Although the flagship feature of Java 9 is Modularity, a large number of other enhancements are planned for this release. This article will provide an overview of those features that are scheduled for Java 9 release but are not as famous and glorious as the Jigsaw.

Reactive Streams

Reactive Streams is a contract for asynchronous stream processing with non-blocking back pressure. Publisher and Subscriber are two key concepts in the Reactive Streams specification. Publisher is a producer of items (and related control messages) that will be received by Subscribers, all these are done in a non-blocking way. Back pressure allows to control the amount of inflight data. That is, it will regulate the transfer between a slow publisher and a fast consumer and a fast publisher and a slow consumer.

Reactive Streams specification is comprised of four interfaces: Publisher, Subscriber, Subscription and Processor.

public interface Publisher<T> {
    void subscribe(Subscriber<? super T> subscriber);

public interface Subscriber<T> {
    void onSubscribe(Subscription subscription);
    void onNext(T item);
    void onError(Throwable throwable);
    void onComplete();

public interface Subscription {
    void request(long n);
    void cancel();

public interface Processor<T, R> extends Subscriber<T>, Publisher<R> {}

Publisher is a producer of items which Subscribers can subscribe to. Subscriber provides four callbacks: onNext will be called every time a new item is available. There are also two terminal signals, onComplete to signal the successful completion of the subscription and onError to signal an unrecoverable error encountered by a Publisher or Subscription. Subscription encapsulates the communication between Publisher and Subscriber: one can request zero or more elements from the publisher or even cancel the subscription altogether. Processor is a component that acts as both a Subscriber and Publisher.

Reactive Streams will be integrated in Java 9. In java.util.concurrent there is a Flow class that contain these four interfaces. Note that these are just the interfaces, Java 9 (or possibly any version of Java) won’t be shipped with an implementation for this specification. So in order to actually use a reactive API, at some point you should use an implementation. Current notable implementations of this specification on the JVM are Project Reactor (which will be integrated in Spring 5), Akka Streams and RxJava.

More Concurrency Updates

Suppose we have a CompletableFuture that going to fetch some movie recommendations from our fictional recommendation service. Now we want to add the capability of loading some static recommendations if the service couldn’t provide the expected result in a timely manner:

Supplier<List<Movie>> callTheRecommendationService = // call the slow service
        .completeOnTimeout(Collections.singletonList(fightClub), 1, TimeUnit.SECONDS)

Here if the recommendation service could provide the recommendations in less than 1 second, we’ll show those recommendations to the end user. Otherwise, we’ll make the poor user to watch Fight Club one more time. The new completeOnTimeout(T value, long timeout, TimeUnit unit) method completes the CompletableFuture with the given value if it’s not completed before the given timeout.

If you don’t want to provide the static recommendation, you can simply raise a TimeoutException with the orTimeout(long timeout, TimeUnit unit) method:

        .orTimeout(1, TimeUnit.SECONDS)

Java 9 also will be shipped with some other enhancements to the CompletableFuture API, which was introduced in Java 8. There was a completedFuture static factory method in Java 8 to create a new CompletableFuture that is already completed with the given value. If you want to create an already failed CompletableFuture, you can use the new failedFuture static factory method:

CompletableFuture.failedFuture(new IllegalArgumentException());

Also there will be completedStage and failedStage methods which will create new CompletionStages that is already completed or failed, respectively.

Convenience Factory Methods for Collections

The goal of JEP 269 is to:

Define library APIs to make it convenient to create instances of collections and maps with small numbers of elements, so as to ease the pain of not having collection literals in the Java programming language.

For example, in order to create a List with few Strings in it:

List<String> geeks = List.of("Fowler", "Beck", "Evans");

Or to create a Map from singer name to his/her band name:

Map<String, String> singerToBand = Map.of(
                "Amy Lee", "Evanescence",
                "Chris Martin", "Coldplay",
                "Jared Leto", "Thirty Seconds to Mars",
                "Matt Bellamy", "Muse"

Same code can be refactored as:

Map<String, String> singerToBand = Map.ofEntries(
                Map.entry("Amy Lee", "Evanescence"),
                Map.entry("Chris Martin", "Coldplay"),
                Map.entry("Jared Leto", "Thirty Seconds to Mars"),
                Map.entry("Matt Bellamy", "Muse")

The Java Shell

Java Shell or JShell is the Read Eval Print Loop (REPL) environment for Java. That is, an interactive tool to evaluate declarations, statements, and expressions of the Java programming language. To start experimenting with JShell, open up a terminal and just type jshell (assuming jshell is accessible from your PATH):

> cd $JAVA9_HOME/bin
> jshell
|  Welcome to JShell -- Version 9-ea
|  For an introduction type: /help intro


JShell is an interactive Java shell that accepts your Java code snippets and evaluates them in real time and tells you the result. So there is no need to create a public static void main to test an API, anymore!

