Generate Java from JSON Schema

Package nameClass name overrideModeling strategyJOJO preferredPOJO preferredPath only (JsonObject preferred)Field generationAllRequired OnlyNoneAccessor strategyLombok annotationsGenerate getters and settersPublic fieldsPath accessorAllRequired OnlyNoneObject leaf mappingJsonObjectMap<String, Object>JOJONumber mappingdoubleDoubleBigDecimalintlongInteger mappingintIntegerlongLongBigIntegerBoolean mappingbooleanBooleanEnum mappingJava enumPlain stringDateTime mappingOffsetDateTimeLocalDateTimeInstantPlain string

Validation annotations

All

Validation namespacejakarta.validationjavax.validationJavaDoc generationDescription onlyTitle onlyNone

JSON Schema Input

51 lines · 1102 chars

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "Order",
  "description": "Checkout order created by the storefront.",
  "type": "object",
  "required": ["id", "amount", "createdAt", "customer", "items"],
  "properties": {
    "id": {
      "type": "string",
      "description": "Business order identifier."
    },
    "amount": {
      "type": "number",
      "description": "Total amount in the settlement currency."
    },
    "createdAt": {
      "type": "string",
      "format": "date-time"
    },
    "paid": {
      "type": "boolean"
    },
    "customer": {
      "type": "object",
      "required": ["id", "email"],
      "properties": {
        "id": {
          "type": "string"
        },
        "email": {
          "type": "string"
        }
      }
    },
    "items": {
      "type": "array",
      "items": {
        "type": "object",
        "required": ["sku", "quantity"],
        "properties": {
          "sku": {
            "type": "string"
          },
          "quantity": {
            "type": "integer"
          }
        }
      }
    }
  }
}

{ "$schema": "https://json-schema.org/draft/2020-12/schema", "title": "Order", "description": "Checkout order created by the storefront.", "type": "object", "required": ["id", "amount", "createdAt", "customer", "items"], "properties": { "id": { "type": "string", "description": "Business order identifier." }, "amount": { "type": "number", "description": "Total amount in the settlement currency." }, "createdAt": { "type": "string", "format": "date-time" }, "paid": { "type": "boolean" }, "customer": { "type": "object", "required": ["id", "email"], "properties": { "id": { "type": "string" }, "email": { "type": "string" } } }, "items": { "type": "array", "items": { "type": "object", "required": ["sku", "quantity"], "properties": { "sku": { "type": "string" }, "quantity": { "type": "integer" } } } } } }

Schema Libraries

Add extra schema documents below the main schema.

No schema libraries yet

Add a supporting schema such as common.json.

Parsed Properties

11 properties

$.idrequired

string

FieldPropertyString

$.amountrequired

number

FieldPropertydoubleDoubleBigDecimalintlong

$.createdAtrequired

string (date-time)

FieldPropertyOffsetDateTimeLocalDateTimeInstantString

$.paid

boolean

FieldPropertybooleanBoolean

$.customerrequired

object

FieldPropertyJOJOPOJOJsonObject

$.customer.idrequired

string

FieldPropertyString

$.customer.emailrequired

string

FieldPropertyString

$.itemsrequired

array

FieldPropertyList<Map<String, Object>>

$.items[*]

object

FieldPropertyJOJOPOJOJsonObject

$.items[*].skurequired

string

FieldPropertyString

$.items[*].quantityrequired

integer

FieldPropertyintIntegerlongLongBigInteger

Java Output

Order.java · 108 lines · 2474 chars

package org.example.generated;

import jakarta.validation.constraints.NotNull;
import java.time.OffsetDateTime;
import java.util.List;
import lombok.Getter;
import lombok.Setter;
import org.sjf4j.JsonObject;
import org.sjf4j.path.JsonPath;

/**
 * Checkout order created by the storefront.
 *
 * JSON shape:
 * <pre>
 * {
 *   "id": "string",
 *   "amount": "number",
 *   "createdAt": "string(date-time)",
 *   "paid": "boolean",
 *   "customer": {
 *     "id": "string",
 *     "email": "string"
 *   },
 *   "items": [
 *     {
 *       "sku": "string",
 *       "quantity": "integer"
 *     }
 *   ]
 * }
 * </pre>
 */
@Getter @Setter
public class Order extends JsonObject {
    /** Business order identifier. */
    @NotNull
    private String id;

