JShell - A Quick Look
jshell
is a new-in-Java-9 tool - a REPL for Java!! I was not sure at first how useful it would be but since I started playing with it I've been finding more and more uses for it, and it's pretty cool. There's a couple of surprising things though. Lets have a look.
A quick look
and we’re at the REPL prompt. We can do a few totally unsurprising (but neat!) things:
There’s also a few meta commands, try starting with /help
The $n
are references to the results:
etc. We can create, modify and replace classes on the fly:
Classes are created
then modified
or replaced
when changed. If the change alters a method signature or adds/removes a field then the class is replaced
, otherwise it is just modified
. What happens to instances when you do that?
Modifying a class
Modifying a class alters existing instances:
Replacing a class
Replacing a class nullifies instance references:
Semicolon insertion
My inner Douglas-Crockford is bristling at this a bit, to be honest. Semi-colons are optional. You might notice I haven’t used any in the code above. The rule seems to be that if an expression can be evaluated, then it will be. So be careful typing code like:
Because you’ll get this:
I use three ways to cope with this:
- Everything on one line. Not nice with long lines.
- Use the backreferences. Usually only used when I forget about semicolon insertion, and accompanied by me kicking myself.
- Move your dots. A bit surprising-looking but works OK. Least-worst option IMHO.
NB there is an open bug to prevent this behaviour during multi-line paste
Pasting content
There seems to be a bug which prevents pasting more than 2 lines of code. It’s reported here. That bug is marked as RESOLVED
but still the bug persists in JDK 9.0.1 which is the latest one (on Linux at least). The workaround is:
This launches your $EDITOR
which you can paste as many lines as you like into, then save & exit and jshell
evaluates it all.
Conclusion
jshell is a nice way to play with Java code. Much nicer than creating dummy classes with a main()
method. Trisha Gee shows us a nice demo of how to use jshell from IntelliJ - looks great, and I assume other IDEs have similar support. Try it out :)
Originally published at mjg123.github.io.