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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<div id="shadow-block"><a class="logo" href="index.html"><img src="images/OSGi.svg" alt="OSGi Working Group Documentation" /><h1>OSGi Compendium Release 8</h1></a></div>
</div>
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<div class="chapter">
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<div>
<div>
<h1 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="util.promise"></a><span xmlns="" class="number">705</span> Promises Specification
</h1>
</div>
<div>
<p xmlns="http://www.w3.org/1999/xhtml" class="releaseinfo"><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise" title="705.11 org.osgi.util.promise">Version 1.3</a></p>
</div>
</div>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h2 xmlns="http://www.w3.org/1999/xhtml" class="title" style="clear: both"><a xmlns="" class="anchor" id="d0e157155"></a><span xmlns="" class="number">705.1</span> Introduction
</h2>
</div>
</div>
</div>
<p>One of the fundamental pieces of an asynchronous programming model
is the mechanism by which clients retrieve the result of the asynchronous
task. Since Java 5, there has been a
<code class="code">java.util.concurrent.Future</code> interface available in the Java
class libraries, which means that it is the <span class="emphasis"><em>de facto</em></span>
API in Java for handling the result of an asynchronous task. Futures have
some limitations however in that they have no mechanism for registering
callbacks. Java 8 introduces the class
<code class="code">java.util.concurrent.CompletableFuture</code> which addresses this
but it is a complex API.
</p>
<p>This specification defines a Promises API which is independent of
all other OSGi specifications including the OSGi Framework and thus can be
easily used outside of the OSGi environment.
</p>
<p>A Promise object holds the result of a potentially asynchronous
task. The receiver of a Promise object can register callbacks on the
Promise to be notified when the result is available or can block on the
result becoming available. Promises can be chained together in powerful
ways to handle asynchronous work flows and recovery.
</p>
<p>Promises capture the effects of latency and errors by making these
explicit in the API signatures. Latency is represented by callbacks which
will eventually be called. Errors are represented by the failure member.
In essence, this is what sets Promises apart from things such as RPC calls
where such effects are not explicitly captured but rather attempted to be
transparently handled.
</p>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h3 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="d0e157175"></a><span xmlns="" class="number">705.1.1</span> Essentials
</h3>
</div>
</div>
</div>
<div class="itemizedlist">
<ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p><span class="emphasis"><em>Common concepts</em></span> - The API is inspired by
the Promises work in JavaScript and uses the same basic concepts.
See <a xmlns="" class="xref" href="util.promise.html#util.promise-ref.javascript" title="JavaScript Promises">[2] <em>JavaScript Promises</em></a>.
</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>Independent</em></span> - The design is independent
of all other OSGi specifications and can be used outside of an OSGi
environment.
</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>Asynchronous</em></span> - The design supports
asynchronous tasks.
</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>Small</em></span> - The API and implementation are
very compact.
</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>Complete</em></span> - The design provides a very
complete set of operations for Promise which are primitives that can
be used to address most use cases.
</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>Monad</em></span> - The design supports monadic
programming. See <a xmlns="" class="xref" href="util.promise.html#util.promise-ref.monad" title="Monad">[4] <em>Monad</em></a>.
</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>Resolution</em></span> - A Promise can be resolved
successfully with a value or unsuccessfully with an
exception.
</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>Generified</em></span> - Generics are used to promote
type safety.
</p>
</li>
</ul>
</div>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h3 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="d0e157223"></a><span xmlns="" class="number">705.1.2</span> Entities
</h3>
</div>
</div>
</div>
<div class="itemizedlist">
<ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p><span class="emphasis"><em>Promise</em></span> - A Promise object holds the
eventual result of a potentially asynchronous task.
</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>Callback</em></span> - The receiver of a Promise can
register callbacks on the Promise to be notified when the task is
completed.
</p>
</li>
<li class="listitem">
<p><span class="emphasis"><em>Deferred</em></span> - A Deferred object represents
the potentially asynchronous task and is used to resolve the
Promise.
</p>
</li>
</ul>
</div>
<div class="figure"><a xmlns="" class="anchor" id="d0e157242"></a><p class="title"><strong>Figure <span xmlns="" class="number">705</span>.1 Class diagram of org.osgi.util.promise</strong></p>
<div class="figure-contents">
<div class="mediaobject" align="center"><img src="images/705-promise-classes.png" align="middle" width="630" height="110" alt="Class diagram of org.osgi.util.promise" /></div>
</div>
</div><br class="figure-break" /></div>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h2 xmlns="http://www.w3.org/1999/xhtml" class="title" style="clear: both"><a xmlns="" class="anchor" id="d0e157248"></a><span xmlns="" class="number">705.2</span> Promise
</h2>
</div>
</div>
</div>
<p>A <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise" title="705.11.5 public interface Promise<T>">Promise</a> object holds the eventual result of a potentially
asynchronous task. A Promise is either unresolved or resolved. An
<span class="emphasis"><em>unresolved</em></span> Promise does not have the result of the
associated task available while a <span class="emphasis"><em>resolved</em></span> Promise
has the result of the associated task available. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.isDone--" title="705.11.5.8 public boolean isDone()">isDone()</a>
method must return <code class="code">true</code> if the Promise is resolved and
<code class="code">false</code> if the Promise is unresolved. A Promise must only be
resolved once.
</p>
<p>A resolved Promise can be either resolved with a value, which means
the associated task <span class="emphasis"><em>completed successfully</em></span> and
supplied a result, or resolved with a failure, which means the associated
task <span class="emphasis"><em>completed unsuccessfully</em></span> and supplied an
exception. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.getFailure--" title="705.11.5.6 public Throwable getFailure() throws InterruptedException">getFailure()</a> method can be called to determine if the resolved
Promise completed successfully with a value or unsuccessfully with a
failure. If the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.getFailure--" title="705.11.5.6 public Throwable getFailure() throws InterruptedException">getFailure()</a> method returns a <code class="code">Throwable</code>, the
Promise resolved unsuccessfully with a failure. If the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.getFailure--" title="705.11.5.6 public Throwable getFailure() throws InterruptedException">getFailure()</a> method returns <code class="code">null</code>, the Promise
resolved successfully with a value that can be obtained from <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.getValue--" title="705.11.5.7 public T getValue() throws InvocationTargetException, InterruptedException">getValue()</a>.
