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?

score 3 · Accepted Answer · edited Nov 11 '21 at 11:10

Try:

bazel run //:my-target -- --debug

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.

That debug flag was all I needed, thanks `bazel run //:my-target -- --debug` — lucasem, Sep 01 '21 at 17:01

score 0 · Answer 2 · answered Dec 09 '20 at 14:36

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.

Turns out `--jvm_flag` is not needed. See the accepted answer. — Gunnar, Nov 11 '21 at 11:11

sbrk · Answer 3 · 2023-05-27T00:10:22.280

The accepted answer works but if you're looking for a more complete command try running

bazel run --java_debug --test_arg=--wrapper_script_flag=--debug=8000 -- //your-target/...

You should see this in the Bazel logs

...
Listening for transport dt_socket at address: 8000
...

If you're running the command on a different host, you can reverse ssh tunnel using

ssh -L 8000:localhost:8000 <remote-host-cname>

Once you do this, just bind your debugger to port 8000 using this JVM flag

-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=8000

This should do what you're looking for.

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

3 Answers3