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README.md
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README.md

JShell

Java REPL (Read-Eval-Print-Loop): previously called Kulla.

The JShell API can be used via applications as well.

Look for the the JShell REPL in the JAVA_HOME/bin folder:

$ ls $JAVA_HOME/bin/

appletviewer  java     javapackager  jdb        jinfo  jps         jstatd        policytool   serialver   wsimport
idlj          javac    javaws        jdeprscan  jjs    jrunscript  jweblauncher  rmic         servertool  xjc
jaotc         javadoc  jcmd          jdeps      jlink  [jshell]    keytool       rmid         tnameserv
jar           javah    jconsole      jhsdb      jmap   jstack      orbd          rmiregistry  unpack200
jarsigner     javap    jcontrol      jimage     jmod   jstat       pack200       schemagen    wsgen

And locate the command from the above list. Ensure JAVA_HOME/bin is in your PATH.

$ echo $PATH
/usr/lib/jvm/jdk-9/bin/:....

or 

$ echo %PATH%
c:\Program Files/jdk-9/bin\;....
    
$ jshell

And you get taken to the JShell prompt:

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

jshell>

The first command to try in the REPL:

jshell> /?

Let's assign a string to a variable and see its effect:

jshell> String name = "ICELAND-JUG"
name ==> "ICELAND-JUG"

Define a void method and execute it:

jshell> void hello() { System.out.println("Hi " + name); }
|  modified method hello()

jshell> hello()
Hi ICELAND-JUG

jshell> hello();
Hi ICELAND-JUG

As you can see there is no need to use a semi-colon at the end of single line commands in the REPL.

By default, the REPL loads with pre-defined imports:

jshell> /i
|    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.*

Now create your own variables, method, etc.. and play with them in the REPL.

Try using the /v or the /m REPL commands, here's a sample output:

jshell> /v
|    String name = "ICELAND-JUG"
|    int i = 10
|    int j = 15
|    int k = 25

jshell> /m
|    void hello()

Now try some more REPL commands, at the jshell prompt:

/vars 
/list
/methods 
/edit hello
/imports

As mentioned earlier, Java 9 provides a JShell API that we can access from within our application:

    JShell shell = JSheel.create();
    shell.eval("int x = 5;");
    Stream<VarSnippet> vars = shell.variables();

The above block is not for the REPL.

We can make http calls via the REPL, lets have a look at how that works. We can use Java 9's new API to create Http calls. Check the JavaDoc for HttpRequest.

Make 50 http GET calls to http://www.google.com via the REPL.

Finally, exit the REPL via one of the two ways:

jshell> /exit
|  Goodbye

Or

jshell> [Ctrl][D]     
(quietly exits)

Sample repositories:

Read all about the Java REPL at

Try some more JShell examples:

Some online Java REPLs to play with:

Credits