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[//]: # TOC

Getting Started

Introduction

J2CL enables seamless use of Java in your JavaScript applications. One of the things that makes J2CL unique is: it gives you the complete freedom of choice! You can use J2CL to just make some Java code accessible from JavaScript or go all the way to create a whole application with it; whatever best suits your needs.

To understand what this means please take a look at the simple hello world sample that demonstrates how extensively Java and JavaScript could work together in J2CL:

https://github.com/google/j2cl/tree/master/samples/helloworld

Building your first application

Tip: If you are not familiar with using Java with Bazel, it might be helpful to start with the Bazel Java Tutorial since J2CL closely follows Bazel Java idioms.

Let's get started with cloning the repository and build a sample app from source:

    $ npm install -g @bazel/bazelisk
    $ alias bazel=bazelisk
  • Clone this repository:
      $ git clone https://github.com/google/j2cl.git
  • Build the sample app:
      $ cd j2cl/samples/helloworld
      $ bazel build src/main/java/com/google/j2cl/samples/helloworld:helloworld

Since we already built our first application, let's take a quick look at how the optimized output looks like:

      $ cat bazel-bin/src/main/java/com/google/j2cl/samples/helloworld/helloworld.js
      Output:
      document.body.innerText='Hello from Java! and JS!';

Amazing, isn't it? When we say J2CL tightly optimizes, we really mean it!

The optimizations in the J2CL world frequently crosses the boundaries of two languages and drop anything you don't need in production to give you the leanest application possible. This is only feasible because J2CL is very closely integrated with Closure Compiler which enables the optimization of the whole app together.

Let's run the development server to see our application in action:

  • Run the development server
      $ bazel run src/main/java/com/google/j2cl/samples/helloworld:helloworld_dev_server

Please take the time to play with the code and get a better feeling of how things are working.

Live-reload

ibazel is file-system watcher that auto-triggers bazel build when needed. To use it, just replace bazel with ibazel in any command:

  • Install ibazel and make sure it is in your path.

  • Run the development server

      $ ibazel run src/main/java/com/google/j2cl/samples/helloworld:helloworld_dev_server

You like build-on-save but you would like to refresh on your own terms? Pass -nolive_reload while running ibazel.

Unit testing

You can also write regular JUnit tests for your application and run them under J2CL.

To run the sample unit test for our sample app:

      $ bazel test src/test/java/com/google/j2cl/samples/helloworldlib:HelloWorldTest

Setting up your first project

To setup your first project you can use the sample as template:

      $ cp -R <j2cl-repo>/samples/helloworld <my-repo>/<app-name>
      $ mv <my-repo>/<app-name>/WORKSPACE.remote <my-repo>/<app-name>/WORKSPACE

and you are done.

Next steps