Concurrency (p2) synchronized (this) { doLecture(part2); } synchronized (this) { doLecture(part2); }

Quick Review  Threads can up performance  Java’s 2 parts for thread safety  Atomicity  Visibility  Java’s Primitives  volatile  synchronized  Library Support  Atomic variables  Concurrent collections  Common synchronizers (BlockingQueue)  Threads can up performance  Java’s 2 parts for thread safety  Atomicity  Visibility  Java’s Primitives  volatile  synchronized  Library Support  Atomic variables  Concurrent collections  Common synchronizers (BlockingQueue)

Outline  Executor Framework  Shutdown and cancellation  Swing and Concurrency  Custom Synchronizers  Executor Framework  Shutdown and cancellation  Swing and Concurrency  Custom Synchronizers

Starting Threads  Executor framework public interface Executor { void execute(Runnable command); }  Executor framework public interface Executor { void execute(Runnable command); }

Executor Example class MyTask implements Runnable { public void run() {…} public static void main(…) { Runnable task1 = new MyTask(); Executor exec = /* */; exec.execute(task1); } class MyTask implements Runnable { public void run() {…} public static void main(…) { Runnable task1 = new MyTask(); Executor exec = /* */; exec.execute(task1); }

1 Thread/ Task class OnePer implements Executor { public void Execute(Runnable r){ new Thread(r).start(); } class OnePer implements Executor { public void Execute(Runnable r){ new Thread(r).start(); }

Single Threaded class Just1 implements Executor { public void Execute(Runnable r){ r.run(); } class Just1 implements Executor { public void Execute(Runnable r){ r.run(); }

Provided Thread Pool Executors  newFixedThreadPool  Bounded size  Replace if thread dies  newCachedThreadPool  Demand driven variable size  newSingleThreadExecutor  Just one thread  Replace if thread dies  newScheduledThreadPool  Delayed and periodic task execution  Good replacement for class Timer  newFixedThreadPool  Bounded size  Replace if thread dies  newCachedThreadPool  Demand driven variable size  newSingleThreadExecutor  Just one thread  Replace if thread dies  newScheduledThreadPool  Delayed and periodic task execution  Good replacement for class Timer

Executor Shut-down  Executor is a serice provider  Executor abstracts thread management  To maintain the abstraction, the service should generally provide methods for shutting down the service  JVM cannot shut down until threads do  Executor is a serice provider  Executor abstracts thread management  To maintain the abstraction, the service should generally provide methods for shutting down the service  JVM cannot shut down until threads do

ExecutorService public interface ExecutorService extends Executor { void shutdown(); List shutdownNow(); boolean isShutdown(); boolean isTerminated(); boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException; } public interface ExecutorService extends Executor { void shutdown(); List shutdownNow(); boolean isShutdown(); boolean isTerminated(); boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException; }

ExecutorService Notes  Implies three states:  Running  Shutting down  Terminated  shutdown() runs tasks in queue  shutdownNow() returns unstarted tasks  awaitTermination() blocks until the service is in the terminated state  Implies three states:  Running  Shutting down  Terminated  shutdown() runs tasks in queue  shutdownNow() returns unstarted tasks  awaitTermination() blocks until the service is in the terminated state

ExecutorService implementation  ExecutorService is an interface  How do you implement shut down and cancellation?  ExecutorService is an interface  How do you implement shut down and cancellation?

Cancellation in Java  Cooperative, not mandated  Traditional method: interruption  Cooperative, not mandated  Traditional method: interruption Public class Thread { public void interrupt(); public void isInterrupted(); public static boolean interrupted(); … }

Interruption  Just a request!  Sets a flag that must be explicitly cleared!  Some methods clear the flag & throw InterruptedException  Others ignore it, leaving other code to handle it  Just a request!  Sets a flag that must be explicitly cleared!  Some methods clear the flag & throw InterruptedException  Others ignore it, leaving other code to handle it

IMPORTANT!  Your code should follow protocol when interrupted  If interrupted(),  Throw exception  Reset the flag for other code  If InterruptedException  Pass it along  Reset the flag for other code  Don’t swallow, unless your code is handles the interruption policy  Your code should follow protocol when interrupted  If interrupted(),  Throw exception  Reset the flag for other code  If InterruptedException  Pass it along  Reset the flag for other code  Don’t swallow, unless your code is handles the interruption policy

Restoring Interrupted Status catch(InterruptedException e) { Thread.currentThread().interrupt(); } // checks (AND CLEARS!) current thread if (Thread.interrupted()) { Thread.currentThread().interrupt(); } catch(InterruptedException e) { Thread.currentThread().interrupt(); } // checks (AND CLEARS!) current thread if (Thread.interrupted()) { Thread.currentThread().interrupt(); }

Handeling Interruption  Long computations “break” to check interrupted status  If interrupted, stop computation, throw InterruptedException or restore the interrupted status  Long computations “break” to check interrupted status  If interrupted, stop computation, throw InterruptedException or restore the interrupted status

Interrupting Non- Blocking Operations  Socket read/writes do not support interruption!  If you detect interruption, close the socket causing read/write to throw an exception  Locks (via synchronized) can not be interrupted  Explicit locks beyond scope of this lecture  Socket read/writes do not support interruption!  If you detect interruption, close the socket causing read/write to throw an exception  Locks (via synchronized) can not be interrupted  Explicit locks beyond scope of this lecture

Future  Interface representing the lifecycle of a task public interface Future { boolean cancel(boolean mayInterruptIfRunning); boolean isCancelled(); boolean isDone(); V get() throws InterruptedException, ExecutionException, CancellationException V get() throws InterruptedException, ExecutionException, CancellationException, TimeoutException } public interface Future { boolean cancel(boolean mayInterruptIfRunning); boolean isCancelled(); boolean isDone(); V get() throws InterruptedException, ExecutionException, CancellationException V get() throws InterruptedException, ExecutionException, CancellationException, TimeoutException }

