Jul 3, 2017
Nov 18, 2018
One Hill Technologies, LLC (onehilltech)
Source code



Promise library for JVM and Android

  • Implements the Promises/A+ specification from JavaScript
  • Built using Java 1.7, but designed for Java 1.8
  • Supports Java Lambda expressions
  • Android module allows resolve/reject callbacks to run on UI thread


This library requires Java 1.8


buildscript {
  repositories {
    maven { url "" }

dependencies {
  // for JVM-only projects
  compile 'com.onehilltech.promises:promises-jvm:x.y.z'
  // for Android projects (includes JVM automatically)
  implementation 'com.onehilltech.promises:promises-android:x.y.z'

Quick Start

The simplest promise to create is one that is already resolved or rejected using Promise.resolve or Promise.reject, respectively.

// resolved promise
Promise.resolve (5);

// rejected promise
Promise.reject (new IllegalStateException ("This is a rejected promise"));

You can also create a promise that is settled in the background. This is good when you need to perform some workload in the background, and notify the caller (or client) when the workload is resolved or rejected.

Promise <Foo> p = new Promise < > (settlement -> {
  // settlement.resolve (foo);
  // settlement.reject (ex);

In this case, you must either invoke settlement.resolve with the resolved value, or settlement.reject with an exception. Any uncaught exceptions will automatically reject the promise with the uncaught exception.

All promises are executed (or settled) when they are first created. To process a promise's settlement, use either then or _catch. It does not matter when you call then or _catch. If the promise is not settled, then the appropriate handler will be called after the promise is settled. If the promise is settled, then the appropriate handler will be called as soon as possible.

Important. All handlers are executed on a separate thread.

Promise.resolve (5)
       .then (n -> {
         // n == 5
         System.out.println ("Resolved value: " + n);
         return null;

Promise.reject (new IllegalStateException ("This is a rejected promise"))
       ._catch (reason -> {
         // reason instanceof IllegalStateException
         reason.printStackTrace ();
         return null;

You may notice that the handlers return null in the example above. This is because the handler has the option of returning a value or a Promise that to used to resolve the value for the next handler in the chain. If the handler does not a value or a Promise, then null is passed to the next handler.

Promise.resolve (5)
       .then (n -> {
         // n == 5
         System.out.println ("Resolved value: " + n);
         return value (10);
       .then (n -> {
         // n == 10
         System.out.println ("Resolved value: " + n);
         return null;

Not all handlers will return a value or Promise. If you are in this situation, then you can use the ResolveNoReturn and RejectNoReturn helper classes, or resolved and rejected helper methods.

import static com.onehilltech.promises.Promise.resolved;
import static com.onehilltech.promises.Promise.rejected;

// ...

Promise.resolve (5)
       .then (resolved (n -> System.out.println ("Resolved value: " + n)))        // n == 5
       ._catch (rejected (reason -> reason.printStackTrace ()));

Chaining Promises

Promises can be chained to create a series of background workloads to be completed in step order. Just use then to chain a series of background workloads, and _catch to handle any rejection from the preceding promises.

Promise.resolve (5)
       .then (resolved (n -> System.out.println ("Resolved value: " + n)))          // n == 5
       .then (resolved (value -> System.out.println ("Resolved value: " + value)))  // value == null
       ._catch (rejected (reason -> { }))
       .then (this::doSomethingElse)
       ._catch (Promise.ignoreReason);

In the example above, we must point our several things. First, execution continues after the first _catch if any of the preceding promises is rejected. If none of the promises are rejected, then the first _catch is skipped. Second, we are using Java method references (i.e., this::doSomethingElse), which improves the readability of the code, and reduces verbosity. Lastly, Promise.ignoreReason is a special handler that will catch the rejection and ignore the reason. This way, you do not have to write a bunch of empty handlers like the first _catch.


JavaScript introduced a concept called async/await for serial execution of promises. The main idea the caller of await blocks until the promises is settled. The advantage of this approach is it makes promise code more readable because it removes promise chaining. We provide a similar with this library, and it is simple to use. Below is example code that uses the await function to serialize promise execution.

import static com.onehilltech.promises.Promise.await;

// ....

  int result = await (Promise.resolve (5));
catch (Exception e)

The await function takes a Promise. The caller of the await function will block until the promise is settled (i.e., resolved or rejected). If the promise is resolved, it will return the resolved value. If the promise is rejected, it will throw the reason for rejection.


The library implements Promise.all, which is resolved if all promises are resolved and rejected if any of the promises is rejected.


The library implements Promise.race, which is settled when the first promise is either resolved or rejected.

Android Support

Running on the UI Thread

All promises are settled on a background thread, and the handlers are called on a background thread. If you attempt to update the UI in the handler, then the Android framework will throw an exception. This is because you are updating the UI on a different thread than the one that create the UI elements (i.e., the main thread). To address the need for updating the UI in the handler methods, the Android module provides onUiThread helper methods for running a handler on the UI thread.

import static com.onehilltech.promises.Promise.resolved;
import static com.onehilltech.promises.Promise.rejected;
import static com.onehilltech.promises.RejectedOnUIThread.onUiThread;
import static com.onehilltech.promises.ResolvedOnUIThread.onUiThread;

// ...

Promise.resolve ("Hello, World!")
       .then (onUiThread (resolved (str -> this.label.setText (str))))
       ._catch (onUiThread (rejected (reason -> reason.printStackTrace ())));

Happy Coding!