JShell: An Interactive Shell for the Java Platform

JShellNew Interactive Java Language Shell for JDK 9

Robert FieldJShell ArchitectCore Language and Tools GroupOracle CorporationNovember 6, 2015

Copyright © 2015, Oracle and/or its affiliates. All rights reserved. |

Copyright © 2015, Oracle and/or its affiliates. All rights reserved. | 3

Safe Harbor StatementThe following is intended to outline our general product direction. It is intended for information purposes only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code, or functionality, and should not be relied upon in making purchasing decisions. The development, release, and timing of any features or functionality described for Oracle’s products remains at the sole discretion of Oracle.


Agenda

What is JShell?

Why JShell?

Using JShell

Demo

Questions

1

2

3

4

5


Agenda

What is JShell?

Why JShell?

Using JShell

Demo

Questions

1

2

3

4

5


Survey

• Who has heard of JShell before JavaDay?


Survey

• Who has heard of JShell before JavaDay?• Who has used JShell?


Survey

• Who has heard of JShell before JavaDay?• Who has used JShell?• For whom is this new?


What is JShell?

• Tool providing a dynamic interaction with the Java™ programming language• Read-Evaluate-Print Loop (REPL) for the Java™ platform– Type in a snippet of Java code, see the results

• Deeply integrated with JDK tool-set– Stays current and compatible

• Also, an API for use within other applications


What is JShell NOT?

• Not a new language– “Snippets” of pure– No new syntax

• Not a replacement for the compiler• Not an IDE


Agenda

What is JShell?

Why JShell?

Using JShell

Demo

Questions

1

2

3

4

5


Who wants JShell?

• New to Java, new to programming– Start with expressions vs classes– Immediate feedback

• Exploring a new API or language feature– Experiment and instantly see results

• Prototyping– Incrementally write complex code


Without JShell: Edit-Compile-Execute

• Write a full program:– Class– Imports–main method

• Cycle the whole program to understand if the behavior is correct:– Edit– Compiler or IDE– Execute


% edit BitShift.javaclass BitShift { public static void main(String[] args) { System.out.println( -8 >> 2 ); System.out.println( -8 >>> 2 ); }}

% javac BitShift.java

% java BitShift -21073741822

% edit BitShift.java -- to print in octal so we can understand

... repeat ...

Without JShell: Edit-Compile-Execute example


With JShell • Type in a “snippet” of code• Immediately see its behavior


• -> -8 >> 2• | Expression value is: -2• | assigned to temporary variable $1 of type int

• -> -8 >>> 2• | Expression value is: 1073741822• | assigned to temporary variable $2 of type int

• -> printf("%o", -8 >>> 2)• 7777777776• ->

With JShell: example


Agenda

What is JShell?

Why JShell?

Using JShell

Demo

Questions

1

2

3

4

5


Using the JShell tool

• With JDK 9 installed, type “jshell” at the command line• The JShell tool takes two kinds of input:– “Snippets” of Java code – declaration and execution– JShell commands – information and control

• All input can be tab completed


-> String foo = "Howdy"| Added variable foo of type String with initial value "Howdy"

Snippets of Java code: variable


-> long fibonacci(long n) {>> if (n == 0) return 0;>> if (n == 1) return 1;>> return fibonacci(n - 1) + fibonacci(n - 2);>> }| Added method fibonacci(long)

Snippets of Java code: method


-> interface Defloat {>> int convert(float x);>> }| Added interface Defloat

Snippets of Java code: class, interface, enum, orannotation interface


-> fibonacci(10)| Expression value is: 55| assigned to temporary variable $4 of type long

Snippets of Java code: expression


-> if ($4 > 20) printf("The value is %s\n", $4)The value is 55

Snippets of Java code: statement


-> import javax.swing.*

• In fact, the only kind of Java code you cannot use is package

Snippets of Java code: import declaration


• -> Defloat lamb = d -> Math.round(d)• | Added variable lamb of type Defloat with ...

