2016-07-24 01:45:49 +05:30
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---
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layout: pattern
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title: Promise
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folder: promise
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permalink: /patterns/promise/
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2016-08-04 18:10:50 +05:30
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categories: Concurrency
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2016-07-24 01:45:49 +05:30
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tags:
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- Java
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2016-08-04 18:10:50 +05:30
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- Functional
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- Reactive
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2016-07-24 01:45:49 +05:30
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- Difficulty-Intermediate
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---
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## Also known as
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CompletableFuture
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## Intent
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A Promise represents a proxy for a value not necessarily known when the promise is created. It
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allows you to associate dependent promises to an asynchronous action's eventual success value or
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failure reason. Promises are a way to write async code that still appears as though it is executing
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in a synchronous way.
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## Applicability
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Promise pattern is applicable in concurrent programming when some work needs to be done asynchronously
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and:
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* code maintainablity and readability suffers due to callback hell.
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* you need to compose promises and need better error handling for asynchronous tasks.
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* you want to use functional style of programming.
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## Real world examples
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* [java.util.concurrent.CompletableFuture](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletableFuture.html)
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* [Guava ListenableFuture](https://github.com/google/guava/wiki/ListenableFutureExplained)
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## Related Patterns
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* Async Method Invocation
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* Callback
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## Credits
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* [You are missing the point to Promises](https://gist.github.com/domenic/3889970)
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* [Functional style callbacks using CompleteableFuture](https://www.infoq.com/articles/Functional-Style-Callbacks-Using-CompletableFuture)
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