Woohoo! Java 9 has a REPL! Getting Started with JShell and Eclipse January

With Java 9 just around the corner, we explore one of its most exciting new features – the Java 9 REPL (Read-Eval-Print Loop). This REPL is called JShell and it’s a great addition to the Java platform. Here’s why.

With JShell you can easily try out new features and quickly check the behaviour of a section of code. You don’t have to create a long-winded dummy main or JUnit test – simply type away.  To demonstrate the versatility of JShell, I am going to use it in conjunction with the Eclipse January package for data structures. Eclipse January is a set of libraries for handling numerical data in Java, think of it as a ‘numpy for Java’.

Install JShell

JShell is part of Java 9, currently available in an Early Access version from Oracle and other sources. Download and install Java 9 JDK from http://jdk.java.net/9/ or, if you have it available on your platform, you can install with your package manager (e.g. sudo apt-get install openjdk-9-jdk).

Start a terminal and run JShell:capture1

As you can see, JShell allows you to type normal Java statements, leave off semi-colons, run expressions, access expressions from previous outputs, and achieve many other short-cuts. (You can exit JShell with Ctrl-D.)

Using JShell with Eclipse January

To use Eclipse January, you need to:

1. Download January:

Get the January 2.0.2 jar ( or older version January 2.0.1 jar).

2. Download the dependency jars:

The January dependencies are available from Eclipse Orbit, they are:

3. Run JShell again, but add to the classpath all the jars you downloaded (remember to be the in the directory you downloaded the jars to):


"c:\Program Files\Java\jdk-9\bin\jshell.exe"  --class-path org.eclipse.january_2.0.2.v201706051401.jar;org.apache.commons.lang_2.6.0.v201404270220.jar;org.apache.commons.math3_3.5.0.v20160301-1110.jar;org.slf4j.api_1.7.10.v20170428-1633.jar;org.slf4j.binding.nop_1.7.10.v20160301-1109.jar


jshell --class-path org.eclipse.january_2.0.2.v201706051401.jar:org.apache.commons.lang_2.6.0.v201404270220.jar:org.apache.commons.math3_3.5.0.v20160301-1110.jar:org.slf4j.api_1.7.10.v20170428-1633.jar:org.slf4j.binding.nop_1.7.10.v20160301-1109.jar

Some notes:
Some version of jshell the command line argument is called -classpath instead of --class-path
If you are using git bash as your shell on Windows, add winpty before calling jshell and use colons to separate the path elements.


Then you can run through the different types of January commands. Note JShell supports completions using the ‘Tab’ key. Also use /! to rerun the last command.

Import classes

Start by importing the needed classes:

import org.eclipse.january.dataset.*

(No need for semi-colons and you can use the normally ill-advised * import)

Array Creation

Eclipse January supports straightforward creation of arrays. Let’s say we want to create a 2-dimensional array with the following data:

[1.0, 2.0, 3.0,
 4.0, 5.0, 6.0,
 7.0, 8.0, 9.0]

First we can create a new dataset:

Dataset dataset = DatasetFactory.createFromObject(new double[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 })

This gives us a 1-dimensional array with 9 elements, as shown below:

[1.0000000, 2.0000000, 3.0000000, 4.0000000, 5.0000000, 6.0000000, 7.0000000, 8.0000000, 9.0000000]

We then need to reshape it to be a 3×3 array:

dataset = dataset.reshape(3, 3)

The reshaped dataset:

 [[1.0000000, 2.0000000, 3.0000000],
 [4.0000000, 5.0000000, 6.0000000],
 [7.0000000, 8.0000000, 9.0000000]]

Or we can do it all in just one step:

Dataset another = DatasetFactory.createFromObject(new double[] { 1, 1, 2, 3, 5, 8, 13, 21, 34 }).reshape(3, 3)

Another dataset:

 [[1.0000000, 1.0000000, 2.0000000],
 [3.0000000, 5.0000000, 8.0000000],
 [13.000000, 21.000000, 34.000000]]

There are methods for obtaining the shape and number of dimensions of datasets


Which gives us:

jshell> dataset.getShape()
$8 ==> int[2] { 3, 3 }

jshell> dataset.getRank()
$9 ==> 2

Datasets also provide functionality for ranges and a random function that all allow easy creation of arrays:

Dataset dataset = DatasetFactory.createRange(15, Dataset.INT32).reshape(3, 5)

