{ String Sound; BellThread (String Sound ) { this.Sound = Sound; } public void run(){ while(true){ System.out.println (Sound + " "); try{sleep(20); } catch(Exception e)

{ return; } } } public static void main(String[] args){ new BellThread(“Ding”).start(); new BellThread(“Dong”).start(); } }

process

Ding Thread

Dong Thread

OS

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Example of Thread Switching

public class threadSwitch extends JApplet { int threadID = 0, star = 0, hash = 0; public void init() { StarThread runner1 = new StarThread();

runner1.start(); HashThread runner2 = new HashThread(); runner2.start(); } public void paint(Graphics g) { super.paint(g); switch(threadID) { case 1: g.drawString("*", star*10, 50); break; case 2: g.drawString("#", hash*10, 100); } }

class StarThread extends Thread {

public void run() {

while (true){

threadID = 1;

if (star >= 50 ) star = 0;

else star++;

repaint(); }

}}

class HashThread extends Thread {

public void run() {

while (true){

threadID = 2;

if (hash >= 50 ) hash = 0;

else hash++;

repaint(); }

}}

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Runnable Interface

• In addition to extending Thread class, an application often needs to extend from another class, e.g., Applet. But Java doesn’t allow multiple inheritance. Runnable interface is for this purpose.

• Runnable interface has only one method run(). You need to implement the run method.

• Thread class constructor creates threads out of Runnable objects. Thread has two constructors:public Thread (Runnable target); the constructed thread object uses target’s run method! public Thread(Runnable target, String name);the save as above, but the new thread object has a name.

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Example of Threads using Runnable Interface

class BellRunnable implements Runnable {

BellRunnable(String sound){

this.sound = sound;

}

public void run() {

for(int i=1;i<=100;i++){

System.out.println(sound + " ");

try{Thread.sleep(20); }

catch (Exception e){ return;}

}

}

public static void main(String[] args) {

BellRunnable Ding = new BellRunnable(“Ding”);

Thread DingThread = new Thread(Ding);

DingThread.start();

BellRunnable Dong = new BellRunnable(“Dong”);

new Thread(Dong).start();

}

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Applets Implement Runnable Interface of Threads

• Runnable interface has only one method run, you can implement the Runnable interface in any class by implementing the run method.

• It is most often used together with Applet class.

class ThreadApplet extends JApplet implements Runnable {private Thread runner;……public void start() {

if (runner == null) {runner = new Thread (this); // run of this classrunner.start (); }

} public void run() {

// thread-code comes here ……

}}

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Thread Control: stop a thread

Methods “stop”, “suspend” and “resume” for thread control were deprecated due to un-safety.

Stop a thread: add the following codes:1. set the thread pointer runner to “null” when you

want to stop it:……runner1 = null; ……

2. add the following code at the beginning of run() of the thread:public void run() Thread thisThread = Thread.currentThread(); while (runner1 == thisThread) {

code-of-runner; } }

N.B. The following two simple methods don’t work:

a) Only set “runner1 = null” without 2nd part, it cannot stop the execution of runner.

b) Simplify the 2nd part as below, it won’t work either:public void run() { while (runner1!= null) {

code-of-runner;} }

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Suspend and Resume a thread

• suspend a thread: use a boolean var to indicate suspending the thread and the thread blocks itself by “wait()” in run method.

• resume a thread: use “notify()” to resume the thread.1. use a boolean var.boolean runnerSuspended;public synchronized void actionPerformed

(ActionEvent e) {switch (id) {case SUSPEND: runnerSuspended = true; break;case RESUME:

runnerSuspended = false; notifyAll(); }

2. use the following code in run().public void run() {

while (runnerSuspended) wait();

code-of-runner;}}

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Example of using wait/notify for suspend/resume

public class threadControl extends JApplet implements Runnable, ActionListener {

public void run() {thisThread = Thread.currentThread();

while (faster == thisThread) { try { synchronized(this) { while (fastSuspended)

wait(); Thread.sleep(20); } catch (InterruptedException e) {} fast++;

repaint(); } while (slower == thisThread) { try { synchronized(this) {

while (slowSuspended) wait();

