Document build profiles and properties in docs/developer/BUILD.md

In doing so, I also noticed a few things in need of improvement. So,
documenting the build also drive those enhancements, such as
  - the new 'fast-build' profile skipping test compilation and execution
    as well as documentation generation,
  - an option to skip generating source assemblies,
  - to skip unzipping source assemblies if javadoc generation for them
    is to be skipped too,
  - activating the 'create-docs' profile by property which is
    true by default instead of using 'activeByDefault=true', because the
    latter does not work reliably if other profiles are activated
    manually according to a Maven bug that was closed as "won't fix",
  - no longer generating separate javadocs for the 'runtime' module,
    because that module is not deployed and the main artifacts recreate
    Javadocs from scratch for all of their constituent sources anyway.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

2 files changed, 204 insertions, 1 deletions

diff --git a/docs/developer/BUILD.md b/docs/developer/BUILD.md
new file mode 100644
index 000000000..119b33832
--- /dev/null
+++ b/docs/developer/BUILD.md
@@ -0,0 +1,200 @@
+# Maven build options
+
+AspectJ is based on a multi-module Maven build with several options influencing
+
+  * what to build, 
+  * whether to run tests,
+  * whether to created documentation for the AspectJ website,
+  * whether to create source and javadoc artifacts,
+  * whether to GPG-sign artifacts for a release.
+
+## Typical build scenarios
+
+As a developer, which modules or artifacts you want to build depends on your situation. For example:
+
+  * When building a stable, non-snapshot release (milestone, release candidate, final), your goal is to publish a full
+    set of artifacts on Maven Central (MC). Because MC requires you to publish javadocs and source code together with
+    the corresponding binaries and to sign all artifacts with GPG, this is the most complex and complete, but also the
+    slowest build you are about to encounter. Probably, you also want to build the AspectJ documentation to be deployed
+    to the website.
+
+  * When building a snapshot, the requirements are less strict, even if you are planning to make the release publicly
+    available in the Sonatype OSSRH snapshots repository. In this case, you can skip creating javadocs and source JARs
+    and also do not need to sign the artifacts. You might want to decide to sign anyway or at least to publish source
+    JARs (which modern IDEs can also use in order to display javadoc information). So even if you are not working with
+    an active Maven release profile, you want to have control over those options by setting properties. Probably, you do
+    not wish to generate the AspectJ documentation for the website, because presently there is not even a dedicated
+    place to deploy snapshot versions of it on the Eclipse webserver.
+
+  * When building during development without the need to publish it, you probably want to skip as many non-essential
+    parts of the build as possible. Firstly, you do not need them. Secondly, you want your build and run development
+    cycles to be as quick as possible. So in this case, definitely you are going to skip javadoc and source JARs,
+    website documentation and GPG signatures - unless you are in the process of changing and/or testing those parts of
+    the build.
+
+  * Independently of the above scenarios, you want to have control over whether to run (or even compile) any tests, and
+    if so, which one(s).
+
+## How to customise the build process
+
+You can customise the Maven build process by using build profiles and/or set corresponding system properties for
+fine-tuning.
+
+### Build profiles
+
+#### Main profiles
+
+The main profiles you are going to use are:
+
+  * By default, when not specifying any profiles or properties, the build skips a few non-essential, time-consuming
+    steps, but runs all tests: no javadocs, no source JARs, no GPG signatures, but generate documentation.
+
+  * `release` - Run tests, create javadoc and source JARs, generate documentation, activate GPG artifact signing.
+    Furthermore, each module creating one of the main build artifacts individually uses Nexus Staging Maven Plugin in
+    order to take care of deploying non-snapshot artifacts to Sonatype OSSRH staging repositories and subsequently
+    releasing them to Maven Central. See [How to release AspectJ](RELEASE.md) for more information. Snapshot artifacts
+    are being deployed normally, using Maven Deploy Plugin. See description of
+    [property `maven.deploy.skip`](#build-properties) below for more information.
