Agent Skills: Nullability Control

Nullability Control

Compile-time null safety for Java using NullAway and JSpecify annotations.

When to use this skill

• Setting up null safety for new Java projects
• Migrating existing code to JSpecify annotations
• Fixing NullAway errors and warnings
• Creating package-info.java files with @NullMarked
• Understanding null safety annotation patterns

Skill Contents

Sections

• When to use this skill
• Quick Start
• Key Principles
• Gradual Adoption Strategy
• Package-info.java Pattern
• Common Patterns
• References
• Lombok Compatibility
• Validation Results
• Related Skills

Available Resources

📚 references/ - Detailed documentation

• annotation guide
• common errors
• gradle setup
• migration openrewrite
• multi module setup
• package info templates
• suppressing warnings

📦 assets/ - Templates and resources

• templates

Quick Start

1. Add Dependencies to Version Catalog

Add to gradle/libs.versions.toml:

[versions]
errorProne = "2.46.0"
nullaway = "0.13.1"
jspecify = "1.0.0"

[libraries]
error-prone-core = { module = "com.google.errorprone:error_prone_core", version.ref = "errorProne" }
nullaway = { module = "com.uber.nullaway:nullaway", version.ref = "nullaway" }
jspecify = { module = "org.jspecify:jspecify", version.ref = "jspecify" }

[plugins]
errorprone = { id = "net.ltgt.errorprone", version = "4.3.0" }

Note: Also register the plugin in settings.gradle under pluginManagement { plugins { ... } }

Versions verified: January 2026. Check for updates at NullAway releases and Error Prone releases.

2. Configure Error Prone + NullAway

In each module's build.gradle (not root project for multi-module):

plugins {
    id 'java'
    alias(libs.plugins.errorprone)
}

dependencies {
    // NullAway + Error Prone
    errorprone libs.error.prone.core
    errorprone libs.nullaway
    implementation libs.jspecify
}

import net.ltgt.gradle.errorprone.CheckSeverity

tasks.withType(JavaCompile).configureEach {
    // Required for Error Prone 2.46.0+ on JDK 21 (OpenJDK only - not supported on Oracle JDK)
    options.compilerArgs += ["-XDaddTypeAnnotationsToSymbol=true"]

    options.errorprone {
        // CRITICAL: Disable all Error Prone checks first to prevent build failures
        // Error Prone enables many strict checks by default that fail on existing code
        disableAllChecks = true
        // Then enable only NullAway
        check("NullAway", CheckSeverity.ERROR)
        option("NullAway:OnlyNullMarked", "true")
    }
}

Important Notes:

• disableAllChecks = true is essential for existing projects. Error Prone enables many strict checks by default (StringCaseLocaleUsage, JavaTimeDefaultTimeZone, ReturnValueIgnored, etc.) that will fail most existing codebases.
• For multi-module projects, apply the Error Prone plugin in each subproject's build.gradle, not via the root subprojects {} block. See multi-module-setup.md for details.

3. Create package-info.java for Each Package

Every package needs a package-info.java file:

@NullMarked
package com.example.mypackage;

import org.jspecify.annotations.NullMarked;

See package-info-templates.md for complete templates.

Key Principles

| Principle | Description | |-----------|-------------| | Non-null by default | In @NullMarked code, all types are non-null unless annotated @Nullable | | Use JSpecify | Use org.jspecify.annotations.Nullable, not javax or JetBrains | | Package-level marking | Apply @NullMarked to packages via package-info.java | | OnlyNullMarked config | Use -XepOpt:NullAway:OnlyNullMarked=true (NullAway 0.12.3+) | | Explicit nullability | Only annotate @Nullable; non-null is implicit | | Type-use placement | Place @Nullable directly before the type (e.g., @Nullable String) |

JDK Compatibility Note: For full JSpecify support with generics, use JDK 22+ or JDK 21.0.8+ with -XDaddTypeAnnotationsToSymbol=true.

Gradual Adoption Strategy

With OnlyNullMarked mode, NullAway only checks code in @NullMarked packages. This enables gradual adoption:

1. Add NullAway to build - Build passes immediately (no @NullMarked code yet)
2. Add package-info.java to one package - Fix null-safety issues in that package
3. Repeat for each package - Continue until all packages are covered

This approach allows incremental migration without blocking the build.

