What do we need for concurrency? • Process creation. – How do you specify
concurrent processes? • Communication. – How do processes exchange ... Page
7 ...
CIT Concurrency
Java and Concurrency
What do we need for concurrency? • Process creation
CIT Concurrency
– How do you specify concurrent processes?
• Communication – How do processes exchange information?
• Synchronization – How do processes maintain consistency?
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Java Threads (1) • A Thread class manages a single sequential thread of control • Threads may be created and deleted dynamically Thread class MyThread extends Thread { public void run() { //...... } } Thread x = new MyThread();
run()
MyThread run()
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Java Threads (2) • Since Java does not permit multiple inheritance, we often implement the run() method in a class not derived from Thread but from the interface Runnable target
Runnable run()
MyRun
Thread public interface Runnable { public abstract void run(); }
class MyRun implements Runnable{ public void run() { run() //..... } } Thread x = new Thread(new MyRun());
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java.lang.Thread public class java.lang.Thread extends java.lang.Object implements java.lang.Runnable { public Thread(); public Thread(Runnable target); public Thread(Runnable target, String name); public Thread(String name); public void run(); public synchronized void start(); public static void sleep(long millis) sleep throws InterruptedException; public static void yield(); yield public final String getName(); … }
Java Thread lifecycle (1) new Thread()
start() causes the thread to call its run() method. start()
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Created st
Alive
op
stop(), or run() returns ()
Terminated The predicate isAlive() can be used to test if a thread has been started but not terminated. Once terminated, it cannot be restarted
Java Thread lifecycle (2) – Alive start() sl ee su p sp en () d( )
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Running yield()
dispatch suspend()
Runnable
Non-Runnable resume() stop(), or run() returns
wait() also makes a Thread Non-Runnable, and notify() Runnable NOTE: suspend(), resume() and stop() are now deprecated
Clock Applet Example (1)
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public class Clock extends java.applet.Applet implements Runnable { Thread clockThread = null; public void start() { if (clockThread == null) { clockThread = new Thread(this, "Clock"); clockThread.start() } } ...
Clock Applet Example (2)
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... public void run() { // stops when clockThread is set to null while(Thread.currentThread()==clockThread) { repaint(); try { clockThread.sleep(1000); } catch (InterruptedException e){ } } } ...
Clock Applet Example (3) ...
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public void paint(Graphics g) { Date now = new Date(); g.drawString(now.getHours() + ":" + now.getMinutes() + ":" + now.getSeconds(), 5, 10); } // When the applet stops, stop its thread public void stop() { clockThread = null; } }
Count Down Timer Example (1)
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Applet
Runnable
CountDown
init() start() stop() run() tick() beep()
counter display
target
Thread
NumberCanvas setvalue()
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Count Down Timer Example (2) public class CountDown extends Applet implements Runnable { Thread counter; int i; final static int N = 10; AudioClip beepSound, tickSound; NumberCanvas display; public void init() public void start() public void stop() public void run() private void tick() private void beep() }
{...} {...} {...} {...} {...} {...}
Count Down Timer Example (3)
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public void start() { counter = new Thread(this); i = N; counter.start(); } public void stop() { counter = null; } public void run() while(true) { if (counter if (i>0) { if (i==0) { } }
{ == null) return; tick(); --i; } beep(); return;}
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ThreadDemo Example (1)
ThreadDemo Example (2) Panel
Applet
GraphicCanvas
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ThreadDemo init() start() stop()
A,B
ThreadPanel
display
rotate() start() stop()
thread
Thread
DisplayThread rotate()
Runnable Rotator run()
target
ThreadDemo Example (3)
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class Rotator implements Runnable { public void run() { try { while(true) ThreadPanel.rotate(); } catch(InterruptedException e) {} } } Rotator implements the runnable interface, calling ThreadPanel.rotate() to move the display
run()finishes if an exception is raised by Thread.interrupt().
ThreadDemo Example (4) ThreadPanel public class ThreadPanel extends Panel { manages the display // construct display with title and segment color c and control buttons for public ThreadPanel(String title, Color c) {…} a thread. CIT Concurrency
// rotate display of currently running thread 6 degrees // return value not used in this example Calls to rotate() public static boolean rotate() are delegated to throws InterruptedException {…} DisplayThread. // create a new thread with target r and start it running public void start(Runnable r) { thread = new DisplayThread(canvas,r,…); Threads are created by thread.start(); the start() method, } and terminated by the // stop the thread using Thread.interrupt() stop() method. public void stop() {Thread.interrupt();} }
ThreadDemo Example (5) public class ThreadDemo extends Applet { ThreadPanel A; ThreadPanel B;
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public void init() { A = new ThreadPanel("Thread A",Color.blue); B = new ThreadPanel("Thread B",Color.blue); add(A); add(B); ThreadDemo creates two } ThreadPanel displays public void start() { when initialized and two A.start(new Rotator()); threads when started. B.start(new Rotator()); } public void stop() { A.stop(); B.stop(); } }
What do we need for concurrency? • Process creation
CIT Concurrency
– How do you specify concurrent processes?
• Communication – How do processes exchange information?
• Synchronization – How do processes maintain consistency?
Interference (1)
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• Ornamental garden problem: – People enter an ornamental garden through either of two turnstiles. Management wish to know how many are in the garden at any time. Garden
West Turnstile
people
East Turnstile
• The concurrent program consists of two concurrent threads and a shared counter object.
Interference (2) Thread
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Applet
Garden
east,west
init() go() eastD, westD, counterD
Turnstile run()
people
Counter increment()
display
display
NumberCanvas setvalue()
The Turnstile thread simulates the periodic arrival of a visitor to the garden every second by sleeping for a second and then invoking the increment() method of the counter object.
Interference (3)
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• The Counter object and Turnstile threads are created by the go() method of the Garden applet: private void go() { counter = new Counter(counterD); west = new Turnstile(westD,counter); east = new Turnstile(eastD,counter); west.start(); east.start(); }
Interference (4) class Turnstile extends Thread { NumberCanvas display; Counter people; CIT Concurrency
Turnstile(NumberCanvas n,Counter c) The run() method { display = n; people = c; } exits and the thread public void run() { terminates after try{ Garden.MAX visitors display.setvalue(0); have entered. for (int i=1;i
