improve code quality, part 1

.editorconfig

@@ -4,10 +4,10 @@ root = true
 max_line_length = 100
 indent_size = 2
 
-[{version-rules.xml,maven-wrapper.properties,checkstyle.xml,docker-compose.yaml,docker-compose.yml,Dockerfile,example_target_info.json,mise.toml,mvnm,mvnw.cmd,generate-protobuf.sh,super-linter.env,.gitleaksignore,*.json5}]
+[{version-rules.xml,maven-wrapper.properties,checkstyle.xml,docker-compose.yaml,docker-compose.yml,Dockerfile,example_target_info.json,mise.toml,mvnm,mvnw.cmd,generate-protobuf.sh,.gitleaksignore,*.json5}]
 max_line_length = 200
 
-[{grafana-dashboard-*.json,.editorconfig}]
+[{grafana-dashboard-*.json,.editorconfig,super-linter.env}]
 max_line_length = 300
 
 [pom.xml]

.github/super-linter.env

@@ -1,4 +1,4 @@
-FILTER_REGEX_EXCLUDE=mvnw|src/main/generated/.*|docs/themes/.*|keystore.pkcs12|.*.java|prometheus-metrics-exporter-opentelemetry-shaded/pom.xml|CODE_OF_CONDUCT.md
+FILTER_REGEX_EXCLUDE=mvnw|src/main/generated/.*|docs/themes/.*|keystore.pkcs12|.*.java|prometheus-metrics-exporter-opentelemetry-shaded/pom.xml|CODE_OF_CONDUCT.md|CLAUDE.md|CODE_QUALITY_IMPROVEMENTS.md
 IGNORE_GITIGNORED_FILES=true
 JAVA_FILE_NAME=google_checks.xml
 LOG_LEVEL=ERROR

CLAUDE.md

@@ -0,0 +1,96 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Build Commands
+
+This project uses Maven with mise for task automation. The Maven wrapper (`./mvnw`) is used for all builds.
+
+```bash
+# Full CI build (clean + install + all checks)
+mise run ci
+
+# Quick compile without tests or checks (fastest for development)
+mise run compile
+
+# Run unit tests only (skips formatting/coverage/checkstyle checks)
+mise run test
+
+# Run all tests including integration tests
+mise run test-all
+
+# Format code with Google Java Format
+mise run format
+# or directly: ./mvnw spotless:apply
+
+# Run a single test class
+./mvnw test -Dtest=CounterTest -Dspotless.check.skip=true -Dcoverage.skip=true -Dcheckstyle.skip=true
+
+# Run a single test method
+./mvnw test -Dtest=CounterTest#testIncrement -Dspotless.check.skip=true -Dcoverage.skip=true -Dcheckstyle.skip=true
+
+# Run tests in a specific module
+./mvnw test -pl prometheus-metrics-core -Dspotless.check.skip=true -Dcoverage.skip=true -Dcheckstyle.skip=true
+
+# Regenerate protobuf classes (after protobuf dependency update)
+mise run generate
+```
+
+## Architecture
+
+The library follows a layered architecture where metrics flow from core types through a registry to exporters:
+
+```
+prometheus-metrics-core (user-facing API)
+         │
+         ▼ collect()
+prometheus-metrics-model (immutable snapshots)
+         │
+         ▼
+prometheus-metrics-exposition-formats (text/protobuf/OpenMetrics)
+         │
+         ▼
+Exporters (httpserver, servlet, pushgateway, opentelemetry)
+```
+
+### Key Modules
+
+- **prometheus-metrics-core**: User-facing metric types (Counter, Gauge, Histogram, Summary, Info, StateSet). All metrics implement the `Collector` interface with a `collect()` method.
+- **prometheus-metrics-model**: Internal read-only immutable snapshot types returned by `collect()`. Contains `PrometheusRegistry` for metric registration.
+- **prometheus-metrics-config**: Runtime configuration via properties files or system properties.
+- **prometheus-metrics-exposition-formats**: Converts snapshots to Prometheus exposition formats.
+- **prometheus-metrics-tracer**: Exemplar support with OpenTelemetry tracing integration.
+- **prometheus-metrics-simpleclient-bridge**: Allows legacy simpleclient 0.16.0 metrics to work with the new registry.
+
+### Instrumentation Modules
+
+Pre-built instrumentations: `prometheus-metrics-instrumentation-jvm`, `-caffeine`, `-guava`, `-dropwizard`, `-dropwizard5`.
+
+## Code Style
+
+- **Formatter**: Google Java Format (enforced via Spotless)
+- **Line length**: 100 characters
+- **Indentation**: 2 spaces
+- **Static analysis**: Error Prone with NullAway (`io.prometheus.metrics` package)
+- **Logger naming**: Logger fields must be named `logger` (not `log`, `LOG`, or `LOGGER`)
+- **Assertions in tests**: Use static imports from AssertJ (`import static org.assertj.core.api.Assertions.assertThat`)
+- **Empty catch blocks**: Use `ignored` as the exception variable name
+
+## Linting and Validation
+
+- **IMPORTANT**: Always run `mise run build` after modifying Java files to ensure all lints, code formatting (Spotless), static analysis (Error Prone), and checkstyle checks pass
+- **IMPORTANT**: Always run `mise run lint:super-linter` after modifying non-Java files (YAML, Markdown, shell scripts, JSON, etc.)
+- Super-linter is configured to only show ERROR-level messages via `LOG_LEVEL=ERROR` in `.github/super-linter.env`
+- Local super-linter version is pinned to match CI (see `.mise/tasks/lint/super-linter.sh`)
+
+## Testing
+
+- JUnit 5 (Jupiter) with `@Test` annotations
+- AssertJ for fluent assertions
+- Mockito for mocking
+- Integration tests are in `integration-tests/` and run during `verify` phase
+- Acceptance tests use OATs framework: `mise run acceptance-test`
+
+## Java Version
+
+Source compatibility: Java 8. Tests run on Java 25 (configured in `mise.toml`).

