Multithreading. Dr. Jens Bennedsen, Aarhus University, School of Engineering Aarhus, Denmark

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1 Multithreading Dr. Jens Bennedsen, Aarhus University, School of Engineering Aarhus, Denmark

2 Submission & Peer feedback peergrade.io/join AJJ452

3 What is going on here?

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

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

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

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

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

9 QUIZ menti.com How many threads are created? public static void main(string[] args) { A. None B. One C. Two D. Three E. I don t know Runnable r1 = new Producer(); Runnable r2 = new Producer(); Thread t1 = new Thread(r1); Thread t2 = new Thread(r2); t1.start(); t2.start();

10 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();

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

12 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

13 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(); 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; Output depends on scheduler: goddag goddag farvel farvel farvel goddag farvel

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

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

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

17 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));

18 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?

19 Queue: Circular Array Implementation public void add(e newvalue) { elements[tail] = newvalue; tail++; size++; if (tail == elements.length) { tail = 0;

20 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++;

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

22 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

23 Consumer or producer thread executing lock() has exclusive access to add/remove methods even when time slice is up until unlock() 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(); public E remove() { alock.lock(); try { E r = (E) elements[head]; head++; size--; if (head==elements.length) head = 0; return r; finally { alock.unlock();

24 Locks and databases What is the equvalent to a Lock-object in JDBC s classes for database managment? A: Transaction B: Statement C: Row D: ResultSet E: Synchronized Go to menti.com!!

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

26 QUIZ Does it work? isfull() check is now inside lock protected add() public void add(e newvalue) throws InterruptedException { alock.lock(); try { while( isfull() ) { Thread.sleep(1000);... finally {alock.unlock(); A: No, race condition may still occur B: Yes, race condition cannot occur, and this code works fine C:??? race condition cannot occur but a new problem arises D: I don t know

27 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)

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

29 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<E> { 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

30 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

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

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

33 It is time for a

Threads. Multithreading Horstmann Chapter 9. Multithreading. 9.1 Thread Basics (Threads in Java) What is a process? What is a thread?

Threads. Multithreading Horstmann Chapter 9. Multithreading. 9.1 Thread Basics (Threads in Java) What is a process? What is a thread? Threads What is a process? Multithreading Horstmann Chapter 9 Unit 6 CS 3354 Spring 2017 Jill Seaman a self-contained running program with its own address space. processes are controlled by the operating