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Nullability Annotations

Using JSpecify annotations and NullAway for compile-time null safety in Vaadin projects.

Vaadin uses JSpecify annotations to express nullability contracts in its APIs. These annotations indicate whether method parameters, return types, and generic type arguments can be null. Combined with a static analysis tool like NullAway, they catch potential NullPointerException errors at compile time.

What Are Nullability Annotations?

Java has no built-in way to express whether a reference can be null. JSpecify fills this gap with a standard set of annotations:

@NullMarked

Marks a class or package as having non-null types by default. All unannotated type usages within the scope are treated as non-null.

@Nullable

Explicitly marks a type as potentially null. Used on parameters, return types, and type arguments that may legitimately be null.

Together, these annotations express nullability contracts across three areas:

Source code
Java
import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

@NullMarked
public class UserService {
    // Return type: non-null by default
    public String getName() {
        return "John";
    }

    // Parameter: explicitly nullable
    public void setNickname(@Nullable String nickname) {
        // nickname can be null
    }

    // Type argument: nullable generic
    public ValueSignal<@Nullable String> optionalName() {
        return new ValueSignal<>(null);
    }
}

How Vaadin Uses Nullability Annotations

Vaadin’s Signal APIs are annotated with JSpecify nullability annotations. All signal types use the bounded type parameter pattern <T extends @Nullable Object>, which allows callers to choose whether a signal holds nullable or non-null values:

Source code
Java
// Non-null signal: get() returns String
ValueSignal<String> name = new ValueSignal<>("John");

// Nullable signal: get() returns @Nullable String
ValueSignal<@Nullable String> optionalName = new ValueSignal<>(null);

Nullability annotations are expected to expand across more Vaadin APIs in the future.

Enabling Null Checking in Your Project

Vaadin’s APIs are annotated with JSpecify nullability information. To take advantage of this, enable NullAway with Error Prone in your project. This gives you compile-time errors when you misuse Vaadin’s nullability contracts — for example, passing null where a non-null parameter is expected, or ignoring a @Nullable return value. This requires JDK 22 or later.

Maven Setup

Use the nullability-maven-plugin to configure Error Prone and NullAway automatically:

Source code
XML
<plugin>
    <groupId>am.ik.maven</groupId>
    <artifactId>nullability-maven-plugin</artifactId>
    <version>0.3.0</version>
    <extensions>true</extensions>
    <executions>
        <execution>
            <goals>
                <goal>configure</goal>
            </goals>
        </execution>
    </executions>
</plugin>

The plugin configures Error Prone and NullAway for you. By default, it enables JSpecify mode and checks all code in @NullMarked scopes.

Gradle Setup

For Gradle, use the gradle-errorprone-plugin:

Source code
groovy
plugins {
    id "net.ltgt.errorprone" version "4.1.0"
}

dependencies {
    errorprone "com.google.errorprone:error_prone_core:2.36.0"
    errorprone "com.uber.nullaway:nullaway:0.12.6"
}

tasks.withType(JavaCompile).configureEach {
    options.errorprone {
        disableAllChecks = true
        error("NullAway")
        option("NullAway:JSpecifyMode", "true")
        option("NullAway:AnnotatedPackages", "com.example")
    }
}

Checking Vaadin Types

After adding the tooling, mark classes that use Vaadin APIs with @NullMarked. NullAway then enforces Vaadin’s nullability contracts in those classes at compile time:

Source code
Java
import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

@NullMarked
public class SignalExample extends Div {
    // Compiler ensures non-null signal is never set to null
    private final ValueSignal<String> name = new ValueSignal<>("John");

    // Compiler requires null checks when reading nullable signals
    private final ValueSignal<@Nullable String> nickname = new ValueSignal<>(null);

    public SignalExample() {
        @Nullable String value = nickname.get();
        // Compiler error if you call value.toUpperCase() without a null check
        add(new Span(value != null ? value : "No nickname"));
    }
}

Extending to Your Own Code

You can go further and apply @NullMarked at the package level to get null-safety defaults for your own APIs. Create a package-info.java file:

Source code
Java
@NullMarked
package com.example.myapp;

import org.jspecify.annotations.NullMarked;

All classes in the package are then non-null by default. Use @Nullable only where null is a valid value:

Source code
Java
package com.example.myapp;

import org.jspecify.annotations.Nullable;

public class CustomerService {
    // Non-null return type (default)
    public Customer findById(long id) { ... }

    // Explicitly nullable return type
    public @Nullable Customer findByEmail(String email) { ... }
}

Nullability with Signals

Signal types use <T extends @Nullable Object>, so the nullability of the value depends on the type argument you provide.

Non-Null and Nullable Signals

By default, signals hold non-null values:

Source code
Java
ValueSignal<String> nameSignal = new ValueSignal<>("John");
String name = nameSignal.get(); // Never null

To allow null values, annotate the type argument with @Nullable:

Source code
Java
import org.jspecify.annotations.Nullable;

ValueSignal<@Nullable String> optionalName = new ValueSignal<>(null);
@Nullable String value = optionalName.get(); // May be null

if (value != null) {
    System.out.println(value.toUpperCase());
}

Handling Null in Signal Operations

When transforming nullable signals, account for null values:

Source code
Java
ValueSignal<@Nullable String> input = new ValueSignal<>(null);

// Transform to non-null
Signal<String> output = input.map(str -> str != null ? str.toUpperCase() : "");

When binding nullable signals to components, convert null to a suitable default:

Source code
Java
ValueSignal<@Nullable String> optionalText = new ValueSignal<>(null);

TextField field = new TextField();
field.bindValue(
    optionalText.map(text -> text != null ? text : ""),
    value -> optionalText.set(value.isEmpty() ? null : value)
);

Collection Type Nullability

Nullability applies independently to a collection and its elements:

Source code
Java
// Non-null list, non-null elements
ValueSignal<List<String>> names = new ValueSignal<>(List.of("a", "b"));

// Non-null list, nullable elements
ValueSignal<List<@Nullable String>> sparse = new ValueSignal<>(new ArrayList<>());

// Nullable list, non-null elements
ValueSignal<@Nullable List<String>> optionalList = new ValueSignal<>(null);

Best Practices

  • Prefer non-null by default. Use @Nullable only when null is a meaningful value in your domain, not as a convenience.

  • Apply @NullMarked at the package level. This provides consistent defaults and reduces annotation noise across your codebase.

  • Handle null explicitly in transformations. When mapping or binding nullable signals, convert to non-null values early rather than propagating null through chains of operations.

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