Package-info.java Pattern

Every package in your codebase should have a package-info.java:

@NullMarked
package com.bitso.myservice.domain;

import org.jspecify.annotations.NullMarked;

Why This Matters

• NullAway only checks code marked as @NullMarked
• Without package-info.java, code is treated as unannotated
• Class-level @NullMarked is an alternative but package-level is preferred

Creating package-info.java Files

For each package directory:

# Find all packages without package-info.java
find src/main/java -type d -exec sh -c '
  if [ -n "$(ls -A "$1"/*.java 2>/dev/null)" ] && [ ! -f "$1/package-info.java" ]; then
    echo "$1"
  fi
' _ {} \;

Common Patterns

Nullable Parameters

public void process(@Nullable String input) {
    if (input != null) {
        // Safe to use input here
        System.out.println(input.length());
    }
}

Nullable Return Values

public @Nullable User findUser(String id) {
    return userMap.get(id);  // May return null
}

Nullable Fields

public class Request {
    private @Nullable String optionalField;
    private String requiredField;  // Non-null by default
}

Collections with Nullable Elements

// List that may contain null elements
List<@Nullable String> items;

// Nullable list (the list itself may be null)
@Nullable List<String> maybeList;

References

| Reference | Description | |-----------|-------------| | gradle-setup.md | Complete Gradle configuration | | multi-module-setup.md | Multi-module project patterns | | package-info-templates.md | Package annotation templates | | annotation-guide.md | Full annotation reference | | migration-openrewrite.md | OpenRewrite migration guide | | common-errors.md | Error messages and solutions | | suppressing-warnings.md | Warning suppression patterns |

Lombok Compatibility

Lombok works well with NullAway but requires configuration.

Required Configuration

Create lombok.config at project root:

lombok.addLombokGeneratedAnnotation = true

This makes NullAway skip Lombok-generated code (constructors, getters, setters, builders).

Lombok @NonNull Annotations

Keep Lombok @NonNull annotations - they serve a different purpose:

• Lombok's @NonNull generates runtime null checks (throws NullPointerException)
• JSpecify's @Nullable provides compile-time null safety (NullAway checks)
• These are complementary: Lombok for runtime protection, JSpecify for compile-time analysis

// Good: Keep Lombok @NonNull for runtime checks
public record User(@NonNull String id, @NonNull String name) {}

// The lombok.config ensures NullAway doesn't analyze generated code

Migration Strategy

| Annotation | Action | |------------|--------| | lombok.NonNull | Keep - provides runtime null checks | | javax.annotation.Nonnull | Remove - redundant in @NullMarked code | | javax.annotation.Nullable | Replace with org.jspecify.annotations.Nullable | | org.jetbrains.annotations.NotNull | Remove - redundant in @NullMarked code | | org.jetbrains.annotations.Nullable | Replace with org.jspecify.annotations.Nullable |

Validation Results

This skill has been validated against multiple real-world multi-module Spring Boot projects with various configurations including protobuf, gRPC, JOOQ, and Lombok.

Key Findings

1. disableAllChecks = true is essential - Error Prone has many strict checks enabled by default that fail most existing code
2. JDK 21 requires -XDaddTypeAnnotationsToSymbol=true - Error Prone 2.46.0+ fails without this flag on JDK 21
3. Apply plugin per-subproject - The Error Prone plugin must be in each module's plugins {} block, not via root subprojects {}
4. Register plugin in settings.gradle - Add id 'net.ltgt.errorprone' version '4.1.0' in pluginManagement { plugins { } }
5. Lombok compatibility - Works well; add lombok.config with lombok.addLombokGeneratedAnnotation = true
6. Keep Lombok @NonNull - These provide runtime null checks; only remove javax.annotation.Nonnull and similar
7. Legacy annotation migration - Remove @Nonnull from legacy packages (javax, JetBrains) since non-null is default in @NullMarked code
8. OnlyNullMarked mode - Enables gradual adoption; build passes immediately with no @NullMarked code

Related Skills

| Skill | Purpose | |-------|---------| | gradle-standards | Gradle configuration and version catalogs | | java-standards | General Java standards | | fix-sonarqube | SonarQube issues including null pointer rules |