NullAway: Fast Annotation-Based Null Checking for Java

NullAway is a tool to help eliminate NullPointerExceptions (NPEs) in your Java code. To use NullAway, first add @Nullable annotations in your code wherever a field, method parameter, or return value may be null. Given these annotations, NullAway performs a series of type-based, local checks to ensure that any pointer that gets dereferenced in your code cannot be null. NullAway is similar to the type-based nullability checking in the Kotlin and Swift languages, and the Checker Framework and Eradicate null checkers for Java.

NullAway is fast. It is built as a plugin to Error Prone and can run on every single build of your code. In our measurements, the build-time overhead of running NullAway is usually less than 10%. NullAway is also practical: it does not prevent all possible NPEs in your code, but it catches most of the NPEs we have observed in production while imposing a reasonable annotation burden, giving a great "bang for your buck." At Uber, we combine NullAway with RAVE to obtain thorough protection against NPEs in our Android apps.

Installation

Overview

NullAway requires that you build your code with Error Prone, version 2.1.1 or higher. See the Error Prone documentation for instructions on getting started with Error Prone and integration with your build system. The instructions below assume you are using the Gradle build system; integration with other systems should require similar steps.

Gradle

To integrate NullAway into your project add the following to your build.gradle file:

buildscript {
  repositories {
    maven {
      url "https://plugins.gradle.org/m2/"
    }
  }
  dependencies {
    classpath "net.ltgt.gradle:gradle-errorprone-plugin:0.0.11"
    classpath "net.ltgt.gradle:gradle-apt-plugin:0.11"
  }
}

apply plugin: "java"
apply plugin: "net.ltgt.errorprone"
apply plugin: "net.ltgt.apt"

configurations.errorprone {
  resolutionStrategy.force "com.google.errorprone:error_prone_core:2.1.1"
}

dependencies {
    apt "com.uber:nullaway:0.1.0"

    compile "com.google.code.findbugs:jsr305:3.0.2"
}

compileJava {
    options.compilerArgs += ["-Xep:NullAway:ERROR", "-XepOpt:NullAway:AnnotatedPackages=com.uber"]
}

Let's walk through this script step by step. The buildscript section of the script pulls in the Gradle Error Prone plugin for Error Prone integration, and the Gradle APT plugin to ease specification of annotation processor dependencies for a build. We need the latter since Error Prone loads plugin checkers from the annotation processor path. Note that the Gradle APT plugin is appropriate for Java projects; for Android projects, use an annotationProcessor dependence with the Android Gradle plugin. The apply plugin lines load the relevant plugins. The configurations.errorprone section forces our desired Error Prone version.

In dependencies, the apt line loads NullAway, and the compile line loads a JSR 305 library which provides a suitable @Nullable annotation (javax.annotation.Nullable). NullAway allows for any @Nullable annotation to be used, so, e.g., @Nullable from the Android support or IntelliJ annotations is also fine.

Finally, in the compileJava section, we pass some configuration options to NullAway as compiler arguments. The first argument -Xep:NullAway:ERROR is a standard Error Prone argument that sets NullAway issues to the error level; by default NullAway emits warnings. The second argument, -XepOpt:NullAway:AnnotatedPackages=com.uber, tells NullAway that source code in packages under the com.uber namespace should be checked for null dereferences and proper usage of @Nullable annotations, and that class files in these packages should be assumed to have correct usage of @Nullable (see the docs for more detail). NullAway requires at least the AnnotatedPackages configuration argument to run, in order to distinguish between annotated and unannotated code. See the configuration docs for other useful configuration options.

Code Example

Let's see how NullAway works on a simple code example:

static void log(Object x) {
    System.out.println(x.toString());
}
static void foo() {
    log(null);
}

This code is buggy: when foo() is called, the subsequent call to log() will fail with an NPE. You can see this error in the NullAway sample app by running:

cp sample/src/main/java/com/uber/mylib/MyClass.java.buggy sample/src/main/java/com/uber/mylib/MyClass.java
./gradlew build

By default, NullAway assumes every method parameter, return value, and field is non-null, i.e., it can never be assigned a null value. In the above code, the x parameter of log() is assumed to be non-null. So, NullAway reports the following error:

warning: [NullAway] passing @Nullable parameter 'null' where @NonNull is required
    log(null);
        ^

We can fix this error by allowing null to be passed to log(), with a @Nullable annotation:

static void log(@Nullable Object x) {
    System.out.println(x.toString());
}

With this annotation, NullAway points out the possible null dereference:

warning: [NullAway] dereferenced expression x is @Nullable
    System.out.println(x.toString());
                        ^

We can fix this warning by adding a null check:

static void log(@Nullable Object x) {
    if (x != null) {
        System.out.println(x.toString());
    }
}

With this change, all the NullAway warnings are fixed.

For more details on NullAway's checks, error messages, and limitations, see our detailed guide.

Contributors

We'd love for you to contribute to our open source projects. Before we can accept your contributions, we kindly ask you to sign our Uber Contributor License Agreement.

• If you find a bug, open an issue or submit a fix via a pull request.
• If you have a feature request, open an issue or submit an implementation via a pull request
• If you want to contribute, submit a pull request.

License

NullAway is licensed under the MIT license. See the LICENSE.txt file for more information.