2. JShell: An Interactive Shell for the Java Platform
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Robert Field
JShell Architect
Core Language and Tools Group
Oracle Corporation
October 4, 2017
Confidential – Oracle Internal/Restricted/Highly Restricted

3. This is a Safe Harbor Front slide, one of two Safe Harbor Statement slides included in this template.
One of the Safe Harbor slides must be used if your presentation covers material affected by Oracle’s Revenue Recognition Policy
To learn more about this policy,
For internal communication, Safe Harbor Statements are not required. However, there is an applicable disclaimer (Exhibit E) that should be used, found in the Oracle Revenue Recognition Policy for Future Product Communications. Copy and paste this link into a web browser, to find out more information.
For all external communications such as press release, roadmaps, PowerPoint presentations, Safe Harbor Statements are required. You can refer to the link mentioned above to find out additional information/disclaimers required depending on your audience.
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4. 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
Dynamic interaction
REPL – as seem, dynamic languages
Snippet: expression, statement, or declaration
Platform: in / built-on
Showing command tool using technology. Encourage other tools built on API.

5. What is JShell NOT? Not a new language
“Snippets” of pure
No new syntax
Nothing added (except allowing snippets)
Nothing removed (except things that don’t make sense…)
Not a replacement for the compiler
Not an IDE
The point: interactive Java – not another language, not a mish-mash.
Snippets: Java Language Specification productions
For interactive exploration – compiler for programs
Slippery slope for any language tool – integrate IDE

6. Who wants JShell? Exploring a new API or language feature
Experiment and instantly see results
New to Java, new to programming
Start with expressions vs classes
Immediate feedback
Prototyping
Incrementally write complex code
Now:: write class declaration, method declaration with access modifiers and array parameters, System.out.println
JShell:: expressions & immediate see results
We are all newbie – new APIs and language features
Prototype seeing results at any granularity

7. 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
When want to try things out in Java now, you need it wrapped in main method and that wrapped in a class. You need imports for any libraries you are using.
You need an editor of something to write the code, then compile it with command line tool or IDE, then execute it.
For a whole program that is fine. For exploration that is slow.

8. Without JShell % cat Test.java import java.util.List; class Test {
  public static void main(String... args) {
      List<String> alpha = List.of("Z", "A", "C");
      alpha.sort(String::compareTo);
      System.out.println( alpha );
  } }
% javac Test.java
% java Test
Exception in thread "main" java.lang.UnsupportedOperationException
at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:70)
at java.base/java.util.ImmutableCollections$AbstractImmutableList.sort(ImmutableCollections.java:85)
at Test.main(Test.java:7) %
This is a sample Title and Code Layout slide, ideal for displaying codes.
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9. Now in JShell…

10. 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
Commands are distinguished from snippets by leading slash: /
All input can be tab completed, has history, and editing
JShell evaluates “snippets” of Java code: an expression, statement, variable definition, method definition, class definition, or import.

11. Snippets Declarations: Statements: Not allowed:
String s = “hello”
int twice(int x) { return x + x; }
class Pair<T> { T a, b; Pair(…
interface Reusable { }
import java.nio.file.*
Statements:
while (mat.find()) { …
if (x < 0) …
switch (val) { case FMT: { format(); break; …
Not allowed:
package foo;
Top-level modifiers: static final
Top-level: break continue return
Expressions:
twice(12)
new Pair<>("red", "blue")
transactions.stream() filter(t -> t.getType() == Transaction.GROCERY) sorted(comparing(Transaction::getValue).reversed()) map(Transaction::getId) collect(toList())
Confidential – Oracle Internal/Restricted/Highly Restricted

12. Commands Snippet information: Snippet control: Session: History:
/list
/vars
/methods
/types
/imports
Snippet control:
/edit
/drop
/save
/open
Session:
/exit
/env
/reset
/reload
History:
/history /!
/id
/-n
General:
/set
/help
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13. /list jshell> /help /li | | /list
| Show the source of snippets, prefaced with the snippet id.
| List the currently active snippets of code that you typed or read with /open
| /list -start
| List the automatically evaluated start-up snippets
| /list -all
| List all snippets including failed, overwritten, dropped, and start-up
| /list <name>
| List snippets with the specified name (preference for active snippets)
| /list <id>
| List the snippet with the specified snippet id
Confidential – Oracle Internal/Restricted/Highly Restricted