+
+  * `fast-build` - In a way, this is the opposite of the release profile, trying to build the product as quickly as
+    possible by skipping all non-essential build steps: no tests (skipping even test compilation), no javadocs, no
+    source JARs, no GPG signatures, no documentation.
+
+  * `create-docs` - If you intend to run a build with all tests, but still wish to skip generating documentation,
+    deactivate this profile by
+    ```shell
+    mvn -P !create-docs ...
+    ```
+    On UNIX-like shells like Bash (also Git Bash under Windows), you probably need to escape the "!":
+    ```shell
+    mvn -P \!create-docs ...
+    ```
+    You can also deactivate the profile using a system property instead:
+    ```shell
+    mvn -DcreateDocs=false ...
+    ```
+
+#### Special profiles for `lib` module
+
+Defined in the `lib` module, there are two special profiles, helping to make the build more efficient: 
+
+  * `provision-libs` - Downloads and installs software used during tests, such as Apache Ant and several libraries. Some
+    are downloaded from Maven Central, others directly from product download sites. Additionally, the build downloads
+    several missing source packages, so developers can use them during development in order to access source code and
+    javadoc. Because this build step is costly and should be performed only once after cloning the AspectJ repository
+    or when other circumstances require re-provisioning at least one of those libraries, it is activated automatically,
+    if marker file `lib/provisioned.marker` does not exist. After successful provisioning, the marker file is created,
+    helping to avoid repeating this build step henceforth.
+
+  * `clean-libs` - By default, `mvn clean` will not delete any of the libraries provisioned in profile `provision-libs`.
+    This is intentional and one of the reasons why the libraries are not provisioned into the `target` directory but
+    directly into `lib` subdirectories. If you wish to re-provision the libraries, simply run
+    ```shell
+    mvn -pl lib -P clean-libs clean
+    ```
+    Now you have a clean slate and during the next build, the libraries will be freshly downloaded and installed into
+    their respective `lib` subdirectories.
+
+Please note: An additional build step using Maven Enforcer Plugin also verifies the existence of several key files which
+ought to exist after a successful download. This heuristic check runs independently of the two build profiles mentioned
+above. It helps to detect accidental corruption of the provisioned libs, e.g., due to manual deletion or a previously
+failed provisioning build step (network problems, manually interrupted build).  
+
+#### Other profiles
+
+Other existing profiles, which developers are less likely to actively use because they are applied automatically, are:
+
+  * `repeat-all-unit-tests` - Maven module `run-all-junit-tests` has the sole purpose of providing a convenient means of
+    running all tests across modules from an IDE instead of from Maven, in order to get JUnit build reporting directly
+    there instead of on the console. As a developer, you simply run test suite `RunTheseBeforeYouCommitTests`. This
+    profile is inactive by default, because in the context of a Maven build it would cause all tests to be run twice
+    (during module build and again when running the big suite), hence the profile name.
+
+  * `jdk-8-to-15` - Activated automatically on JDKs 8-15, setting property `jvm.arg.addOpens` to an empty value, because
+    it is only needed on JDK 16+, see next bullet point.
+
+  * `jdk-16-to-xx` - Activated automatically on JDKs 16+, setting property `jvm.arg.addOpens` to value
+    `--add-opens java.base/java.lang=ALL-UNNAMED`, which is needed in order to run LTW tests.
+
+### Build properties
+
+The following properties and their default values in different profile are used in order to activate or skip Maven
+plugin executions:
+
+  * `maven.deploy.skip` (default `true`) - By default, do not deploy artifacts, because only the main AspectJ artifacts
+    are meant to be shared with the general public, i.e. deployed to Sonatype OSSRH Snapshots or Maven Central artifact
+    repositories. The main AspectJ artifact modules override the default, setting the value to `false`. This property is
+    used independently of build profiles, it simply has a global default and module-specific overrides.