Thread.sleep(40); } catch (InterruptedException e) {} slow++;

repaint(); }

public synchronized void actionPerformed (Event e) { if (e.getSource() == fsus) fastSuspended = true; else if (e.getSource() == fres) { fastSuspended = false; notifyAll(); } else if (e.getSource() == fstp) faster = null;

………………

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Concurrent accesses may cause inconsistencyclass unsynchBankTest {

public static void main (String [] args) {Bank b = new Bank ();

for (int i = 1; i <= Ntrans; i++)new Trans (b).start(); }//concurrent threads

}class Bank {

……public void transfer ( int from, int to, int amnt) {

…… accounts[from] -= amnt;accounts[to] += amnt;

} ……} class Transaction extends Thread {

public Trans (Bank b) { bank = b;}public void run () {

while (true) {int from = (Bank.Nacct - 1)*

Math.random();// random numbers for to and amnt;bank.transfer ( from, to, amnt); }

}

load r0, acct[to]add r0, amntstore r0, acct[to]

Bank

Thread1 Thread2 Thread n….

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synchronized Method for access shared objects

• When multiple threads access shared object concurrently, a monitor is required to guarantee each data access operation is atomic (non-interruptable).

• Java provides a modifier “synchronized” for methods which requires mutual exclusion. When a method is declared as synchronized, the monitor ensures that at any time no more than one thread is active in the monitor (i.e. accesses the object).

• When there is a possibility for two threads to modify a data object in a method, declare the method (or the code-segment) as synchronized, which makes you free of mutual exclusion problem.

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Example of synchronized Methods

class unsynchBankTest {public static void main (String [] args) {

Bank b = new Bank ();for (int i = 1; i <= Ntrans; i++)

new Trans (b).start();}

}class Bank {

public synchronized void transfer ( int from, to, amnt) {……accounts[from] -= amnt; // it’s safe now!accounts[to] += amnt;

} ……}class Trans extends Thread {

public void run () {while (true) {

int from = (Bank.Nacct - 1)* Math.random();// random numbers for to and amnt;bank.transfer ( from, to, amnt);

}}}

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Thread Coordination

• Threads often need to coordinate with each other to get a job done, such as the Producer-Consumer problem.

• The coordination among threads can be achieved by wait and notify methods. When a thread cannot proceed further and waits for the change of a state, it calls wait() to block itself; when a thread changes a state which may unblock others, it calls notify().

• In Java, there is only one wait-queue, a FIFO queue, for each application. If more than one thread waiting, notify wakes up the head of wait-queue. Threads cannot wait() on specific events. Therefore, notifyAll() is often used to make sure the right thread is waken up.

Box

Boy Monkey

Put Get

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Example of Threads Coordination

class Box {

boolean banana = false; //no banana

synchronized void get() {

while (!banana) // ? not use if

wait(); // wait for banana

System.out.println("take banana away”);

banana = false; // no more banana

notifyAll(); //notify box is empty

}

synchronized void put() {

while (banana)

wait(); // wait for box empty

System.out.println(“put banana in”);

banana = true;

notifyAll(); // notify banana is ready

}

Box

Boy Monkey

Put Get

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Example of Threads Coordination (Cont.)

class Monkey extends Thread { Monkey(Box box) { this.box = box; } public void run() {

while(true) { box.get();

} }class Boy extends Thread { Boy (Box box) { this.box = box; } public void run() { while(true){

box.put(); }

}

class MonkeyEatBanana { public static void main(String args[]) { Box box = new Box(); new Monkey (box).start(); new Boy (box).start(); } }

Summary of producer-consumer problem:

• define a class for the share objects, with methods (synchronized if necessary) for accessing the objects.

• define classes for producer threads and consumer threads, using the methods on the shared objects to access them.

• define a main routine which instantiates shared objects and threads, and starts the threads.