[[0, 1, 2, 3, 4],
 [5, 6, 7, 8, 9],
 [10, 11, 12, 13, 14]]

import org.eclipse.january.dataset.Random //specify Random class (see this is why star imports are normally bad)
Dataset another = Random.rand(new int[]{3,5})

[[0.27243843, 0.69695728, 0.20951172, 0.13238926, 0.82180144],
 [0.56326222, 0.94307839, 0.43225034, 0.69251040, 0.22602319],
 [0.79244049, 0.15865358, 0.64611131, 0.71647195, 0.043613393]]

Array Operations

The org.eclipse.january.dataset.Maths provides rich functionality for operating on the Dataset classes. For instance, here’s how you could add 2 Dataset arrays:

Dataset add = Maths.add(dataset, another)

Or you could do it as an inplace addition. The example below creates a new 3×3 array and then adds 100 to each element of the array.

Dataset inplace = DatasetFactory.createFromObject(new double[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 }).reshape(3, 3)

[[101.0000000, 102.0000000, 103.0000000],
 [104.0000000, 105.0000000, 106.0000000],
 [107.0000000, 108.0000000, 109.0000000]]


Datasets simplify extracting portions of the data, known as ‘slices’. For instance, given the array below, let’s say we want to extract the data 2, 3, 5 and 6.

[1, 2, 3,
 4, 5, 6,
 7, 8, 9]

This data resides in the first and second rows and the second and third columns. For slicing, indices for rows and columns are zero-based. A basic slice consists of a start and stop index, where the start index is inclusive and the stop index is exclusive. An optional increment may also be specified. So this example would be expressed as:

Dataset dataset = DatasetFactory.createFromObject(new double[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 }).reshape(3, 3)
Dataset slice = dataset.getSlice(new Slice(0, 2), new Slice(1, 3))

slice of dataset:

[[2.0000000, 3.0000000],
 [5.0000000, 6.0000000]]

Slicing and array manipulation functionality is particularly valuable when dealing with 3-dimensional or n-dimensional data.


For more on Eclipse January see the following examples and give them a go in JShell:

  • NumPy Examples shows how common NumPy constructs map to Eclipse Datasets.
  • Slicing Examples demonstrates slicing, including how to slice a small amount of data out of a dataset too large to fit in memory all at once.
  • Error Examples demonstrates applying an error to datasets.
  • Iteration Examples demonstrates a few ways to iterate through your datasets.
  • Lazy Examples demonstrates how to use datasets which are not entirely loaded in memory.

Eclipse January is a ‘numpy for Java’ but until now users have not really been able to play around with it in the same way you would numpy in Python.

JShell provides a great way to test drive libraries like Eclipse January. There are a couple of features that would be nice-to-have such as a magic variable for the last result (maybe $_ or $!) and maybe a shorter way to print a result (maybe /p :-). But overall, it is great to have and finally gives Java the REPL and ability to be used interactively that many have gotten so used to with other programming languages.

In fact we will be making good use of JShell for the Eclipse January workshop being held at EclipseCon France, see details and register here:  https://www.eclipsecon.org/france2017/session/eclipse-january


Launching with EASE

Launching with EASE - using auto completion
Launching with EASE – using auto completion

I have been doing some experiments with Eclipse EASE in preparation for determining the suitability – and creating a prototype – of using EASE with non-JVM scripting languages, in particular Python (CPython specifically).

To achieve this end goal I am working on a new module, /System/Launch, to allow me to explore that functionality. As a practical goal I am trying to use EASE to solve a long term problem of how to launch more complicated systems, especially those that run out of steam with the Launch Groups. For instance, multiple launches for debugging a multi-core system (especially with customizations and interdependencies) or parametrization of job launches.

Scripted Launch Group

As a starting point I want to simply re-create what Launch Groups can do in Javascript. So with EASE installed, including my new module, this is what a simple Launch Group replacement looks like.

The Launch Module

The Launch Module is a new module under consideration for EASE (Bug 478397). Its key method is the “launch” method which takes a Launch Configuration Name and an optional Launch Mode. The method then loads the named launch configuration and launches it, finally returning an ILaunch to allow future interaction with.

By using the launch method multiple times, with some additional control around it, allows complex launch sequences to be created.


