False positive SING_SINGLETON_HAS_NONPRIVATE_CONSTRUCTOR with reused instances about spotbugs

[rovarga]

In this particular case, the fact that the class is NOT a singleton can easily be inferred:

1. the base class is abstract, but defines hashCode()/equals() methods which are final. The designer was clearly thinking about subclassing and defined the equality contract

2. the implementation of equals() performs instanceof checks (as opposed to .getClass() equality check), further reinforcing the fact the class is supposed to be subclassed

[xzel23]

What heuristics does SpotBugs use to detect singleton classes? After upgrading to the new release, I also get several warnings of this type for classes that are clearly not singletons. For example one class has several factory methods that each return new instances. And there are several package private constructors declared.

Also AFAIK synchronization is not always needed, i. e. if the instance is held in a static field that is unconditionally initialized either at declaration or in a static initializer block.

This rule needs some rework.

[JuditKnoll]

@rovarga Thank you for the example and your explanation. I proposed a PR fixing this FP.

@xzel23 Yes, you are right about the synchronization. It was a bug in the detector in SpotBugs, which was reported in #2932 and the PR fixing it was already merged to master, so it will be in the next release.
If you could share some (hopefully minimal) examples, that could help identifying the underlying issues.

[xzel23]

@JuditKnoll Here is an example. Even if you say the error reported by spotbugs is already fixed, the important thing is: this simply is not a singleton class, so none of the singleton rules should apply. This class provides public factory methods to create new instances, this should be detected and the class not be marked as singleton:

package issues.spotbugs;

public class A {

    private static final A INSTANCE = new A("1");

    private final String text;

    private A(String text) {
        this.text = text;
    }

    public static A instance1() {
        return INSTANCE;
    }

    public static A create(String text) {
        return new A("text");
    }

    public String getText() {
        return text;
    }
}

[JuditKnoll]

@xzel23 Thank you for your example. The #2951 modifies the logic for identifying Singletons as well, which was indeed not strict enough. Your example (named NotSingletonWithFactoryMethod in the PR) isn't be considered Singleton in it.