</p>
<p>If the Promise is unresolved, then calling <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.getFailure--" title="705.11.5.6 public Throwable getFailure() throws InterruptedException">getFailure()</a> or <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.getValue--" title="705.11.5.7 public T getValue() throws InvocationTargetException, InterruptedException">getValue()</a>
must block until the Promise is resolved. In general, these two methods
should not be used outside of a callback. Use callbacks to be notified
when the Promise is resolved. See <a xmlns="" class="xref" href="util.promise.html#util.promise-callbacks" title="705.4 Callbacks">Callbacks</a>.
</p>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h2 xmlns="http://www.w3.org/1999/xhtml" class="title" style="clear: both"><a xmlns="" class="anchor" id="d0e157299"></a><span xmlns="" class="number">705.3</span> Deferred
</h2>
</div>
</div>
</div>
<p><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise" title="705.11.5 public interface Promise<T>">Promise</a> is an interface which can allow for many Promise
implementations. This API contains the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Deferred" title="705.11.2 public class Deferred<T>">Deferred</a> class
which provides access to the standard Promise implementation. A Deferred
object can be created by calling the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory.deferred--" title="705.11.6.5 public Deferred<T> deferred()">deferred()</a> method on a <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory" title="705.11.6 public class PromiseFactory">PromiseFactory</a>
object.
</p>
<p>A <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory" title="705.11.6 public class PromiseFactory">PromiseFactory</a> object is created with a specified callback
executor and a specified scheduled executor to use for created Promise
objects and the Promise objects associated with created Deferred objects.
If the callback executor or the scheduled executor is not specified or is
specified as <code class="code">null</code>, then implementation default executors will
be used. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Deferred.Deferred--" title="705.11.2.1 public Deferred()">Deferred()</a> constructor will create a Deferred whose
associated Promise uses the default Promise Factory which uses the
implementation default executors. All Promise objects created by a Promise
must use the same Promise Factory as the creating Promise. Callbacks must
be called using the callback executor of the Promise Factory associated
with the Promise if the Promise Factory was created with the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory.Option.CALLBACKS_EXECUTOR_THREAD" title="705.11.7.1 CALLBACKS_EXECUTOR_THREAD">CALLBACKS_EXECUTOR_THREAD</a> option. Otherwise, when the Promise is already
resolved, the current thread may be used to call the callback to avoid a
thread context switch. If the callback executor rejects the execution, the
current thread will be used to call the callback to avoid loss of the
callback operation. The scheduled executor must be used by the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.timeout-long-" title="705.11.5.21 public Promise<T> timeout(long milliseconds)">timeout(long)</a> and <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.delay-long-" title="705.11.5.1 public Promise<T> delay(long milliseconds)">delay(long)</a> operations. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory.inlineExecutor--" title="705.11.6.8 public static Executor inlineExecutor()">inlineExecutor()</a> method can be used to obtain an executor which
runs callbacks immediately on the thread calling the
<code class="code">Executor.execute</code> method. This behavior is similar to how
callbacks were executed in the default Promise implementation of Promise
1.0 specification.
</p>
<p>The Promise associated with a Deferred object can be obtained using
<a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Deferred.getPromise--" title="705.11.2.3 public Promise<T> getPromise()">getPromise()</a>. This Promise can then be supplied to other
parties who can use it to be notified of and obtain the eventual
result.
</p><pre xmlns="" class="programlisting"><code>public Promise<String> getTimeConsumingAnswer() {
Deferred<String> deferred = factory.deferred();
asynchronously(() -> doTask(deferred));
return deferred.getPromise();
}</code></pre><p>A Deferred object can later be used to resolve the associated
Promise successfully by calling <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Deferred.resolve-T-" title="705.11.2.4 public void resolve(T value)">resolve(T)</a> or unsuccessfully by calling <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Deferred.fail-Throwable-" title="705.11.2.2 public void fail(Throwable failure)">fail(Throwable)</a>.
</p><pre xmlns="" class="programlisting"><code>private void doTask(Deferred<String> deferred) {
try {
String answer = computeTimeConsumingAnswer();
deferred.resolve(answer); // successfully resolve with value
} catch (Exception e) {
deferred.fail(e); // unsuccessfully resolve with exception
}
}</code></pre><p>A Deferred object can also be used to resolve the associated Promise
with the eventual result of another Promise by calling <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Deferred.resolveWith-Promise-" title="705.11.2.5 public Promise<Void> resolveWith(Promise<? extends T> with)">resolveWith(Promise)</a> or the result of a CompletionStage by calling
<a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Deferred.resolveWith-CompletionStage-" title="705.11.2.6 public Promise<Void> resolveWith(CompletionStage<? extends T> with)">resolveWith(CompletionStage)</a>.
</p><pre xmlns="" class="programlisting"><code>private void doTask(Deferred<String> deferred) {
try {
Promise<String> promise = getPromiseWithTheAnswer();
deferred.resolveWith(promise); // resolve with another Promise
} catch (Exception e) {
deferred.fail(e); // unsuccessfully resolve with exception
}
}</code></pre><p>If <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Deferred.resolve-T-" title="705.11.2.4 public void resolve(T value)">resolve(T)</a> or <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Deferred.fail-Throwable-" title="705.11.2.2 public void fail(Throwable failure)">fail(Throwable)</a> is called when the Promise associated with the
Deferred is already resolved, then an Illegal State Exception must be
thrown.
</p>
<p>Care must be taken in sharing a Deferred object with other parties
since the other parties can resolve the associated Promise. A Deferred
object should be made available only to the party that will responsible
for resolving the associated Promise.
</p>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h2 xmlns="http://www.w3.org/1999/xhtml" class="title" style="clear: both"><a xmlns="" class="anchor" id="util.promise-callbacks"></a><span xmlns="" class="number">705.4</span> Callbacks
</h2>
</div>
</div>
</div>
<p>To be notified when a Promise has been resolved, callbacks are used.
