javax.annotation: @Nullable vs @CheckForNull

Question

What is the difference between the two? Both seem to mean that the value may be null and should be dealt with accordingly i.e. checked for null.

Update: The two annotations above are part of JSR-305/FindBugs: http://findbugs.sourceforge.net/manual/annotations.html

Answer 1

I think it is pretty clear from the link you added: if you use @CheckForNull and the code that uses the value does not check for null, FindBugs will show it as an error.

FindBugs will ignore @Nullable.

In practice this annotation is useful only for overriding an overarching NonNull annotation.

Use @CheckForNull in the cases when the value must always be checked. Use @Nullable where null might be OK.

EDIT: it seems that @CheckForNull is not well supported at the moment, so I suggest avoiding it and using @NonNull (also see Which @NotNull Java annotation should I use?). Another idea would be to get in touch directly with the FindBugs developers, and ask their opinion about the inconsistency in the documentation.

Answer 2

@Nonnull and @Nullable are correctly handled by IntelliJ IDEA. FindBugs found the problem with @Nonnull but missed those for @Nullable and @CheckForNUll. Problems which were detected by IDEA and FindBugs are marked with comments.

package com.db.icestation;

import javax.annotation.CheckForNull;
import javax.annotation.Nonnull;
import javax.annotation.Nullable;

public class test {

    @Nullable public String nullable() {
        return "";
    }

    @Nonnull public String nonnull() {
        return null; // IDEA, findbugs
    }

    @CheckForNull public String checkForNull() {
        return null;
    }

    public static void main(String[] args) {
        System.out.println(new test().nullable().length()); // IDEA
        System.out.println(new test().nonnull().length());
        System.out.println(new test().checkForNull().length());
    }
}

Answer 3

In the IntelliJ Idea @javax.annotation.Nullable is supported by default and any attempts to dereference @Nullable arguments or return values will result in warning.

@alexander-pavlov, You could add @javax.annotation.CheckForNull in configuration of "Constant conditions & exceptions" inspection. Go File->Settings->Inspections->Probable bugs->Constant conditions & exceptions->Configure annotations.

I prefer doing this as @CheckForNull has more clear meaning than @Nullable as @lbalazscs mentioned in his answer above.