    /** Total amount in the settlement currency. */
    @NotNull
    private double amount;

    @NotNull
    private OffsetDateTime createdAt;

    private boolean paid;

    @NotNull
    private Customer customer;

    @NotNull
    private List<ItemsItem> items;

    private static final JsonPath PATH_CUSTOMER_ID = JsonPath.parse("$.customer.id");
    private static final JsonPath PATH_CUSTOMER_EMAIL = JsonPath.parse("$.customer.email");

    public String getCustomerId() {
        return PATH_CUSTOMER_ID.getString(this);
    }

    public void setCustomerId(String value) {
        PATH_CUSTOMER_ID.ensurePut(this, value);
    }

    public String getCustomerEmail() {
        return PATH_CUSTOMER_EMAIL.getString(this);
    }

    public void setCustomerEmail(String value) {
        PATH_CUSTOMER_EMAIL.ensurePut(this, value);
    }

    public String getItemsSku(int itemsIndex) {
        return getStringByPath("$.items[" + itemsIndex + "].sku");
    }

    public void setItemsSku(int itemsIndex, String value) {
        ensurePutByPath("$.items[" + itemsIndex + "].sku", value);
    }

    public int getItemsQuantity(int itemsIndex) {
        return getIntByPath("$.items[" + itemsIndex + "].quantity", 0);
    }

    public void setItemsQuantity(int itemsIndex, int value) {
        ensurePutByPath("$.items[" + itemsIndex + "].quantity", value);
    }

    @Getter @Setter
    public static class Customer extends JsonObject {
        @NotNull
        private String id;

        @NotNull
        private String email;
    }

    @Getter @Setter
    public static class ItemsItem extends JsonObject {
        @NotNull
        private String sku;

        @NotNull
        private int quantity;
    }
}

Generation Rules

Draft v2026-04-20. These rules define the current baseline and regression target, and should be sufficient for an AI to reproduce an equivalent generation tool.

1. Input contract

• Input must be valid JSON.
• Top-level schema must be a JSON object.
• The playground accepts one main schema plus zero or more schema-library documents.
• Schema-library documents are identified by their own $id values.
• Relative external $ref targets are resolved against the current document $id when that $id is a URI-like base.
• If className is empty, the generator uses schema.title; otherwise it uses the explicit override.
• If neither is available, the fallback class name is GeneratedType.

2. Current baseline mappings

JSON Schema shape	Java type baseline
string	String
string + format: date	LocalDate
string + format: date-time	OffsetDateTime, LocalDateTime, Instant, String
integer	int, Integer, long, Long, BigInteger
number	double, Double, BigDecimal, int, long
boolean	boolean, Boolean
enum	Java enum, String
object leaf	JsonObject, Map<String, Object>, JOJO
object nested	JOJO, POJO
array<T>	List<T>
unsupported / unknown	Object

3. Rendering baseline

• Root properties are rendered in schema declaration order. Whether a property becomes a Java field or a JsonObject-backed property depends on field-generation rules and per-property overrides.
• Nested object schemas are rendered as public static inner classes inside the containing class unless modelingStrategy = pathOnly; no separate Java files are generated for nested objects.
• Imports are deduplicated and sorted lexicographically.
• packageName is emitted only when non-empty.
• When validation is enabled, selected validation annotations are generated from supported schema constraints in the current baseline:
  • @NotNull from required
  • @Size from minLength / maxLength and minItems / maxItems
  • @Pattern from pattern
  • @Min / @Max from integral minimum / maximum
• JavaDoc comes from description or title, depending on the selected option.
• Accessors currently support three modes: Lombok annotations, explicit getter/setter methods, or no generated accessors.
• When accessorMode = lombok:
  • JOJO classes use @Getter and @Setter
  • POJO classes use @Data
  • No other Lombok annotations are generated in the current baseline.

4. JSON shape in JavaDoc

• Every generated root class includes a simplified JSON shape block in JavaDoc.
• The JSON shape describes payload structure, not full schema semantics.
• It includes object nesting, arrays, primitive kinds, string formats, and string-enum summaries.
• It intentionally omits validation details, required, allOf, $ref, and other schema-level metadata.
• Nested inner classes do not repeat the JSON shape block in the current baseline.

