how to propagate data down build chain using bazel aspects

Question

Let's say I have a simple java program including 2 classes:

Example, Example2

and another class that uses both classes:

ExamplesUsage

and I have corresponding bazel build targets of kind java_library: example, example2, examples_usage

so example and example2 need to be compiled before examples_usage is built.

I want to accumulate information from all three targets using bazel aspects propagation technique, how do I go about doing that?

Answer 1

Here's an example for accumulating the number of source files in this build chain:

def _counter_aspect_impl(target, ctx):
  sources_count = len(ctx.rule.attr.srcs)
  print("%s: own amount - %s" % (target.label.name , sources_count))

  for dep in ctx.rule.attr.deps:
      sources_count = sources_count + dep.count

  print("%s: including deps: %s" % (target.label.name , sources_count))

  return struct(count = sources_count)

counter_aspect = aspect(implementation = _counter_aspect_impl,
  attr_aspects = ["deps"]
)

if we run it on the hypothetical java program we get the following output:

example2: own amount - 1.
example2: including deps: 1.
example: own amount - 1.
example: including deps: 1.
examples_usage: own amount - 1.
examples_usage: including deps: 3.

As you can see the 'dependencies' targets' aspects were run first, and only then the 'dependant' target aspect was run.

Of course in order to actually utilize the information some ctx.action or ctx.file_action needs to be called in order to persist the gathered data