Code quality improvements

[zeitlinger]

Code Quality Improvement Plan

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

High Priority

1. Add Missing Test Coverage for Exporter Modules

• prometheus-metrics-exporter-common - base module, no tests
• prometheus-metrics-exporter-servlet-jakarta - no tests
• prometheus-metrics-exporter-servlet-javax - no tests
• prometheus-metrics-exporter-opentelemetry-otel-agent-resources - no tests

2. Eliminate Dropwizard Module Duplication

• Create shared base class or use generics for prometheus-metrics-instrumentation-dropwizard and prometheus-metrics-instrumentation-dropwizard5 (~297 lines each, nearly identical)

3. Address Technical Debt (TODOs)

• prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/Histogram.java:965 - "reset interval isn't tested yet"
• prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/Summary.java:205 - "Exemplars (are hard-coded as empty)"
• prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/SlidingWindow.java:18 - "synchronized implementation, room for optimization"
• prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/PrometheusPropertiesLoader.java:105 - "Add environment variables like EXEMPLARS_ENABLED"
• prometheus-metrics-exporter-opentelemetry/src/main/java/io/prometheus/metrics/exporter/opentelemetry/PrometheusMetricProducer.java:44 - "filter configuration for OpenTelemetry exporter"
• prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/ExporterOpenTelemetryProperties.java:7 - "JavaDoc missing"

4. Improve Exception Handling

Replace broad catch (Exception e) with specific exception types:

• prometheus-metrics-instrumentation-dropwizard5/src/main/java/.../DropwizardExports.java:237
• prometheus-metrics-instrumentation-caffeine/src/main/java/.../CacheMetricsCollector.java:229
• prometheus-metrics-exporter-opentelemetry/src/main/java/.../PrometheusInstrumentationScope.java:47
• prometheus-metrics-exporter-opentelemetry/src/main/java/.../OtelAutoConfig.java:115
• prometheus-metrics-instrumentation-jvm/src/main/java/.../JvmNativeMemoryMetrics.java:166
• prometheus-metrics-exporter-httpserver/src/main/java/.../HttpExchangeAdapter.java:115

Medium Priority

5. Add Branch Coverage to JaCoCo

• Update pom.xml to add branch coverage requirement (~50% minimum)

<limit>
  <counter>BRANCH</counter>
  <value>COVEREDRATIO</value>
  <minimum>0.50</minimum>
</limit>

6. Raise Minimum Coverage Thresholds

Current thresholds to review:

• prometheus-metrics-exporter-httpserver - 45% (raise to 60%)
• prometheus-metrics-instrumentation-dropwizard5 - 50% (raise to 60%)
• prometheus-metrics-exposition-textformats - 50% (raise to 60%)
• prometheus-metrics-instrumentation-jvm - 55% (raise to 60%)

7. Add SpotBugs

• Add spotbugs-maven-plugin to pom.xml
• Configure with appropriate rule set

8. Narrow Checkstyle Suppressions

• Review checkstyle-suppressions.xml - currently suppresses ALL Javadoc checks globally
• Narrow to specific packages/classes that need exceptions

Lower Priority

9. Refactor Large Classes

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

10. Document Configuration Classes

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

11. Consolidate Servlet Exporter Duplication

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

12. Add Mutation Testing

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

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Progress Notes

Add notes here as items are completed:

Date	Item	Notes
2026-01-24	Missing Test Coverage for Exporter Modules	Added 55 tests across 4 modules: exporter-common (22 tests), servlet-jakarta (14 tests), servlet-javax (14 tests), otel-agent-resources (5 tests). All tests passing.
2026-01-24	Eliminate Dropwizard Module Duplication	Created AbstractDropwizardExports base class (267 lines) with generic type parameters. Reduced dropwizard module from 297 to 209 lines (-88 lines, -30%), dropwizard5 module from 297 to 212 lines (-85 lines, -29%). All tests passing (32 tests dropwizard5, 13 tests dropwizard).