Page JSON Serializer

PageJsonSerializer.kt

import com.fasterxml.jackson.core.JsonGenerator
import com.fasterxml.jackson.databind.JsonSerializer
import com.fasterxml.jackson.databind.SerializerProvider
import org.springframework.boot.jackson.JsonComponent
import org.springframework.data.domain.Page

/**
 * JSON serializer for [Page].
 *
 * @since 2023-11-27
 */
@JsonComponent
class PageJsonSerializer<T> : JsonSerializer<Page<T>>() {

  /**
   * Method that can be called to ask implementation to serialize values of a type this serializer
   * handles. This serializer replicates pre-`Spring Boot 3.2.0` JSON structure.
   *
   * @param page Value to serialize; CANNOT be null.
   * @param gen Generator used to output resulting Json content
   * @param serializers Provider that can be used to get serializers for serializing Objects value
   *   contains, if any.
   */
  override fun serialize(page: Page<T>, gen: JsonGenerator, serializers: SerializerProvider?) {
    gen.writeStartObject() // means start, like '{}'

    gen.writeObjectField("content", page.content)

    gen.writeBooleanField("empty", page.isEmpty)
    gen.writeBooleanField("first", page.isFirst)
    gen.writeBooleanField("last", page.isLast)
    gen.writeNumberField("number", page.number)
    gen.writeNumberField("numberOfElements", page.numberOfElements)
    gen.writeNumberField("size", page.size)
    gen.writeNumberField("totalPages", page.totalPages)
    gen.writeNumberField("totalElements", page.totalElements)

    // mostly duplicate data (pageable.pageSize -> size, pageable.offset -> number, etc.)
    // need special care in `Spring Boot 3.2.0` if `pageable` object is `unpaged`; otherwise error
    if (page.pageable.isUnpaged) {
      gen.writeStringField("pageable", "INSTANCE")
    } else {
      gen.writeObjectField("pageable", page.pageable)
    }

    gen.writeObjectField("sort", page.sort)

    gen.writeEndObject()
  }
}