java-9-repl
Java 9 – Exploring the REPL
1. Introduction
This article is about jshell, an interactive REPL (Read-Evaluate-Print-Loop) console that is bundled with the JDK for the upcoming Java 9 release. For those not familiar with the concept, a REPL allows to interactively run arbitrary snippets of code and evaluate their results.
A REPL can be useful for things such as quickly checking the viability of an idea or figuring out e.g. a formatted string for String or SimpleDateFormat.
2. Running
$JAVA_HOME/bin/jshellIf more detailed messaging from the shell is desired, a -v flag can be used:
$JAVA_HOME/bin/jshell -vOnce it is ready, we will be greeted by a friendly message and a familiar Unix-style prompt at the bottom.
3. Defining and Invoking Methods
jshell> void helloWorld() { System.out.println("Hello world");}
| created method helloWorld()Here we defined the ubiquitous “hello world” method. It can be invoked using normal Java syntax:
jshell> helloWorld()
Hello world4. Variables
jshell> int i = 0;
i ==> 0
| created variable i : int
jshell> String company = "Baeldung"
company ==> "Baeldung"
| created variable company : String
jshell> Date date = new Date()
date ==> Sun Feb 26 06:30:16 EST 2017
| created variable date : DateNote that semicolons are optional. Variables can also be declared without initialization:
jshell> File file
file ==> null
| created variable file : File5. Expressions
Any valid Java expression is accepted and the result of the evaluation will be shown. If no explicit receiver of the result is provided, “scratch” variables will be created:
jshell> String.format("%d of bottles of beer", 100)
$6 ==> "100 of bottles of beer"
| created scratch variable $6 : StringThe REPL is quite helpful here by informing us that it created a scratch variable named $6 which value is “100 of bottles of beer on the wall” and its type is String.
Multi-line expressions are also possible. Jshell is smart enough to know when an expression is incomplete and will prompt the user to continue on a new line:
jshell> int i =
...> 5;
i ==> 5
| modified variable i : int
| update overwrote variable i : intNote how the prompt changed to an indented …> to signify the continuation of an expression.
6. Commands
Jshell provides quite a few meta-commands that aren’t related to evaluating Java statements. They all start with a forward-slash (/) to be distinguished from normal operations. For example, we can request a list of all available commands by issuing /help or /?.
Let’s take a look at some of them.
6.1. Imports
jshell> /import
| import java.io.*
| import java.math.*
| import java.net.*
| import java.nio.file.*
| import java.util.*
| import java.util.concurrent.*
| import java.util.function.*
| import java.util.prefs.*
| import java.util.regex.*
| import java.util.stream.*As we can see, the shell starts with quite a few useful imports already added.
6.2. Lists
Working in a REPL is not nearly as easy as having a full-featured IDE at our fingertips: it is easy to forget what variables have which values, what methods have been defined and so on. To check the state of the shell we can use /var, /methods, /list or /history:
jshell> /var
| int i = 0
| String company = "Baeldung"
| Date date = Sun Feb 26 06:30:16 EST 2017
| File file = null
| String $6 = "100 of bottles of beer on the wall"
jshell> /methods
| void helloWorld()
jshell> /list
1 : void helloWorld() { System.out.println("Hello world");}
2 : int i = 0;
3 : String company = "Baeldung";
4 : Date date = new Date();
5 : File file;
6 : String.format("%d of bottles of beer on the wall", 100)
jshell> /history
void helloWorld() { System.out.println("Hello world");}
int i = 0;
String company = "Baeldung"
Date date = new Date()
File file
String.format("%d of bottles of beer on the wall", 100)
/var
/methods
/list
/historyThe difference between /list and /history is that the latter shows commands in addition to expressions.
6.3. Saving
jshell> /save repl.javaThis saves our expression history into repl.java in the same directory from which we ran the jshell command.
6.4. Loading
jshell> /open repl.javaA loaded session can then be verified by issuing /var, /method or /list.
