JSON to Code: Types and Models

Runs locally

Turn an API payload into typed models for your service layer.

JSON payload

Java (Lombok)

Generated models will appear here…
Guide

Generating a Java class, TypeScript interface or Go struct from a payload

Paste a JSON response and pick a target — Java with or without Lombok, TypeScript, Go, Python or C# — and the generator walks the payload, names a type for each nested object, and emits the model. It is the fastest way to stop hand-typing a DTO from an API you are integrating against; if the payload is minified, format the sample JSON first so you can check it.

Like any inference from a single sample, it is right about the shape and guessing about everything else. The list below is what to check before the generated class goes anywhere near production. When you need a contract rather than classes, generate a JSON Schema from the same sample.

JSON

{
  "order_id": 9007199254740993,
  "placed_at": "2026-09-15T09:00:00Z",
  "total": 12,
  "items": [],
  "customer": {
    "id": 41,
    "email": "[email protected]"
  }
}

Java (Lombok)

@Data
public class Order {
    private Long orderId;
    private String placedAt;
    private Integer total;
    private List<Object> items;
    private Customer customer;
}

@Data
public class Customer {
    private Integer id;
    private String email;
}

Three fields in that class need a human: orderId needs @JsonProperty("order_id") because the name was rewritten, placedAt is really an Instant, and total is an Integer only because this sample had no decimal point.

Where people get caught

Renamed fields stop deserialising

order_id is not a legal Java field name in the house style, so it becomes orderId — and Jackson then looks for a JSON key called orderId. Keep an @JsonProperty annotation, or configure a snake_case naming strategy globally.

One sample cannot tell optional from required

A field absent from your sample does not exist in the generated class, and the first response that includes it is silently dropped. Generate from the fullest payload you have, not the tidiest one.

Large integers do not fit a double

An ID above 2^53 loses precision the moment it passes through a JavaScript number or a float field. Type it as Long, or as a String if the API sends it as one.

Whole numbers become integers

A total of 12 in the sample produces Integer; the first 12.50 then fails to parse. Money should be BigDecimal in Java and a string in TypeScript, never a float.

Dates are strings until you say otherwise

Nothing in JSON marks a timestamp. Every generator emits String for an ISO-8601 value; swapping it for Instant, OffsetDateTime or Date is a manual step, and forgetting it is why timezone bugs surface weeks later.

Empty and mixed arrays have no element type

[] gives List<Object>, and [1, "a"] has no clean typed representation at all. Supply a sample with a populated, homogeneous array, or write that member by hand.

About the JSON to code generator

How it works in 4 steps · 4 common use cases · 4 questions answered

How it works

  1. 1.Paste a real API response — the more representative the sample, the better the generated types.
  2. 2.Pick a target language. Java is the default and can be generated with Lombok annotations or as a plain POJO.
  3. 3.Name the root class; nested objects are emitted as their own types named after their keys.
  4. 4.Copy the generated models straight into your project.

Common use cases

  • •Writing the DTO layer for a third-party API you are integrating against
  • •Turning a sample webhook payload into typed models before writing the handler
  • •Generating TypeScript interfaces from a backend response so the front end stops guessing
  • •Producing Go structs or Python dataclasses for a quick client library

FAQ

What is the difference between the Lombok and plain Java output?

The Lombok version annotates each class with @Getter, @Setter and constructor annotations and leaves the fields bare. The plain version writes explicit getters and setters, so it compiles in a project that does not have Lombok on the classpath.

How are arrays typed?

By their first element. An empty array cannot be typed from a sample, so it becomes a list of Object — replace it by hand once you know the element type.

What happens to keys that are not valid Java identifiers?

A key in snake_case or kebab-case is converted to camelCase and given a @JsonProperty annotation carrying the original name, so Jackson still binds it correctly.

Are date strings detected?

ISO 8601 date-time strings are mapped to Instant in Java and to the closest equivalent in the other languages. Other date formats come through as strings.

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