TLDR: like other REPLs, JShell provides an easy way to test Java one-liners, and, like the Rails console, a handy ad hoc CLI.
I appreciate a REPL for quickly checking the validity of small snippets. For example, I can improve the quality of my code reviews by verifying an idea works in a REPL before recommending it in a review.
My first exposure to a Java-esque REPL was the Scala REPL, which could also interpret Java. This was handy, but only easily available when Scala is installed.
When Scala wasn’t installed, I used Repl.it for Java, but this is a public site , so I need to be mindful not to use it for anything confidential, and it can take some time to load.
Recently, I learned about JShell, which is included in the JDK as of version 9.
Aside, in case you’re on a Chromebook, Google’s Cloud shell is great ad hoc terminal.
Per the JShell docs, I can start/stop the shell:
$ jshell
| Welcome to JShell -- Version 11.0.6
| For an introduction type: /help intro
jshell> /exit
| Goodbye
Hello world:
jshell> System.out.println("hi")
hi
Note implicit semicolons for simple code. Return statements appear to need explicit semicolons, though, eg:
jshell> String get(){
...> return "s"
...> }
| Error:
| ';' expected
| return "s"
| ^
From the docs, I see there are “scratch variables”, which reminds me of the Scala REPL’s res variables:
jshell> 2+2
$3 ==> 4
jshell> $3
$3 ==> 4
jshell> $3 + 2
$5 ==> 6
The feedback is comparable to javac, eg:
$ cat MyMap.java
import java.util.HashMap;
import java.util.Map;
class MyMap {
static Map<String, String> m = new HashMap<>();
public void put(String k, String v){
m.put(k,v);
}
public void get(String k){
return m.get(k);
}
}
$ javac MyMap.java
MyMap.java:10: error: incompatible types: unexpected return value
return m.get(k);
^
1 error
$ jshell
| Welcome to JShell -- Version 11.0.6
| For an introduction type: /help intro
jshell> /open MyMap.java
| Error:
| incompatible types: unexpected return value
| return m.get(k);
| ^------^
We can use tab completion:
jshell> List
List ListIterator ListResourceBundle
Signatures:
java.util.List<E>
<press tab again to see documentation>
jshell> List.
class copyOf( of(
jshell> List.of("1", "2")
$3 ==> [1, 2]
jshell> $3.stream().forEach(System.out::println)
1
2
The up arrow scrolls back through history. We can also print it:
jshell> /list
1 : List.of("1", "2")
2 : $3.stream().forEach(n->System.out.println(n))
We can also search history, eg Ctrl + R for reverse search:
(reverse-i-search)`hi': System.out.println("hi")
Editing multi-line code is cumbersome, eg:
jshell> class Foo {
...> String get(){}
...> }
| created class Foo
// Up arrow to edit method definition
jshell> String get(){
...> return "s";
...> }
| created method get()
// Creates new function instead of editing Foo.get
jshell> get()
$10 ==> "s"
jshell> Foo f = new Foo();
f ==> Foo@1e88b3c
jshell> f.get();
jshell>
I’d recommend using an external editor for anything non-trivial.
JShell has an /edit command to launch an external editor, but it doesn’t appear to save the output.
jshell> /set editor vim
| Editor set to: vim
jshell> class Foo {}
| created class Foo
jshell> /edit Foo // add bar method to Foo
| replaced class Foo
jshell> Foo f = new Foo()
f ==> Foo@56ac3a89
jshell> f.bar()
| Error:
| cannot find symbol
| symbol: method bar()
| f.bar()
| ^---^
jshell> /edit Foo // Observe bar method is undefined
I’d recommend just having an editor open in a separate terminal, and using JShell’s /open command to load the file after changes.
For folks using Google Cloud Shell, it appears to have an implicit tmux session, which makes it easy to edit in one pane and use JShell in another.
In practice, I’m guessing there’s little use for JShell when editing complex code, but it does provide a handy CLI for exploring complex code. We could have a build target, like pants repl, or a CLI for our app, like rails console.
For example, given a naive script build_to_repl.sh:
javac -d bin src/main/com/example/* \
&& jar cf bin/MyMap.jar -C bin com \
&& jshell --class-path bin/MyMap.jar
We could:
$ ./build_to_repl.sh
| Welcome to JShell -- Version 11.0.6
| For an introduction type: /help intro
jshell> import com.example.MyMap;
jshell> MyMap m = new MyMap();
m ==> com.example.MyMap@6a41eaa2
jshell> m.put("k", "v")
jshell> m.get("k")
$4 ==> "v"
A software engineering toolkit 🛠 