Micronaut NullAway and JSPecify

Nov 2025, 21. by Sergio del Amo

In this blog post, I show you how to configure a Micronaut 4 application to use JSpecify and NullAway to protect you from NPEs (Null Pointer Exceptions).

NullAway:

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.

JSpecify Dependency

Since Micronaut 4.10, Micronaut BOM contains the JSpecify dependency. Thus, you can add the following dependency to your project without specifying the version:

<dependency>
   <groupId>org.jspecify</groupId>
   <artifactId>jspecify</artifactId>
</dependency>

NullAway with Gradle

GitHub Repository with a Gradle Micronaut Application which uses NullAway

To use NullAway in a Gradle Micronaut Application. You need to add the Gradle error prone plugin and such configuration:

plugins {
...
..
.
    id("net.ltgt.errorprone") version "4.3.0"
}
dependencies {
...
..
.
    implementation("org.jspecify:jspecify")
    errorprone("com.uber.nullaway:nullaway:0.12.12")
    errorprone("com.google.errorprone:error_prone_core:2.44.0")
}
tasks.withType<JavaCompile>().configureEach {
    options.errorprone {
        check("NullAway", net.ltgt.gradle.errorprone.CheckSeverity.ERROR)
        option("NullAway:AnnotatedPackages", example.micronaut")
        if (name.lowercase().contains("test")) {
            disable("NullAway")
        }
    }
}

check("NullAway", CheckSeverity.ERROR) sets NullAway issues to the error level.
option("NullAway:AnnotatedPackages", “example.micronaut”) tells NullAway that source code in packages under the example.micronaut 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.
Then, it disables NullAway on test code.

These instructions are described in the NullAway README.md file.

NullAway with Maven

GitHub Repository with a Maven Micronaut Application which uses NullAway


<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
...
..
.
  <properties>
...
..
.
    <error-prone.version>2.29.2</error-prone.version>
    <nullaway.version>0.12.12</nullaway.version>
  </properties>
...
..
.
  <dependencies>
      <dependency>
          <groupId>org.jspecify</groupId>
          <artifactId>jspecify</artifactId>
          <version>1.0.0</version>
      </dependency>
...
..
.
  </dependencies>
  <build>
    <plugins>
...
..
.
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <configuration>
          <compilerArgs>
            <arg>-XDcompilePolicy=simple</arg>
            <arg>--should-stop=ifError=FLOW</arg>
            <arg>-Xplugin:ErrorProne -Xep:NullAway:ERROR -XepOpt:NullAway:AnnotatedPackages=example.micronaut</arg>
            <arg>-Amicronaut.processing.group=mn.maven.jspecify</arg>
            <arg>-Amicronaut.processing.module=mn-maven-jspecify</arg>
          </compilerArgs>
          <annotationProcessorPaths combine.children="append">
            <path>
              <groupId>com.google.errorprone</groupId>
              <artifactId>error_prone_core</artifactId>
              <version>${error-prone.version}</version>
            </path>
            <path>
              <groupId>com.uber.nullaway</groupId>
              <artifactId>nullaway</artifactId>
              <version>${nullaway.version}</version>
            </path>
            <!-- Existing Micronaut processors -->
...
..
.
          </annotationProcessorPaths>
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

The above example adds two annotation processors. Error Prone and NullAway.

The previous code sample configures NullAway:

-XepOpt:NullAway:AnnotatedPackages sets the list of packages that should be considered properly annotated according to the NullAway convention
-Xep:NullAway:ERROR sets the NullAway check severity to ERROR, which should cause compilation to fail when NullAway violations are found.

As described in the error prone Maven documentation, I added a .mvn/jvmconfig file with the following content:

--add-exports jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED
--add-exports jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED
--add-exports jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED
--add-exports jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED
--add-exports jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED
--add-exports jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED
--add-exports jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED
--add-exports jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED
--add-opens jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED
--add-opens jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED

A failure looks like:

[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.14.1:compile (default-compile) on project mn-maven-jspecify: Compilation failure
[ERROR] /Users/sdelamo/github/sdelamo/micronaut-maven-nullaway-demo/src/main/java/example/micronaut/GreetingController.java:[21,39] [NullAway] dereferenced expression greetingService.greet() is @Nullable
[ERROR]     (see http://t.uber.com/nullaway )
[ERROR] 
[ERROR] -> [Help 1]
[ERROR]

Micronaut Fundamentals

Oct 2025, 29. by Sergio del Amo Permalink

During this year, I recorded Micronaut Fundamentals, a video course available for free on Oracle MyLearn with an OCI Free Learning Subscription.

It is a 7-hour and 37-minute course covering the following topics:

Introduction to Micronaut
Micronaut Documentation
Creating your first Micronaut application
Using Micronaut in popular IDEs
Exploring a Micronaut application
Micronaut Build Plugins
Micronaut Test
Micronaut Core Concepts
Serialization
Application Events
Source Code Generation
Configuration
Dependency Injection
HTTP Client
HTTP Server
Filters
Management Endpoints
Logging
Validation
Error formats
Error handling
OpenAPI
Packaging and distribution
GraalVM Native Image
Graal Development Kit for Micronaut

Announcement in Oracle Blog.

jCasbin

Oct 2025, 23. by Sergio del Amo Permalink

jCasbin is a powerful and efficient open-source access control library for Java projects. It provides support for enforcing authorization based on various access control models.

System Rules

Oct 2025, 21. by Sergio del Amo Permalink

A collection of JUnit rules for testing code that uses java.lang.System.