For these examples I have already created three launch configurations for my individual tools (the name of the launch configuration is the quoted string in the bullet list):

  • “Prepare” – An External Tools configuration which prepares my test environment
  • “Server” – A PyDev supplied, Python Run configuration to launch my Python server
  • “Client” – A Java configuration to launch my Java client

Running The Examples

  1. Create three launch configurations named as above.
  2. Clone/Import “JavaScript Snippets” project. The examples are located in org.eclipse.ease.scripts git repository (or will be soon, follow Bug 478397 to find out more).
  3. Open the scripts in “Launch Module Examples” folder.
  4. Right-click on the desired script
  5. Choose Run As –> EASE Script

Alternatively paste the lines from the examples into the Rhino Script Shell console (as in the screenshot above). You get auto-completion of the names of the launch configurations and the launch modes too!

Example 1 – Fire and forget

In this first example, we simply launch the Client in Debug mode.


launch("Client", "debug")

Line 1: load the Launch module, this populates the namespace with all the methods defined in the Launch module.
Line 3: launch the existing launch configuration “Client” in debug mode.

Example 2 – Replicate Functionality from Launch Groups

In this example, we prepare our environment with the “Prepare” configuration, then launch the “Server” and “Client” configurations.


prepare = launch("Prepare")
while (!prepare.isTerminated()) {


launch("Client", "debug")

Line 1: load the Launch module
Line 3: launch the Prepare configuration
Line 4-6: Busy-wait until the Prepare launch has terminated[1]
Line 8: launch the server
Line 9: Wait 3 seconds for the server to be ready
Line 11: launch the client in Debug mode

Example 3 – Terminating the Server Automatically

Example 1 and 2 are a promising start, but do not yet add any new functionality to Eclipse. So what do you do if you want the server to stop automatically when you finish debugging your client. Well that is really easy now, just monitor the client launch and terminate the server.


prepare = launch("Program prepare")
while (!prepare.isTerminated()) {

server = launch("Python Server")

client = launch("Java Client", "debug")
while (!client.isTerminated()) {

Line 1-7: the same as Example 2
Line 8: launch the Server, but keep a handle of the ILaunch
Line 9: Wait 3 seconds for the server to be ready
Line 11: launch the Client
Line 12-14:Busy-wait until the Client debug session launch has terminated[1]
Line 15: terminate the server

This is a screenshot that shows what the Debug View looks like when we are busy-waiting on line 12.
At the top is the EASE Script launch of the example.
Then is the now terminated Prepare launch.
Followed by the still running Server in Run mode.
And finally, the Java Client, in Debug mode stopped at a breakpoint in main.

Terminating the Server Automatically – Debug View

More Advanced Options

With the full power of the scripting language you can take these examples to the next step. A good place to start would be to remove the 3 second delay on Line 9 and replace that with some logic that actually determines if the server is ready to accept connections.

Other Functionality in the Launch Module

The Launch module is very new and I invite additional contributions to it to make it more useful. For now this is a quick overview of what it does:

String[] getLaunchConfigurationNames()

Returns an array of all the Launch Configuration Names known to the Launch Manager. These names can be used as the argument to the getLaunchConfiguration, launch and launchUI methods.

ILaunchConfiguration[] getLaunchConfigurations()

Returns an array of all the Launch Configurations known to the Launch Manager. These can be used as the argument to launch and launchUI methods.

ILaunchConfiguration getLaunchConfiguration(String name)

Return the launch configuration given by name parameter. The launch configuration can be edited or otherwise operated on. See ILaunchConfiguration.getWorkingCopy().

ILaunch launch(String launchConfigurationName, String mode)
ILaunch launch(ILaunchConfiguration configuration, String mode)

Launch the configuration either given by name or a launch configuration and return the ILaunch for further processing. This is the way to launch a configuration within a script which is itself launched.

ILaunch launchUI(String launchConfigurationName, String mode)
ILaunch launchUI(ILaunchConfiguration configuration, String mode)

Launch the configuration in the UI thread. This method respects the workspace settings for things like building before launching.

ILaunchManager getLaunchManager()

Obtain the platform launch manager. This allows access to the Eclipse debug core launch manager, allowing control over all non-UI aspects of launches. The most valuable of these should be wrapped for ideal script usage and made available in the module itself.[2]


1. A better API than busy-waiting is probably desired here, but that is for another day (and more Javascript knowledge).
2. Additional UI functionality is within the DebugUITools class, enabling access to this class directly from within the launch module is an option. Additionally, a Debug module would be very useful.