Java 9 - Trivias

This article summarises a couple of Java 9 features we did not talk about in our previous articles. As most of them are really rather small, we will go quickly through them - section by section.

Let’s start with enhancements to Project Coin, initially introduced in Java 7.

Project Coin Enhancements

Project Coin introduced rather small language changes in JDK/SE 7. Java 9 addresses some rough edges of those changes:


The try-with-resources language construct, introduced with Java 7, gains another extension in Java 7. You do not have to declare resources within the try instead you can use an effectively-final variable from within the try expression: myCloseable = // ...

try (myCloseable) {
    // ... do stuff

You can do that for multiple resources too: myCloseable = // ... anotherCloseable = // ...

try (myCloseable, anotherCloseable) {
    // ... do even more stuff

Private methods in interfaces

Private methods are now allowed in interfaces to share common functionality between either static or default method implementations in the interface:

public interface SomeInterface {

    default String saySomething(String text) {
        return buildString(text);

    default String shoutSomething(String text) {
        return buildString(text);

    // private method is allowed since Java 9:
    private String buildString(String text)  {
        StringBuilder stringBuilder = new StringBuilder();
        stringBuilder.append("This is your text: ");
        return stringBuilder.toString();

The following features are independent of Project Coin changes addressing other areas in JDK/SE.

Flow API

Java 9 comes with a nice API addition supporting the implementation of flow-controlled components: the Flow API, found in package java.util.concurrent.

The new interfaces from the Flow API implement the reactive streams specification. The following interfaces come with Java 9:

So those classes are basically interfaces only. In JDK 9 itself, concrete interface implementations can be found in the java.util.concurrent package and the jdk.incubator.http package. For the new Java 9 HTTP client, there is for example a HttpResponse.BodyProcessor interface which allows for the subscriber to work on the incoming byte-stream.

HTTP 2 Client

As mentioned in the last section, Java 9 comes with a new HTTP 2 client found in package jdk.incubator.httpclient in module jdk.incubator.httpclient. The classes therein define high-level HTTP and WebSocket APIs.

The HttpClient.Builder can be used to create an instance of HttpClient. Those instances are immutable and can be used for multiple HttpRequests. Besides synchronous sending with send, HttpClient also has an asynchronous sendAsync method returning a CompletableFuture<HttpResponse>.

JavaDoc Search is a nice new addition to the generated JavaDoc documentation files. It adds a search box in the top right corner for the user to lookup program elements or tagged words/phrases within the documentation. This is all done with client-side JavaScript. When you navigate to official the Java 9 JavaDoc documentation, you can have a look at the search box and play with it.

Besides providing a new search box, JavaDoc has also been enhanced to generate a nicer HTML 5 markup.

Enhanced Depreciation

Enhanced Depreciation is another small feature, being quite useful though. It is about extending the @Deprecated annotation with new attributes for providing better and more detailed information about the status and the intended disposition of APIs.

@Deprecated(since = "9") allows for specifying the version in which the annotated element became deprecated. The given version string should be in the same format as the version used in the JavaDoc @since tag. @Deprecated(forRemoval = true) indicates whether the annotated element is subject to removal in a future version.

Several Java SE APIs have been refined with more detailed @Deprecated annotations.


Class java.lang.ProcessHandle (JavaDoc) has been introduced in Java 9 to get more information about the currently running JVM process and its children. It provides methods for querying the operating system process that the JVM is currently running in.

For example, querying the current process ID (PID) is now as easy as


ProcessHandle::inf() returns a ProcessHandle.Info instance containing various meta-data about the current process, like the command, command-line arguments, process user, etc.

UTF-8 Property Files

The default encoding for property files changes from ISO-8859-1 to UTF-8 in Java 9. Applications therefore no longer need to convert characters whose code points are over U+00FF to escaped characters. If, for some reason, this change needs to be overridden, there is a new system property java.util.PropertyResourceBundle.encoding which allows for that.

jlink is a new tool distributed with the JDK that can assemble and optimise the set of modules into a custom run-time image. We will cover jlink in a separate blog post, for now you can gather more information here or here.


This article summarises Java 9 features not being covered in our previous articles. Java 9 comes with a couple of nice though smaller features, enhancing Project Coin or adding new Flow reactive-stream APIs. This article gives an overview about these features.