What changed with Jackson in Spring Boot 4 and what are the migration considerations?

// Jackson 3: package migration
// Old (Jackson 2 / Spring Boot 3):
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.annotation.JsonProperty;

// New (Jackson 3 / Spring Boot 4):
import tools.jackson.databind.ObjectMapper;
import tools.jackson.annotation.JsonProperty;

// Date serialisation change:
// Jackson 2 default: {"createdAt": 1732099200000}
// Jackson 3 default: {"createdAt": "2025-11-20T10:00:00Z"}

// If your API clients expect timestamps, configure explicitly:
@Configuration
public class JacksonConfig {
    @Bean
    public Jackson3BuilderCustomizer timestampCustomizer() {
        return builder -> builder
            .featuresToEnable(DateTimeFeature.WRITE_DATES_AS_TIMESTAMPS);
    }
}

// Automated migration using OpenRewrite:
// Run this recipe to handle package renames:
// org.openrewrite.java.jackson.UpgradeJackson_2_3
// mvn rewrite:run -Drewrite.activeRecipes=org.openrewrite.java.jackson.UpgradeJackson_2_3

// Property ordering change:
// Jackson 2: {"name":"Alice","id":"1","email":"a@b.com"}
// Jackson 3: {"email":"a@b.com","id":"1","name":"Alice"} <- alphabetical!
// Fix tests that compare exact JSON strings character by character