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

NullAway isfast. It is built as a plugin toError 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 alsopractical: 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."

NullAway requires that you build your code with JDK 17 or higher andError Prone, version 2.36.0 or higher. See theError Prone documentation for instructions on getting started with Error Prone and integration with your build system. The instructions below assume you are using Gradle; seethe docs for discussion of other build systems. If you are building with JSpecify mode enabled, we recommend building with the most recent JDK available; seethe wiki docs on JSpecify support for more details.

To integrate NullAway into your non-Android Java project, add the following to yourbuild.gradle file:

plugins {// we assume you are already using the Java plugin  id"net.ltgt.errorprone" version"<plugin version>"}dependencies {  errorprone"com.uber.nullaway:nullaway:<NullAway version>"// Some source of nullability annotations; JSpecify recommended,// but others supported as well.  api"org.jspecify:jspecify:1.0.0"  errorprone"com.google.errorprone:error_prone_core:<Error Prone version>"}importnet.ltgt.gradle.errorprone.CheckSeveritytasks.withType(JavaCompile) {  options.errorprone {    check("NullAway",CheckSeverity.ERROR)    option("NullAway:AnnotatedPackages","com.uber")  }// Include to disable NullAway on test codeif (name.toLowerCase().contains("test")) {    options.errorprone {      disable("NullAway")    }  }}

Let's walk through this script step by step. Theplugins section pulls in theGradle Error Prone plugin for Error Prone integration.

Independencies, the firsterrorprone line loads NullAway, and theapi line loads theJSpecify library which provides suitable nullability annotations, e.g.,org.jspecify.annotations.Nullable. NullAway allows for any@Nullable annotation to be used, so, e.g.,@Nullable from the AndroidX annotations Library or JetBrains annotations is also fine. The seconderrorprone line sets the version of Error Prone is used.

Finally, in thetasks.withType(JavaCompile) section, we pass some configuration options to NullAway. Firstcheck("NullAway", CheckSeverity.ERROR) sets NullAway issues to the error level (it's equivalent to the-Xep:NullAway:ERROR standard Error Prone argument); by default NullAway emits warnings. Then,option("NullAway:AnnotatedPackages", "com.uber") (equivalent to the-XepOpt:NullAway:AnnotatedPackages=com.uber standard Error Prone argument) tells NullAway that source code in packages under thecom.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 (seethe docs for more detail). NullAway requires exactly one of theAnnotatedPackages orOnlyNullMarked configuration arguments to run, in order to distinguish between annotated and unannotated code. Seethe configuration docs for more details and other useful configuration options. For even simpler configuration of NullAway options, use theGradle NullAway plugin. Finally, we show how to disable NullAway on test code, if desired.

We recommend addressing all the issues that Error Prone reports, particularly those reported as errors (rather than warnings). But, if you'd like to try out NullAway without running other Error Prone checks, you can useoptions.errorprone.disableAllChecks (equivalent to passing"-XepDisableAllChecks" to the compiler, before the NullAway-specific arguments).

Versions 3.0.0 and later of the Gradle Error Prone Pluginno longer support Android. So if you're using a recent version of this plugin, you'll need to add some further configuration to run Error Prone and NullAway. Oursample appbuild.gradle file shows one way to do this, but your Android project may require tweaks. Alternately, 2.x versions of the Gradle Error Prone Plugin still support Android and may still work with your project.

Beyond that, compared to the Java configuration, the JSpecify dependency can be removed; you can use theandroidx.annotation.Nullable annotation from the AndroidX annotation library instead.

Some annotation processors likeDagger andAutoValue generate code into the same package namespace as your own code. This can cause problems when setting NullAway to theERROR level as suggested above, since errors in this generated code will block the build. Currently the best solution to this problem is to completely disable Error Prone on generated code, using the-XepExcludedPaths option added in Error Prone 2.1.3 (documentedhere, useoptions.errorprone.excludedPaths= in Gradle). To use, figure out which directory contains the generated code, and add that directory to the excluded path regex.

Note for Dagger users: Dagger versions older than 2.12 can have bad interactions with NullAway; seehere. Please update to Dagger 2.12 to fix the problem.

As of version 33.4.1,Guava usesJSpecify@NullMarked annotations for most of its packages, and hence NullAway will treat those packages as annotated by default. This treatment may lead to some false positives (due to handling of type variables), which can be mitigated by running NullAway in its (under-development) JSpecify mode. Seethe wiki docs on JSpecify support for more details.

Unlike other annotation processors above, Lombok modifies the in-memory AST of the code it processes, which is the source of numerous incompatibilities with Error Prone and, consequently, NullAway.

We do not particularly recommend using NullAway with Lombok. However, NullAway encodes some knowledge of common Lombok annotations and we do try for best-effort compatibility. In particular, common usages like@lombok.Builder and@Data classes should be supported.

In order for NullAway to successfully detect Lombok generated code within the in-memory Java AST, the following configuration option must be passed to Lombok as part of an applicablelombok.config file:

lombok.addLombokGeneratedAnnotation = true

This causes Lombok to add@lombok.Generated to the methods/classes it generates. NullAway will ignore (i.e. not check) the implementation of this generated code, treating it as unannotated.

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

staticvoidlog(Objectx) {System.out.println(x.toString());}staticvoidfoo() {log(null);}

This code is buggy: whenfoo() is called, the subsequent call tolog() 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 isnon-null, i.e., it can never be assigned anull value. In the above code, thex parameter oflog() 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 allowingnull to be passed tolog(), with a@Nullable annotation:

staticvoidlog(@NullableObjectx) {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:

staticvoidlog(@NullableObjectx) {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, seeour detailed guide.

Please feel free toopen a GitHub issue if you have any questions on how to use NullAway. Or, you canjoin the NullAway Discord server and ask us a question there.

We'd love for you to contribute to NullAway! Please note that onceyou create a pull request, you will be asked to sign ourUber Contributor License Agreement.

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