Java - SonarQube，关于单例中“实用程序类不应具有公共构造函数”（squid：S1118）的问题

Question

I am performing static code analysis on old code using a SonarLint analysis. I cannot paste the code here but it is similar to:

@SuppressWarnings("static-access")
public class SuperClass {

    private SuperClass() {
    }

    public static SuperClass getInstance() {
        return InstanceHolder.instance;
    }

    private static class InstanceHolder {
        public final static SuperClass instance = new SuperClass();
    }

    public void doSomething() {
        //do something
    }

}

SonarQube (sonar-java: 4.2.1.6971), reports an issue on S1118 .

Adding a private constructor to InstanceHolder has no solving effect here, since SuperClass is the only class that can create an instance of it due to its private modifier.

SuperClass can still create an instance, even with ÌnstanceHolder having a private constructor.

BTW: adding the constructor removes the sonar-issue, so I think the analyzer marked this as a rule violation because of the internal 'UtilityClass' without further investigation.

Is this a bug? Instead of a design flaw, this is an example of a thread-safe singleton.

Answer 1

Make your class final so that Instance creation can be avoided.

    @SuppressWarnings("static-access")
    public final class SuperClass {

        private SuperClass() {
        }
    }