14. Completion / Information / Control
▷ -- snippet and command completion ▷ -- method parameters ▷ ▷ -- javadoc ⇧▷v -- expression to variable declaration ⇧▷i -- add import ^C -- interrupt running process or abort line
▷ == tab
@ method(
⇧▷ == shift-tab
^C == Ctrl-C
Confidential – Oracle Internal/Restricted/Highly Restricted

15. Line Editing (based on JLine 2.0)
Navigation
⏎ -- enters typed or history entry RETURN
-- navigate line and history
Editing – emacs/readline bindings ^A ^E ^L ^K ◆D ^Y ^R
^X ( …
-- move to start of line
-- move to end of line
-- clear screen
-- kill rest of line
-- kill word (Meta-D)
-- yank: paste last killed text
-- search history (reverse)
-- begin keyboard macro
Confidential – Oracle Internal/Restricted/Highly Restricted

16. Demo

17. JShell Guiding Principles
Compatibility: current and future
Avoid chasing corner-cases
Build on the platform (JDK)
Easy and natural to use
One size does not fit all (API)
I've seen language tools that “cleverly” implemented functionality their own way, only to to be continually slightly out of sync, playing catch-up, and eventually falling too far behind.
JShell built on and in the platform.
API makes functionality available to any tool.

18. JShell Architecture Compilation: built on Compiler API
Execution: standard JVM controlled by Java Platform Debug Architecture
All core functionality provided by JShell API.
The jshell tool just one possible client.
The jshell tool uses the popular jline2 API for command line interaction.

19. JShell module: JShell APIs, SPI, and library
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20. JShell API
An instance of JShell is the starting point. Doing an eval() of an input string returns SnippetEvents. Which hold several things, like value. But also created Snippets. Queries can be made for information about the Snippet.
SourceCodeAnalysis provides information like completion.
[API Demo?]

21. Meta-Demo

22. JDK 9 Information JShell released with JDK 9!
Download now:
Documentation:
Join the OpenJDK mailing lists: openjdk.java.net
Follow us on and #Java9
Source code: hg.openjdk.java.net/jdk9/dev/
JShell now in JDK 9

23. JShell Documentation jshell tool tutorial “User Guide”:
jshell tool Reference:
JShell APIs:
JShell now in JDK 9

24. JShell Information
JShell JEP: The Java Shell:
JShell project / mailing list:
JShell / my
JShell now in JDK 9

25. The Team Engineering OpenJDK Commiters Documentation:
Robert Field
Jan Lahoda
OpenJDK Commiters
Shinya Yoshida
You?
Documentation:
Joni Gordon
Advisors / Cheerleaders / Reviewers
Brian Goetz
Maurizio Cimadamore
Joe Darcy
Paul Sandoz
Jonathan Gibbons
Michel Trudeau
Sundararajan Athijegannathan
Rémi Forax
Arun Gupta
Mani Sarkar
Daniel Daugherty
Jan and I are the Oracle engineering team. Shinya Yoshida has contributed significantly as an OpenJDK Commiter, as could you, …
The project was Brian's inspiration. Many people have advised, reviewed, and cheered us on.

26. Q & A

27. Stay connected
Join us: DevOps Corner (Developer Lounge – Moscone West)
Learn more: openjdk.java.net | wercker.com/java
Follow: #JavaOne #DevOps

28. This is a Safe Harbor Front slide, one of two Safe Harbor Statement slides included in this template.
One of the Safe Harbor slides must be used if your presentation covers material affected by Oracle’s Revenue Recognition Policy
To learn more about this policy,
For internal communication, Safe Harbor Statements are not required. However, there is an applicable disclaimer (Exhibit E) that should be used, found in the Oracle Revenue Recognition Policy for Future Product Communications. Copy and paste this link into a web browser, to find out more information.
For all external communications such as press release, roadmaps, PowerPoint presentations, Safe Harbor Statements are required. You can refer to the link mentioned above to find out additional information/disclaimers required depending on your audience.
Confidential – Oracle Internal/Restricted/Highly Restricted