Support -D system properties for compiler options (#275)

[AndreasIgel]

Summary

mvn compile -Dsimplebuilder.<option>=<value> (and the Gradle equivalent) now works without a -Asimplebuilder.<option>=${simplebuilder.<option>} mapping in the build file. Previously the docs suggested it did, but the processor only read processingEnv.getOptions() (i.e. -A args), so -D alone was silently ignored (#275).

CompilerArgumentsReader.readValue now resolves:

value = System.getProperty("simplebuilder.<opt>");   // -D  (new, highest)
if (value == null) value = options.get("simplebuilder.<opt>");  // -A, prefixed
if (value == null) value = options.get("<opt>");                // -A, bare (legacy)

The system property is checked first: -A args from pom.xml/build.gradle and from the CLI land in the same javac option map, so a -D given on the command line would otherwise never be able to override a setting configured in the build file. Only the prefixed name is looked up as a system property — a bare -Dverbose=true is deliberately ignored to avoid colliding with unrelated JVM properties. Since every option goes through readValue, this covers all of readBuilderConfiguration, verbose, performanceTracking* and deactivateGenerationComponents at once.

Effective precedence: @SimpleBuilder.Options > -D system property > -A compiler arg > default.

Verified end-to-end (not committed): Maven mvn -pl example clean compile -D… changes the generated builders; Gradle 8.10 with the default in-process compiler (-D… on the CLI or systemProp.simplebuilder.… in gradle.properties) does too. It does not work with forked javac (<fork>true</fork> / options.fork = true) or IDE builds — -A remains required there — and Gradle does not treat the property as a task input (compileJava stays UP-TO-DATE without clean/--rerun-tasks). All of this is documented in docs/CONFIGURATION.md / docs/DEBUG_LOGGING.md.

Tests: three CompilerArgumentsReaderTest cases (fallback, -D wins over -A, bare-name property ignored).

Closes #275.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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