Download Task, Cont.

CMPE212 – Section 001 - Reminders
• Assignment 5 due Today.
• Quiz marking not started yet...
• Just after the final exam – make sure you have all
assignment marks and that all quizzes are fully
graded.
Winter 2017
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Today
• Multi-Threading, Cont.
• Multi-Core Threading.
• What’s Left in Java?
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Example Program
•
Suppose that we want to have a window that:
1. Has a timer counting seconds.
2. Buttons to start and stop the timer.
3. A TextArea node to store comments, provided one at
a time.
4. A Close button to close the window.
– No sweat, right? See LoggerNoThreads
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So, What’s the Problem?
• The program cannot move beyond the timer loop.
• Events such as paint() calls will not get processed
– they just pile up!
• What’s the solution?
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Multi-Threading
• In a JavaFX program Application runs on its own
thread.
• Our timing loop is still on the Application thread
and is blocking other thread processes.
• The easiest way is to use the existing animation
thread that is already available through the use of
the Timeline object.
• See LoggerTimeline
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Timeline
• From before:
timeline = new Timeline(new
KeyFrame(Duration.ZERO, actionEvent ->
incrementClock()),
new KeyFrame(Duration.millis(1000)));
timeline.setCycleCount(Timeline.INDEFINITE);
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Timeline, Cont.
• Use timeline.playFromStart() to start the
timer and timeline.stop() to halt it.
• Invoking Platform.exit() will also stop all
Application threads when you exit the program.
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Multi-Threading, Cont.
• Suppose you wish to create your own thread
instead of piggybacking on an existing thread?
• Use classes from the javafx.concurrent package.
• These include Service and Task.
• Threads based on the Thread class cannot work
with the Application thread.
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Task
• Task is designed to work with JavaFX
applications.
• It cannot directly access scenegraph nodes on the
Application thread, because Node objects are not
“thread safe”.
• It is a one-shot deal. Once the task is finished,
you cannot re-start it.
• It is generic!
• You must override the T call() method.
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Task, Cont.
• call() contains the process that will run on a
separate thread.
• call() returns a type T, but if it does not return
anything then you can return null and use “Void”
for the type.
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Task, Cont.
• The class uses “update” methods to update node
contents:
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updateMessage(String message)
updateProgress(…)
updateTitle(…)
updateValue(T value)
• These are to be bound to specific node
properties:
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Task, Cont.
• Binding, example:
lblTime.textProperty().bind(myService.messageProperty());
• “myService” is a Service object, but use the same
syntax to bind a Task property to a node property.
• Inside the Task call method:
updateMessage("" + timeCounter);
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Task, Cont.
• These update calls are immediately “coalesced”
on the Application thread. They may not run at
once, but will run in the order in which they have
been added to the main thread.
• One way to update multiple nodes is to add a
change listener to the bound node which updates
the other nodes of interest.
• Another is to use an AnimationTimer to trigger
updates when a frame is drawn.
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Task, Cont.
• If you need to do more to the scenegraph that just
update the property of a single node, then
consider using:
Platform.runLater(Runnable r)
• The Runnable is a process (which can be a Task,
for example) which will be piled onto the
Application thread.
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Service
• A Task cannot be re-started.
• Use a Service instead.
• Service creates and manages a Task in a way
that is compatible with the Application thread.
• Generic!
• Override the method:
Task<T> createTask()
• This method creates the Task object for the
Service.
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Service, Cont.
• Has many useful methods to monitor and
manipulate the task, including:
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.cancel()
.cancelled()
.isRunning()
.reset()
.restart()
.start()
// Stops the task
// Returns true if the task is cancelled
// Returns true if the task is running
// Resets the service
// Cancels and re-starts the task
// Starts the task
• See LoggerThreaded
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The Thread Class, Cont.
• Threads have a priority.
• This helps the processor decided which thread
gets a larger slice of processor time.
• Threads with a lower priority can be delayed by
CPU-intensive threads with a higher priority.
