This project demonstrates a JaCoCo false-negative coverage measurement bug that affects Kotlin interfaces containing a companion object with a @JvmStatic-annotated method.

All tests pass and fully exercise the method under test, yet JaCoCo reports it as 0% covered with 1 missed instruction.

• JDK 17+
• Gradle 8.x on PATH (only needed once to bootstrap the wrapper)

cd jacoco-jvmstatic-interface-bug
gradle wrapper --gradle-version 8.12

This generates gradlew, gradlew.bat, and gradle/wrapper/gradle-wrapper.jar.

./gradlew test jacocoTestReport

IConfigTest > forUser returns EnterpriseConfig when isConsumer is false  PASSED
IConfigTest > forUser returns ConsumerConfig when isConsumer is true      PASSED
IConfigTest > EnterpriseConfig returns expected base URL                  PASSED
IConfigTest > ConsumerConfig strips query parameters from URL             PASSED

BUILD SUCCESSFUL

Open the HTML report:

build/reports/jacoco/test/html/index.html

Navigate to com.example.coverage → click IConfig.

You will see:

In the source view, the closing } of forUser is highlighted red (not covered). The function declaration and return statement have no highlight at all — they are not tracked in this class entry (see Root Cause below for why).

In build/reports/jacoco/test/jacocoTestReport.xml, find <class name="com/example/coverage/IConfig">:

<method name="forUser" desc="(Z)Lcom/example/coverage/IConfig;" line="43">
  <counter type="INSTRUCTION" missed="1" covered="0"/>
  <counter type="BRANCH"      missed="0" covered="0"/>
  <counter type="LINE"        missed="1" covered="0"/>
  <counter type="COMPLEXITY"  missed="1" covered="0"/>
  <counter type="METHOD"      missed="1" covered="0"/>
</method>

And in <sourcefile name="IConfig.kt">:

<!-- Only the closing brace line appears; the declaration and return lines have no entry -->
<line nr="47" mi="1" ci="0" mb="0" cb="0"/>

(nr is the line number of the closing } of forUser in IConfig.kt.)

When @JvmStatic is placed on a method inside a companion object of a Kotlin interface, kotlinc emits two separate class files:

The bridge in IConfig.class is roughly equivalent to:

// Synthetic — generated by kotlinc for Java interop
public static IConfig forUser(boolean isConsumer) {
    return IConfig.Companion.forUser(isConsumer);
}

Its bytecode LineNumberTable maps every instruction to the closing brace line of the forUser method body in the Kotlin source — that closing } is the only line number entry the bridge carries.

When a Kotlin call site writes IConfig.forUser(...), the Kotlin compiler resolves the call directly to IConfig$Companion.forUser(...). The static bridge is never invoked. Only a Java call site (e.g., IConfig.forUser(true) in a .java file) goes through it.

All tests in this project are Kotlin, so the bridge instruction is never executed.

JaCoCo maps the bridge's single missed instruction to the closing } line, producing the false "1 missed line" report under IConfig.kt.

Lines corresponding to the function declaration and return statement appear in the IConfig$Companion class entry (as covered), not in the IConfig entry, which is why they show no coverage highlight at all in the IConfig.kt source view.

Search the XML for IConfig$Companion:

<class name="com/example/coverage/IConfig$Companion" ...>
  <method name="forUser" desc="(Z)Lcom/example/coverage/IConfig;">
    <counter type="INSTRUCTION" missed="0" covered="5"/>
    <counter type="METHOD"      missed="0" covered="1"/>
  </method>
</class>

The companion class entry shows full coverage. The bug is that JaCoCo does not suppress the uninvocable synthetic bridge from the coverage metrics of IConfig.

The unique combination here is:

• @JvmStatic annotation
• Inside a companion object
• Inside an interface (not a class)
• Called only from Kotlin (so the Java-interop bridge is never reached)

1. Add a Java test that calls IConfig.forUser(...) from a .java file. This triggers the static bridge and removes the false negative — but requires adding an otherwise-unnecessary Java caller.

2. Exclude IConfig.class via a JaCoCo class-directory filter (e.g., **/IConfig.class). This over-excludes if the interface has other coverable code such as default method implementations.

3. Remove @JvmStatic if Java interop is not required. The companion method remains callable from Kotlin as IConfig.forUser(...) regardless; Kotlin resolves that to the companion without needing the bridge.

4. Wait for a JaCoCo fix that either:

   • Suppresses synthetic interop bridges on interface companion classes from line/instruction/method coverage (preferred), or
   • Merges bridge coverage with the companion method so that covering IConfig$Companion.forUser counts the bridge as covered.

src/
├── main/kotlin/com/example/coverage/
│   ├── IConfig.kt          # Interface with @JvmStatic companion — triggers the bug
│   ├── EnterpriseConfig.kt # Enterprise implementation
│   ├── ConsumerConfig.kt   # Consumer implementation
│   └── ConfigProvider.kt   # Two-branch factory that selects the implementation
└── test/kotlin/com/example/coverage/
    └── IConfigTest.kt      # 4 tests, all Kotlin callers, all pass

See ISSUE.md for the upstream JaCoCo bug report text.