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1. Overview

Java 9 comes with a rich feature set. Although there are no new language concepts, new APIs and diagnostic commands will definitely be interesting to developers.

In this writeup we’re going to have quick, high level look at some of the new features; a full list of new features is available here.

2. Modular System – Jigsaw Project

Let’s start with the big one – bringing modularity into the Java platform.

A modular system provides capabilities similar to OSGi framework’s system. Modules have a concept of dependencies, can export a public API and keep implementation details hidden/private.

One of the main motivations here is to provide modular JVM, which can run on devices with a lot less available memory. The JVM could run with only those modules and APIs which are required by the application. Check out this link for a description of what these modules are.

Also, JVM internal (implementation) APIs like com.sun.* are no longer accessible from application code.

Simply put, the modules are going to be described in a file called located in the top of java code hierarchy:

module {
    requires com.baeldung.java9.modules.engines;

Our module car requires module engine to run and exports a package for handling.

For more in-depth example check OpenJDK Project Jigsaw: Module System Quick-Start Guide.

3. A New HTTP Client

A long-awaited replacement of the old HttpURLConnection.

The new API is located under the package.

It should support both HTTP/2 protocol and WebSocket handshake, with performance that should be comparable with the Apache HttpClient, Netty and Jetty.

Let have a look at this new functionality by creating and sending a simple HTTP request.

Update: The HTTP Client JEP is being moved to Incubator module, so it is no longer available in the package and instead is available under jdk.incubator.http.

3.1. Quick GET Request

The API uses the Builder pattern, which makes it really easy for quick use:

HttpRequest request = HttpRequest.newBuilder()
  .uri(new URI(""))

HttpResponse<String> response = HttpClient.newHttpClient()
  .send(request, HttpResponse.BodyHandler.asString());

4. Process API

The process API has been improved for controlling and managing operating-system processes.

4.1. Process Information

The class java.lang.ProcessHandle contains most of the new functionalities:

ProcessHandle self = ProcessHandle.current();
long PID = self.getPid();
ProcessHandle.Info procInfo =;
Optional<String[]> args = procInfo.arguments();
Optional<String> cmd =  procInfo.commandLine();
Optional<Instant> startTime = procInfo.startInstant();
Optional<Duration> cpuUsage = procInfo.totalCpuDuration();

The current method returns an object representing a process of currently running JVM. The Info subclass provides details about the process.

4.2. Destroying Processes

Now – let’s stop all the running child processes using destroy():

childProc = ProcessHandle.current().children();
childProc.forEach(procHandle -> {
    assertTrue("Could not kill process " + procHandle.getPid(), procHandle.destroy());

5. Small Language Modifications

5.1. Try-With-Resources

In Java 7, the try-with-resources syntax requires a fresh variable to be declared for each resource being managed by the statement.

In Java 9 there is an additional refinement: if the resource is referenced by a final or effectively final variable, a try-with-resources statement can manage a resource without a new variable being declared:

MyAutoCloseable mac = new MyAutoCloseable();
try (mac) {
    // do some stuff with mac
try (new MyAutoCloseable() { }.finalWrapper.finalCloseable) {
   // do some stuff with finalCloseable
} catch (Exception ex) { }

5.2. Diamond Operator Extension

Now we can use diamond operator in conjunction with anonymous inner classes:

FooClass<Integer> fc = new FooClass<>(1) { // anonymous inner class
FooClass<? extends Integer> fc0 = new FooClass<>(1) { 
    // anonymous inner class
FooClass<?> fc1 = new FooClass<>(1) { // anonymous inner class

5.3. Interface Private Method

Interfaces in the upcoming JVM version can have private methods, which can be used to split lengthy default methods:

interface InterfaceWithPrivateMethods {
    private static String staticPrivate() {
        return "static private";
    private String instancePrivate() {
        return "instance private";
    default void check() {
        String result = staticPrivate();
        InterfaceWithPrivateMethods pvt = new InterfaceWithPrivateMethods() {
            // anonymous class
        result = pvt.instancePrivate();

6. JShell Command Line Tool

JShell is read–eval–print loop – REPL for short.

Simply put, it’s an interactive tool to evaluate declarations, statements, and expressions of Java, together with an API. It is very convenient for testing small code snippets, which otherwise require creating a new class with the main method.