The Promise API provides two forms of callbacks: the basic
<code class="code">Runnable</code> and <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Consumer" title="705.12.2 public interface Consumer<T>">Consumer</a> callbacks and the more specialized <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Success" title="705.11.9 public interface Success<T, R>">Success</a> and <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Failure" title="705.11.4 public interface Failure">Failure</a>
callbacks.
</p>
<p>A callback may be called on a different thread than the thread which
registered the callback. So the callback must be thread safe but can rely
upon that the registration of the callback
<span class="emphasis"><em>happens-before</em></span> the callback is called.
</p>
<p>Resolving a Promise <span class="emphasis"><em>happens-before</em></span> any
registered callback is called. That is, for the resolved Promise, in a
registered callback <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.isDone--" title="705.11.5.8 public boolean isDone()">isDone()</a> must return <code class="code">true</code> and <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.getValue--" title="705.11.5.7 public T getValue() throws InvocationTargetException, InterruptedException">getValue()</a>
and <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.getFailure--" title="705.11.5.6 public Throwable getFailure() throws InterruptedException">getFailure()</a> must not block.
</p>
<p>Callbacks may be registered at any time including before and after a
Promise has been resolved. If a callback is registered before the Promise
is resolved, it will be called later when the Promise is resolved. If a
callback is registered on an already resolved Promise, it will be called
right away.
</p>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h3 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="d0e157396"></a><span xmlns="" class="number">705.4.1</span> Runnable
</h3>
</div>
</div>
</div>
<p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.onResolve-Runnable-" title="705.11.5.12 public Promise<T> onResolve(Runnable callback)">onResolve(Runnable)</a> method is used to register a
<code class="code">Runnable</code> with the Promise which must be called when the
Promise is resolved either successfully with a value or unsuccessfully
with a failure. The resolved Promise is not passed to the Runnable, so
if the Runnable implementation needs access to the resolved Promise, it
must take care to ensure it has access.
</p><pre xmlns="" class="programlisting"><code>final Promise<String> answer = getTimeConsumingAnswer();
answer.onResolve(() -> doSomethingWithAnswer(answer));</code></pre><p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.onResolve-Runnable-" title="705.11.5.12 public Promise<T> onResolve(Runnable callback)">onResolve(Runnable)</a> method returns the Promise object upon which it
is called.
</p>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h3 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="util.promise-callbacks.consumer"></a><span xmlns="" class="number">705.4.2</span> Consumer
</h3>
</div>
</div>
</div>
<p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.thenAccept-Consumer-" title="705.11.5.20 public Promise<T> thenAccept(Consumer<? super T> consumer)">thenAccept(Consumer)</a> method is used to register a <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Consumer" title="705.12.2 public interface Consumer<T>">Consumer</a> with
the Promise which must be called when the Promise is resolved
successfully with a value. The value of the resolved Promise is passed
to the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Consumer" title="705.12.2 public interface Consumer<T>">Consumer</a>.
</p><pre xmlns="" class="programlisting"><code>final Promise<String> answer = getTimeConsumingAnswer().thenAccept(s ->
doSomethingWithAnswer(s)
);</code></pre><p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.thenAccept-Consumer-" title="705.11.5.20 public Promise<T> thenAccept(Consumer<? super T> consumer)">thenAccept(Consumer)</a> method returns a new Promise which will be
resolved with either the exception thrown from the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Consumer" title="705.12.2 public interface Consumer<T>">Consumer</a>, if
one is thrown, or with the Promise.
</p>
<p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.onSuccess-Consumer-" title="705.11.5.13 public Promise<T> onSuccess(Consumer<? super T> success)">onSuccess(Consumer)</a> method is used to register a <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Consumer" title="705.12.2 public interface Consumer<T>">Consumer</a> with
the Promise which must be called when the Promise is resolved
successfully with a value. The value of the resolved Promise is passed
to the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Consumer" title="705.12.2 public interface Consumer<T>">Consumer</a>. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.onSuccess-Consumer-" title="705.11.5.13 public Promise<T> onSuccess(Consumer<? super T> success)">onSuccess(Consumer)</a> method returns the Promise object upon which it
is called.
</p>
<p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.onFailure-Consumer-" title="705.11.5.10 public Promise<T> onFailure(Consumer<? super Throwable> failure)">onFailure(Consumer)</a> method is used to register a <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Consumer" title="705.12.2 public interface Consumer<T>">Consumer</a> with
the Promise which must be called when the Promise is resolved
unsuccessfully with a failure. The failure of the resolved Promise is
passed to the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Consumer" title="705.12.2 public interface Consumer<T>">Consumer</a>. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.onFailure-Consumer-" title="705.11.5.10 public Promise<T> onFailure(Consumer<? super Throwable> failure)">onFailure(Consumer)</a> method returns the Promise object upon which it
is called.
</p>
<p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.onFailure-Consumer-Class-" title="705.11.5.11 public Promise<T> onFailure(Consumer<? super F> failure, Class<? extends F> failureType)">onFailure(Consumer,Class)</a> method is used to register a <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Consumer" title="705.12.2 public interface Consumer<T>">Consumer</a> with
the Promise which must be called when the Promise is resolved
unsuccessfully with a failure of a specified type. The failure of the
resolved Promise is passed to the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Consumer" title="705.12.2 public interface Consumer<T>">Consumer</a>. If
the Promise is unsuccessfully resolved with a failure which is not of
the specified type, the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Consumer" title="705.12.2 public interface Consumer<T>">Consumer</a> argument is not called. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.onFailure-Consumer-Class-" title="705.11.5.11 public Promise<T> onFailure(Consumer<? super F> failure, Class<? extends F> failureType)">onFailure(Consumer,Class)</a> method returns the Promise object upon which it
is called.
</p>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h3 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="d0e157463"></a><span xmlns="" class="number">705.4.3</span> Success and Failure
</h3>
</div>
</div>
</div>
<p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.then-Success-" title="705.11.5.19 public Promise<R> then(Success<? super T, ? extends R> success)">then(Success)</a> and <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.then-Success-Failure-" title="705.11.5.18 public Promise<R> then(Success<? super T, ? extends R> success, Failure failure)">then(Success,Failure)</a> methods can be used to register the more
specialized <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Success" title="705.11.9 public interface Success<T, R>">Success</a> and <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Failure" title="705.11.4 public interface Failure">Failure</a>
callbacks. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Success" title="705.11.9 public interface Success<T, R>">Success</a> callback is only called if the Promise is
successfully resolved with a value. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Failure" title="705.11.4 public interface Failure">Failure</a> callback
is only called if the Promise is unsuccessfully resolved with a
failure.
</p><pre xmlns="" class="programlisting"><code>Promise<String> answer = getTimeConsumingAnswer();
answer.then(p -> processResult(p.getValue()), p -> handleFailure(p.getFailure()));</code></pre><p>The <code class="code">then</code> methods return a new Promise which can be
used to chain Promises together.
</p>
</div>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h2 xmlns="http://www.w3.org/1999/xhtml" class="title" style="clear: both"><a xmlns="" class="anchor" id="util.promise-chaining.promises"></a><span xmlns="" class="number">705.5</span> Chaining Promises
</h2>
</div>
</div>
</div>
<p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.then-Success-" title="705.11.5.19 public Promise<R> then(Success<? super T, ? extends R> success)">then(Success)</a>, <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.then-Success-Failure-" title="705.11.5.18 public Promise<R> then(Success<? super T, ? extends R> success, Failure failure)">then(Success,Failure)</a>, and <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.thenAccept-Consumer-" title="705.11.5.20 public Promise<T> thenAccept(Consumer<? super T> consumer)">thenAccept(Consumer)</a> methods also provide a means to chain Promises
together. These methods return a new Promise which is chained to the
original Promise upon which the method was called. The returned Promise
must be resolved when the original Promise is resolved after the specified
Success, Failure, or Consumer callback is executed. The result of the
executed callback must be used to resolve the returned Promise. A sequence
of calls to the <code class="code">then</code> methods can be used to create a chain of
promises which are resolved in sequence.
</p>
<p>For the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.then-Success-" title="705.11.5.19 public Promise<R> then(Success<? super T, ? extends R> success)">then(Success)</a> or <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.then-Success-Failure-" title="705.11.5.18 public Promise<R> then(Success<? super T, ? extends R> success, Failure failure)">then(Success,Failure)</a> methods, if the original Promise is successfully
resolved, the Success callback is executed and the Promise returned by the
Success callback, if any, or thrown exception is used to resolve the
Promise returned from the method. If the original Promise is resolved with
a failure, the Failure callback is executed and the Promise returned from
the method is resolved with a failure.
</p>
<p>For the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.thenAccept-Consumer-" title="705.11.5.20 public Promise<T> thenAccept(Consumer<? super T> consumer)">thenAccept(Consumer)</a> method, if the original Promise is successfully
resolved, the Consumer callback is executed and the value of the original
Promise or thrown exception is used to resolve the Promise returned from
the method. If the original Promise is resolved with a failure, the
Consumer callback is not executed and the Promise returned from the method
is resolved with the failure of the original Promise.
</p>
<p>In the following example, a Promise which will supply the name of
the file to download is chained to a Promise which will return a mirror
URL to use to download the file which is then further chained to a Promise
which will return an Input Stream from which to read the download
file.
</p><pre xmlns="" class="programlisting"><code>Promise<String> name = getDownloadName();
Promise<URL> mirror = name.then(p -> getMirror(p.getValue()));
Promise<InputStream> in = mirror.then(p -> getStream(p.getValue()));</code></pre><p>Since we probably do not need the intermediate Promises, we can
collapse the chain into a single statement.
</p><pre xmlns="" class="programlisting"><code>Promise<InputStream> in = getDownloadName().then(p -> getMirror(p.getValue()))
.then(p -> getStream(p.getValue()));</code></pre><p>The chain of Promises will also propagate any exceptions that occur
to resolve the last Promise in the chain which means we do not need to do
any exception handling in the intermediate tasks. Promises can also be
chained by using the monadic programming methods in <a xmlns="" class="xref" href="util.promise.html#util.promise-monad.programming" title="705.6 Monad">Monad</a>.
</p>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h2 xmlns="http://www.w3.org/1999/xhtml" class="title" style="clear: both"><a xmlns="" class="anchor" id="util.promise-monad.programming"></a><span xmlns="" class="number">705.6</span> Monad
</h2>
</div>
</div>
</div>
<p>The Promise API supports monadic programming. See <a xmlns="" class="xref" href="util.promise.html#util.promise-ref.monad" title="Monad">[4] <em>Monad</em></a>. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise" title="705.11.5 public interface Promise<T>">Promise</a> interface
defines a number of interesting methods including <code class="code">map</code>,
<code class="code">flatMap</code> and <code class="code">filter</code>.
</p>
<div class="itemizedlist">
<ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.filter-Predicate-" title="705.11.5.4 public Promise<T> filter(Predicate<? super T> predicate)">filter(Predicate)</a> - Filter the value of the Promise.
</p>
<p>If the Promise is successfully resolved, the predicate argument
is called with the value of the Promise. If the predicate accepts the
value, then the value is used to successfully resolve the Promise
returned by the filter method. If the predicate does not accept the
value, the Promise returned by the filter method is unsuccessfully
resolved with a No Such Element Exception. If the predicate throws an
exception, the Promise returned by the filter method is unsuccessfully
resolved with that exception.
</p>
<p>If the Promise is unsuccessfully resolved, the predicate
argument is not called and the Promise returned by the filter method
is unsuccessfully resolved with the failure of the Promise.
</p>
</li>
<li class="listitem">
<p><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.map-Function-" title="705.11.5.9 public Promise<R> map(Function<? super T, ? extends R> mapper)">map(Function)</a> - Map the value of the Promise.
</p>
<p>If the Promise is successfully resolved, the function argument
is called with the value of the Promise. The value returned by the
function is used to successfully resolve the Promise returned by the
map method. If the function throws an exception, the Promise returned
by the map method is unsuccessfully resolved with that
exception.
</p>
<p>If the Promise is unsuccessfully resolved, the function argument
is not called and the Promise returned by the map method is
unsuccessfully resolved with the failure of the Promise.
</p>
</li>
<li class="listitem">
<p><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.flatMap-Function-" title="705.11.5.5 public Promise<R> flatMap(Function<? super T, Promise<? extends R>> mapper)">flatMap(Function)</a> - FlatMap the value of the Promise.
</p>
<p>If the Promise is successfully resolved, the function argument
is called with the value of the Promise. The Promise returned by the
function is used to resolve the Promise returned by the flatMap
method. If the function throws an exception, the Promise returned by
the flatMap method is unsuccessfully resolved with that
exception.
</p>
<p>If the Promise is unsuccessfully resolved, the function argument
is not called and the Promise returned by the flatMap method is
unsuccessfully resolved with the failure of the Promise.
</p>
</li>
<li class="listitem">
<p><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.recover-Function-" title="705.11.5.14 public Promise<T> recover(Function<Promise<?>, ? extends T> recovery)">recover(Function)</a> - Recover from the unsuccessful resolution of
the Promise with a recovery value.
</p>
<p>If the Promise is successfully resolved, the function argument
is not called and the Promise returned by the recover method is
resolved with the value of the Promise.
</p>
<p>If the Promise is unsuccessfully resolved, the function argument
is called with the Promise to supply a recovery value. If the recovery
value is not <code class="code">null</code>, the Promise returned by the recover
method is successfully resolved with the recovery value. If the
recovery value is <code class="code">null</code>, the Promise returned by the
recover method is unsuccessfully resolved with the failure of the
Promise. If the function throws an exception, the Promise returned by
the recover method is unsuccessfully resolved with that
exception.
</p>
</li>
<li class="listitem">
<p><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.recover-Function-Class-" title="705.11.5.15 public Promise<T> recover(Function<Promise<?>, ? extends T> recovery, Class<?> failureType)">recover(Function,Class)</a> - Recover from the unsuccessful resolution of
the Promise with a recovery value when the failure is of a specified
type.
</p>
<p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.recover-Function-Class-" title="705.11.5.15 public Promise<T> recover(Function<Promise<?>, ? extends T> recovery, Class<?> failureType)">recover(Function,Class)</a> method provides the same behavior as <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.recover-Function-" title="705.11.5.14 public Promise<T> recover(Function<Promise<?>, ? extends T> recovery)">recover(Function)</a> but only when the failure is of the specified
type. If the Promise is unsuccessfully resolved with a failure which
is not of the specified type, the function argument is not called and
the Promise returned by the recover method is resolved with the
failure of the Promise.
</p>
</li>
<li class="listitem">
<p><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.recoverWith-Function-" title="705.11.5.16 public Promise<T> recoverWith(Function<Promise<?>, Promise<? extends T>> recovery)">recoverWith(Function)</a> - Recover from the unsuccessful resolution of
the Promise with a recovery Promise.
</p>
<p>If the Promise is successfully resolved, the function argument
is not called and the Promise returned by the recover method is
resolved with the value of the Promise.
</p>
<p>If the Promise is unsuccessfully resolved, the function argument
is called with the Promise to supply a recovery Promise. If the
recovery Promise is not <code class="code">null</code>, the Promise returned by the
recover method is resolved with the recovery Promise. If the recovery
Promise is <code class="code">null</code>, the Promise returned by the recover
method is unsuccessfully resolved with the failure of the Promise. If
the function throws an exception, the Promise returned by the recover
method is unsuccessfully resolved with that exception.
</p>
</li>
<li class="listitem">
<p><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.recoverWith-Function-Class-" title="705.11.5.17 public Promise<T> recoverWith(Function<Promise<?>, Promise<? extends T>> recovery, Class<?> failureType)">recoverWith(Function,Class)</a> - Recover from the unsuccessful resolution of
the Promise with a recovery Promise when the failure is of a specified
type.
</p>
<p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.recoverWith-Function-Class-" title="705.11.5.17 public Promise<T> recoverWith(Function<Promise<?>, Promise<? extends T>> recovery, Class<?> failureType)">recoverWith(Function,Class)</a> method provides the same behavior as <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.recoverWith-Function-" title="705.11.5.16 public Promise<T> recoverWith(Function<Promise<?>, Promise<? extends T>> recovery)">recoverWith(Function)</a> but only when the failure is of the specified
type. If the Promise is unsuccessfully resolved with a failure which
is not of the specified type, the function argument is not called and
the Promise returned by the recover method is resolved with the
failure of the Promise.
</p>
</li>
<li class="listitem">
<p><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.fallbackTo-Promise-" title="705.11.5.2 public Promise<T> fallbackTo(Promise<? extends T> fallback)">fallbackTo(Promise)</a> - Fall back to the value of the Promise
argument if the Promise unsuccessfully resolves.
</p>
<p>If the Promise is successfully resolved, the Promise argument is
not used and the Promise returned by the fallbackTo method is resolved
with the value of the Promise.
</p>
<p>If the Promise is unsuccessfully resolved, the Promise argument
is used to provide a fallback value when it becomes resolved. If the
Promise argument is successfully resolved, the Promise returned by the
fallbackTo method is resolved with the value of the Promise argument.
If the Promise argument is unsuccessfully resolved, the Promise
returned by the fallbackTo method is unsuccessfully resolved with the
failure of the Promise.
</p>
</li>
<li class="listitem">
<p><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.fallbackTo-Promise-Class-" title="705.11.5.3 public Promise<T> fallbackTo(Promise<? extends T> fallback, Class<?> failureType)">fallbackTo(Promise,Class)</a> - Fall back to the value of the Promise
argument if the Promise unsuccessfully resolves with a failure of a
specified type.
</p>
<p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.fallbackTo-Promise-Class-" title="705.11.5.3 public Promise<T> fallbackTo(Promise<? extends T> fallback, Class<?> failureType)">fallbackTo(Promise,Class)</a> method provides the same behavior as <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.fallbackTo-Promise-" title="705.11.5.2 public Promise<T> fallbackTo(Promise<? extends T> fallback)">fallbackTo(Promise)</a> but only when the failure is of the specified
type. If the Promise is unsuccessfully resolved with a failure which
is not of the specified type, the Promise argument is not used and the
Promise returned by the fallbackTo method is resolved with the failure
of the Promise.
</p>
</li>
</ul>
</div>
<p>These functions can be used to build pipelines of chained Promises
that are processed in sequence. For example, in the following chain, the
value of the original promise, once resolved, is filtered for acceptable
values. If the filter says the value is not acceptable, the recover method
will be used to replace it with a default value.
</p><pre xmlns="" class="programlisting"><code>return promise.filter(v -> isValueOk(v)).recover(p -> getDefaultValue())</code></pre><p>With these chains, one can write powerful programs without the need
to resort to complex if/else and try/catch logic.
</p>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h2 xmlns="http://www.w3.org/1999/xhtml" class="title" style="clear: both"><a xmlns="" class="anchor" id="util.promise-timing.methods"></a><span xmlns="" class="number">705.7</span> Timing
</h2>
</div>
</div>
</div>
<p>The Promise API provides methods to affect the timing of resolving
Promises.
</p>
<div class="itemizedlist">
<ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.timeout-long-" title="705.11.5.21 public Promise<T> timeout(long milliseconds)">timeout(long)</a> - Time out the resolution of the
Promise.
</p>
<p>If the Promise is successfully resolved before the timeout, the
returned Promise is resolved with the value of the Promise. If the
Promise is resolved with a failure before the timeout, the returned
Promise is resolved with the failure of the Promise. If the timeout is
reached before the Promise is resolved, the returned Promise is failed
with a <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.TimeoutException" title="705.11.10 public class TimeoutException extends Exception">TimeoutException</a>.
</p>
</li>
<li class="listitem">
<p><a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.delay-long-" title="705.11.5.1 public Promise<T> delay(long milliseconds)">delay(long)</a> - Delay after the resolution of the
Promise.
</p>
<p>Once the Promise is resolved, resolve the returned Promise with
the Promise after the specified delay.
</p>
</li>
</ul>
</div>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h2 xmlns="http://www.w3.org/1999/xhtml" class="title" style="clear: both"><a xmlns="" class="anchor" id="d0e157658"></a><span xmlns="" class="number">705.8</span> Functional Interfaces
</h2>
</div>
</div>
</div>
<p>In Java 8, the concept of Functional Interfaces is introduced. See
<a xmlns="" class="xref" href="util.promise.html#util.promise-ref.functional.interfaces" title="Function Interfaces">[5] <em>Function Interfaces</em></a>. Functional
interfaces are types with a single abstract method. Instances of
functional interfaces can be created with lambda expressions, method
references, or constructor references. Many methods on <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise" title="705.11.5 public interface Promise<T>">Promise</a> take
functional interface arguments and so are suitable for use with lambda
expressions and method references in Java 8.
</p>
<p>Four of these functional interfaces are <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Function" title="705.12.3 public interface Function<T, R>">Function</a>, <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Predicate" title="705.12.4 public interface Predicate<T>">Predicate</a>, <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Supplier" title="705.12.5 public interface Supplier<T>">Supplier</a>, and
<a xmlns="" class="xref" href="util.promise.html#org.osgi.util.function.Consumer" title="705.12.2 public interface Consumer<T>">Consumer</a>.
These are equivalent to functional interfaces which are part of the
<code class="code">java.util.function</code> package introduced in Java 8 with
additional static methods to support interoperation. OSGi defines these
interfaces to allow throwing checked exceptions which can be propagated in
a chain of Promises.
</p>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h2 xmlns="http://www.w3.org/1999/xhtml" class="title" style="clear: both"><a xmlns="" class="anchor" id="d0e157680"></a><span xmlns="" class="number">705.9</span> Utility Methods
</h2>
</div>
</div>
</div>
<p>The API also provides several useful utility methods when working
with Promises.
</p>
<p>Often, you may need to create an already resolved Promise to return
or chain with another Promise. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory.resolved-T-" title="705.11.6.9 public Promise<T> resolved(T value)">resolved(T)</a> method can be used to create a new Promise already
successfully resolved with the specified value. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory.failed-Throwable-" title="705.11.6.7 public Promise<T> failed(Throwable failure)">failed(Throwable)</a> method can be used to create a new Promise already
unsuccessfully resolved with the specified exception. These methods also
exists as static methods on the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promises" title="705.11.8 public class Promises">Promises</a> class
returning Promises which use the implementation default executors.
</p><pre xmlns="" class="programlisting"><code> return getTimeConsumingAnswer().fallbackTo(factory.resolved("Fallback Value"));</code></pre><p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory.resolvedWith-Promise-" title="705.11.6.11 public Promise<T> resolvedWith(Promise<? extends T> with)">resolvedWith(Promise)</a> method can be used to return a new Promise that
will be resolved with the specified Promise.
</p>
<p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory.submit-Callable-" title="705.11.6.13 public Promise<T> submit(Callable<? extends T> task)">submit(Callable)</a> method can be used to return a new Promise that
will hold the result of the specified task. The task will be executed on
the callback executor.
</p>
<p>The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory.all-Collection-" title="705.11.6.4 public Promise<List<T>> all(Collection<Promise<S>> promises)">all(Collection)</a> method returns a Promise that is a latch on the
specified Promises. The returned Promise must resolve only when all of the
specified Promises have resolved. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory.toPromise--" title="705.11.6.14 public Collector<Promise<S>, ?, Promise<List<T>>> toPromise()">toPromise()</a> method returns a <code class="code">Collector</code> which can
be used on a <code class="code">Stream</code> of Promises to collect the results of the
Promises into a latch using the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory.all-Collection-" title="705.11.6.4 public Promise<List<T>> all(Collection<Promise<S>> promises)">all(Collection)</a> method as the finisher. The <code class="code">all</code>
method also exists as a static method on the <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promises" title="705.11.8 public class Promises">Promises</a> class
returning a Promise which uses the implementation default
executors.
