Java Null Safety Skill

Prerequisites

This skill requires JSpecify annotations:

• org.jspecify:jspecify

  (NullMarked, Nullable, NonNull)

Required Imports

// JSpecify Null Safety Annotations
import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;
import org.jspecify.annotations.NonNull;

Maven Dependency

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

Core Annotations

• @NullMarked

  - Marks a package or class where all types are non-null by default

• @Nullable

  - Marks a type as nullable (exception to @NullMarked default)

• @NonNull

  - Explicitly marks a type as non-null (only needed without @NullMarked)

Package-Level Configuration (PREFERRED)

Always prefer

@NullMarked

in

package-info.java

for consistent null-safety across the entire package.

Correct package-info.java Structure

The

package-info.java

file has a unique syntax that differs from regular Java classes:

// package-info.java
/*
 * Copyright headers and license...
 */

/**
 * Token validation and authentication services.
 *
 * <p>All types in this package are non-null by default due to {@code @NullMarked}.
 * Use {@code @Nullable} to explicitly mark nullable types.
 */
@NullMarked
package de.cuioss.portal.authentication;

import org.jspecify.annotations.NullMarked;

CRITICAL: Unique package-info.java Syntax

The structure is special and MUST follow this exact order:

1. File header comment (copyright, license)
2. Package JavaDoc comment (describes the package)
3. Package annotations (like

   @NullMarked

   )

4. package

   declaration

5. import

   statements (AFTER the package declaration)

Why This Is Different:

In regular Java classes, imports come BEFORE the class declaration:

import java.util.List;  // Import first

public class MyClass {  // Then class
}

In

package-info.java

, imports come AFTER the package declaration:

@NullMarked            // Annotation first
package com.example;   // Then package

import org.jspecify.annotations.NullMarked;  // Import last

This reverse ordering is the Java Language Specification requirement for package-info.java files. Placing imports before the package declaration will cause compilation errors.

Benefits:

• Consistent null-safety across entire package
• Less annotation noise (default is non-null)
• Clear contract for package APIs
• Easier to maintain

API Return Type Guidelines

Pattern 1: Guaranteed Non-Null Return (Default)

Methods return non-null by default with package-level

@NullMarked

:

/**
 * Validates the JWT token and returns the result.
 *
 * @param token the token to validate, must not be null
 * @return validation result, never null
 */
public ValidationResult validate(String token) {
    // Implementation must ensure non-null return
    return new ValidationResult(token, checkSignature(token));
}

Pattern 2: Optional Result

Use

Optional<T>

when the method may not have a result to return:

/**
 * Finds a user by their unique identifier.
 *
 * @param userId the user identifier, must not be null
 * @return the user if found, or Optional.empty() if not found
 */
public Optional<User> findById(String userId) {
    User user = repository.get(userId);
    return Optional.ofNullable(user);
}

CRITICAL RULE: Never Use @Nullable for Return Types

NEVER use

@Nullable

for return types. Either guarantee a non-null return or use Optional.

// ❌ BAD - Never do this
public @Nullable ValidationResult validate(String token) {
    // Nullable returns are forbidden
}

// ✅ GOOD - Guaranteed non-null
public ValidationResult validate(String token) {
    // Must return non-null
}

// ✅ GOOD - Use Optional for "no result" scenarios
public Optional<ValidationResult> tryValidate(String token) {
    // Returns Optional.empty() when no result
}

Null-Safe Implementation Patterns

With @NullMarked (Package Level)

// With @NullMarked at package level, everything is non-null by default
public class TokenValidator {

    // Field is non-null by default
    private final TokenConfig config;

    // Parameter is non-null by default
    public TokenValidator(TokenConfig config) {
        // No null check needed if caller respects contract
        // But defensive programming is still acceptable:
        this.config = Objects.requireNonNull(config, "config must not be null");
    }

    // Parameter and return are non-null by default
    public ValidationResult validate(String token) {
        Objects.requireNonNull(token, "token must not be null");
        // Implementation must return non-null
        return new ValidationResult(/*...*/);
    }

    // Mark nullable parameters explicitly
    public String processWithDefault(@Nullable String input) {
        return input != null ? input.toUpperCase() : "DEFAULT";
    }

    // Use Optional instead of @Nullable returns
    public Optional<UserInfo> extractUserInfo(String token) {
        return parseToken(token)
            .map(this::extractUser);
    }
}

