1

Threads

Mehdi Einali

Advanced Programming in Java

2

AgendaConcurrencyThreads in java

Problems in concurrencySolution for problems in concurrencyProblems in Solutions for problems in concurrency

Java High-level API for concurrency

3

concurrency

4

Sequential ProgrammingUp to this point, we learned sequential programming. Everything in a program happens one step at a time.What is wrong with this approach?

5

Multitasking vs single task

A single-tasking system can only run one program at a time, while a multi-tasking operating system allows more than one program to be running in concurrency.

Single task OS:

Multiple task OS:

6

OS Support

7

Concurrency vs. Parallelism

CPU CPU1 CPU2

8

Concurrency vs parallelism

9

concurency

10

Thread vs processA process is an executing instance of an program.

Processes contain their own state information, use their own address spaces, and only interact with each other via inter-process communication mechanisms (generally managed by the operating system)

A thread is a single sequence of execution within a program

Threads within a process share the same state and same memory space, and can communicate with each other directly, because they share the same variables

11

Thread vs process

12

Jvm is multi threaded

CPU

Process 1 Process 3Process 2 Process 4

main

run

GC

13

Multithreading

A thread is a single sequence of execution within a program

Multithreading refers to multiple threads of control within a single program

each program can run multiple threads of control within it, e.g., Web Browser

14

What are Threads Good For?

To maintain responsiveness of an application during a long running task.To enable cancellation of separable tasks.Some problems are intrinsically parallel.To monitor status of some resource (DB).

15

Parallel ProcessingMulti-Processor SystemsMulti-core CPUs

Dual coreCore2duoCorei7, corei5

Even with no multi-core processors, Multithreading is useful

How?I/O bounded tasksResponsive UISimulated multi-threading

16

Language SupportSome languages have no built-in mechanism for muli-threading

C, C++, …QT as a solution

OS-dependent librariespthread in linuxWindows API

Java has multi-threading in its core language

Pros and cons

17

Threads in java

18

Application ThreadWhen we execute an application:

The JVM creates a Thread object whose task is defined by the main() method It starts the threadThe thread executes the statements of the program one by one until the method returns and the thread dies

19

Multiple Threads in an Application

Each thread has its private run-time stack If two threads execute the same method, each will have its own copy of the local variables the methods usesHowever, all threads see the same dynamic memory (heap)Two different threads can act on the same object and same static fields concurrently

20

Creating ThreadsThere are two ways to create our own Thread object

1. Subclassing the Thread class and instantiating a new object of that class

2. Implementing the Runnable interface

In both cases the run() method should be implemented

21

Extending Thread

public class ThreadExample extends Thread {public void run() {

for (int i = 1; i <= 100; i++) { System.out.println("Thread: " + i);}

}}

22

Thread Methods

void start()Creates a new thread and makes it runnable

This method can be called only once

void run()The new thread begins its life inside this method

23

Thread Methods

sleep(int m)/sleep(int m,int n)  The thread sleeps for m milliseconds, plus n nanoseconds

yield()Causes the currently executing thread object to temporarily pause and allow other threads to execute

Allow only threads of the same priority to run

Nothing is guaranteed for this method

24

Implementing Runnablepublic class RunnableExample implements Runnable {

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

System.out.println ("Runnable: " + i);

} }

}

25

A Runnable Object

The Thread object’s run() method calls the Runnable object’s run() method

Allows threads to run inside any object, regardless of inheritance

26

Starting the Threads

public class ThreadsStartExample { public static void main (String argv[]) {

new ThreadExample ().start (); new Thread(new RunnableExample ()).start ();

}}

27

Scheduling Threads

I/O operation completes

start()

Currently executedthread

Ready queue

• Waiting for I/O operation to be completed• Waiting to be notified• Sleeping• Waiting to enter a synchronized section

Newly createdthreads

What happens when a program with aServerSocket callsaccept()?

28

More Thread States

29

Thread State Diagram

Alive

New Thread Dead Thread

Running

Runnable

new ThreadExample();

run() method returns

while (…) { … }

BlockedObject.wait()Thread.sleep()blocking IO callwaiting on a monitor

thread.start();

30

class ThreadExample extends Thread {public void run() {

MultiThreading.task("Thread");}

}class RunnableExample implements Runnable{

public void run() {MultiThreading.task("Runnable");

