object that both initiates the rendering thread and keeps track of the returned result in Java

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object that both initiates the rendering thread and keeps track of the returned result
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While it is common practice, the use of unsynchronized (or direct) access of internal result fields as seen in the waiter object is a bit delicate Since access is not synchronized, correctness relies on the fact that both thread termination and the join method intrinsically employ synchronized methods or blocks (see 227)
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interface Pic { byte[] getImage(); } interface Renderer {
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Pic render(URL src); } class PictureApp { // Code sketch // private final Renderer renderer = new StandardRenderer(); public void show(final URL imageSource) { class Waiter implements Runnable { private Pic result = null; Pic getResult() { return result; } public void run() { result = rendererrender(imageSource); } }; Waiter waiter = new Waiter(); Thread t = new Thread(waiter); tstart(); displayBorders(); displayCaption(); // do other things // while rendering
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try { tjoin(); } catch(InterruptedException e) { cleanup(); return; } Pic pic = waitergetResult(); if (pic != null) displayImage(picgetImage()); else // deal with assumed rendering failure } } Threadjoin returns control to the caller whether the thread completed successfully or abnormally For simplicity of illustration, nullness of the result field is used here to indicate any kind of failure, including cancellation of the renderer The version in 4331 illustrates a more responsible approach
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433 Futures
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The operations underlying join-based constructions can be packaged in a more convenient and structured fashion by:
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Creating Futures "virtual" data objects that automatically block when clients try to invoke their field accessors before their computation is complete A Future acts as an "IOU" for a given data object Creating versions of service methods that start up one or more threads and then return Future objects that are unblocked when computations complete
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Because the mechanics surrounding futures are built into data access and service methods, they can be applied in a general fashion only if both the data objects and the service methods are defined using interfaces, not classes However, if the associated interfaces are defined, Futures are easy to set up For example, a Future-based AsynchRenderer can employ proxies around concrete implementation classes (see 142):
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class AsynchRenderer implements Renderer { private final Renderer renderer = new StandardRenderer(); static class FuturePic implements Pic { // inner class private Pic pic = null; private boolean ready = false; synchronized void setPic(Pic p) { pic = p;
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ready = true; notifyAll(); } public synchronized byte[] getImage() { while (!ready) try { wait(); } catch (InterruptedException e) { return null; } return picgetImage(); }
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public Pic render(final URL src) { final FuturePic p = new FuturePic(); new Thread(new Runnable() { public void run() { psetPic(rendererrender(src)); } })start(); return p; }
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For illustration, AsynchRenderer uses explicit waiting and notification operations based on a ready flag rather than relying on Threadjoin Applications relying on this class can be written in a simple fashion:
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class PicturAppWithFuture { // Code sketch private final Renderer renderer = new AsynchRenderer(); public void show(final URL imageSource) { Pic pic = rendererrender(imageSource); displayBorders(); displayCaption(); // do other things
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byte[] im = picgetImage(); if (im != null) displayImage(im); else // deal with assumed rendering failure }
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4331 Callables Most designs based on Futures take exactly the form illustrated in class AsynchRenderer The construction and use of such classes can be further standardized and automated by stepping up to a blander interface
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In the same way that interface Runnable describes any pure action, a Callable interface can be used to describe any service method that accepts an Object argument, returns an Object result, and may throw an Exception:
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interface Callable { Object call(Object arg) throws Exception; }
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The use of Object here (awkwardly) accommodates, for example, adaptation of methods accepting multiple arguments by stuffing them into array objects While there are other options, it is most convenient to package up support mechanics via a single class that coordinates usage The following FutureResult class shows one set of choices (It is a streamlined version of one in the utilconcurrent package available from the online supplement) The FutureResult class maintains methods to get the result Object that is returned, or the Exception that is thrown by a Callable Unlike our Pic versions where all failures were just indicated via null values, it deals with interruptions more honestly by throwing exceptions back to clients attempting to obtain results To differentiate properly between exceptions encountered in the service versus those encountered trying to execute the service, exceptions thrown by the Callable are repackaged using javalangreflectInvocationTargetException, a general-purpose class for wrapping one exception inside another Also, for the sake of generality, the FutureResult does not itself create threads Instead, it supports method setter that returns a Runnable that users can then execute within a thread or any other code Executor (see 414) This makes Callables usable within lightweight executable frameworks that are otherwise set up to handle tasks initiated via oneway messages As an alternative strategy, you could set up a Caller framework that is otherwise similar to Executor, but is more specialized to the needs of service-style tasks, for example supporting methods to schedule execution, check status, and control responses to exceptions
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class FutureResult { // Fragments protected Object value = null; protected boolean ready = false; protected InvocationTargetException exception = null; public synchronized Object get() throws InterruptedException, InvocationTargetException { while (!ready) wait(); if (exception != null) throw exception; else return value; }
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public Runnable setter(final Callable function) { return new Runnable() { public void run() { try { set(functioncall()); } catch(Throwable e) { setException(e); } } }; } synchronized void set(Object result) { value = result; ready = true; notifyAll(); } synchronized void setException(Throwable e) { exception = new InvocationTargetException(e); ready = true; notifyAll(); } // other auxiliary and convenience methods }
The FutureResult class can be used directly to support generic Futures or as a utility in constructing more specialized versions As an example of direct use: