Promise for Java 8

Write a clean and intuitive asynchronous code in Java without a pain, the same way as ECMAScript 6.0 does:

getJSON("http://github.com").
        then(System.out::println).
        catchVoid(Throwable::printStackTrace);
}

which is an asynchronous alternative to the following piece of blocking code:

try {
    json = getJSON("http://github.com");
    System.out.println(json);
} catch (Throwable e) {
    e.printStackTrace();
}

What is a Promise?

placeholder for a value (or an exception) returned from an asynchronous operation
internally a promise has three states: pending, resolved, rejected
after a promise is resolved or rejected its state and value can never be changed
asynchronous code can be written by chaining promises together
exception thrown by a promise is propagated through promise chains

Why would I use it?

because it is better suited for writing asynchronous code than Java 8's CompletableFuture
- API is much cleaner and easier to understand - just 2 instance method types (thenXXX, catchXXX) and 4 static factory methods (all, race, resolve, reject) solving most of the problems
- much better handling of exceptions
  - exceptions thrown are not wrapped
  - then/catch callbacks could throw checked exceptions
  - Promise's executor code could throw an exception which will be handled by the Promise (no need to manually try/catch)
  - therefore exceptions can be caught and processed similar way as they are normally handled in a blocking code
- Promise API enforces programmer to use an asynchronous programming, no way to run get() and block the current thread
- code written using Promises is a little bit less verbose and easier to understand
well tested (100% code covered by unit tests)

When it should not be used?

Promise is for one-shot operations, that is, you can execute some method and get a self-contained response (or error), i.e. get some REST resource
When you need to monitor progress of the execution or process a stream of events then use something like RxJava instead

Examples

public void loadFakeJSON() {
    getJSON("http://github.com").
            thenReturn(json -> json.get("someProperty")).
            then(System.out::println).
            catchVoid(Throwable::printStackTrace);
}

public void loadJSONSequentially() {
    getJSON("http://github.com").
            thenPromise(json -> getJSON(json.get("otherURL"))).
            then(System.out::println).
            catchVoid(Throwable::printStackTrace);
}

public void catchFallback() {
    getJSON("http://unreliable-url.com").
            catchReturn(json -> {
                Map<String, String> fallbackJSON = new HashMap<>();
                fallbackJSON.put("someProperty", "defaultValue");
                fallbackJSON.put("otherURL", "http://default-url.com");
                return fallbackJSON;
            }).
            then(System.out::println).
            catchVoid(Throwable::printStackTrace);
}

public void all() {
    Promise.all(
            getJSON("https://fake-url.com/resources/1"),
            getJSON("https://fake-url.com/resources/2"),
            getJSON("https://fake-url.com/resources/3")
    ).then(jsons -> {
        System.out.println(jsons[0]);
        System.out.println(jsons[1]);
        System.out.println(jsons[2]);
    });
}

public void race() {
    Promise.race(
            getJSON("https://fake-url.com/resources/1"),
            getJSON("https://fake-url.com/resources/2"),
            getJSON("https://fake-url.com/resources/3")
    ).then(System.out::println);
}

public void timers() {
    timeout(getJSON("http://github.com"), 100).
            then(System.out::println).
            catchVoid(Throwable::printStackTrace);

    delay(100).then(v -> getJSON("http://github.com"));
}

private Promise<Map<String, String>> getJSON(String url) {
    return new Promise<>(p -> {
        // execute HTTP request asynchronously here (some Netty based client etc.)
        Map<String, String> json = new HashMap<>();
        json.put("someProperty", "value");
        json.put("otherURL", "http://google.com");
        p.resolve(json);
    });
}

Source code: PromiseSamples.java

How to use with Maven

Add repository:

<repositories>
    <repository>
        <id>jacekolszak</id>
        <url>https://dl.bintray.com/jacekolszak/maven</url>
    </repository>
</repositories>

And use the artifact like this:

<dependency>
	    <groupId>com.github.jacekolszak</groupId>
	    <artifactId>promises</artifactId>
	    <version>1.0</version>
</dependency>

How to use with Gradle

Add repository to your build.gradle file:

repositories {
    maven {
        url  "https://dl.bintray.com/jacekolszak/maven" 
    }    
}