}}public class MultiThreading {

public static void task(String taskName){for (int i = 1; i <= 10; i++) {

System.out.println(taskName + ": " + i); try {

Thread.sleep(new Random().nextInt(10)); } catch (InterruptedException e) {

e.printStackTrace(); }}}

31

Running the ThreadsThreadExample thr1 = new ThreadExample();thr1.start();RunnableExample run1 = new RunnableExample();

new Thread(run1).start();ThreadExample thr2 = new ThreadExample();thr2.start();RunnableExample run2 = new RunnableExample();

new Thread(run2).start();

32

Output

First Run • Second Run…

1. Thread: 7

2. Runnable: 7

3. Thread: 9

4. Runnable: 9

5. Thread: 10

6. Thread: 8

7. Runnable: 8

8. Runnable: 10

9. Thread: 9

10. Runnable: 9

11. Runnable: 10

12. Thread: 10

…

1. Thread: 8

2. Runnable: 9

3. Thread: 9

4. Runnable: 7

5. Thread: 8

6. Runnable: 8

7. Thread: 9

8. Thread: 10

9. Runnable: 10

10. Thread: 10

11. Runnable: 9

12. Runnable: 10

33

responsiveness

34

GUI ExampleStart Counting starts counting the counterStop Counting stops counting the counter

35

Unresponsive UIStartButton:

startButton.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent evt) { this.stop = false; for (int i = 0; i < 100000; i++) { if (this.stop) break; tfCount.setText("" + countValue); countValue++; } } });

36

Unresponsive UI (2)

StopButton:

stopButton.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {

this.stop = true; }

});

37

Responsive UIbtnStart.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent evt) { this.stop = false; Thread counter = new CounterThread();

counter.start(); }});

Inner classclass CounterThread extends Thread { public void run() { for (int i = 0; i < 100000; i++) { if (this.stop){ break;} tfCount.setText("" + countValue); countValue++;

try { sleep(10); } catch (InterruptedException ex) {} } } }

38

Responsive UIbtnStart.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent evt) {

this.stop = false;new Thread() {

public void run() { for (int i = 0; i < 100000; i++) { if (this.stop){ break;} tfCount.setText("" + countValue); countValue++;

try { sleep(10); } catch (InterruptedException ex) {} } } }.start();

}}); • Define anonymous inner class

• Create object • Start thread

39

Thread scheduling

40

Java SchedulingThread scheduling is the mechanism used to determine how runnable threads are allocated CPU timeScheduler is based on priority of threadsThe priority of a thread : the importance of a thread to the schedulerUses fixed-priority scheduling:

Threads are scheduled according to their priority Priority is compared with other threads in the ready queue

41

Thread PriorityThe scheduler will lean toward running the waiting thread with the highest priority firstLower-priority threads just tend to run less oftenThe exact behavior depends on the platformUsually, all threads should run at the default priorityTrying to manipulate thread priorities is usually a mistake

42

Thread Priority (2)Every thread has a priorityWhen a thread is created, it inherits the priority of the thread that created itThe priority values range from 1 to 10, in increasing priority

43

Thread Priority (3)The priority can be adjusted subsequently using the setPriority() methodThe priority of a thread may be obtained using getPriority()

Priority constants are defined: MIN_PRIORITY=1MAX_PRIORITY=10NORM_PRIORITY=5

44

Some NotesThread implementation in Java is actually based on operating system supportSome Windows operating systems support only 7 priority levels, so different levels in Java may actually be mapped to the same operating system level

45

Daemon ThreadsDaemon threads are “background” threads, that provide services to other threads, e.g., the garbage collection threadThe Java VM will not exit if non-Daemon threads are executingThe Java VM will exit if only Daemon threads are executingDaemon threads die when the Java VM exitsA thread becomes a daemon with setDaemon() method

46

quiz

Which one is impossible output?

A) 1234567B) 3416527C) 1334567D) 7654321E) 0124567F) none of them

47

Problems in concurrency

48

Shared dataAn object in a program can be changed by more than one threadQ: Is the order of changes that were preformed on the object important?

49

Race ConditionA race condition – the outcome of a program is affected by the order in which the program's threads are allocated CPU timeTwo threads are simultaneously modifying a single object

Both threads “race” to store their value

50

Race Condition Example

A

B

C

Read A

Decrement A

Check Condition(B)

Increment(B)

Check Condition(C)

Increment(C)

Read A

Decrement A

Check Condition(B)

Increment(A)

51

Dirty read

Read A

Decrement A

Read A

Decrement A

Time Thread IIThread I

52

Lost updateTime Thread IIThread I

Read A

Decrement A

Read A

Decrement A

Check Condition(B)

Increment(A)

53

Solution for problems in concurrency

54

MonitorsEach object has a “monitor” that is a token used to determine which application thread has control of a particular object instanceIn execution of a synchronized method (or block), access to the object monitor must be gained before the executionAccess to the object monitor is queued

55

Monitor (cont.)

