Why does the main Spring Boot application always trigger PMD's HideUtilityClassConstructorCheck?

The standard Spring Boot application has some main method class file, say SampleApplication.java, that looks like this:

SampleApplication.java

@SpringBootApplication
@RestController
public class SampleApplication

public static void main(final String args)
SpringApplication.run(SampleApplication.class, args);





But PMD static analysis flags it as an error (HideUtilityClassConstructorCheck):

Utility classes
should not have a public or default constructor.

Makes sure that utility classes (classes that contain only static
methods or fields in their API) do not have a public constructor.

Rationale: Instantiating utility classes does not make sense. Hence
the constructors should either be private or (if you want to allow
subclassing) protected. A common mistake is forgetting to hide the
default constructor.

If you make the constructor protected you may want to consider the
following constructor implementation technique to disallow
instantiating subclasses:

public class StringUtils // not final to allow subclassing protected
StringUtils() // prevents calls from subclass throw new
UnsupportedOperationException(); public static int count(char c,
String s) // ...

Why is this? Should I be suppressing this PMD error?

2 Answers
2

The inspection speaks for itself.

By default any code inspector (IntelliJ IDEA, FindBugs, PMD, Sonar) assumes that if class has only static methods then it's utility class. Example of utility class is java.lang.Math, which looks like this:

static

java.lang.Math

public final class Math

/**
* Don't let anyone instantiate this class.
*/
private Math()

public static double exp(double a)
...


// More helper methods



Such classes are designed for using it as a bag of static functions: it's a good practice to declare private constructor for it, so no one will ever instantiate it by mistake and declare class final, because extending it makes no sense.

final

In your case (and in case almost every entry point of Spring Boot applications) SampleApplication class has one public static void main method, so PMD decides its utility class, checks private construction and final modifier and flags the error. It's not the problem, PMD just don't know about Spring Boot or any other frameworks and their entry points, so it makes perfect sense to suppress this warning and exclude your class from PMD: for me its more semantically correct than adding private constructor to application entry point.

SampleApplication

public static void main

The PMD UseUtilityClass rule can be suppressed just for classes with @SpringBootApplication annotation using the following snippet in the PMD ruleset XML file:

@SpringBootApplication

<rule ref="category/java/design.xml/UseUtilityClass">
<properties>
<property name="violationSuppressXPath" value="//ClassOrInterfaceDeclaration/preceding-sibling::Annotation/MarkerAnnotation/Name[@Image='SpringBootApplication']" />
</properties>
</rule>


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