JSON mapping with Jackson

Key points

• Boot auto-configures one ObjectMapper, registers the JavaTimeModule, and writes dates as ISO-8601 text.
• Tune it globally with spring.jackson.*, or add a Jackson2ObjectMapperBuilderCustomizer bean.
• @JsonProperty renames a field on the wire; a naming strategy switches the whole API to snake_case.
• @JsonInclude(NON_NULL) omits empty fields instead of shipping a wall of null.
• @JsonIgnore hides a field, but a DTO that never had it is safer - you cannot forget the annotation.
• Records bind through their canonical constructor, and unknown JSON fields are ignored by default in Boot.

Example

public record OrderResponse(
        long id,
        @JsonProperty("customer_email") String customerEmail,  // renamed on the wire
        BigDecimal total,                                      // 42.00, a JSON number
        @JsonInclude(JsonInclude.Include.NON_NULL)
        BigDecimal discount,                                   // key omitted when null
        LocalDateTime createdAt) {                             // "2026-05-01T10:00:00"
}

// An entity should never be the response type. If you must expose one, hide the secrets:
@Entity
class User {
    @JsonIgnore
    private String passwordHash;
}

// The same choices as properties, in application.yml:
//   spring.jackson.default-property-inclusion: non_null
//   spring.jackson.property-naming-strategy: SNAKE_CASE
//   spring.jackson.serialization.write-dates-as-timestamps: false

The JSON your API emits is a contract - decide it deliberately instead of inheriting it from your entities.