The Cleansing Flame of Null Analysis

May 21, 2016, 10:18 AM

Tags: java maven
  1. The Cleansing Flame of Null Analysis
  2. Quick Tip: JDK Null Annotations for Eclipse
  3. The Joyful Utility of Optionals in Java

Though most of my work lately has been on sprawling, platform-level stuff or other large existing codebases, part of it has involved a new small app. I decided to take this opportunity to dive more aggressively than previously into automated null analysis and other potential-bugs tools.

What I mean by "null analysis" is letting the IDE or compiler try to help you avoid NullPointerExceptions. Though there are plenty of other programming mistakes you could still make, these are among the most common, and so a little extra work up front to avoid them should pay dividends. Eclipse has some handy options in its Java → Compiler → Errors/Warnings preferences to assist with this:

The first option will pick up on some pretty basic instances, such as:

Object foo = null;

Since this is clearly going to always cause an NPE, Eclipse is able to point this out as an error. The next level gets a little more nebulous: "potential" null pointer access. This crops up when Eclipse can't reliably determine whether a value will be null, either because there is no way to know at compile time (say, database access) or because the compiler's tooling is too limited. Here's a contrived example:

Object foo = Math.random() > 0.5 ? new Object() : null;

This situation is clearly untenable, but there are other situations where you as a programmer can be very confident that the value will not be null (say, if you swap out the > 0.5 for >= 0.0), but the compiler doesn't know that. That's why it often makes sense to leave that as a warning instead of an error.

That's all stuff I've done before, but now I've decided to dive into annotation-based null analysis as well. Unfortunately, in stock Java, this is something of a hot mess (that list even leaves out Eclipse's home-grown version). Since Java didn't grow up with this sort of capability, it's been shoehorned in by various parties over the years. There are other tools to assist you in Java 8, but, unfortunately, I can only target 7 as the highest. For now, I've settled on the "sort-of standard" javax.validation.constraints package. It wasn't really intended for this specific purpose, but it's flexible enough to suit and can be used in Eclipse and FindBugs (though I have my reservations about the choice).

In Eclipse, this type of analysis can be enabled by checking "Enable annotation-based null analysis" below the other options and, unless you're using Eclipse's known annotations, adjusting the "Configure" options next to "Use default annotations for null specifications":

In any event, regardless of the choice of tooling, the "this shouldn't be null" annotations work the same way: you use them to decorate things that you either require not be null when provided to you (method parameters) or you promise to not be null when providing to others (method return values). For example:

public @NotNull Object doSomething(@NotNull Object otherObject) {
	return otherObject.toString();

This highlights three things, two good and one bad:

  • Good: The @NotNull in the method parameter means that, as long as the calling code is also checked for null use, the method can be confident that there won't be a NullPointerException when calling a method on otherObject.
  • Good: The @NotNull on the return value means that other code calling this method can be confident that they will not get a null value from it, and so can skip extra null checks.
  • Bad: Eclipse flags otherObject.toString() as a potential problem because it doesn't know for sure that Object#toString doesn't return null, because it has no nullability annotations. As programmers (or as a compiled-code analysis tool), we can be fairly confident that it will be non-null because any object returning null for that is essentially broken on its own.

That last one is a common problem when adopting annotation-based null analysis, at least in Eclipse (I hear it may be better in IntelliJ): its logic doesn't go very deep. If everything is gussied up with these annotations, you're clear - but as soon as you step outside of the project you're working on, you have to add in likely-unnecessary checks. Fortunately, these checks don't realistically hurt (a null check at runtime in a normal app is negligible performance-wise), but they can grate to have to add in.

Glutton for punishment that I am, I decided to go a step further and enable FindBugs processing as an integral step of my build. Though FindBugs can be very picky about the types of things it complains about, it is blessedly more thorough in its analysis than Eclipse, so you generally end up conceding that it is correct when it yells at you. Since the project is Maven-based, I added the check in the project's pom file:


For most uses, that's all that's required. Now, when the project is compiled, FindBugs will give it a once-over and halt the build if it finds anything it doesn't like. This can be tweaked a great deal - for example, changing the checks to run or the severity of the problem needed to fail the build - but the defaults will likely suit.

Adding these extra checks involves a lot of plusses and minuses. The big minus is that you may end up spending a lot of time "fixing" bugs that don't really exist, time that you could instead spend actually writing your application (and writing new bugs that the tools won't find anyway). There's really nothing to be gained by carefully explaining to Eclipse for the hundredth time that toString always returns non-null.

Still, particularly when tested out in a small, low-surface-area app, this can be a good practice to learn and refine. Eventually, a move to Java 8 will help this more, and it certainly doesn't hurt to add in nullability annotations in the mean time. Overall, I think having the tooling help you avoid a whole suite of common "brain fart" bugs like this is worthwhile.

Commenter Photo

Nathan T Freeman - Jun 7, 2016, 12:52 PM

Stop using otherObject.toString() and start using String.valueOf(otherObject) instead! :)

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