How do I debug a 'java_binary' target executed by a Bazel rule via 'ctx.actions.run(...)'?

Question

I have a java_binary target in my workspace that I'm later passing as an executable to ctx.actions.run inside the rule. So far so good.

Now I want to debug this java_binary while Bazel is executing the rule. In order to attach a debugger I need the java_binary run in debug mode. So far, the only thing I came up with is setting jvm_flags on the java_binary . I was able to get that to work. But I was wondering if there is a way to achieve it from the command line instead of baking it into the java_binary .

java_binary(
    ...
    jvm_flags = [
        "-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=8000"
    ],
)

Is it possible to achieve this from the command line without hard coding jvm_flags ?

Answer 1

It looks like you can pass the --jvm_flag option as part of the program options after the -- .

BUILD:

java_binary(
    name = "extract",
    main_class = "com.pkg.Main",
    resources = glob(["src/main/resources/**/*"]),
)

CLI:

bazel run //:extract -- --jvm_flag="-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=7942" -path %cd%\config.json

It seems that the --jvm_flag option needs to come immediately after the -- , before the program options ( -path in the example). This is with Bazel 3.7.0.

Answer 2

One strategy is to run the build with --subcommands , which will tell bazel to print out all the commands it's running during the build. Then find the command line corresponding to the invocation of the java_binary you're interested in. Then you can copy/paste that command (including the cd part) and modify it to include the debug flags, and debug it as you would any other process.

Note also that java_binary outputs a wrapper script that includes a --debug[=<port>] flag, so that should be all that needs to be added to the command line.

Note also that --subcommands will only print the commands that are actually executed during the build, so a fully cached / fully incremental build will print nothing. You may need to do a clean , or delete some of the outputs of the action you're interested in so that bazel runs that command.