• Use getPriority() and setPriority(int)
methods.
• Priorities lie between 1 and 10, with 5 being the
default.
• (Use Thread.MAX_PRIORITY,
Thread.MIN_PRIORITY and
Thread.NORM_PRIORITY.)
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The Thread Class, Cont.
• Other methods are listed in the API.
• In particular, sleep(long sleeptime) can put
the current thread to sleep for a specified number
of milliseconds. This gives other threads a
chance to do something. You should always
place a sleep() command inside any loops in your
thread.
• sleep() can throw the InterruptedException too.
• Or you can simply invoke the yield() method to
pause the current thread and allow other threads
to execute.
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Multi-Threading, Cont.
• The thread in a task can be halted (externally!) in
two ways:
• A cancel has been issued.
• An InterruptedException has been thrown.
• Inside a loop in your call method, you need to
invoke .isCancelled() to see if you need to
stop the loop.
• Put any blocking calls like Thread.sleep() inside
a try catch to catch the InterruptedException and
use the catch to stop the loop.
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Use of Multi-Core Processors in Java >7
• Look up the “Fork/Join” framework in the Java
Tutorial, which is in the Concurrency section:
http://docs.oracle.com/javase/tutorial/essential/concurrency/forkjoin.html
• Designed to work best with recursive processes, it
allows you to split your tasks over the multiple
cores of your processor.
• The framework handles all the multiple threads
needed in a thread “pool”.
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Multi-Core Processing, Cont.
• The idea is straight-forward:
if the task is small then:
Do the task normally, non-recursively
otherwise:
Split the task into two parts and recurse.
• The size of “small” depends on what you are
trying to do – some experimentation will be
required...
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Multi-Core Processing, Cont.
• You need to do this in such a way that the
recursed tasks are given to “Worker” threads that
are part of a thread pool controlled by the fork/join
framework.
• The framework spreads the threads over the
available cores using a “Work stealing” algorithm:
• If a core thread is idle a task can be stolen from
a threaded core that is still busy.
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Multi-Core Processing, Cont.
• Your task must be built as a class that extends
java.util.concurrent.RecursiveAction.
• You need to write a concrete version of
void compute()
• This is where you carry out the tasks.
• Recurse your task by instantiating your class
twice and then supply these objects to a call to
invokeAll(task1, task2)
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Multi-Core Processing, Cont.
• In order to start the process, instantiate your class
for the first time and supply this object to the
invoke() method of an instantiated ForkJoinPool
object.
• Sound complicated? Not really!
• See the demo program
MultiCoreQuicksortingExperiment.java.
• Compares a normal quicksort to a multi-core
quicksort.
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Multi-Core Processing, Cont.
• Note the use of:
Runtime.getRuntime().availableProcessors()
to get the number of available cores.
• Even without any further experimentation to
optimize the code, the multi-core version is
definitely faster than a normally-coded quicksort!
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Want to Know More about MultiThreading?
• Read over the Concurrency section in the Java
Tutorial:
http://docs.oracle.com/javase/tutorial/essential/conc
urrency/index.html
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Other JavaFX Topics
• Packaging, Deployment & Distribution. Web page
embedded, stand-alone, etc.
• 3D graphics.
• Images and image manipulation.
• 2D and 3D Animation.
• Lots of other controls!
• Charts.
• Visual effects and transformation.
• Media (sound and video) playback.
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Other Java Topics
• Other Collection Types. Iterators.
• Streams. Sockets. Internet streams and web
pages.
• Regular expressions.
• Working with databases.
• Security.
• JDBC, JSP, JMX, JNDI, JAXP, JAXB, RMI, …
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More Acronyms!
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Java Database Connectivity
Java Server Pages
Java Management Extensions
Java Naming and Directory Interface
Java API for XML Processing
Java Architecture for XML Binding
Remote Method Invocation
• Look for ‘Specialized Trails and Lessons’ in the
Java Tutorial.
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