• -> lamb.convert(2.5)• | Error:• | incompatible types: possible lossy conversion from • | double to float• | lamb.convert(2.5)• | ^-^

• -> lamb.convert((float) 2.5)• | Expression value is: 3• | assigned to temporary variable $8 of type int

Examples


JShell Commands

• Commands are distinguished from snippets by leading slash: /• Commands – Show state– Configure display, environment, and start-up– Access history

• Have tab-completion and short-cuts


-> /help…/l or /list [all] -- list the source you have typed /seteditor <executable> -- set the external editor command to use/e or /edit <name or id> -- edit a source entry referenced by name or id/d or /drop <name or id> -- delete a source entry referenced by name or id/s or /save [all|history] <file> -- save the source you have typed/o or /open <file> -- open a file as source input/v or /vars -- list the declared variables and their values/m or /methods -- list the declared methods and their signatures/c or /classes -- list the declared classes/x or /exit -- exit the REPL/r or /reset -- reset everything in the REPL/f or /feedback <level> -- feedback information: off, concise, normal, verbose, …/p or /prompt -- toggle display of a prompt/cp or /classpath <path> -- add a path to the classpath/h or /history -- history of what you have typed /setstart <file> -- read file and set as the new start-up definitions /savestart <file> -- save the default start-up definitions to the file/? or /help -- this help message /! -- re-run last snippet /<n> -- re-run n-th snippet /-<n> -- re-run n-th previous snippet

JShell Command: /help


-> /list

1 : String foo = "Howdy"; 2 : long fibonacci(long n) { if (n == 0) return 0; if (n == 1) return 1; return fibonacci(n - 1) + fibonacci(n - 2); } 3 : interface Defloat { int convert(float x); } 4 : fibonacci(10) 5 : if ($4 > 20) printf("The value is %s\n", $4); 6 : import javax.swing.*; 7 : Defloat lamb = d -> Math.round(d); 8 : lamb.convert((float) 2.5)

JShell Command: /list


JShell Commands: /vars, /methods, /classes

• Get a list of variables with /vars-> /vars| String foo = "Howdy"| long $4 = 55| Defloat lamb = $Lambda$1/1170794006@5ba23b66| int $8 = 3

• For information about methods there is /methods• For information about classes, interfaces, enums, and annotations

interfaces there is /classes


-> /save mycode.jsh

-> /exit| Goodbye

% jshell | Welcome to JShell -- Version 1.9.0-internal| Type /help for help

-> /open mycode.jshThe value is 55

-> /list

1 : String foo = "Howdy";…

JShell Commands: /save, /open


-> /list all

s1 : import java.util.*; s2 : import java.io.*; s3 : import java.math.*; s4 : import java.net.*; s5 : import java.util.concurrent.*; s6 : import java.util.prefs.*; s7 : import java.util.regex.*; s8 : void printf(String format, Object... args) { System.out.printf(format, args); }

Start-up File


JShell Commands: start-up

• Use /setstart to set your own start-up• Use /savestart to get the current start-up• Both commands and snippets allowed in start-up• Too verbose? /feedback concise


Command Line Editing

• Navigable history (up/down, enter)• Editing (emacs bindings)• Snippet and command completion (tab)• Argument type query (shift-tab)• Interruption (control-C)• Based on jline


Agenda

What is JShell?

Why JShell?

Using JShell

Demo

Questions

1

2

3

4

5


Demo


JDK 9 / JShell Information

• Current schedule: GA September 22, 2016• Early access binaries + docs, updated weekly: https://jdk9.java.net/• Source code: http://hg.openjdk.java.net/jdk9/dev/ • -- Kulla (JShell) Project: http://openjdk.java.net/projects/kulla/• JEP 222: jshell: The Java Shell: http://openjdk.java.net/jeps/222

https://jdk9.java.net/

https://jdk9.java.net/

http://hg.openjdk.java.net/jdk9/dev/

http://openjdk.java.net/projects/kulla/

http://openjdk.java.net/jeps/222


API & Architecture• Tomorrow @ 16:10 • Interactive Java Support to your tool — the JShell API and Architecture


The Team

• Engineering– Robert Field– Jan Lahoda

• OpenJDK Commiters– Shinya Yoshida– You?

• Testing– Andrei Eremeev

– Brian Goetz–Maurizio Cimadamore– Joe Darcy– Paul Sandoz– Jonathan Gibbons–Michel Trudeau

– Sundararajan Athijegannathan– Rémi Forax– Arun Gupta–Mani Sarkar– Daniel Daugherty

• Advisors / Cheerleaders / Reviewers


Agenda

What is JShell?

Why JShell?

Using JShell

Demo

Questions

1

2

3

4

5


Q & A


Safe Harbor StatementThe preceding is intended to outline our general product direction. It is intended for information purposes only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code, or functionality, and should not be relied upon in making purchasing decisions. The development, release, and timing of any features or functionality described for Oracle’s products remains at the sole discretion of Oracle.

Software

JShell: An Interactive Shell for the Java Platform