1. Slide design: Dr. Mark L. Hornick
SE3910
5/24/2019
SE3910 Week 6, Class 1
Week 6, Class 2 (Wednesday)
Quiz – to be graded still
Week 7, Class 2 (Wednesday)
Half-Exam 2
Bring Calculator!!
Qt and Java: Multithreading
Full agenda:
Return Exam
Questions about lab due tomorrow in class?
Threads: Locking on null object
Threads: invokeLater
Threads: The squares example
Decorator Class Diagram
More on Java IO
Class diagrams
Design Principles
in the patterns we’ve seen so far
Compare with alternatives
Decorator vs. array approach suggested in class
Non-decorator array – decorator can be added on without modifying the original hierarchy
Decorator has “before-after” and possibly other combinatorial control that would be hard-coded in array
[Show “screenshot” of discussion from class? Or just re-type?]
Strategy vs. Decorator class diagrams side-by-side
Structural difference (inheritance optional in Strategy pattern?)
Decorator vs. “Strategy” array
Perhaps next:
Coding Starbuzz coffee (?)
Add real patterns
ArrayList – null-checking
Java I/O: Students do coding examples
SE-2811
Slide design: Dr. Mark L. Hornick
Content: Dr. Hornick
Errors: Dr. Yoder
Dr. Josiah Yoder

2. Multi-core programming
SE3910
5/24/2019
Multi-core programming
SE-2811
Dr.Yoder
Dr. Josiah Yoder

3. Slide design: Dr. Mark L. Hornick
SE3910
5/24/2019
Volatile
SE-2811
Slide design: Dr. Mark L. Hornick
Content: Dr. Hornick
Errors: Dr. Yoder
Dr. Josiah Yoder

4. Volatile readily vaporizable at a relatively low temperature
SE3910
5/24/2019
Volatile
readily vaporizable at a relatively low temperature
flying or having the power to fly
lighthearted, lively
tending to erupt into violence : explosive
unable to hold the attention fixed because of an inherent lightness or fickleness of disposition
difficult to capture or hold permanently
SE-2811
Dr.Yoder
Dr. Josiah Yoder

5. SE3910
5/24/2019
Based on these definitions, which do you think is a closer fit for volatile?
A variable which is cached – at any time, value could be incorrect because other threads change it.
A variable which is not cached. Must always go to main memory to read value, because other threads may change it at any time.
Note: I am NOT asking which is volatile in Java or other languages.
SE-2811
Dr.Yoder
Dr. Josiah Yoder

6. SE3910
5/24/2019
Volatile (in Java)
A variable which is not cached. Must always go to main memory to read or write the value
Why might these variables be called volatile?
Why might we want a volatile variable?
SE-2811
Dr.Yoder
Dr. Josiah Yoder

7. SE3910
5/24/2019
Volatile (in C)
SE-2811
Dr.Yoder
Dr. Josiah Yoder

8. Task States (more essential rambling)
5/24/2019
Task States (more essential rambling)
Wiki:Process (computing)
See also Laplante and Ovaske 4E p. 97
Talk about task states and how they relate to mutex locks
Draw an example UML timing diagram on the board.
SE-2811
Dr.Yoder
Dr. Josiah Yoder

9. Threading – pthreads (review)
SE3910
5/24/2019
Threading – pthreads (review)
Java
Pthreads
java.lang.Thread
#include <pthread.h>
No external jar needed
link with -pthread
Thread t = new Thread(r)
t.start();
pthread_create(t,r,sr,a)
interface Runnable {
void run(); }
Parameter:
void* (*sr) (void *)
t.join();
pthread_join(*t, &p)
Object o;
pthread_mutex_init(m,null)
synchronized(o) { …
} … /* Garbage coll. */
pthread_mutex_lock(…)
pthread_mutex_destroy(…)
View links in class
Dr. Josiah Yoder

