`UnusedMethod` flags `@JsonValue` methods as unused

[hisener]

We have a couple of private static nested classes and enums with @JsonValue methods, mostly in tests. The UnusedMethod bug pattern flags those as unused, unlike the @JsonCreator methods.

Steps to reproduce

JsonValueExample.java

import java.util.Locale;
import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonValue;

public class JsonValueExample {
  private JsonValueExample() {}

  private enum FooBar {
    FOO,
    BAR;

    @JsonValue
    String value() {
      return name().toLowerCase(Locale.ROOT);
    }

    @JsonCreator
    static FooBar of(String value) {
      return valueOf(value.toUpperCase(Locale.ROOT));
    }
  }
}

curl -LO https://repo1.maven.org/maven2/com/google/errorprone/error_prone_core/2.13.1/error_prone_core-2.13.1-with-dependencies.jar
curl -LO https://repo1.maven.org/maven2/com/fasterxml/jackson/core/jackson-annotations/2.13.1/jackson-annotations-2.13.1.jar

javac \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED \
  -J--add-opens=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED \
  -J--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED \
  -cp jackson-annotations-2.13.1.jar \
  -XDcompilePolicy=simple \
  -processorpath error_prone_core-2.13.1-with-dependencies.jar \
  '-Xplugin:ErrorProne -XepDisableAllChecks -Xep:UnusedMethod:ERROR' \
  JsonValueExample.java

JsonValueExample.java:13: error: [UnusedMethod] Method 'value' is never used.
    String value() {
           ^
    (see https://errorprone.info/bugpattern/UnusedMethod)
  Did you mean 'BAR'?
1 error

A potential solution would be adding com.fasterxml.jackson.annotation.JsonValue to exempted annotations similar to JsonCreator.

error-prone/core/src/main/java/com/google/errorprone/bugpatterns/UnusedMethod.java

Line 97 in f489035

"com.fasterxml.jackson.annotation.JsonCreator",

Let me know if that makes sense; I can create a PR, as well.

[rickie]

We have the same situation in some of our tests for the following annotations:

• org.junit.jupiter.params.ParamaterizedTest
• org.junit.jupiter.api.Test
• org.junit.jupiter.api.RepeatedTest

For completeness we can also add the following JUnit 5 variants (the JUnit 4 equivalents are already listed):

• org.junit.jupiter.api.BeforeAll
• org.junit.jupiter.api.AfterAll
• org.junit.jupiter.api.AfterEach
• org.junit.jupiter.api.BeforeEach

And while we are at it, we could also make the JUnit 4 list complete by adding:

• org.junit.Before
• org.junit.After
• org.junit.Test

One last thing to consider is that instead of the org.junit.jupiter.api.Test annotation we could add support for the meta-annotation org.junit.platform.commons.annotation.Testable. That annotation is used on both org.junit.jupiter.api.Test and org.junit.jupiter.api.TestTemplate. TestTemplate is in turn also used in the org.junit.jupiter.api.RepeatedTest annotation.
Supporting this meta annotation requires some more work, but we are also willing to help out.

What do you think of adding these as well?

[retrodaredevil]

I think there's an endless number of annotations that should be allow a method to be except from UnusedMethod, so I think the better solution would be to allow the configuration of these annotations.