JAVA.DEEPNULL.RET.METH : Return Value may be null (Java)

Summary

A method might return null.

This checker looks for program points where, at runtime, a null pointer might be dereferenced and the execution stopped with a NullPointerException. It also looks for calls to library methods where null, or a collection or array containing a null element, might be passed as a parameter in a place where this is not allowed.

Since nullness is undecidable, it is impossible to find all and only the nullness bugs, without any false positive. As a consequence, the analyzer will often issue spurious warnings, where a fake bug is signalled because the analyzer is not clever enough to understand that some value is never actually null. However, the analyzer is currently the only analyzer that does not miss nullness bugs: if there is a real bug, it will be signalled. If the number of false alarms is too high and soundness is not paramount, it is also possible to use the corresponding basic warning class, which is unsound but yields its results much more quickly and with fewer false alarms.

Manually-provided Annotations

In order to reduce the number of false positives, you can instruct the analyzer with nullness annotations, placed in the code that you analyze. Usually, one performs a first nullness analysis and gets many fake warnings (false positives), but most of them have the same origin: a field that the analyzer does not identify as non-null or a method that the analyzer does not identify as always returning a non-null value. Hence, you can use this first analysis to place annotations in your code and then start the analysis again. In particular, the analyzer can be helped by putting the following annotations in the code under analysis:

• @NonNull for fields and return values: states that the field is never null when it is read, or that the method always returns a non-null value;
• @Inner0NonNull for fields and return values of generic type: states that the first generic type variable of the values held in the field or returned by the method is non-null. For instance, this allows one to state that a set bound to a field contains non-null elements only or that a method returns a map whose keys are all non-null;
• @Inner1NonNull for fields and return values of generic type: as in the case above, but predicates over the second generic type variable. For instance, this allows one to state that the values of a map held in a field are all non-null.
• @Nullable for the formal parameters of a method or constructor in a library: it states that it is allowed to pass null for that parameter.

It is also possible to state that a given expression is non-null at a specific program point. For that, use NullnessAssertions.assertNonNull(expression). Note that the fact that a variable is non-null at a program point does not prevent it from holding null later.

Properties

WARNING_FILTER += allow class="Return Value may be null (Java)"

Resolution

Check if the value is actually null and modify the program in such a way to be sure that it is never null again. In most cases, a nullness bug is the consequence of a more complex bug, where some field is not always initialized before being used. Try to define as many as possible of your fields as final. Do not use null as a special value returned by a method.

Relevant Configuration File Parameters

The following configuration file parameters affect checks for this warning class.

• REPORT_SIMILAR_WARNINGS
• WARNING_FILTER