5. Modeling strategy

• additionalProperties defaults to true when omitted.
• modelingStrategy supports three modes:
  • jojo
  • pojo
  • pathOnly (UI label: Path only (JsonObject preferred))
• In jojo mode, every generated object class is a JOJO, meaning the class extends org.sjf4j.JsonObject.
• In pojo mode:
  • if an object has additionalProperties: true (or omits the keyword), it is generated as a JOJO
  • if an object has additionalProperties: false, it is generated as a plain POJO and does not extend JsonObject
• In jojo mode, if an object has additionalProperties: false, it is still generated as a JOJO, but with @NodeBinding(readDynamic = false) to disable dynamic reads of undeclared properties.
• In pathOnly mode:
  • only the root object is materialized as a generated class
  • the root class is always generated as a JOJO
  • descendant object schemas do not generate inner classes
  • root direct object properties with nested properties default to JsonObject
  • root direct arrays of object default to List<JsonObject>
  • descendant object-valued paths use JsonObject for path accessor typing
• Scope: jojo and pojo rules apply to all generated object classes, including nested objects; pathOnly suppresses descendant object-class generation entirely.

6. Field generation

• All: all properties default to field.
• Required Only: required properties default to field; non-required properties default to property.
• None: all properties default to property.
• property generation is only valid for JOJO classes because property access is backed by JsonObject APIs.
• A generated property has no backing field. It is rendered as explicit getter/setter methods that read and write through JsonObject methods such as getXxx(key) and put(key, value).
• Primitive boolean getters use JavaBean-style isXxx() naming; boxed Boolean getters continue to use getXxx().
• Getter generation prefers dedicated JsonObject APIs when available (for example getString(key), getInt(key), getJsonObject(key)); otherwise it falls back to typed access such as get(key, LocalDateTime.class).
• For List<T> properties, getter generation prefers getList(key, T.class) when T is a concrete non-generic item type; for complex generic item types it falls back to getList(key).
• property generation always emits explicit getter/setter methods, even when accessorMode = lombok or accessorMode = none.
• Per-property settings in Parsed Properties override the global field-generation strategy.
• When modelingStrategy = pathOnly, field-generation rules apply only to root direct properties.
• In pathOnly mode, every non-root property is fixed to property: it never generates a field, it is exposed only through root-level path accessors, and it may still expose type and path configuration in Parsed Properties but not field/property switching.
• For nested object properties with their own properties, the parsed-property type selector defaults to JOJO or POJO according to the effective modeling strategy, except under pathOnly where root direct object properties default to JsonObject.
• If such a nested object property is changed to JsonObject, the nested class is not generated and descendant members no longer expose field/property configuration; only eligible root-level by-path getter/setter configuration remains.

7. Path accessor generation

• Path accessors are generated only on the root class.
• Root direct members do not generate path accessors.
• Path accessors without index parameters use pre-parsed static final JsonPath constants via JsonPath.parse(...) on the root class.
• Path accessors with one or more index parameters continue to use JsonObject *ByPath APIs and ensurePutByPath(path, value) directly.
• Primitive boolean path getters use JavaBean-style isXxx() naming; boxed Boolean path getters continue to use getXxx().
• Getter generation prefers dedicated *ByPath APIs when available; otherwise it falls back to typed access such as getByPath(path, LocalDateTime.class).
• For primitive cached-path getters backed by JsonPath, generation uses default-value overloads such as getInt(this, 0), getLong(this, 0L), getDouble(this, 0d), and getBoolean(this, false) instead of manual null checks.
• Path setter generation uses ensurePut semantics so missing intermediate containers are created when needed.
• For List<T> path accessors, getter generation prefers getListByPath(path, T.class) when T is a concrete non-generic item type; for complex generic item types it falls back to getListByPath(path).
• Eligible descendant paths use flattened method names such as getCustomerEmail() and setCustomerEmail(String value).
• If a path contains arrays, one int index parameter is generated for each array segment, in path order, for example getItemsSku(int itemsIndex) or getGroupsUsersName(int groupsIndex, int usersIndex).
• Path accessors require the root class to be a JOJO; if the root class is generated as a POJO, no path accessors are emitted.
• Per-property settings in Parsed Properties can enable or disable root path methods for eligible descendant paths.
• When modelingStrategy = pathOnly, root-level path accessors are the only generated access surface for non-root properties.
• In pathOnly mode, object-valued descendant paths may also generate path accessors, using JsonObject.
• In pathOnly mode, descendant property JavaDoc is attached to the generated path getter method.