Entering a monitor is also referred to as locking the monitor, or acquiring ownership of the monitor

If a thread A tries to acquire ownership of a monitor and a different thread has already entered the monitor, the current thread (A) must wait until the other thread leaves the monitor

56

Critical SectionThe synchronized methods define critical sectionsExecution of critical sections is mutually exclusive. Why?

57

Examplepublic class BankAccount {

private float balance;

public synchronized void deposit(float amount) { balance += amount; } public synchronized void withdraw(float amount)

{ balance -= amount; } }

58

Static Synchronized Methods

Marking a static method as synchronized, associates a monitor with the class itself The execution of synchronized static methods of the same class is mutually exclusive.

Why?

59

Synchronized Statements

A monitor can be assigned to a block

It can be used to monitor access to a data element that is not an object, e.g., array

byte[] array;

void arrayShift(int count) {

synchronized(array) {

System.arraycopy (array,

count,array, 0, array.size - count);

}

}

60

Two Identical Methods

private synchronized void g() {h();

}

private void g() {synchronized(this){

h();}

}

61

Join()A method can wait for finishing another threadUsing thread.join()

62

Wait and NotifyAllows two threads to cooperateBased on a single shared lock object

Marge put a cookie notify Homer and waitHomer eat a cookie notify Marge and wait

63

The wait() MethodThe wait() method is part of the java.lang.Object interfaceIt requires a lock on the object’s monitor to executeIt must be called from a synchronized method, or from a synchronized segment of code. In other words: The current thread should have acquired a lock on this object before calling any of the wait methodsOtherwise: IllegalMonitorStateException

64

The wait() Method

wait() causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object

Upon call for wait(), the thread releases ownership of this monitor and waits until another thread notifies the waiting threads of the object

65

The wait() Method

wait() is also similar to yield() Both take the current thread off the execution stack and force it to be rescheduled

However, wait() is not automatically put back into the scheduler queue

notify() must be called in order to get a thread back into the scheduler’s queue

66

notify() and notifyAll()The need to the clockSimilar to wait() method:The current thread should have acquired a lock on this object before calling notify() or notifyAll()Otherwise: IllegalMonitorStateException

the task calling wait( ), notify( ), or notifyAll( ) must "own" (acquire) the lock for the object before it can call any of those methods.

67

wait() and the LockThe object lock is released during the wait( )

68

Problems in Solutions for problems in concurrency

69

deadlock

70

starvationSome locks exchanged by specific threads and other ones starve

71

Example: Producer/Consumer

The producer–consumer problem is a classic example of a multi-process synchronization problemThe problem:

two processes, the producer and the consumer, who share a common, fixed-size buffer used as a queue.

72

Thread safeThread-safe Classes

E.g, StringBuffer is thread-safe public synchronized StringBuffer append(String str){…}

and StringBuilder is NOT thread-safe public StringBuilder append(String str) {…}

73

Concurrent collectionsArrayBlockingQueue<E> A bounded blocking queue backed by an array.

ConcurrentHashMap<K,V>A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates.

ConcurrentLinkedQueue<E> An unbounded thread-safe queue based on linked nodes.

CopyOnWriteArrayList<E>A thread-safe variant of ArrayList in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array.

CopyOnWriteArraySet<E> A Set that uses CopyOnWriteArrayList for all of its operations.

DelayQueue<E extends Delayed>

An unbounded blocking queue of Delayed elements, in which an element can only be taken when its delay has expired.

LinkedBlockingQueue<E> An optionally-bounded blocking queue based on linked nodes.

PriorityBlockingQueue<E>An unbounded blocking queue that uses the same ordering rules as class PriorityQueue and supplies blocking retrieval operations.

SynchronousQueue<E>A blocking queue in which each put must wait for a take, and vice versa.

74

New Concurrency Classes in Java

75

High-level Concurrency APIs

Low-level threads management:synchronization, wait, notify, …

Since 5.0:Java also supports high-level concurrency APIs In its java.util.concurrent packageThese high-level APIs exploit today’s multi-core hardware

76

Synchronizer ClassesSemaphoreCountDownLatchExchangerCyclicBarrier

77

SemaphoreA semaphore controls access to shared resources. It maintains a counter to specify the number of resources that the semaphore controls.Main methods: acquire() and release()

78

exampleSemaphore semaphore = new Semaphore(1);

semaphore.acquire(); //critical section

...

semaphore.release();

10

1

79

Semaphore Exampleint i=1;Semaphore sem = new Semaphore(1);sem.release();System.out.println(i++);sem.release();System.out.println(i++);sem.acquire();System.out.println(i++);sem.acquire();System.out.println(i++);sem.acquire();System.out.println(i++);sem.acquire();System.out.println(i++);sem.acquire();System.out.println(i++);

The output:

Prints 1..5 and then waits…

80

Applications of SemaphoreA useful way to think of a semaphore:How many units of a particular resource are availableE.g., in Producer/Consumer problem:

The consumer checks a semaphore (initialized by zero)The producer checks another semaphore

(initialized by the buffer size)

81

CountDownLatch This synchronizer allows one or more threads to wait for a countdown to complete.

