AWS Flow Framework for Java
Developer Guide (API Version 2012-01-25)

Passing Data to Asynchronous Methods

The use of Promise<T> has been explained in previous sections. Some advanced use cases of Promise<T> are discussed here.

Passing Collections and Maps to Asynchronous Methods

The framework supports passing arrays, collections, and maps as Promise types to asynchronous methods. For example, an asynchronous method may take Promise<ArrayList<String>> as an argument as shown in the following listing.

@Asynchronous public void printList(Promise<List<String>> list) { for (String s: list.get()) { activityClient.printActivity(s); } }

Semantically, this behaves as any other Promise typed parameter and the asynchronous method will wait until the collection becomes available before executing. If the members of a collection are Promise objects, then you can make the framework wait for all members to become ready as shown in the following snippet. This will make the asynchronous method wait on each member of the collection to become available.

@Asynchronous public void printList(@Wait List<Promise<String>> list) { for (Promise<String> s: list) { activityClient.printActivity(s); } }

Note that the @Wait annotation must be used on the parameter to indicate that it contains Promise objects.

Note also that the activity printActivity takes a String argument but the matching method in the generated client takes a Promise<String>. We are calling the method on the client and not invoking the activity method directly.


Settable<T> is a derived type of Promise<T> that provides a set method that allows you to manually set the value of a Promise. For example, the following workflow waits for a signal to be received by waiting on a Settable<?>, which is set in the signal method:

public class MyWorkflowImpl implements MyWorkflow{ final Settable<String> result = new Settable<String>(); //@Execute method @Override public Promise<String> start() { return done(result); } //Signal @Override public void manualProcessCompletedSignal(String data) { result.set(data); } @Asynchronous public Promise<String> done(Settable<String> result){ return result; } }

A Settable<?> can also be chained to another promise at a time. You can use AndPromise and OrPromise to group promises. You can unchain a chained Settable by calling the unchain() method on it. When chained, the Settable<?> automatically becomes ready when the promise that it is chained to becomes ready. Chaining is especially useful when you want to use a promise returned from within the scope of a doTry() in other parts of your program. Since TryCatchFinally is used as a nested class, you cannot declare a Promise<> in the parent's scope and set it in doTry(). This is because Java requires variables to be declared in parent scope and used in nested classes to be marked final. For example:

@Asynchronous public Promise<String> chain(final Promise<String> input) { final Settable<String> result = new Settable<String>(); new TryFinally() { @Override protected void doTry() throws Throwable { Promise<String> resultToChain = activity1(input); activity2(resultToChain); // Chain the promise to Settable result.chain(resultToChain); } @Override protected void doFinally() throws Throwable { if (result.isReady()) { // Was a result returned before the exception? // Do cleanup here } } }; return result; }

A Settable can be chained to one promise at a time. You can unchain a chained Settable by calling the unchain() method on it.


When you pass a Promise to an asynchronous method, by default, the framework will wait for the Promise(s) to become ready before executing the method (except for collection types). You may override this behavior by using the @NoWait annotation on parameters in the declaration of the asynchronous method. This is useful if you are passing in Settable<T>, which will be set by the asynchronous method itself.


Dependencies in asynchronous methods are implemented by passing the Promise returned by one method as an argument to another. However, there may be cases where you want to return void from a method, but still want other asynchronous methods to execute after its completion. In such cases, you can use Promise<Void> as the return type of the method. The Promise class provides a static Void method that you can use to create a Promise<Void> object. This Promise will become ready when the asynchronous method finishes execution. You can pass this Promise to another asynchronous method just like any other Promise object. If you are using Settable<Void>, then call the set method on it with null to make it ready.

AndPromise and OrPromise

AndPromise and OrPromise allow you to group multiple Promise<> objects into a single logical promise. An AndPromise becomes ready when all promises used to construct it become ready. An OrPromise becomes ready when any promise in the collection of promises used to construct it becomes ready. You can call getValues() on AndPromise and OrPromise to retrieve the list of values of the constituent promises.