The jshell executable itself can be found in <JAVA_HOME>/bin folder:

|  Welcome to JShell -- Version 9
|  For an introduction type: /help intro
jshell> "This is my long string. I want a part of it".substring(8,19);
$5 ==> "my long string"

The interactive shell comes with history and auto-completion; it also provides functionality like saving to and loading from files, all or some of the written statements:

jshell> /save c:\develop\JShell_hello_world.txt
jshell> /open c:\develop\JShell_hello_world.txt
Hello JShell!

Code snippets are executed upon file loading.

7. JCMD Sub-Commands

Let’s explore some of the new subcommands in jcmd command line utility. We will get a list of all classes loaded in the JVM and their inheritance structure.

In the example below we can see the hierarchy of java.lang.Socket loaded in JVM running Eclipse Neon:

jdk-9\bin>jcmd 14056 VM.class_hierarchy -i -s
|  implements (declared intf)
|  implements java.lang.AutoCloseable/null (inherited intf)
|  |--org.eclipse.ecf.internal.provider.filetransfer.httpclient4.CloseMonitoringSocket
|  |  implements java.lang.AutoCloseable/null (inherited intf)
|  |  implements (inherited intf)
|  |
|  |  implements java.lang.AutoCloseable/null (inherited intf)
|  |  implements (inherited intf)

The first parameter of jcmd command is the process id (PID) of the JVM on which we want to run the command.

Another interesting subcommand is set_vmflag. We can modify some JVM parameters online, without the need of restarting the JVM process and modifying its startup parameters.

You can find out all the available VM flags with subcommand jcmd 14056 VM.flags -all

8. Мulti-Resolution Image API

The interface java.awt.image.MultiResolutionImage encapsulates a set of images with different resolutions into a single object. We can retrieve a resolution-specific image variant based on a given DPI metric and set of image transformations or retrieve all of the variants in the image.

The java.awt.Graphics class gets variant from a multi-resolution image based on the current display DPI metric and any applied transformations.

The class java.awt.image.BaseMultiResolutionImage provides basic implementation:

BufferedImage[] resolutionVariants = ....
MultiResolutionImage bmrImage
  = new BaseMultiResolutionImage(baseIndex, resolutionVariants);
Image testRVImage = bmrImage.getResolutionVariant(16, 16);
assertSame("Images should be the same", testRVImage, resolutionVariants[3]);

9. Variable Handles

The API resides under java.lang.invoke and consists of VarHandle and MethodHandles. It provides equivalents of java.util.concurrent.atomic and sun.misc.Unsafe operations upon object fields and array elements with similar performance.

With Java 9 Modular system access to sun.misc.Unsafe will not be possible from application code.

10. Publish-Subscribe Framework

The class java.util.concurrent.Flow provides interfaces that support the Reactive Streams publish-subscribe framework. These interfaces support interoperability across a number of asynchronous systems running on JVMs.

We can use utility class SubmissionPublisher to create custom components.

11. Unified JVM Logging

This feature introduces a common logging system for all components of the JVM. It provides the infrastructure to do the logging, but it does not add the actual logging calls from all JVM components. It also does not add logging to Java code in the JDK.

The logging framework defines a set of tags – for example, gc, compiler, threads, etc. We can use the command line parameter -Xlog to turn on logging during startup.

Let’s log messages tagged with ‘gc’ tag using ‘debug’ level to a file called ‘gc.txt’ with no decorations:

java -Xlog:gc=debug:file=gc.txt:none ...

-Xlog:help will output possible options and examples. Logging configuration can be modified runtime using jcmd command. We are going to set GC logs to info and redirect them to a file – gc_logs:

jcmd 9615 VM.log output=gc_logs what=gc

12. New APIs

12.1. Immutable Set

java.util.Set.of() – creates an immutable set of a given elements. In Java 8 creating a Set of several elements would require several lines of code. Now we can do it as simple as:

Set<String> strKeySet = Set.of("key1", "key2", "key3");

The Set returned by this method is JVM internal class: java.util.ImmutableCollections.SetN, which extends public java.util.AbstractSet. It is immutable – if we try to add or remove elements, an UnsupportedOperationException will be thrown.

You can also convert an entire array into a Set with the same method.

12.2. Optional to Stream gives us an easy way to you use the power of Streams on Optional elements:

List<String> filteredList =

13. Conclusion

Java 9 will come with a modular JVM and lots of other new and diverse improvements and features.

You can find the source code for the examples over on GitHub.

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