requireSpecifiedNullness

Category: Rules Reference
Scope: Fields, Methods, Parameters, Type parameters
Purpose: Ensures that nullness is explicitly specified where required.

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What does it check?

The requireSpecifiedNullness rule verifies that fields, method parameters, return types, and type parameters have explicitly specified nullness.

Explicit nullness is typically defined by:

• Being under a @NullMarked context
• Being individually annotated with @Nullable or @NonNull

If neither is present, the element is considered to have unspecified nullness, and an error is reported.

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Why is it important?

• Promotes strict null-safety and removes ambiguities.
• Helps projects enforce JSpecify-compliant code practices.
• Clarifies API contracts and prevents null-related bugs.
• Improves the reliability of static nullness analysis tools.

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Examples

❌ Invalid Example: Unspecified nullness

package com.example;

public class UserService {

    public String findUsernameById(Integer id) {
        // missing explicit nullness on parameter and return type
        return "user123";
    }
}

Problems detected:

[ERROR] com.example.UserService#findUsernameById(java.lang.Integer): Unspecified nullness found:
[ERROR]     java.lang.String* findUsernameById(java.lang.Integer*)
[ERROR]                     ^                                   ^ 

✅ Valid Example: Specified nullness

package com.example;

import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

@NullMarked
public class UserService {

    public String findUsernameById(@Nullable Integer id) {
        return id != null ? "user" + id : "unknown";
    }
}

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How to configure it?

You can enable the requireSpecifiedNullness rule by adding it to your plugin configuration:

<configuration>
    <rules>
        <requireSpecifiedNullness/>
    </rules>
</configuration>

Optional parameters:

Parameter	Type	Default	Description
exclusionsFile	String	(none)	Path to a text file listing classes to exclude, see Exclusions File Format

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When to enable?

Recommended for:

• Projects migrating to JSpecify who want to enforce strict nullness over time.
• New codebases starting from scratch with null-safety first principles.
• CI/CD pipelines to block unspecified nullness in pull requests.

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Limitations

• Does not validate runtime behavior — only checks for presence of annotations.