README.md

Case 2: Hello, Java!

About the case

This case uses the native java_binary and java_library rules. The java_library rule compiles and links sources into a .jar file The java_binary rule builds a jar file, plus a wrapper shell script. See bazel-bin/workshop/case2/hello.runfiles/_main/workshop/case2/hello for the wrapper script.

This is also the first example that uses external depencencies from Maven, for example @maven//:org_assertj_assertj_core which is a dependency for the greeter-test test-target.

All dependencies are resolved using the MODULE.bazel and the maven_install.json lockfile. This generates the @maven// namespace and the Starlark-targets for the dependencies.

Things to try out

Build the lib

bazel build //workshop/case2:greeter-lib - builds the greeter-lib target and outputs the location for the resulting jar file.

Run the binary

bazel run //workshop/case2:hello - runs the hello target, which depends on the greeter-lib target and outputs the result.

Modifying the different parts, and see what recompiles

If you modify the Greeter.java file and run bazel run //workshop/case2:hello, the greeter-lib target will recompile and then the hello will rebuild before executing.

If you modify the HelloWorld.java file and then run bazel run //workshop/case2:hello, the hello target will recompile.

You can see how many processes were needed before the build was complete by looking at the output:

bazel run //workshop/case2:hello
INFO: Invocation ID: 1ca27de2-08e0-4ff0-93b9-a83b6f518f70
INFO: Analyzed target //workshop/case2:hello (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //workshop/case2:hello up-to-date:
  bazel-bin/workshop/case2/hello
  bazel-bin/workshop/case2/hello.jar
INFO: Elapsed time: 0.712s, Critical Path: 0.56s
INFO: 2 processes: 1 internal, 1 worker.
INFO: Build completed successfully, 2 total actions
INFO: Running command line: bazel-bin/workshop/case2/hello
Hello, World!

Here you can see the line INFO: 2 processes: 1 internal, 1 worker. which shows that 2 processes were used to build the target.

If you run it again without modifying anything, you will see that the target is up-to-date and only 1 process are used:

INFO: 1 process: 1 internal.

Additional things to try out

Query Bazel for the dependency graph of the hello target:

bazel query "deps(//workshop/case2:hello)" Here you can see the full dependency graph of the hello target (including all the tools etc bazel use)

vs

bazel query "deps('//workshop/case2:hello') intersect //..." Here you only see the dependencies that are part of the workspace (e.g //...)

Show files generated by a target:

bazel cquery //workshop/case2:hello --output=files

Create another library yourself and consume it in the hello target