Thread
Advanced Programming
SUT • Spring 2019
Contents
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Need for multi-thread programming
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Threads in java
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Samples
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Synchronization
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synchronized
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wait & notify
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join
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Program Executation
Sequential Programming
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Up to this point, we learned sequential programming.
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Everything in a program happens one step at a time.
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What is wrong with this approach?
Multitasking and Multithreading
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Multitasking refers to a computer's ability to perform multiple jobs concurrently
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more than one program are running concurrently, e.g., UNIX
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A thread is a single sequence of execution within a program
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Multithreading refers to multiple threads of control within a single program
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each program can run multiple threads of control within it, e.g., Web Browser
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Concurrency vs. Parallelism
What are Threads Good For?
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To maintain responsiveness of an application during a long running task.
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To enable cancellation of separable tasks.
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Some problems are intrinsically parallel.
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To monitor status of some resource (DB).
Parallel Processing
- Multi-Processor Systems
- Multi-core CPUs
- Dual core
- Core2duo
- Corei7, corei5
- Even with no multi-core processors, Multithreading is useful
- How?
- I/O bounded tasks
- Responsive UI
- Simulated multi-threading
- How?
OS Support
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Multi-task OS
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Windows & Unix
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Multi-thread OS
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Single task OS
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DOS
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Language Support
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Some languages have no built-in mechanism for muli-threading
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C, C++, …
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QT as a solution
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OS-dependent libraries
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pthread in linux
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Windows API
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Java has multi-threading in its core language
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Pros and cons
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ISA experience
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Application Thread
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When we execute an application:
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The JVM creates a Thread object whose task is defined by the main() method
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It starts the thread
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The thread executes the statements of the program one by one until the method returns and the thread dies
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Multiple Threads in an Application
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Each thread has its private run-time stack
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If two threads execute the same method, each will have its own copy of the local variables the methods uses
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However, all threads see the same dynamic memory (heap)
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Two different threads can act on the same object and same static fields concurrently
Creating Threads
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There are two ways to create our own Thread object
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Subclassing the Thread class and instantiating a new object of that class
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Implementing the Runnable interface
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In both cases the run() method should be implemented
Extending Thread
public class ThreadExample extends Thread {
public void run() {
for (int i = 1; i <= 100; i++) {
System.out.println("Thread: " + i);
}
}
}
Thread Methods
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void start()
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Creates a new thread and makes it runnable
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This method can be called only once
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void run()
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The new thread begins its life inside this method
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Thread Methods
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sleep(int m)/sleep(int m,int n)
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The thread sleeps for m milliseconds, plus n nanoseconds
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yield()
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Causes the currently executing thread object to temporarily pause and allow other threads to execute
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Allow only threads of the same priority to run
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Nothing is guaranteed for this method
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Implementing Runnable
public class RunnableExample implements Runnable {
public void run () {
for (int i = 1; i <= 100; i++) {
System.out.println ("Runnable: " + i);
}
}
}
A Runnable Object
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The Thread object’s run() method calls the Runnable object’s run() method
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Allows threads to run inside any object, regardless of inheritance
Starting the Threads
public class ThreadsStartExample {
public static void main (String argv[]) {
new ThreadExample().start();
new Thread(new RunnableExample()).start();
}
}
Scheduling Threads
More Thread States
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getState() method in Thread
Thread State Diagram
Example
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();
}
}
}
}
Running the Threads
public class Main {
public static void main(String[] args){
ThreadExample 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();
}
}
GUI Example
- Start Counting = starts counting the counter
- Stop Counting = stops counting the counter
UnresponsiveUI
startButton.addActionListener(
new ActionListener() {
public void actionPerformed(ActionEvent evt) {
stop = false;
for (int i = 0; i < 100000; i++) {
if (stop)
break;
tfCount.setText("" + countValue);
countValue++;
}
}
});
UnresponsiveUI (2)
StopButton:
stopButton.addActionListener(
new ActionListener() {
public void actionPerformed(ActionEvent evt) {
stop = true;
}
});
ResponsiveUI
btnStart.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent evt) {
stop = false;
Thread t = new Thread() {
public void run() {
for (int i = 0; i < 100000; i++) {
if (stop)
break;
tfCount.setText("" + countValue);
countValue++;
try {
sleep(10);
} catch (InterruptedException ex) {}
}
}
};
t.start();
}
});
Java Scheduling
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Thread scheduling is the mechanism used to determine how runnable threads are allocated CPU time
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Scheduler is based on priority of threads
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The priority of a thread: the importance of a thread to the scheduler
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Uses fixed-priority scheduling:
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Threads are scheduled according to their priority
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Priority is compared with other threads in the ready queue
