What are JSpecify nullability annotations in Spring Boot 4 and why are they important?

// package-info.java: declare the whole package as null-safe
@NullMarked
package com.example.service;

import org.jspecify.annotations.NullMarked;

// Service class: unannotated types are non-null by default
@Service
public class OrderService {

    // Non-null by default (from @NullMarked)
    public Order createOrder(String productId, int quantity) {
        return new Order(productId, quantity);
    }

    // Explicitly mark where null is possible:
    public @Nullable Order findOrder(@Nullable String orderId) {
        if (orderId == null) return null;
        return orderRepository.findById(orderId).orElse(null);
    }
}

// Maven dependency:
<dependency>
    <groupId>org.jspecify</groupId>
    <artifactId>jspecify</artifactId>
    <!-- Version managed by Spring Boot 4 BOM -->
</dependency>

// Kotlin benefit: JSpecify annotations are automatically
// translated into Kotlin nullability by Kotlin 2.2 compiler
// val order: Order? = service.findOrder(null)  <- correct type
// val order: Order  = service.createOrder("p1", 2) <- non-null inferred