Code Quality Improvement Plan

[zeitlinger]

This issue tracks code quality improvements for the Prometheus Java Client library. Work through these items incrementally.

Remaining Items

Add SpotBugs (Medium Priority)

• Add spotbugs-maven-plugin to pom.xml
• Configure with appropriate rule set
• Note: Deferred due to Java 25 compatibility issues

Refactor Large Classes (Lower Priority)

• prometheus-metrics-core/src/main/java/.../Histogram.java (978 lines) - consider extracting native histogram logic

Document Configuration Classes (Lower Priority)

• PrometheusPropertiesLoader - add JavaDoc
• ExporterProperties and related classes - add JavaDoc
• ExporterOpenTelemetryProperties - add JavaDoc (noted in TODO)

Consolidate Servlet Exporter Duplication (Lower Priority)

• Extract common logic from servlet-jakarta and servlet-javax into exporter-common

Add Mutation Testing (Lower Priority)

• Add Pitest (pitest-maven) for critical modules
• Start with prometheus-metrics-core and prometheus-metrics-model

Completed Items

• Add Missing Test Coverage for Exporter Modules — Added 55 tests across 4 modules: exporter-common (22), servlet-jakarta (14), servlet-javax (14), otel-agent-resources (5).
• Eliminate Dropwizard Module Duplication — Created AbstractDropwizardExports base class with generics. Reduced both modules by ~30%.
• Address Technical Debt (TODOs) — Completed all 6 TODO items: JavaDoc for ExporterOpenTelemetryProperties, Histogram reset interval tests, environment variable support in PrometheusPropertiesLoader, OpenTelemetry filter documentation, Summary exemplars documentation, SlidingWindow synchronization documentation.
• Improve Exception Handling — Replaced broad catch (Exception e) with specific exception types in 6 locations.
• Add Branch Coverage to JaCoCo — Added BRANCH counter check with 50% minimum threshold.
• Raise Minimum Coverage Thresholds — Raised 4 modules to consistent 60% line coverage.
• Narrow Checkstyle Suppressions — Replaced global Javadoc suppression with targeted rules for internal/example/benchmark/test packages.