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Thread Priority
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The scheduler will lean toward running the waiting thread with the highest priority first
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Lower-priority threads just tend to run less often
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The exact behavior depends on the platform
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Usually, all threads should run at the default priority
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Trying to manipulate thread priorities is usually a mistake
Thread Priority (2)
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Every thread has a priority
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When a thread is created, it inherits the priority of the thread that created it
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The priority values range from 1 to 10, in increasing priority
Thread Priority (3)
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The priority can be adjusted subsequently using the setPriority() method
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The priority of a thread may be obtained using getPriority()
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Priority constants are defined:
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MIN_PRIORITY=1
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MAX_PRIORITY=10
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NORM_PRIORITY=5
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Some Notes
- Thread implementation in Java is actually based on operating system support
- Some Windows operating systems support only 7 priority levels, so different levels in Java may actually be mapped to the same operating system level
Daemon Threads
- Daemon threads are “background” threads, that provide services to other threads, e.g., the garbage collection thread
- The Java VM will not exit if non-Daemon threads are executing
- The Java VM will exit if only Daemon threads are executing
- Daemon threads die when the Java VM exits
- A thread becomes a daemon with setDaemon() method
Concurrency
Concurrency
- An object in a program can be changed by more than one thread
- Q: Is the order of changes that were preformed on the object important?
Race Condition
- A race condition – the outcome of a program is affected by the order in which the program's threads are allocated CPU time
- Two threads are simultaneously modifying a single object
- Both threads “race” to store their value
Monitors
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Each object has a “monitor” that is a token used to determine which application thread has control of a particular object instance
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In execution of a synchronized method (or block), access to the object monitor must be gained before the execution
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Access to the object monitor is queued
Monitor (cont.)
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Entering a monitor is also referred to as locking the monitor, or acquiring ownership of the monitor
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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
Critical Section
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The synchronized methods define critical sections
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Execution of critical sections is mutually exclusive. Why?
Example
public class BankAccount {
private float balance;
public synchronized void deposit(float amount) {
balance += amount;
}
public synchronized void withdraw(float amount) {
balance -= amount;
}
}
Static Synchronized Methods
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Marking a static method as synchronized, associates a monitor with the class itself
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The execution of synchronized static methods of the same class is mutually exclusive.
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Why?
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Synchronized Statements
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A monitor can be assigned to a block
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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);
}
}
Two Identical Methods
private synchronized void g() {
h();
}
Private void g() {
synchronized(this){
h();
}
}
Join()
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A method can wait for finishing another thread
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Using thread.join()
Wait and Notify
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Allows two threads to cooperate
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Based on a single shared lock object
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Marge put a cookie notify Homer and wait
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Homer eat a cookie notify Marge and wait
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The wait() Method
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The wait() method is part of the java.lang.Object interface
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It requires a lock on the object’s monitor to execute
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It must be called from a synchronized method, or from a synchronized segment of code.
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In other words: The current thread should have acquired a lock on this object before calling any of the wait methods
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Otherwise: IllegalMonitorStateException
The wait() Method
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wait() causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object
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Upon call for wait(), the thread releases ownership of this monitor and waits until another thread notifies the waiting threads of the object
The wait() Method
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wait() is also similar to yield()
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Both take the current thread off the execution stack and force it to be rescheduled
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However, wait() is not automatically put back into the scheduler queue
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notify() must be called in order to get a thread back into the scheduler’s queue
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notify() and notifyAll()
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The need to the clock
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Similar to wait() method:
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The current thread should have acquired a lock on this object before calling notify() or notifyAll()
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Otherwise: IllegalMonitorStateException
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the task calling wait(), notify(), or notifyAll() must "own" (acquire) the lock for the object before it can call any of those methods.
wait() and the Lock
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The object lock is released during the wait()
Some Notes
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Thread-safe Classes
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E.g, StringBuffer is thread-safe
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public synchronized StringBuffer append(String str){…}
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and StringBuilder is NOT thread-safe
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public StringBuilder append(String str) {…}
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Threading Problems
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Starvation
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Deadlock
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Example: Producer/Consumer
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The producer–consumer problem
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is a classic example of a multi-process synchronization problem
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The problem:
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two processes, the producer and the consumer,
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who share a common, fixed-size buffer used as a queue.
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Title Text
Thread
By Behnam Hatami
Thread
Thread / Advanced Programming Course @ SUT, Spring 2019
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