Datanucleus enhancement with Bazel

bazel, datanucleus

Solution

As Neil Stockton mentioned, you cannot enhance classes in a jar. So, the basic strategy will be:

- Create the jar.

- Unjar the class files.

- Run the enhancements.

- Jar it back up.

Steps 2 & 3 have to be rolled into 4, as Bazel insists that you declare all inputs & outputs to a build rule (and you cannot know what .class files a .java file will generate, so Bazel always jars them up).

Create a `datanucleus.bzl` file to declare your enhancement rule in. It should look something like:

# Run datastore enhancements on the java_library named "jarname".
def enhance(jarname):
  # src is the name of the jar file your java_library rule generates.
  src = "lib" + jarname + ".jar"
  native.genrule(
      name = jarname + "-enhancement",
      srcs = [
          src, 
          "//third_party:datanucleus_core"
      ],
      outs = [jarname + "-enhanced.jar"],
      cmd = """
# Un-jar the .class files.
jar tf $(location {0})
# Run the enhance.
classes=""
for $$class in $$(find . -name *.class); do
  java -cp {0}:$(location //third_party:datanucleus_core) $$class
  classes="$$classes $$class"
done
# jar them back up.
jar cf $@ $$classes""".format(src),
  )

(I'm not too familiar with datastore so the `cmd` might need some modification, but it should be that general idea.)

Then, in your BUILD file, you'd do:

java_library(
    name = "my-lib",
    srcs = glob(["*.java"]),
    deps = ["..."],
)

# import the rule you wrote.
load('//:datanucleus.bzl', 'enhance')
enhance("my-lib")

Now you can do:

bazel build //:my-lib-enhanced.jar

and use `my-lib-enhanced.jar` as a dependency in other `java_` rules.

More info on .bzl files: https://bazel.build/versions/master/docs/skylark/concepts.html.

Edited to add more info on depending on a jar:

There are a couple of options to get a jar that contains the content of datanucleus. First, you don't need the layer of indirection: you can just say:

      srcs = [
          src, 
          "@datanucleus_core//jar"
      ],

This will give you the actual jar.

If, for some reason, you need the jar to be in third_party, you can modify third_party/BUILD to create a deploy jar, which is a java binary that bundles up all of its dependencies for deployment (since you're not actually going to using it as a binary, you can use whatever you want for the main class name):

java_binary(
    name = "datanucleus-core",
    main_class = "whatever",
    runtime_deps = ["@org_datanucleus_datanucleus_core//jar"],
)

genrule(
    name = "your-lib",
    srcs = [":datanucleus-core_deploy.jar", ...],
)

The `:datanucleus-core_deploy.jar` is called an implicit target: it's only built if requested, but it can be generated from your `java_binary` declaration.

Problem

Original source