</p>
<p>Interoperation with CompletionStage is supported. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.PromiseFactory.resolvedWith-CompletionStage-" title="705.11.6.10 public Promise<T> resolvedWith(CompletionStage<? extends T> with)">resolvedWith(CompletionStage)</a> method returns a Promise that is resolved by the
specified CompletionStage. The <a xmlns="" class="xref" href="util.promise.html#org.osgi.util.promise.Promise.toCompletionStage--" title="705.11.5.22 public CompletionStage<T> toCompletionStage()">toCompletionStage()</a> method returns a CompletionStage that is resolved
by the receiving Promise.
</p>
</div>
<div class="section">
<div xmlns="" class="titlepage">
<div>
<div>
<h2 xmlns="http://www.w3.org/1999/xhtml" class="title" style="clear: both"><a xmlns="" class="anchor" id="d0e157728"></a><span xmlns="" class="number">705.10</span> Security
</h2>
</div>
</div>
</div>
<p>The Promise API does not define any OSGi services nor does the API
perform any privileged actions. Therefore, it has no security
considerations.
</p>
</div>
<div class="section package">
<div xmlns="" class="titlepage">
<div>
<div>
<h2 xmlns="http://www.w3.org/1999/xhtml" class="title" style="clear: both"><a xmlns="" class="anchor" id="org.osgi.util.promise"></a><span xmlns="" class="number">705.11</span> org.osgi.util.promise
</h2>
</div>
<div>
<p xmlns="http://www.w3.org/1999/xhtml" class="releaseinfo">Version 1.3</p>
</div>
</div>
</div>
<p>
Promise Package Version 1.3.
</p>
<p>
Bundles wishing to use this package must list the package in the
Import-Package header of the bundle's manifest.
</p>
<p>
Example import for consumers using the API in this package:
</p>
<p>
<code class="code">Import-Package: org.osgi.util.promise; version="[1.3,2.0)"</code>
</p>
<p>
Example import for providers implementing the API in this package:
</p>
<p>
<code class="code">Import-Package: org.osgi.util.promise; version="[1.3,1.4)"</code>
</p>
<div class="section summary">
<div xmlns="" class="titlepage">
<div>
<div>
<h3 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="d0e157759"></a><span xmlns="" class="number">705.11.1</span> Summary
</h3>
</div>
</div>
</div>
<div class="itemizedlist">
<ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p>
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Deferred" title="705.11.2 public class Deferred<T>">
<code xmlns="http://www.w3.org/1999/xhtml" class="code">Deferred</code>
</a> -
A Deferred Promise resolution.
</p>
</li>
<li class="listitem">
<p>
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.FailedPromisesException" title="705.11.3 public class FailedPromisesException extends RuntimeException">
<code xmlns="http://www.w3.org/1999/xhtml" class="code">FailedPromisesException</code>
</a> -
Promise failure exception for a collection of failed Promises.
</p>
</li>
<li class="listitem">
<p>
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Failure" title="705.11.4 public interface Failure">
<code xmlns="http://www.w3.org/1999/xhtml" class="code">Failure</code>
</a> -
Failure callback for a Promise.
</p>
</li>
<li class="listitem">
<p>
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise" title="705.11.5 public interface Promise<T>">
<code xmlns="http://www.w3.org/1999/xhtml" class="code">Promise</code>
</a> -
A Promise of a value.
</p>
</li>
<li class="listitem">
<p>
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.PromiseFactory" title="705.11.6 public class PromiseFactory">
<code xmlns="http://www.w3.org/1999/xhtml" class="code">PromiseFactory</code>
</a> -
Promise factory to create Deferred and Promise objects.
</p>
</li>
<li class="listitem">
<p>
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.PromiseFactory.Option" title="705.11.7 enum PromiseFactory.Option">
<code xmlns="http://www.w3.org/1999/xhtml" class="code">PromiseFactory.Option</code>
</a> -
Defines the options for a Promise factory.
</p>
</li>
<li class="listitem">
<p>
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promises" title="705.11.8 public class Promises">
<code xmlns="http://www.w3.org/1999/xhtml" class="code">Promises</code>
</a> -
Static helper methods for <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise" title="705.11.5 public interface Promise<T>">Promise</a>s.
</p>
</li>
<li class="listitem">
<p>
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Success" title="705.11.9 public interface Success<T, R>">
<code xmlns="http://www.w3.org/1999/xhtml" class="code">Success</code>
</a> -
Success callback for a Promise.
</p>
</li>
<li class="listitem">
<p>
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.TimeoutException" title="705.11.10 public class TimeoutException extends Exception">
<code xmlns="http://www.w3.org/1999/xhtml" class="code">TimeoutException</code>
</a> -
Timeout exception for a Promise.
</p>
</li>
</ul>
</div>
</div>
<div class="section class">
<div xmlns="" class="titlepage">
<div>
<div>
<h3 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="org.osgi.util.promise.Deferred"></a><span xmlns="" class="number">705.11.2</span> public class Deferred<T>
</h3>
</div>
</div>
</div>
<p xmlns="" class="parameter"><label><T></label><span>The value type associated with the created Promise.</span></p>
<p>
A Deferred Promise resolution.
</p>
<p>
Instances of this class can be used to create a <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise" title="705.11.5 public interface Promise<T>">Promise</a> that can be
resolved in the future. The <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Deferred.getPromise--" title="705.11.2.3 public Promise<T> getPromise()">associated</a> Promise can be
successfully resolved with <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Deferred.resolve-T-" title="705.11.2.4 public void resolve(T value)">resolve(Object)</a> or resolved with a
failure with <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Deferred.fail-Throwable-" title="705.11.2.2 public void fail(Throwable failure)">fail(Throwable)</a>. It can also be resolved with the
resolution of another promise using <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Deferred.resolveWith-Promise-" title="705.11.2.5 public Promise<Void> resolveWith(Promise<? extends T> with)">resolveWith(Promise)</a>.
