In our previous article, we explored the world of ListenableFuture. Now, let's dive deeper into more advanced techniques, specifically focusing on transformations and chaining. Imagine we have a ListenableFuture obtained from an asynchronous service. Additionally, we have a simple method:
Document parse(String xml) {//...
Instead of working with the String itself, we need the Document. One approach would be to resolve the Future and process the String. However, a more elegant solution is to apply a transformation once the results are available, making our method behave as if it always returned ListenableFuture. This is remarkably straightforward:
final ListenableFuture future = //... final ListenableFuture documentFuture = Futures.transform(future, new Function() { @override public Document apply(String contents) { return parse(contents); } });
Or, for better readability:
final Function parseFun = new Function() { @override public Document apply(String contents) { return parse(contents); } }; final ListenableFuture future = //... final ListenableFuture documentFuture = Futures.transform(future, parseFun);
While Java syntax may have its limitations, let's focus on what we've achieved. Futures.transform() doesn't wait for the underlying ListenableFuture to apply the parse() transformation. Instead, it registers a callback, ready to be notified whenever the given future completes. This transformation is applied dynamically and transparently for us at the right moment. We still have a Future, but this time wrapping a Document.
Now, let's take it a step further. We also have an asynchronous, possibly long-running method that calculates the relevance (whatever that means in this context) of a given Document:
ListenableFuture calculateRelevance(Document pageContents) {//...
Can we somehow chain it with the ListenableFuture we already possess? First attempt:
final Function> relevanceFun = new Function>() { @override public ListenableFuture apply(Document input) { return calculateRelevance(input); } }; final ListenableFuture future = //... final ListenableFuture documentFuture = Futures.transform(future, parseFun); final ListenableFuture> relevanceFuture = Futures.transform(documentFuture, relevanceFun);
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