The Java Shell

JDK9 is on its way. Meanwhile, the early access version is available for playing around and experimenting.

JDK had so far a huge monolithic nature which makes hard to tackle today’s challenges and demands, such as optimization of the software for a cloud, or running a java class on a hardware with small memory. Having this, it’s not a surprise that main talks and discussions are around the new major feature which comes with JDK9 - project Jigsaw. I’ll talk about Jigsaw later though and my today’s wander is the REPL for java - The Java Shell.

So far we couldn’t try java without having a proper class with a main() method. Moreover, in order to see java working, we had to compile and execute the class. Sometimes though, what we need is simply immediate feedback. Especially when learning/teaching or prototyping we are not searching for a fully working program, instead we want to get immediate feedback. The motivation behind the java shell (jshell from now on) is exactly this - to provide immediate feedback and save us from the whole bureaucracy required to get simple expression evaluated.

jshell is a REPL for Java. It continually reads the user input, evaluates it and prints the value of the input or a description of the state change the input caused. Java of course is not the first language which comes with it’s own REPL; Scala, Ruby, Clojure and many other programming languages have their REPLs already. If you are not familiar with the REPL concept, treat it as a command-line tool with typical features such as history with editing and tab-completion.

Before jumping into and trying jshell in practice, here is the necessary things to know

  • “snippets” are the code fragments which user inputs in the command-line
  • A snippet should correspond to one of the following from JLS
  • Package Declarations are not allowed; jshell code is placed under transient jshell package.
  • Access modifiers (public, protected, and private) and the modifiers final and static are not allowed in a top-level declarations, if provided they are ignored by warning.
  • The modifiers default and synchronized are not allowed at all in a top-level declarations, however they are allowed in a nested context.
  • abstract modifier is allowed only on classes.
  • When user input is incomplete (e.g. you type only Sytem.out and skip the println) jshell autocompletion API prompts for a more user input.
  • If the input is complete but there is no semicolon jshell will append it automatically.

jshell in practice

To try it we need to install JDK9 EA first (below example is for Ubuntu/Linux Mint environments).

// Add the repo
➜ sudo add-apt-repository ppa:webupd8team/java
➜ sudo apt-get update
// Install JDK9
➜ sudo apt-get install oracle-java9-installer
// Setup env variables automatically
➜ sudo apt-get install oracle-java9-set-default

Let’s make sure that we have proper setup

➜ java -version
java version "9-ea"
Java(TM) SE Runtime Environment (build 9-ea+134)
Java HotSpot(TM) 64-Bit Server VM (build 9-ea+134, mixed mode)

To enter the jshell, simply type jshell in the console

➜  jshell
|  Welcome to JShell -- Version 9-ea
|  For an introduction type: /help intro


As usual, let’s start by saying hello to the world!

jshell> System.out.println("Hello World")
Hello World

Note that I skipped the semicolon, and as promised jshell has autocompleted it and printed the result to the output. Let’s challenge autocompletion more;

jshell> System.out.
   ...> println("Hello World")
Hello World

As I skipped the println() part it prompted me for more input and only after adding the println("Hello World") I got the correct output. jshell is smart enough also to notice the errors.

jshell> System.out.hello
|  Error:
|  cannot find symbol
|    symbol:   variable hello
|  System.out.hello
|  ^--------------^

What about expressions?

jshell> int x = 9 % 2
x ==> 1

jshell> System.out.println(x)

As you can see we could use x variable and print it to the output. Last but not least, let’s try to write a traditional class with properties and methods.

jshell> class HelloWorld {
   ...>     private String name;
   ...>     HelloWorld(String _name) {
   ...> = _name;
   ...>     }
   ...>     String greet() {
   ...>        return "Hello " + name;
   ...>     }
   ...> }
|  created class HelloWorld

jshell> HelloWorld hw = new HelloWorld("human");
hw ==> HelloWorld@754ba872

jshell> hw.greet();
$11 ==> "Hello human"

I find jshell an effective tool for learning and for a quick feedback. Unfortunately there isn’t something like javap which we could use to analyze the generated bytecode for the input, as I think this would make jshell ideal for java explorers.

Let /help to show you more ;)

Written on October 9, 2016