Multithreading Horstmann ch.9

Multithreading Threads Thread states Thread interruption Race condition Lock Built-in lock java.util.concurrent library Animation example

Single vs. Multiple Threads Finish each line of bullets before starting new line Allow simultaneous drawing of several lines ThreadTester1 ThreadTester2

Single vs. Multiple Threads Using single thread: in method actionPerformed:... for (int i=0; i<10; i++) { ”draw single bullet”; Thread.sleep(500); }...

Running Threads public class MyRunnable implements Runnable { public void run() { thread action } }... Runnable r = new MyRunnable(); Thread t = new Thread(r); t.start(); Specify action Construct thread from Runnable object Start thread

Single vs. Multiple Threads Using single thread: in method actionPerformed:... for (int i=0; i<10; i++) { ”draw single bullet”; Thread.sleep(500); }... Using multiple threads: In method actionPerformed: Runnable r = new Runnable() { public void run() { } Thread t = new Thread(r); t.start();

QUIZ public static void main(String[] args) { Runnable r1 = new Producer(); Runnable r2 = new Producer(); Thread t1 = new Thread(r1); Thread t2 = new Thread(r2); t1.start(); t2.start(); } How many threads? 1.None 2.One 3.Two 4.Three 5.I don’t know

Starting Two Threads public static void main(String[] args) { Runnable r1 = new Producer(); Runnable r2 = new Producer(); Thread t1 = new Thread(r1); Thread t2 = new Thread(r2); t1.start(); t2.start(); }

Multithreading Threads Thread states Thread interruption Race condition Lock Built-in lock java.util.concurrent library Animation example

Thread states Each thread has –State (new, runnable, blocked, dead), and –priority Thread blocked when Sleeping Waiting for I/O Waiting to acquire lock Waiting for condition Scheduler activates the runnable thread of max priority if a thread has completed its time slice a thread has blocked itself a thread with higher priority has become runnable

Thread scheduling public static void main(String[] args) { Runnable r1 = new Producer(“goddag”); Runnable r2 = new Producer(“farvel”); Thread t1 = new Thread(r1); Thread t2 = new Thread(r2); t1.start(); t2.start(); } Output depends on scheduler: “goddag” “farvel” “goddag” “farvel” … public class Producer implements Runnable { public Producer(String a) { greeting = a; } public void run() { try { for (int i = 1; i <= 10; i++) { System.out.println(i + ": " + greeting); Thread.sleep(100); } } catch (InterruptedException e) {} } private String greeting; }

QUIZ Thread T1 based on run method public void run { try { Thread.sleep(100000); } catch (InterruptedException e) {} } Thread T2 based on run method public void run { factor( ) } Which thread(s) become blocked? 1.None 2.T1 3.T2 4.Both 5.I don’t know factor method from ”noter” ch. 1

Multithreading Threads Thread states Thread interruption Race condition Lock Built-in lock java.util.concurrent library Animation example

Thread interaction Recursive Fibonacci –May take long time –Window freezes Compute in separate threads –Old slow thread may overwrite result from new fast thread? –Interrupt old threads! ThreadTester3

Terminating Threads Thread terminates when run exits Or ask thread to finish by calling interrupt Thread may check for interrupt by call Thread.currentThread().isInterr upted() sleep, wait throw InterruptedException when thread interrupted

private Thread t = null; public void actionPerformed(ActionEvent e) { if (t!=null) t.interrupt(); Runnable r = new Runnable() { public void run() { try { long res = fib(Integer.parseInt(input.getText())); if (!Thread.currentThread().isInterrupted()) recResult.setText(res+""); } catch (InterruptedException e) {} } }; t = new Thread(r); t.start(); } private long fib(int n) throws InterruptedException { if (Thread.currentThread().isInterrupted()) throw new InterruptedException(); if (n <= 1) return 1; else return fib(n - 1) + fib(n - 2); } Code of anonymous class implementing ActionListener

QUIZ public class Test1 implements Runnable { public void run() { try { for (int i=1; i<=10; i++) { System.out.println(i); Thread.sleep(1000); } } catch (InterruptedException e) { } } } The catch statement is empty? 1.Compiler error 2.Legal code, but better style to omit try-catch 3.Legal code, try-catch is necessary, but empty catch is bad style 4.Legal code, try-catch is necessary, empty catch is good style 5.I don’t know