Add an MCP Server to GitHub Copilot in IntelliJ IDEA

Oct 2025, 03. by Sergio del Amo

To register a MCP Server in GitHub Copilot for IntelliJ IDEA, change to Agent model, click the Tools icon and click Add More Tools...

Enter your MCP Server in the mcp.json configuration file. The following example shows an MCP Server packaged as a FAT jar which uses STDIO transport.

{
	"servers": {
	  "diskspace": {
		"type": "stdio",
		"command": "java",
		"args": ["-jar", "/Users/sdelamo/bin/diskspace-0.1-all.jar"]
	  }

	}
}

Add an MCP Server to Claude Desktop

Oct 2025, 03. by Sergio del Amo

To register a MCP Server in Claude Desktop, click on Settings, click the Developer, and click Edit Config

Enter your MCP Server in the claude_desktop_config.json configuration file. The following example shows an MCP Server packaged as a FAT jar which uses STDIO transport.

{
  "mcpServers": {
    "diskspace": {
      "command": "java",
      "args": ["-jar", "/Users/sdelamo/bin/diskspace-0.1-all.jar"]
    }
  }
}

Add an MCP Server to Claude Code

Oct 2025, 03. by Sergio del Amo Permalink

You can connect Claude Code to tools via MCP. You can use the claude mcp add command to add an MCP server.

claude mcp add --transport http micronautfun https://micronaut.fun/mcp

The previous command adds an entry in the mcpServers of the project:

"mcpServers": {
   "micronautfun": {
     "type": "http",
     "url": "https://micronaut.fun/mcp"
   }
},

These commands modify the vi $HOME/.claude.json configuration file.

You can list your project MCP servers with the claude mcp list command.

Add an MCP Server to VS Code

Oct 2025, 03. by Sergio del Amo

To register a MCP Server in VS Code open the command palette with (CMD + Shift + P) and search for "Add MCP"

Select your MCP Server transport (STDIO or HTTP)

If HTTP, enter your MCP Server URL.

Select whether you want to install the MCP Server globally or for the current workspace only.

Enter your MCP Server in the mcp.json configuration file.

The following example shows an MCP Server which uses Streamable HTTP transport.

{
  "servers": {
    "micronautfun": {
      "url": "https://micronaut.fun/mcp",
      "type": "http"
    }
  },
  "inputs": []
}

You should see your MCP listed as installed.

Devoxx Talk - Build an MCP Server with Java

Aug 2025, 17. by Sergio del Amo Permalink

I am excited to be talking at Devoxx Belgium, arguably the most important Java conference in Europe, about how to build an MCP Server with Java.

Moving my Mastodon account to foojay.social

Aug 2025, 04. by Sergio del Amo

My new handle is @sdelamo@foojay.social.

I followed this video to migrate from jvm.social to foojay.social.

📼 Micronaut Constraints Validation

Aug 2025, 03. by Sergio del Amo Permalink

In this video, I show you how to set up Micronaut Validation to validate the constraints of a Java Record.

📼 Micronaut Development environment

Aug 2025, 03. by Sergio del Amo Permalink

In this video, I show you how to set up a Micronaut Development environment.

📼 Micronaut @EachProperty

Aug 2025, 03. by Sergio del Amo Permalink

In this video, I show you how to use the @EachProperty to load configuration into your Micronaut application.

OSS-Fuzz

Aug 2025, 02. by Sergio del Amo Permalink

OSS-Fuzz aims to make common open source software more secure and stable by combining modern fuzzing techniques with scalable, distributed execution.

Micronaut is using this great initiative by Google.

Kuvasz is an open-source, self-hosted uptime & SSL monitoring service, designed to help you keep track of your websites and services

MCP Resources

Jul 2025, 17. by Sergio del Amo

Collection of links to Model Context Protocol (MCP) resources, including videos, articles, and tools.

The Model Context Protocol (MCP) was open sourced by Anthropic in November 2024 to provide users and developers with an easy way to extend the capabilities of AI-powered apps by integrating them with data sources and applications.

Model Context Protocol (MCP)

Courses

Anthropic Academy

Introduction to Model Context Protocol

Courses

Model Context Protocol with Agentic AI

Videos

Why MCP really is a big deal with Tim Berglund

MCP Java SDK

The official Java SDK for Model Context Protocol servers and clients. Maintained in collaboration with Spring AI. I have contributed some code as well.
MCP Java Github Organization
Java MCP Server Configurator Generator

Spring

Spring AI MCP

MCP Server Spring Boot Starter Documentation

Micronaut NullAway and JSPecify

NullAway:

JSpecify Dependency

NullAway with Gradle

NullAway with Maven

Micronaut Fundamentals

jCasbin

System Rules

Add an MCP Server to GitHub Copilot in IntelliJ IDEA

Add an MCP Server to Claude Desktop

Add an MCP Server to Claude Code

Add an MCP Server to VS Code

Devoxx Talk - Build an MCP Server with Java

Moving my Mastodon account to foojay.social

📼 Micronaut Constraints Validation

📼 Micronaut Development environment

📼 Micronaut @EachProperty

OSS-Fuzz

📼 Micronaut CLI upgrade via SDKMan

📼 Micronaut Gradle GitHub Actions

Add junit-platform-launcher to your Gradle JUnit 5 builds

Install Ruby in MacOS

Kuvasz - Monitoring Service built with Micronaut

MCP Resources

Model Context Protocol (MCP)

Courses

Anthropic Academy

Courses

Videos

MCP Java SDK

Spring