This countdown could be for a set of events to happen or until a set of operations being performed in other threads completes.

Main methods: await() and countDown()

82

use

83

84

Note: in countDown() method, if the current count is already zero, nothing happens.

85

ExchangerThe Exchanger class is meant for exchanging data between two threads.a kind of rendezvous point where two threads can exchange objectsVery simple: it waits until both the threads have called the exchange() method.

86

Example for Exchanger

87

88

output

89

CyclicBarrierAllows a set of threads to all wait for each other to reach a common barrier pointCyclicBarriers are useful when a fixed number of threads must occasionally wait for each other

Constructor: CyclicBarrier(int numThreads)Main method: await()

90

CyclicBarrier

The barrier is called cyclic, because it can be re-used after the waiting threads are released.

91

More You can also specify a timeout for the waiting thread.

barrier.await(10, TimeUnit.SECONDS);

The CyclicBarrier supports a barrier action, which is a Runnable that is executed once the last thread arrives

Runnable barrierAction = ... ; CyclicBarrier barrier = new CyclicBarrier(2, barrierAction);

92

usage

93

94

Note that the sequence in which the threads gets to write to the console may vary from execution to execution.

Sometimes Thread-0 prints first, sometimesThread-1 prints first etc.

output

95

Concurrent CollectionsA number of classes in java.util.concurrent package:Are thread-safe equivalents of collections framework classes in the java.util package For example: java.util.concurrent.ConcurrentHashMap

96

BlockingQueueBlockingQueue

An interface that extends the Queue interface if the queue is empty, it waits (i.e., blocks) for an element to be insertedand if the queue is full, it waits for an element to be removed

ArrayBlockingQueueFixed-sized array-based implementation of BlockingQueue

LinkedBlockingQueuea linked-list-based implementation of BlockingQueue

97

Other Concurrent Collections

DelayQueuePriorityBlockingQueueSynchronousQueueLinkedBlockingDequeConcurrentHashMapConcurrentSkipListMapConcurrentSkipListSetCopyOnWriteArrayListCopyOnWriteArraySet…

98

Atomic VariablesSuppose a multi-threaded program that acquires and releases locks for implementing primitive/simple operations like incrementing a variable, decrementing a variable, and … Such acquiring and releasing of locks for such primitive operations is not efficientan efficient alternative: atomic variablesProvided in java.util.concurrent.atomic package

99

Atomic Variable ClassesAtomicBooleanAtomicInteger (extends from Number)AtomicIntegerArrayAtomicLong (extends from Number)AtomicLongArrayAtomicReferenceAtomicReferenceArray

100

Example: Atomic Integer methods

AtomicInteger()AtomicInteger(int initVal)int get() void set(int newVal)int getAndSet(int newValue)boolean compareAndSet (int expect, int update)int getAndIncrement()int getAndDecrement()…

101

LocksThe java.util.concurrent.locks package Provides facilities that are similar to “synchronized”, but are more sophisticated Using synchronized: locking implicitly Using Lock objects: locking explicitly

102

Locks (2)Using a Lock object is similar to obtaining implicit locks using the synchronized keyword. The aim of both constructs is the same:

to ensure that only one thread accesses a shared resource at a time

However, unlike the synchronized keyword, Locks also support the wait/notify mechanism Along with its support for Condition objects

103

Lock vs. synchronized (2)You can do a “non-blocking attempt”Using the tryLock()

104

Implementations of Lock interface

ReentrantLockE.g.:

Lock lockObject = new ReentrantLock();

ReadLockThe lock returned by method ReentrantReadWriteLock.readLock()

WriteLockThe lock returned by method ReentrantReadWriteLock.writeLock()

105

ReadWriteLock interfaceMethods:

Lock readLock();Lock writeLock();

Implementation Class: ReentrantReadWriteLock

ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();rwl.readLock().lock();rwl.writeLock().lock();

106

More on java high level concurrency api

http://tutorials.jenkov.com/java-util-concurrent/index.html

107

end