Java REPL with JShell and Bazel

One great new feature of the upcoming Java 9 release is JShell, a Java REPL. Java is long overdue for an official REPL, so I was eager to try it out. I found this article to be a great starting point: Java 9 Early Access: A Hands-on Session with JShell – The Java REPL.

It’s easy to interact with the built in Java classes out of the box, but JShell becomes a lot more useful if you can interact with your existing application code. We use Bazel at Braintree, so I decided to add a Bazel target that would let me run my app code in JShell.

First, I downloaded the JShell (codenamed kulla) jar from their Jenkins and added it to the third_party directory:

% cd third_party
% wget https://adopt-openjdk.ci.cloudbees.com/view/OpenJDK/job/langtools-1.9-linux-x86_64-kulla-dev/lastSuccessfulBuild/artifact/kulla-0.819-20150913005850.jar

Then, I imported this jar in Bazel by adding this to third_party/BUILD:

java_import(
  name = "kulla_jshell",
  jars = ["kulla-0.819-20150913005850.jar"],
)

Finally, I added a java_binary target to the app. This target sets the main class to JShell and adds both the app and kulla as dependencies:

java_binary(
  name = "repl",
  main_class = "jdk.internal.jshell.tool.JShellTool",
  runtime_deps = [
    ":app",
    "//third_party:kulla_jshell",
  ],
)

One issue is that JShell requires Java 9, but Bazel does not currently support it. The Bazel built jars are compatible, however, so we can build a deploy jar with Bazel and Java 8, and then run it with Java 9:

% bazel build //app:repl_deploy.jar

% /usr/lib/jvm/java-9-oracle/bin/java -jar bazel-bin/app/repl_deploy.jar
|  Welcome to JShell -- Version 0.819
|  Type /help for help

-> import app.*

-> App a = new App()
|  Added variable a of type App with initial value app.App@1761e840

-> a.go()
|  Expression value is: "go go go"
|    assigned to temporary variable $3 of type String

Now, I can interactively play with app code in a REPL. Once Bazel adds support for Java 9, it will be even easier.