8. Enum generation

• The current baseline supports javaEnum generation for string enums only.
• Generated enums are nested inside the class that owns the property unless modelingStrategy = pathOnly removes that owning nested class; no separate Java files are generated for enums.
• Enum constants use normalized UPPER_SNAKE_CASE names.
• If normalization produces duplicates, numeric suffixes are appended to keep names unique.
• Generated enums keep only constant names; no raw-value field, constructor, or custom codec metadata is emitted.
• Enum properties use typed access such as get(key, StatusEnum.class) and getByPath(path, StatusEnum.class).
• When modelingStrategy = pathOnly removes the owning nested class for an enum property, the enum is hoisted to the root class.
• In pathOnly mode, hoisted enum names are derived from the flattened property path, for example CustomerStatusEnum.

9. allOf normalization

• The current baseline supports allOf flattening for object schemas only.
• allOf is normalized before type mapping and rendering.
• Local $ref values of the form #/... are resolved during normalization.
• External $ref values such as common.json#/$defs/Type are resolved against the current document $id, then matched to a supplied schema-library document $id.
• Circular local $ref chains are rejected with an error.
• Missing external $ref document ids are rejected with an error.
• Supported merge behavior:
  • properties: merged by property name
    • identical property schemas are preserved as-is
    • compatible object-property definitions are recursively merged as an inner object-level allOf, allowing later branches to narrow an earlier type: object declaration with nested properties, required, or additionalProperties
    • incompatible property definitions still produce an error instead of guessing
  • required: union of all entries
  • title: prefer the outer schema, otherwise the first non-empty branch title
  • description: prefer the outer schema, otherwise the first non-empty branch description
• additionalProperties: false wins; otherwise compatible schema values are preserved, or true when explicitly enabled
• Nested object schemas may also use allOf; they are rendered as inner classes after normalization unless modelingStrategy = pathOnly suppresses descendant class generation.
• Conflicting definitions for the same property name produce an error instead of guessing when they cannot be reconciled through object-schema narrowing.
• Non-object allOf composition is not supported in the current baseline.

10. Determinism and diagnostics

• Imports are sorted lexicographically and deduplicated.
• Fields, properties, nested enums, and nested inner classes preserve schema declaration order after normalization.
• required values merged from allOf are emitted in first-seen order.
• Path accessor methods are emitted in schema traversal order.
• Enum constants preserve source enum value order after normalization.
• Enum constant name collisions are resolved by appending numeric suffixes such as _2, _3, and so on.
• Property-definition conflicts in allOf are hard errors.
• Circular local $ref chains are hard errors.
• Duplicate schema-library $id values are hard errors.
• Missing external $ref document ids are hard errors.
• Unsupported non-object allOf composition is a hard error.
• Other unsupported or unrecognized schema keywords are ignored in the current baseline unless a rule explicitly states otherwise.

11. Smoke-test regression scope

The first regression suite locks down these behaviors:

• invalid JSON / invalid top-level schema handling
• class-name resolution
• a stable single-file happy path (simple-order)
• nested object inner-class generation
• pathOnly generation without nested inner classes
• nested field-path discovery for object and array shapes
• path accessor generation on the root JOJO class
• nested enum generation and typed enum access
• pathOnly enum hoisting and typed path access
• object allOf flattening, object-property narrowing, and conflict detection
• local $ref expansion and schema-library $id-based external $ref resolution

12. Deferred rules to refine next

These are intentionally not frozen yet and will be revised before full implementation:

• nullable semantics and primitive vs boxed types
• non-object and non-local $ref-based allOf composition
• object leaf mapping semantics when JOJO is selected
• multi-file output layout and filename conventions