+
+  * `maven.gpg.skip` (default: `true`) - By default, do not GPG-sign artifacts, because only the main AspectJ artifacts
+    need to be signed before publishing them on Maven Central. The main AspectJ artifact modules override the default,
+    setting the value to `false`. This property is used independently of build profiles, it simply has a global default
+    and module-specific overrides. Given the additional fact that Maven GPG Plugin is only active in the `release`
+    profile, it also means that the build globally skips signing if that profile is inactive. So if you wish to sign
+    snapshot artifacts, you need to activate the `release` profile (also activating all the other build steps that
+    profile has).
+  
+  * `maven.javadoc.skip` (default: `true`) - By default, do not create javadoc. Overridden in the `release` profile.
+    When javadoc generation is skipped while producing the uber JAR assemblies for the main AspectJ artifacts, also
+    unzipping of source uber JARs is skipped, because that step is only needed in order to create uber JAR javadocs in
+    the first place. (Do not worry too much, if you do not fully understand what I just wrote.)
+
+  * `maven.source.skip` (default: `true`) - By default, do not create source JARs. Overridden in the `release` profile.
+    Actually, this property is meant to be used in order to skip execution of Maven Source Plugin, but currently the
+    AspectJ build does not even use that plugin, because the build does not create source JARs for individual modules.
+    That might change in the future, though, so we use this property to also influence Maven Assembly Plugin, which is
+    responsible for creating source uber JARs for the main AspectJ artifacts.
+
+  * `skipTests` (default: `false`) - By default, execute tests. Profile `fast-build` overrides this property.
+
+  * `maven.test.skip` (default: `false`) - By default, compile and execute tests. Profile `fast-build` overrides this
+    property. Actually, activating this property also implies `skipTests`, but `fast-build` sets both of them in order
+    to be explicit about its intentions.
+
+  * `createDocs` (default: `true`) - By default, create user documentation for the AspectJ website. Profile `fast-build`
+    overrides this property.
+
+### Examples
+
+In addition to the examples above, concerning how to skip website documentation generation in the `docs` module and how
+to clean downloaded libraries in the `lib` module, here are a few more:
+
+  * Run a clean default build including tests and generating Aspect documentation: 
+    ```shell
+    mvn clean verify
+    ```
+    If you wish to install all artifacts in the local Maven repository, because subsquently maybe you want to run builds
+    for submodules you are working on and which need to find other artifacts in the repository for a successful build,
+    you rather use:
+    ```shell
+    mvn clean install
+    ```
+
+  * Run a fast build, no test compilation and execution, no AspectJ documentation, no javadoc, no source JARs 
+    ```shell
+    mvn -P fast-build package
+    ```
+
+  * Run a release build incl. tests, GPG artifact signing and deployment:
+    ```shell
+    mvn -P release clean deploy
+    ```
+
+  * Run a release build incl. deployment, but without compiling and running tests because you ran all tests before
+    successfully already:
+    ```shell
+    mvn -P release,fast-build clean deploy
+    ```
+    This is effectively the same as:
+    ```shell
+    mvn -P release -Dmaven.test.skip=true clean deploy
+    ```
+    In a UNIX shell, you probably have to double-quote when using properties containing dots:  
+    ```shell
+    mvn -P release "-Dmaven.test.skip=true" clean deploy
+    ```
+
+In general, you should not combine profiles setting the same properties in contradictory ways. If you need a very
+specific build configuration, you might want to use a profile matching your needs most closely and override specific
+properties. However, I am not going to share examples for this approach, because generally it is not necessary and also
+both error-prone and sensitive to evan small changes in Maven POMs.  
diff --git a/docs/pom.xml b/docs/pom.xml
index f2caccaf8..57baa3603 100644
--- a/docs/pom.xml
+++ b/docs/pom.xml
@@ -14,7 +14,10 @@
     <profile>
       <id>create-docs</id>
       <activation>
-        <activeByDefault>true</activeByDefault>
+        <property>
+          <name>createDocs</name>
+          <value>true</value>
+        </property>
       </activation>
 
       <build>