Multithreading Threads Thread states Thread interruption Race condition Lock Built-in lock java.util.concurrent library Animation example

Shared Resource Producer Thread ”goddag” Producer Thread ”farvel” Consumer Thread Queue object Bounded Queue: max 10 elements

Producer Thread int i = 1; while (i <= 100) { if (!queue.isFull()) { queue.add(i + ": " + greeting); i++; } Thread.sleep((int)(Math.random() * DELAY)); }

Consumer Thread int i = 1; while (i <= 100) { if (!queue.isEmpty()) { Object greeting = queue.remove(); System.out.println(greeting); i++; } Thread.sleep((int)(Math.random() * DELAY)); }

Expected Program Output 1: goddag 1: farvel 2: goddag 3: goddag... 99: farvel 100: farvel Possible output... 10: goddag 11: farvel... 15: goddag 11: farvel... Why?

Queue: Circular Array Implementation public void add(E newValue) { elements[tail] = newValue; tail++; size++; if (tail == elements.length) { tail = 0; }

Race Condition First thread calls add and executes elements[tail] = newValue; First thread at end of time slice Second thread calls add and executes elements[tail] = newValue; tail++; Second thread at end of time slice First thread executes tail++;

QUIZ Producer Thread: BoundedQueue queue = new BoundedQueue (10); for (int i = 1; i <= 100; i++) { queue.add(i + ": " + greeting); Thread.sleep((int)(Math.random() * DELAY)); } Consumer Thread: BoundedQueue queue = new BoundedQueue (10); for (int i = 1; i <= 100; i++) { Object greeting = queue.remove(); System.out.println(greeting); Thread.sleep((int)(Math.random() * DELAY)); } Could race condition occur when running these thread in parallel? 1.Yes 2.No 3.I don’t know

Multithreading Threads Thread states Thread interruption Race condition Lock Built-in lock java.util.concurrent library Animation example

Locks Thread can acquire lock When another thread tries to acquire same lock, it blocks When first thread releases lock, other thread is unblocked and tries again Two kinds of locks –Objects of class implementing java.util.concurrent.Lock interface type, usually ReentrantLock –Locks that are built into every Java object

private Lock aLock = new ReentrantLock();... public void add(E newValue) { aLock.lock(); try { elements[tail] = newValue; tail++; size++; if (tail==elements.length) tail = 0; } finally { aLock.unlock(); } Consumer or producer thread executing lock() has exclusive access to add/remove methods – even when time slice is up – until unlock() public E remove() { aLock.lock(); try { E r = (E) elements[head]; head++; size--; if (head==elements.length) head = 0; return r; } finally { aLock.unlock(); }

Producer Thread int i = 1; while (i <= 100) { if (!queue.isFull()) { queue.add(i + ": " + greeting); i++; } Thread.sleep((int)(Math.random() * DELAY)); } Good: add is protected by lock Bad: Another producer thread may fill up the queue between isFull() check and add() Solution: Must move isFull() check inside protected add()

QUIZ public void add(E newValue) throws InterruptedException { aLock.lock(); try { while( isFull() ) { Thread.sleep(1000); }... } finally {aLock.unlock();} } Does it work? 1: No, race condition may still occur 2: Yes, race condition cannot occur, and this code works fine 3: ??? – race condition cannot occur – but a new problem arises… 4: I don’t know isfull() check is now inside lock protected add()

QUIZ public void add(E newValue) throws InterruptedException { aLock.lock(); try { while( isFull() ) { Thread.sleep(1000); }... } finally {aLock.unlock();} } Does it work? 1: No, race condition may still occur 2: Yes, race condition cannot occur, and this code works fine 3: ??? – race condition cannot occur – but a new problem arises… 4: I don’t know isfull() check is now inside lock protected add() ”Deadlock”: Since the queue remains locked, no consumer thread can execute remove() and make isFull() false