Without @NullMarked (Explicit Annotations)

If not using package-level

@NullMarked

, you must explicitly annotate:

import org.jspecify.annotations.NonNull;
import org.jspecify.annotations.Nullable;

public class TokenValidator {

    @NonNull
    private final TokenConfig config;

    public TokenValidator(@NonNull TokenConfig config) {
        this.config = config;
    }

    @NonNull
    public ValidationResult validate(@NonNull String token) {
        return new ValidationResult(/*...*/);
    }

    @NonNull
    public String processWithDefault(@Nullable String input) {
        return input != null ? input.toUpperCase() : "DEFAULT";
    }
}

Implementation Requirements

1. Package-Level Configuration

• Always prefer

  @NullMarked

  in

  package-info.java

• Document the null-safety contract in package documentation
• Use

  @Nullable

  only for exceptions to the non-null default

2. Default Non-Null

• With

  @NullMarked

  , all types are non-null by default
• Only use

  @Nullable

  for exceptions
• Never annotate with

  @NonNull

  when using

  @NullMarked

  (redundant)

3. Implementation Responsibility

• The implementation MUST ensure that non-nullable methods never return null
• Add defensive null checks at API boundaries
• Use

  Objects.requireNonNull()

  for parameter validation

public ValidationResult validate(String token) {
    // Defensive programming at API boundary
    Objects.requireNonNull(token, "token must not be null");

    // Implementation ensures non-null return
    if (isValid(token)) {
        return ValidationResult.valid();
    }
    return ValidationResult.invalid("Token validation failed");
}

Nullable Parameters

Use

@Nullable

sparingly for parameters that genuinely accept null:

// Good - null has clear meaning (use default)
public String format(@Nullable Locale locale) {
    Locale effectiveLocale = locale != null ? locale : Locale.getDefault();
    return formatter.format(effectiveLocale);
}

// Best - overload methods instead of nullable parameters
public String format() {
    return format(Locale.getDefault());
}

public String format(Locale locale) {
    return formatter.format(locale);
}

Collections and Generics

Apply null-safety to collection types:

// With @NullMarked, all elements are non-null
public List<User> getActiveUsers() {
    // Returns non-null list of non-null User objects
    return users.stream()
        .filter(User::isActive)
        .toList();
}

// Use @Nullable for nullable elements
public List<@Nullable String> getOptionalValues() {
    // List is non-null, but elements can be null
    return Arrays.asList("value1", null, "value3");
}

Unit Testing

Test that non-nullable methods never return null under any valid input conditions:

@Test
void shouldNeverReturnNull() {
    // With @NullMarked, non-nullable methods must never return null
    assertNotNull(service.processToken("valid"));
    assertNotNull(service.processToken(""));

    // Non-nullable methods should handle edge cases without returning null
    assertNotNull(service.processToken("edge-case"));
}

@Test
void shouldUseOptionalForMissingValues() {
    // Use Optional.empty() instead of null returns
    assertTrue(service.findUser("unknown").isEmpty());
    assertTrue(service.findUser("existing").isPresent());
}

@Test
void shouldRejectNullParameters() {
    // Non-nullable parameters should be validated
    assertThrows(NullPointerException.class,
        () -> service.processToken(null));
}

Migration Strategy

For New Code

1. Add

   @NullMarked

   to

   package-info.java

2. Write code assuming non-null by default
3. Use

   @Nullable

   only where null is explicitly allowed
4. Use

   Optional<T>

   for "no result" return types
5. Validate with unit tests

For Existing Code

1. Add

   @NullMarked

   to package
2. Review all public APIs
3. Add

   @Nullable

   where null is currently accepted/returned
4. Refactor nullable returns to Optional where appropriate
5. Add null checks with

   Objects.requireNonNull()

   at API boundaries
6. Update tests to verify null-safety contracts

Quality Checklist

• Package has @NullMarked in package-info.java
• No @Nullable used for return types (use Optional instead)
• Nullable parameters documented and justified
• Defensive null checks at API boundaries
• Unit tests verify non-null contracts (see

  pm-dev-java:junit-core

  skill)
• Static analysis configured and passing
• JavaDoc documents null-safety contract
• Collections specify element nullability if needed

Related Skills

• pm-dev-java:java-core

  - Core Java patterns

• pm-dev-java:java-lombok

  - Lombok patterns (interop with null safety)