For starters, let’s declare a variable:

jshell> String txt = "foo bar"
txt ==> "foo bar"

As you can see semicolons are optional. JShell supports Tab completion, just type a few chars and hit Tab and JShell tries its best to complete that name:

jshell> txt.sub
subSequence(   substring(

When you did choose a method name, you can hit the Shift and Tab to see all overloaded versions of the chosen method:

jshell> txt.substring(
String String.substring(int beginIndex)
String String.substring(int beginIndex, int endIndex)
<press shift-tab again to see javadoc>

Hit it one more time to see the Javadoc:

jshell> txt.substring(
String String.substring(int beginIndex)
Returns a string that is a substring of this string.The substring begins with
the character at the specified index and extends to the end of this string.
     "unhappy".substring(2) returns "happy"
     "Harbison".substring(3) returns "bison"
     "emptiness".substring(9) returns "" (an empty string)

beginIndex - the beginning index, inclusive.

the specified substring.
-- Press space for next javadoc, Q to quit. --

If you evaluate an expression and don’t assign it to a variable, it will be assigned to a Pseudo Variable, which starts with $:

jshell> txt.substring(0, 3)
$3 ==> "foo"

Here the $3 pseudo variable will hold the foo value, you can make sure of that fact by:

jshell> $3
$3 ==> "foo"

Here’s a List of three programming languages:

jshell> List<String> langs = List.of("Java", "JS", "Scala")
langs ==> [Java, JS, Scala]

Now we’re gonna just keep those languages that their name starts with the letter J:

jshell> -> n.startsWith("J")).collect(Collectors.toList())

|  Error:
|  cannot find symbol
|    symbol:   variable Collectors
| -> n.startsWith("J")).collect(Collectors.toList())
|                                                        ^--------^

Here JShell couldn’t find the Collectors symbol. By default JShell provides a set of common imports:

jshell> /imports
|    import java.util.*
|    import*
|    import java.math.*
|    import*
|    import java.util.concurrent
|    import java.util.prefs.*
|    import java.util.regex.*

Since Collectors isn’t part of those default imports, we should bring it to the scope:

jshell> import*
jshell> -> n.startsWith("J")).collect(Collectors.toList())
$11 ==> [Java, JS]

Anyway, JShell is all about exploratory programming with features to ease interaction, including: a history with editing, tab-completion, automatic addition of needed terminal semicolons, and configurable predefined imports and definitions.

New IO Features

Probably the most familiar code snippet for every single Java developer is the following:

InputStream is = ...
ByteArrayOutputStream buffer = new ByteArrayOutputStream();
int nRead;
byte[] data = new byte[16384];
while ((nRead =, 0, data.length)) != -1) {
  buffer.write(data, 0, nRead);

byte[] bytes = buffer.toByteArray();

In Java 9 there is a readAllBytes method for InputStream which reads all remaining bytes from the input stream and returns a byte array containing the bytes read from that input stream. Finally After 20 years we can simply write:

InputStream is = ...
byte[] bytes = is.readAllBytes();

There is also a readNBytes(byte[] b, int off, int len) method which reads the requested number of bytes from the input stream into the given byte array.

Do you ever want an easy way to write contents of a Java InputStream to an OutputStream? The newly added transferTo(OutputStream out) method will read all bytes from the input stream and writes the bytes to the given output stream in the order that they are read. So in order to write contents of a Java InputStream to an OutputStream, one can:

InputStream is = ...
OutputStream os = ...

New HTTP Client

The new HTTP Client will be shipped with Java 9 but as part of an Incubator module named jdk.incubator.httpclient. Incubator modules are a means of putting non-final APIs in the hands of developers while the APIs progress towards either finalization or removal in a future release.

The other new addition to Java 9 is the new HTTP client API which supports HTTP/2 and WebSocket. This client isn’t in the java.base module, so we should declare a dependency to the jdk.incubator.httpclient module in our

 module me.alidg {
     // Other declarations
     requires jdk.incubator.httpclient;

There is a HttpRequest to represent an HTTP request which can be sent to a server:

HttpRequest request = HttpRequest
                           .create(new URI(""))

By calling the response() method you can send the request and block until a response comes from the server encapsulated as a HttpResponse:

HttpResponse response = request.response();

For more details on this new API, you can checkout the Java 9: High level HTTP and WebSocket API post.

And More!

Stream and Optional, which first were introduced in Java 8, hasn’t undergone any significant changes but there will be some new and useful methods there. Stream would have dropWhile and takeWhile methods to drop or take only elements that match the given Predicate. New stream() method in Optional can be used to convert the Optional to a Stream of at most one element. Also, ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) method is useful for scenarios when you want to consume the optional value if it’s present or supply a empty action if it’s not.

LocalDate will have a datesUntil(LocalDate endExclusive) method which returns a sequential ordered stream of dates, starting from this date and goes to endExclusive (exclusive) by an incremental step of 1 day. For example, in order to count number of leap years between my birthday and current date:

LocalDate birthday = LocalDate.of(1989, Month.FEBRUARY, 11);
long leapYears = birthday
                    .map(d -> Year.of(d.getYear()))

For this particular scenario, using a 1 year step is more plausible (instead of default 1 day):

long leapYears = birthday
                    .datesUntil(, Period.ofYears(1))
                    .map(d -> Year.of(d.getYear()))

Sometimes in Java we had to write statements like:

String media = request.getHeader("Accept");
media = media != null ? media : "Default Value";

languages like groovy are providing an Elvis operator which is a shortening of the ternary operator:

def media = request.getHeader("Accept") ?: "Default Value";

Java doesn’t provide such a syntactic sugar but Objects utility class will have requireNonNullElse and requireNonNullElseGet methods to kinda (not as smooth and cool as Elvis operator) alleviates that problem:

import static java.util.Objects.requireNonNullElse;
// Omitted stuff
String media = requireNonNullElse(request.getHeader("Accept"), "Default Value");