Using condition object to avoid deadlock private Lock aLock = new ReentrantLock(); private Condition spaceAvailableCondition = aLock.newCondition(); public void add(E newValue) throws InterruptedException { aLock.lock(); try { while (isFull()) spaceAvailableCondition.await();... } finally {aLock.unlock();} } public E remove() throws InterruptedException { aLock.lock(); try {... spaceAvailableCondition.signalAll(); return r; } finally {aLock.unlock();} } Calling await blocks the thread and releases the lock Calling signalAll() unblocks all threads awaiting this condition (they remain blocked until getting the lock)

Multithreading Threads Thread states Thread interruption Race condition Lock Built-in lock java.util.concurrent library Animation example

Object Locks Each object has a lock Calling a synchronized method acquires lock of implicit parameter Leaving the synchronized method releases lock Easier than explicit Lock objects public class BoundedQueue { public synchronized void add(E newValue) {... } public synchronized E remove() {... }... }

Object Locks Each implicit lock has one associated (anonymous) condition object wait() blocks current thread and adds it to wait set notifyAll() unblocks waiting threads public synchronized void add(E newValue) throws InterruptedException { while (isFull()) wait(); elements[tail] = anObject;... notifyAll(); } Just one condition object Used by both threads waiting due to full queue and due to empty queue

QUIZ What is not reason for blocking a thread? 1.Sleeping 2.Waiting for I/O 3.Time slice is up 4.Waiting to acquire lock 5.Waiting for a condition 6.I don’t know

QUIZ When is InterruptedException thrown? 1.when Thread.sleep(..) returns 2.when.await() returns 3.when other thread calls this threads interrupt method while this thread is blocked 4.I don’t know

Multithreading Threads Thread states Thread interruption Race condition Lock Built-in lock java.util.concurrent library Animation example

Use java.util.concurrent library Main thread: BlockingQueue queue = new LinkedBlockingQueue (10); Producer Thread: for (int i = 1; i <= 100; i++) { queue.put(i + ": " + greeting); Thread.sleep((int)(Math.random() * DELAY)); } Consumer Thread: for (int i = 1; i <= 100; i++) { Object greeting = queue.take(); System.out.println(greeting); Thread.sleep((int)(Math.random() * DELAY)); } Synchronized queue with space for 10 elements Producer thread is made to wait until queue has available space Consumer thread is made to wait until queue is nonempty

Multithreading Threads Thread states Thread interruption Race condition Lock Built-in lock java.util.concurrent library Animation example

Algorithm Animation Use thread to make progress in algorithm Display algorithm state Example: Animate MergeSorter (similar to java.util.Arrays.sort ) Pause inside compare method Pass custom comparator Comparator comp = new Comparator () { public int compare(Double d1, Double d2) { draw current state pause thread return d1.compareTo(d2); } };

Algorithm Animation

public class Sorter implements Runnable { public Sorter(Double[] values, ArrayComponent panel) { this.values = values; this.panel = panel; } public void run() { Comparator comp = new Comparator () { public int compare(Double d1, Double d2) { panel.setValues(values, d1, d2); try { Thread.sleep(DELAY); } catch (InterruptedException exception) { Thread.currentThread().interrupt(); } return d1.compareTo(d2); }; MergeSorter.sort(values, comp); panel.setValues(values, null, null); } private Double[] values; private ArrayComponent panel; private static final int DELAY = 100; } array being sorted

Pausing and Running the Animation Want to pause animation until "Run" or "Step" button is clicked Need to coordinate UI thread, animation thread Try to use built-in thread-safe construct in java.util.concurrent Trick: Use a blocking queue Button click adds string "Run" or "Step" to queue Animation thread calls take on the queue, blocks if no string inserted BlockingQueue queue;

User Interface Thread stepButton.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent event) { queue.add("Step"); runButton.setEnabled(true); } }); runButton.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent event) { runButton.setEnabled(false); queue.add("Run"); } });

Animation Thread public int compare(Double d1, Double d2) { try { String command = queue.take(); if (command.equals("Run")) { Thread.sleep(DELAY); if (!"Step".equals(queue.peek())) queue.add("Run"); } } catch (InterruptedException exception) { Thread.currentThread().interrupt(); } panel.setValues(values, d1, d2); return d1.compareTo(d2); }... private BlockingQueue queue;