Spotbugs: CheckForNull does not override ParametersAreNonnullByDefault

[danielb987]

While trying to understand the problem in #6510 (comment), I found that @CheckForNull for a parameter does not overrides @ParametersAreNonnullByDefault for the class.

package jmri;

import javax.annotation.*;

@ParametersAreNonnullByDefault
class ClassA {
    
    public void testA(@CheckForNull Object param) {
        // Do nothing
    }
    
    public void testB(@Nullable Object param) {
        // Do nothing
    }
}

This means that Spotbugs will give an error on new ClassA().testA(null); but not on new ClassA().testB(null);, since the later is annotated with @Nullable.

The JMRI documentation about Spotbugs say that @CheckForNull can be used for parameters, but that's not true if the class is annotated with @ParametersAreNonnullByDefault.

The JMRI documentation says that @Nullable shouldn't be used and that @CheckForNull should be used instead. But if the class is annotated with @ParametersAreNonnullByDefault, @Nullable works but @CheckForNull doesn't.

My branch nonnull_4_test has a working example of the above.

[danielb987]

@bobjacobsen @pabender
Any comments on this issue?

The JMRI documentation says that @Nullable shouldn't be used since SpotBugs doesn't really check anything when this is used. But I don't really understand what Spotbugs should check when @Nullable is used, so I don't see the problem with using it.

[danielb987]

I have done additional testing of this class:

package jmri;

import javax.annotation.*;

@ParametersAreNonnullByDefault
class ClassA {
    
    public void testA(@CheckForNull Object param) {
        testC(param);
    }
    
    public void testB(@Nullable Object param) {
        testC(param);
    }
    
    private void testC(Object param) {
        // Do nothing
    }
    
}

The annotation @CheckForNull doesn't seem to do anything. Spotbugs complains if I call testA(null) and Spotbugs does not complains about the call testC(param) from the method testA. So Spotbugs does not treat the param in the method declaration for testA as a parameter that can be null.

The annotation @Nullable makes Spotbugs be happy with the call testB(null) but Spotbugs complains about the call testC(param) from the method testB. So Spotbugs clearly treats the param in the method declaration for testB as a parameter that can be null.

So my testing shows that @Nullable works fine but @CheckForNull does not work at all when the class is annotated with @ParametersAreNonnullByDefault.

The comment in the JMRI documentation about spotbugs which tells that @CheckForNull should be used instead of @Nullable was written for FindBugs before we switched over to Spotbugs, so it may be a difference between Spotbugs and Findbugs. But the comment is not valid today.

[bobjacobsen]

A bit short of time right now, but see java/src/apps/FindBugsCheck.java

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