[java] AtLeastOneConstructor

[dgroup]

Affects PMD Version:
6.44.0+
7.0.0

Rule: AtLeastOneConstructor

Please provide the rule name and a link to the rule documentation:
https://pmd.github.io/latest/pmd_rules_java_codestyle.html#atleastoneconstructor

Description:

Code Sample demonstrating the issue:

• Case 1: when class implement functional interface that doesn't need the constructor

public class ToString implements java.util.Function<Object, String> {
     @Override
     public String apply(Object arg) {
         return arg.toString();
    }
}

• Case 2: When methods in class have some particular annotations like @org.junit.Test, etc.

public class AbstractForSomeReason { // violation at this line
     
     @org.junit.Test / @Before / @After / @BeforeSuite / @BeforeGroup / @etc.
     public void test() {
          // implementation
    }
}

• Case 3: When we have unit test class, thus we don't need the constructor

public class ClassTest { // violation at this line, but here is workaround below
     
     public void test() {
          // implementation
    }
}

could be fixed by the following configuration

<rule ref="category/java/codestyle.xml/AtLeastOneConstructor">
    <properties>
      <property name="violationSuppressXPath" value=".[ends-with(@SimpleName,'Test')]"/>
      <property name="ignoredAnnotations" value="lombok.Data|lombok.Value|lombok.Builder|lombok.NoArgsConstructor|lombok.RequiredArgsConstructor|lombok.AllArgsConstructor|org.springframework.context.annotation.Configuration|org.springframework.web.bind.annotation.ControllerAdvice|org.springframework.web.bind.annotation.RestController|lombok.experimental.SuperBuilder|org.springframework.stereotype.Component|org.springframework.stereotype.Service|org.springframework.stereotype.Repository|org.springframework.boot.context.properties.ConfigurationProperties|org.mapstruct.Mapper|javax.xml.bind.annotation.XmlRootElement|org.junit.jupiter.api.extension.ExtendWith|org.mockserver.junit.jupiter.MockServerSettings|org.springframework.test.context.ContextConfiguration|org.springframework.test.context.web.WebAppConfiguration|org.junit.runner.RunWith" />
    </properties>
  </rule>

Expected outcome:

PMD reports a violation in case1 and case2, but that's wrong. That's a false positive.
The possible ways of resolution:

1. Make ignoredAnnotations to support method-level (not only class-level as implemented now)

   <property name="ignoredAnnotations" value="lombok.Data|org.junit.Test" />

   where @lombok.Data is class-level annotation, and @org.junit.Test is method-level annotation
2. Make a new property 'allowedInterfaces' where you can specify full class name or package like

   <property name="allowedInterfaces" value="java.lang|org.apache.camel.Processor"/>

3. Deprecate the rule if there is no exact reason to keep it

Running PMD through: Gradle

[cemuener]

Rule AtLeastOneConstructor does not make sense for @UtilityClass annotations.
Interferres with IDE rule: No constructors for UtilityClasses.