Code Quality Improvement Plan

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.

This is a multi-item plan rather than a single task. Start by selecting one unchecked item, then inspect the named pom.xml, Histogram.java, configuration classes, or servlet modules and account for the SpotBugs Java 25 compatibility note. Done means the selected checklist item is implemented and its scope is verified.