10. Threading – pthreads (updated with links)
CS2852
5/24/2019
Threading – pthreads (updated with links)
Java
Pthreads
Object o;
o.notify();
phread_cond_t c = PTHREAD_COND_INITIALIZER;
pthread_cond_broadcast(c);
o.wait();
pthread_cond_wait(c,m);
phtread_cond_signal(c);
o.notifyAll();
phtread_cond_broadcast(c);
View links in class.
See Java coding example NotifyWaitExample
Caveat: “POSIX threads can wait at condition variables of a greater generality than available in Java, but the corresponding queues may be leaky.”
SE Dr. Josiah Yoder
Slide style: Dr. Hornick
Much Material: Dr. Schilling
Dr. Yoder

11. Ex: Edit to include notify/wait to interleave these threads.
5/24/2019
Ex: Edit to include notify/wait to interleave these threads.
Consider this code-snippet Queue q = … … synchronized(q) { while(true) { img = getImage(); q.offer(img); }
// on another thread synchronized(q) { while(true) { img = q.poll(); showImage(); }
SE-2811
Dr.Yoder
Dr. Josiah Yoder

12. Threading – qthreads (with corrections)
SE3910
5/24/2019
Threading – qthreads (with corrections)
Java
qthreads
java.lang.Thread
#include <QThread>
No external jar needed
(moc and friends take care of this)
Thread t = new Thread(r)
t.start();
QThread *t = new QThread;
moveToThread(t); // note here
interface Runnable {
void run(); }
QObject (e.g. QWidget (e.g. QMainWindow))
t.join();
connect the QThread::finish() signal to a slot that checks if all threads are done.
Object o;
QMutex
synchronized(o) { …
} … /* Garbage coll. */
Avoid sharing memory entirely…
… see code example…
????
Dr. Josiah Yoder

13. Useful if you are into Qt slots/signals == events
5/24/2019
Useful if you are into Qt slots/signals == events
“A QThread should be used much like a regular thread instance: prepare an object (QObject) class with all your desired functionality in it. Then create a new QThread instance, push the QObject onto it using moveToThread(QThread*) of the QObject instance and call start() on the QThread instance. That’s all.”
I have successfully used this approach.
SE-2811
Dr.Yoder
Dr. Josiah Yoder

14. Qt’s connect method http://doc.qt.io/qt-5/qobject.html#connect
SE-2811
Dr.Yoder

15. Qt Connection types 1 2 3 0x80 Constant Value Description
Constant
Value
Description
Qt::AutoConnection
(Default) If the receiver lives inthe thread that emits the signal, Qt::DirectConnection is used. Otherwise, Qt::QueuedConnection is used. The connection type is determined when the signal is emitted.
Qt::Direct Connection 1
The slot is invoked immediately when the signal is emitted. The slot is executed in the signalling thread.
Qt::Queued Connection 2
The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed inhe t receiver's thread.
Qt::Blocking QueuedConnection 3
Same as Qt::QueuedConnection, except that the signalling thread blocks until the slot returns. This connection must not be used if the receiver lives in the signalling thread, or else the application will deadlock.
Qt::Unique Connection
0x80
This is a flag that can be combined with any one of the above connection types, using a bitwise OR. When Qt::UniqueConnection is set,QObject::connect() will fail if the connection already exists (i.e. if the same signal is already connected to the same slot for the same pair of objects). This flag was introduced in Qt 4.6.
SE-2811
Dr.Yoder

16. Coding example. Exercise:
Both the Java and Qt solutions will behave poorly if I start multiple threads.
Predict how each solution will behave if I:
Click start
// what happens? (For Java? For Qt?)
Click stop
SE-2811
Dr.Yoder

17. Slide design: Dr. Mark L. Hornick
SE3910
5/24/2019
References
EB: Derek Malloy, Exploring Beaglebone, Wiley, 2015
SE-2811
Slide design: Dr. Mark L. Hornick
Content: Dr. Hornick
Errors: Dr. Yoder
Dr. Josiah Yoder