CompletionService  Combines Executor with BlockingQueue  ExecutorCompletionService  Combines Executor with BlockingQueue  ExecutorCompletionService public interface CompletionService { Future poll(); Future poll(long timeout, TimeUnit unit); Future submit(Callable task); Future submit(Runnable task); Future take(); } public interface CompletionService { Future poll(); Future poll(long timeout, TimeUnit unit); Future submit(Callable task); Future submit(Runnable task); Future take(); }

Futures Again  ExecutorService interface also provides public interface ExecutorService { … Future submit(Callable task); Future submit(Runnable task); Future submit(Runnable task, T result); } public interface ExecutorService { … Future submit(Callable task); Future submit(Runnable task); Future submit(Runnable task, T result); }

GUI’s  Are almost always single threaded  That is, they have a dedicated thread for the GUI, but just 1!  Multithreaded has been tried, but has generally failed  Are almost always single threaded  That is, they have a dedicated thread for the GUI, but just 1!  Multithreaded has been tried, but has generally failed

Event Processing  The Swing apps expect short events that can be processed quickly in sequence  So, long running operations must be executed in another thread  Swing troubles are often related to communication between these threads  The Swing apps expect short events that can be processed quickly in sequence  So, long running operations must be executed in another thread  Swing troubles are often related to communication between these threads

Swing Manipulation  Swing does NOT use synchronization, it uses thread confinement  You should almost NEVER create, modify, or query swing components or data models outside the event-dispatching thread  Swing does NOT use synchronization, it uses thread confinement  You should almost NEVER create, modify, or query swing components or data models outside the event-dispatching thread

(A Few Exceptions)  Some components (see JavaDoc)  SwingUtilities.isEventDispatchThrea d  SwingUtilities.invokeLater  SwingUtilities.invokeAndWait  Methods to enqueue repaint or revalidation  Methods for add/remove listeners  Some components (see JavaDoc)  SwingUtilities.isEventDispatchThrea d  SwingUtilities.invokeLater  SwingUtilities.invokeAndWait  Methods to enqueue repaint or revalidation  Methods for add/remove listeners

Short-running Tasks  Short, GUI-confined operations can be programmed entirely in the EDThread  Trivial Example: button that changes color when clicked  Example: information between model and view  Short, GUI-confined operations can be programmed entirely in the EDThread  Trivial Example: button that changes color when clicked  Example: information between model and view

Long-running Tasks  Proposed Handling:  GuiExecutor  newCachedThreadPool  Proposed Handling:  GuiExecutor  newCachedThreadPool

GuiExecutor  import java.util.*;  import java.util.concurrent.*;  public class GuiExecutor extends AbstractExecutorService {  // Singletons have a private constructor and a public factory  private static final GuiExecutor instance = new GuiExecutor();  private GuiExecutor() {}  public static GuiExecutor instance() { return instance; }  public void execute(Runnable r) {  if (SwingUtilities.isEventDispatchThread())  r.run();  else  SwingUtilities.invokeLater(r);  }  /* shutdown, shutdownNow, and awaitTermination throw  UnsupportedOperationException for now */  public boolean isShutdown() {return false; }  public boolean isTerminated() {return false; }  }  import java.util.*;  import java.util.concurrent.*;  public class GuiExecutor extends AbstractExecutorService {  // Singletons have a private constructor and a public factory  private static final GuiExecutor instance = new GuiExecutor();  private GuiExecutor() {}  public static GuiExecutor instance() { return instance; }  public void execute(Runnable r) {  if (SwingUtilities.isEventDispatchThread())  r.run();  else  SwingUtilities.invokeLater(r);  }  /* shutdown, shutdownNow, and awaitTermination throw  UnsupportedOperationException for now */  public boolean isShutdown() {return false; }  public boolean isTerminated() {return false; }  }

Long Running Tasks: Starting  Fire off tasks to thread pool  When completed, use the GuiExecutor to send a task for updating gui  Simple example: status text  Fire off tasks to thread pool  When completed, use the GuiExecutor to send a task for updating gui  Simple example: status text

Long-running Tasks: Cancellation  Instead of execute(), use submit()  Returns a Future that is cancelable  You still have to make your task interruptable  Instead of execute(), use submit()  Returns a Future that is cancelable  You still have to make your task interruptable

Long-running Tasks: Visual Feedback  The books method:  Create a task subclass with some methods callable from EDthread, & others callable elsewhere  I think this is dangerous. If you’re going to do this, you should  Clearly document the methods  Check if you’re in the Edthread  The books method:  Create a task subclass with some methods callable from EDthread, & others callable elsewhere  I think this is dangerous. If you’re going to do this, you should  Clearly document the methods  Check if you’re in the Edthread

BackgroundTask  Code is confusing, hard to follow  If you want to look at it:  Code is confusing, hard to follow  If you want to look at it:

Simpler Solution  Design LR tasks with “hooks”  The hook can be a special communication class  When checking for cancellation, update the hook  Hook schedules update to GUI  Design LR tasks with “hooks”  The hook can be a special communication class  When checking for cancellation, update the hook  Hook schedules update to GUI

Last Notes  Writing your own synchronizers // BLOCKS-UNTIL: not-full public synchronized void put(V v) throws InterruptedException { while (isFull()) wait(); doPut(v); notifyAll(); } // BLOCKS-UNTIL: not-empty public synchronized V take() throws InterruptedException { while (isEmpty()) wait(); V v = doTake(); notifyAll(); return v; }