</p>
<p>
The associated Promise can be provided to any one, but the Deferred object
should be made available only to the party that will responsible for
resolving the Promise.
</p>
<p xmlns="" class="parameter"><label>Concurrency</label><span>Immutable </span></p>
<p xmlns="" class="parameter"><label>Provider Type</label><span>Consumers of this API must not implement this type</span></p>
<div class="section method">
<div xmlns="" class="titlepage">
<div>
<div>
<h4 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="org.osgi.util.promise.Deferred.Deferred--"></a><span xmlns="" class="number">705.11.2.1</span> public Deferred()
</h4>
</div>
</div>
</div>
<p xmlns="" class="description"><label>□</label><span>
Create a new Deferred.
</span></p>
<p>
The <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Deferred.getPromise--" title="705.11.2.3 public Promise<T> getPromise()">associated promise</a> will use the default
callback executor and default scheduled executor.
</p>
<p xmlns="" class="parameter"><label>See Also</label><span>
<a class="link" href="util.promise.html#org.osgi.util.promise.PromiseFactory.deferred--" title="705.11.6.5 public Deferred<T> deferred()">PromiseFactory.deferred()</a>
</span></p>
</div>
<div class="section method">
<div xmlns="" class="titlepage">
<div>
<div>
<h4 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="org.osgi.util.promise.Deferred.fail-Throwable-"></a><span xmlns="" class="number">705.11.2.2</span> public void fail(Throwable failure)
</h4>
</div>
</div>
</div>
<p xmlns="" class="parameter"><label>failure</label><span>The failure of the resolved Promise. Must not be
<code xmlns="http://www.w3.org/1999/xhtml" class="code">null</code>.</span></p>
<p xmlns="" class="description"><label>□</label><span>
Fail the Promise associated with this Deferred.
</span></p>
<p>
After the associated Promise is resolved with the specified failure, all
registered <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise.onResolve-Runnable-" title="705.11.5.12 public Promise<T> onResolve(Runnable callback)">callbacks</a> are called and
any <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise.then-Success-Failure-" title="705.11.5.18 public Promise<R> then(Success<? super T, ? extends R> success, Failure failure)">chained</a> Promises are resolved.
This may occur asynchronously to this method.
</p>
<p>
Resolving the associated Promise <span class="emphasis"><em>happens-before</em></span> any registered
callback is called. That is, in a registered callback,
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise.isDone--" title="705.11.5.8 public boolean isDone()">Promise.isDone()</a> must return <code class="code">true</code> and
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise.getValue--" title="705.11.5.7 public T getValue() throws InvocationTargetException, InterruptedException">Promise.getValue()</a> and <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise.getFailure--" title="705.11.5.6 public Throwable getFailure() throws InterruptedException">Promise.getFailure()</a> must not
block.
</p>
<p xmlns="" class="parameter"><label>Throws</label><span>
<code xmlns="http://www.w3.org/1999/xhtml" class="code">IllegalStateException</code>– If the associated Promise was already
resolved.</span></p>
</div>
<div class="section method">
<div xmlns="" class="titlepage">
<div>
<div>
<h4 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="org.osgi.util.promise.Deferred.getPromise--"></a><span xmlns="" class="number">705.11.2.3</span> public Promise<T> getPromise()
</h4>
</div>
</div>
</div>
<p xmlns="" class="description"><label>□</label><span>
Returns the Promise associated with this Deferred.
</span></p>
<p>
All Promise objects created by the associated Promise will use the
executors of the associated Promise.
</p>
<p xmlns="" class="parameter"><label>Returns</label><span>The Promise associated with this Deferred.</span></p>
</div>
<div class="section method">
<div xmlns="" class="titlepage">
<div>
<div>
<h4 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="org.osgi.util.promise.Deferred.resolve-T-"></a><span xmlns="" class="number">705.11.2.4</span> public void resolve(T value)
</h4>
</div>
</div>
</div>
<p xmlns="" class="parameter"><label>value</label><span>The value of the resolved Promise.</span></p>
<p xmlns="" class="description"><label>□</label><span>
Successfully resolve the Promise associated with this Deferred.
</span></p>
<p>
After the associated Promise is resolved with the specified value, all
registered <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise.onResolve-Runnable-" title="705.11.5.12 public Promise<T> onResolve(Runnable callback)">callbacks</a> are called and
any <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise.then-Success-Failure-" title="705.11.5.18 public Promise<R> then(Success<? super T, ? extends R> success, Failure failure)">chained</a> Promises are resolved.
This may occur asynchronously to this method.
</p>
<p>
Resolving the associated Promise <span class="emphasis"><em>happens-before</em></span> any registered
callback is called. That is, in a registered callback,
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise.isDone--" title="705.11.5.8 public boolean isDone()">Promise.isDone()</a> must return <code class="code">true</code> and
<a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise.getValue--" title="705.11.5.7 public T getValue() throws InvocationTargetException, InterruptedException">Promise.getValue()</a> and <a xmlns="" class="link" href="util.promise.html#org.osgi.util.promise.Promise.getFailure--" title="705.11.5.6 public Throwable getFailure() throws InterruptedException">Promise.getFailure()</a> must not
block.
</p>
<p xmlns="" class="parameter"><label>Throws</label><span>
<code xmlns="http://www.w3.org/1999/xhtml" class="code">IllegalStateException</code>– If the associated Promise was already
resolved.</span></p>
</div>
<div class="section method">
<div xmlns="" class="titlepage">
<div>
<div>
<h4 xmlns="http://www.w3.org/1999/xhtml" class="title"><a xmlns="" class="anchor" id="org.osgi.util.promise.Deferred.resolveWith-Promise-"></a><span xmlns="" class="number">705.11.2.5</span> public Promise<Void> resolveWith(Promise<? extends T> with)
</h4>
</div>