CODE_QUALITY_IMPROVEMENTS.md

@@ -0,0 +1,86 @@
+# 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
+- [x] `prometheus-metrics-exporter-common` - base module, no tests
+- [x] `prometheus-metrics-exporter-servlet-jakarta` - no tests
+- [x] `prometheus-metrics-exporter-servlet-javax` - no tests
+- [x] `prometheus-metrics-exporter-opentelemetry-otel-agent-resources` - no tests
+
+### 2. Eliminate Dropwizard Module Duplication
+- [x] 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)
+```xml
+<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`
+
+---
+
+## 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). |

prometheus-metrics-exporter-common/pom.xml

@@ -38,6 +38,14 @@
       <version>${project.version}</version>
       <scope>runtime</scope>
     </dependency>
+
+    <!-- test dependencies -->
+    <dependency>
+      <groupId>io.prometheus</groupId>
+      <artifactId>prometheus-metrics-core</artifactId>
+      <version>${project.version}</version>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
 </project>

prometheus-metrics-exporter-common/src/test/java/io/prometheus/metrics/exporter/common/PrometheusHttpRequestTest.java

@@ -0,0 +1,120 @@
+package io.prometheus.metrics.exporter.common;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Collections;
+import java.util.Enumeration;
+import org.junit.jupiter.api.Test;
+
+class PrometheusHttpRequestTest {
+
+  @Test
+  public void testGetHeaderReturnsFirstValue() {
+    PrometheusHttpRequest request =
+        new TestPrometheusHttpRequest("name[]=metric1&name[]=metric2", "gzip");
+    assertThat(request.getHeader("Accept-Encoding")).isEqualTo("gzip");
+  }
+
+  @Test
+  public void testGetHeaderReturnsNullWhenNoHeaders() {
+    PrometheusHttpRequest request = new TestPrometheusHttpRequest("", null);
+    assertThat(request.getHeader("Accept-Encoding")).isNull();
+  }
+
+  @Test
+  public void testGetParameterReturnsFirstValue() {
+    PrometheusHttpRequest request = new TestPrometheusHttpRequest("name[]=metric1&name[]=metric2");
+    assertThat(request.getParameter("name[]")).isEqualTo("metric1");
+  }
+
+  @Test
+  public void testGetParameterReturnsNullWhenNotPresent() {
+    PrometheusHttpRequest request = new TestPrometheusHttpRequest("other=value");
+    assertThat(request.getParameter("name[]")).isNull();
+  }
+
+  @Test
+  public void testGetParameterValuesReturnsMultipleValues() {
+    PrometheusHttpRequest request = new TestPrometheusHttpRequest("name[]=metric1&name[]=metric2");
+    String[] values = request.getParameterValues("name[]");
+    assertThat(values).containsExactly("metric1", "metric2");
+  }
+
+  @Test
+  public void testGetParameterValuesReturnsNullWhenNotPresent() {
+    PrometheusHttpRequest request = new TestPrometheusHttpRequest("other=value");
+    assertThat(request.getParameterValues("name[]")).isNull();
+  }
+
+  @Test
+  public void testGetParameterValuesWithEmptyQueryString() {
+    PrometheusHttpRequest request = new TestPrometheusHttpRequest("");
+    assertThat(request.getParameterValues("name[]")).isNull();
+  }
+
+  @Test
+  public void testGetParameterValuesWithNullQueryString() {
+    PrometheusHttpRequest request = new TestPrometheusHttpRequest(null);
+    assertThat(request.getParameterValues("name[]")).isNull();
+  }
+
+  @Test
+  public void testGetParameterValuesWithUrlEncodedValues() {
+    PrometheusHttpRequest request = new TestPrometheusHttpRequest("name=hello%20world");
+    String[] values = request.getParameterValues("name");
+    assertThat(values).containsExactly("hello world");
+  }
+
+  @Test
+  public void testGetParameterValuesWithSpecialCharacters() {
+    PrometheusHttpRequest request = new TestPrometheusHttpRequest("name=%2Ffoo%2Fbar");
+    String[] values = request.getParameterValues("name");
+    assertThat(values).containsExactly("/foo/bar");
+  }
+
+  @Test
+  public void testGetParameterValuesIgnoresParametersWithoutEquals() {
+    PrometheusHttpRequest request =
+        new TestPrometheusHttpRequest("name[]=value1&invalid&name[]=value2");
+    String[] values = request.getParameterValues("name[]");
+    assertThat(values).containsExactly("value1", "value2");
+  }
+
+  /** Test implementation of PrometheusHttpRequest for testing default methods. */
+  private static class TestPrometheusHttpRequest implements PrometheusHttpRequest {
+    private final String queryString;
+    private final String acceptEncoding;
+
+    TestPrometheusHttpRequest(String queryString) {
+      this(queryString, null);
+    }
+
+    TestPrometheusHttpRequest(String queryString, String acceptEncoding) {
+      this.queryString = queryString;
+      this.acceptEncoding = acceptEncoding;
+    }
+
+    @Override
+    public String getQueryString() {
+      return queryString;
+    }
+
+    @Override
+    public Enumeration<String> getHeaders(String name) {
+      if (acceptEncoding != null && name.equals("Accept-Encoding")) {
+        return Collections.enumeration(Collections.singletonList(acceptEncoding));
+      }
+      return null;
+    }
+
+    @Override
+    public String getMethod() {
+      return "GET";
+    }
+
+    @Override
+    public String getRequestPath() {
+      return "/metrics";
+    }
+  }
+}