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Change Detection

pomgen tracks whether an artifact has changed since it was last released. It also tracks the last released version of each artifact. It uses this information to decide whether an artifact needs to be released and to determine which artifact version to use in <dependency> references in generated poms.

Change detection is most useful for libraries that have a large number of transitives, because it prevents the (unchanged) transitives from being released over and over again when the main Library changes.

Example

A1 -> A2 -> A3

A1 references A2, A2 references A3. These are all source artifacts in the same repository, and their BUILD files look similar to this:

A1's BUILD file

java_library(
    name = "a1",
    deps = ["projects/libs/a2"],
    ...

A2's BUILD file

java_library(
    name = "a2",
    deps = ["projects/libs/a3"],
    ...

When pomgen runs for A1, it will use change detection to determine whether A2 and A3 actually need to be released:

  • If A2 and A3 have not changed since they were last released, then the pom.xml file generated for A1 will contain references to the previously released version of A2.
  • If A2 has changed since it was last released, but A3 has not, then pomgen will generate a new pom.xml for both A1 and A2. A2's reference to A3 will use the last released version.
  • If A3 has changed but not A2, then pomgen generates a new pom.xml for A3, and also a new pom.xml for A2, which has the updated reference (version) to A3.

Release Snapshoting

pomgen uses a hash based on the git repository state to track the state of an artifact. Updating this hash requires running the pomgen update command. This has to happen as part of the artifact release process.

In order to update the artifact's hash and the released artifact's version, use the following command:

bazel run @pomgen//:update -- --package <path/to/bazel/package> \ 
    --update_released_artifact_hash_to_current \
    --new_released_version <released version>

For example, the following command updates the release state of all artifacts under the hello-world directory.

bazel run @pomgen//:update -- --package examples/hello-world \
    --update_released_artifact_hash_to_current \
    --new_released_version 1.0.0

For more details on working with snapshot jars, see this doc.

Release Reasons

pomgen query can show information about the Libraries being relased and their "release reason". For example:

bazel run //:query -- --package examples/hello-world/juicer --library_release_plan_tree

Output:

examples/hello-world/juicer ++ 3.0.0-SNAPSHOT
  examples/hello-world/healthyfoods ++ 1.0.0-SNAPSHOT
  examples/hello-world/wintervegetables ++ 2.0.0-SNAPSHOT
    examples/hello-world/healthyfoods ++ 1.0.0-SNAPSHOT

++ artifact has never been released

The output above shows that the juicer Library, and all libraries referenced by juicer, need to be released, because they have never been released previously (and so changes are not tracked yet).

Now simulate a release:

# this script creates release state metadata:
examples/hello-world/update_release_metadata.sh
# those changes need to be committed:
git add examples/hello-world
git commit -m "Juicer released to Maven Central"

Modify a file:

echo "// a comment" >> examples/hello-world/juicer/src/main/java/com/pomgen/example/Main.java
git add examples/hello-world
git commit -m "fix Juicer bug"

Re-run pomgen query:

bazel run //:query -- --package examples/hello-world/juicer --library_release_plan_tree

Output:

examples/hello-world/juicer + 3.0.0-SNAPSHOT
  examples/hello-world/healthyfoods - 0.0.1
  examples/hello-world/wintervegetables - 0.0.1
    examples/hello-world/healthyfoods - 0.0.1

 + binary artifact changed
 - no changes to release

The output shows that juicer needs to be released because it has changed. However the Libraries that the juicer Library references do not need to be released, since they have not changed (the pom.xml generated for the jar artifact belonging to juicer will reference the previous released versions of healthyfoods and wintervegetables).

In order to make local development more convenient, pomgen also releases Libraries when local, uncomitted changes are present:

# undo previous commit:
git reset --hard HEAD~ 
# modify a file, but don't commit
echo "// a comment" >> examples/hello-world/juicer/src/main/java/com/pomgen/example/Main.java
# run pomgen query:
bazel run //:query -- --package examples/hello-world/juicer --library_release_plan_tree

Output:

examples/hello-world/juicer <> 3.0.0-SNAPSHOT
  examples/hello-world/healthyfoods - 0.0.1
  examples/hello-world/wintervegetables - 0.0.1
    examples/hello-world/healthyfoods - 0.0.1

 - no changes to release
<> artifact has uncommitted changes (dev)

A typical dev workflow is:

  • Make changes to one or more libraries
  • Generate pom(s) and install the library/ies into ~/.m2/repository so that other Maven-based builds can find them:
bazel run //maven -- -a pomgen -t examples/hello-world/juicer
bazel run //maven -- -a install

# or to install jar artifacts and also source and javadoc jars:
bazel run //maven -- -a install_all

Disabling Change Detection

Change detection is enabled by default. It can be disabled:

  • On a per-artifact basis by setting the change_detection attribute of the maven_install rule in the BUILD.pom file to False
  • By setting the --force argument when running pomgen
  • By setting the -f argument when running //maven