Transcript Multithreading in Java 2

More Multithreaded Programming
in Java
Multimedia Technology Programming
http://mea.create.aau.dk/course/view.php?id=26
Medialogy, Semester 7
Aalborg University, Aalborg
2010
David Meredith
[email protected]
Sources
• Chapter 14 of The Java Programming
Language (Fourth Edition) by Ken Arnold,
James Gosling and David Holmes (AddisonWesley, 2006)
• Sun Java Tutorial on Concurrency
http://java.sun.com/docs/books/tutorial/essential/c
oncurrency/
Synchronized methods and statements
• The two code fragments
to the right are
equivalent
• In both cases, the object
referenced by this is
locked while doThis()
and doThat() are carried
out
• Remember that this
refers to the object on
which the method is
called
public synchronized void myMethod() {
doThis();
doThat();
}
public void myMethod() {
synchronized(this) {
doThis();
doThat();
}
}
Client-side and server-side
synchronization
• Server-side synchronization is where object to be
locked takes responsibility for synchronizing code
that uses it
– often by using a synchronized method
• Client-side synchronization is where responsibility
for synchronizing critical regions of code lies with
the code that uses the object to be locked
– Means every client has to agree to use synchronized
code
– Less safe than server-side synchronization
– Often done using synchronized statements
Synchronized statements vs.
synchronized methods
• Synchronized statement can protect code
region smaller than a whole method
• Synchronized statement can lock an object
other than the one whose method contains
the code being protected
• Allows for holding a lock for as short a time as
possible (increases performance)
• Allows for increasing concurrency by using a
finer granularity of locking
Interrupts
• If b is a Thread, then we interrupt b (“send an
interrupt request to b”) by using the message
b.interrupt();
• If Thread b frequently calls methods that throw
InterruptedException (e.g., Thread.sleep()), then
the first occurrence of such a method after the
interrupt is received will throw an
InterruptedException
– In this case, we define how the Thread responds to
the interrupt in a catch clause (see
PingPongRunnable.java)
– Usually the run() method will be made to return
Manually responding to an interrupt
public void run() {
for (int i = 0; i < 300; i++) {
doSomeCalc(i);
if (Thread.interrupted()) {
return;
}
}
}
public void run() {
try {
for (int i = 0; i < 300; i++) {
doSomeCalc(i);
if (Thread.interrupted()) {
throw new InterruptedException();
}
}
} catch (InterruptedException e) {
doSomeStuff();
return;
}
}
• Sometimes a run() method might not contain any call to a method that
throws an InterruptedException
• In this case, need to regularly check whether the Thread has received an
interrupt
• Using a try-catch clause allows you to centralize the interrupt handling
code in one place and separate it clearly from the run code
Interrupt status flag
• Message, thrA.interrupt() causes interrupt status
flag to be set to true for Thread thrA
• If thrA checks for interrupt status by calling static
method, Thread.interrupted(), then interrupt
status flag set to false (“cleared”)
– thrA ceases to be in an interrupted state
• If Thread thrB wants to know if Thread thrA has been
interrupted, then call thrA.isInterrupted() from
inside thrB
– Does not clear interrupt status flag
– That is, if thrA has been interrupted, it remains in an
interrupted state, as indicated by the true value of the interrupt
status flag
• When InterruptedException thrown, interrupt
status flag is cleared (i.e., set to false)
– thread ceases to be in an interrupted state
The join method
• The call, t.join() causes the current Thread to
block (i.e., stop doing anything) until Thread t
has stopped running
• The join method throws an
InterruptedException if the current Thread is
interrupted while it is waiting for another
Thread to complete
• See HrOgFruJensen2.java
SimpleThreads.java
Guarded Blocks
public void guardedJoy() {
while (!joy) {}
System.out.println(“Joy has been achieved!”);
}
• A guarded block suspends the current thread until another
thread makes some condition become true, which restarts
the current thread
• guardedJoy() method must not execute its last line until the
shared variable joy has been made true by some other
thread
• The while loop is keeping the processor busy while it is
waiting (busy waiting) – DON’T DO THIS!
Guarded blocks using wait and notifyAll
AchievingJoy.java