/kotlin-tooling-java-to-kotlin

Jackson Conversion Guide

When This Applies

Detected when imports match com.fasterxml.jackson.*.

Key Rules

1. Annotation site targets:

   • @JsonProperty on a Java field → @field:JsonProperty in Kotlin.
   • @JsonProperty on a Java getter → @get:JsonProperty in Kotlin.
   • When converting to Kotlin properties, apply BOTH @field: and @get: targets to match Java's dual annotation on field + getter.

2. @JsonCreator: Java's @JsonCreator static factory or constructor → Kotlin primary constructor. The @JsonCreator annotation is often unnecessary on Kotlin's primary constructor if using the Jackson Kotlin module, but preserve it for safety.

3. @JsonIgnore: Preserve exactly. Use @get:JsonIgnore or @field:JsonIgnore depending on original target.

4. @JsonDeserialize / @JsonSerialize: Preserve exactly with correct site targets.

5. @JsonInclude: Preserve on class or property level.

6. @JsonFormat: Preserve with @field:JsonFormat site target.

7. Jackson Kotlin Module: Note that projects using Jackson with Kotlin should add jackson-module-kotlin for proper Kotlin support (data classes, default values, nullable types). This is NOT something to add during conversion — just note it if missing.

8. Builder pattern with @JsonPOJOBuilder: Replace with primary constructor + @JsonCreator if converting to data class. Otherwise preserve.

Example 1: DTO with Various Jackson Annotations

Java Input

package com.acme.dto;

import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonFormat;

/**
 * Data transfer object for an order summary.
 */
@JsonInclude(JsonInclude.Include.NON_NULL)
public class OrderSummaryDto {

    @JsonProperty("order_id")
    private final String orderId;

    @JsonProperty("total_amount")
    private final double totalAmount;

    @JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd")
    private final String createdDate;

    @JsonIgnore
    private String internalNote;

    public OrderSummaryDto(String orderId, double totalAmount, String createdDate) {
        this.orderId = orderId;
        this.totalAmount = totalAmount;
        this.createdDate = createdDate;
    }

    @JsonProperty("order_id")
    public String getOrderId() {
        return orderId;
    }

    @JsonProperty("total_amount")
    public double getTotalAmount() {
        return totalAmount;
    }

    @JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd")
    public String getCreatedDate() {
        return createdDate;
    }

    @JsonIgnore
    public String getInternalNote() {
        return internalNote;
    }

    public void setInternalNote(String internalNote) {
        this.internalNote = internalNote;
    }
}

Kotlin Output

package com.acme.dto

import com.fasterxml.jackson.annotation.JsonIgnore
import com.fasterxml.jackson.annotation.JsonInclude
import com.fasterxml.jackson.annotation.JsonProperty
import com.fasterxml.jackson.annotation.JsonFormat

/**
 * Data transfer object for an order summary.
 *
 * @property orderId unique identifier for the order, serialized as `"order_id"`
 * @property totalAmount total monetary amount, serialized as `"total_amount"`
 * @property createdDate date the order was created, formatted as `yyyy-MM-dd`
 */
@JsonInclude(JsonInclude.Include.NON_NULL)
open class OrderSummaryDto(
    @field:JsonProperty("order_id")
    @get:JsonProperty("order_id")
    val orderId: String?,

    @field:JsonProperty("total_amount")
    @get:JsonProperty("total_amount")
    val totalAmount: Double,

    @field:JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd")
    @get:JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd")
    val createdDate: String?
) {
    @field:JsonIgnore
    @get:JsonIgnore
    var internalNote: String? = null
}

Key points:

• @JsonInclude stays at class level — no site target needed.
• @JsonProperty gets both @field: and @get: to match the Java field + getter annotations.
• @JsonFormat also gets both @field: and @get: since Java had it on both.
• @JsonIgnore gets both @field: and @get: to suppress serialization fully.

Example 2: Class with @JsonCreator Factory Method

Java Input

package com.acme.model;

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;

/**
 * Immutable configuration entry deserialized from JSON.
 */
public class ConfigEntry {

    private final String key;
    private final String value;
    private final boolean enabled;

    @JsonCreator
    public static ConfigEntry create(
            @JsonProperty("key") String key,
            @JsonProperty("value") String value,
            @JsonProperty("enabled") boolean enabled) {
        return new ConfigEntry(key, value, enabled);
    }

    private ConfigEntry(String key, String value, boolean enabled) {
        this.key = key;
        this.value = value;
        this.enabled = enabled;
    }

    @JsonProperty("key")
    public String getKey() {
        return key;
    }

    @JsonProperty("value")
    public String getValue() {
        return value;
    }

    @JsonProperty("enabled")
    public boolean isEnabled() {
        return enabled;
    }
}

Kotlin Output

package com.acme.model

import com.fasterxml.jackson.annotation.JsonCreator
import com.fasterxml.jackson.annotation.JsonProperty

/**
 * Immutable configuration entry deserialized from JSON.
 *
 * @property key the configuration key
 * @property value the configuration value
 * @property enabled whether this entry is active
 */
data class ConfigEntry @JsonCreator constructor(
    @field:JsonProperty("key")
    @get:JsonProperty("key")
    val key: String?,

    @field:JsonProperty("value")
    @get:JsonProperty("value")
    val value: String?,

    @field:JsonProperty("enabled")
    @get:JsonProperty("enabled")
    val enabled: Boolean
) {
    companion object {
        /**
         * Factory method preserved for documentation; the primary constructor
         * with [JsonCreator] handles deserialization directly.
         */
        @JsonCreator
        @JvmStatic
        fun create(
            @JsonProperty("key") key: String?,
            @JsonProperty("value") value: String?,
            @JsonProperty("enabled") enabled: Boolean
        ): ConfigEntry = ConfigEntry(key, value, enabled)
    }
}

Key points:

• The Java @JsonCreator static factory is converted to a Kotlin primary constructor with @JsonCreator. The companion object factory is preserved for backward compatibility but the primary constructor handles deserialization.
• The class becomes a data class since it is immutable and value-oriented.
• @JsonCreator is kept on the primary constructor for safety, ensuring Jackson can deserialize even without the Jackson Kotlin module.
• String parameters remain nullable (String?) since Java strings are nullable by default and there is no @NonNull or Objects.requireNonNull evidence.