JSON to Code

How to use JSON to Code

1. Paste a sample of your JSON into Input JSON, drop a file on it, or use Open file. Try an example loads a sample.
2. Pick a Language: Go, Python, Rust, Java, C# or Kotlin. Python, Java and C# also offer a style (dataclass or Pydantic v2, record or POJO, record or class).
3. Set the Root name and choose Field names: the language’s own convention, or the JSON names kept as they are.
4. Copy the code, use Download (the file gets the right extension), or send it to another tool.

How it works

The JSON is read with the same strict parser as the JSON Formatter and a shape is inferred from the values, using the same inference as JSON to TypeScript. All items of an array are merged into one shape. A key missing from some items is optional, a key that is sometimes null is nullable, and a position that holds different kinds of values (for example a string and a number) becomes the language’s “any” type. Whole numbers become integers and numbers with a fraction or exponent become floating point.

Each object becomes a named type. Nested types are named after their key in PascalCase and array items use the singular form of the key. Objects with exactly the same fields share one type. Each language then renders that model: Go structs with json tags (omitempty for optional keys, pointers for nullable ones), Python dataclasses or Pydantic v2 models, Rust structs with serde derives and Option<T>, Java records or POJOs with Jackson annotations, C# records or classes with System.Text.Json attributes, and Kotlin data classes with kotlinx.serialization. Keys that aren’t valid identifiers, or whose idiomatic name differs from the key, get a rename annotation (json tag, alias, serde(rename), @JsonProperty, JsonPropertyName, @SerialName).

Limits

• Input is limited to about 5 MB (5 million characters); opened or dropped files to 10 MB.
• Types only describe the sample you paste. A field that is always null or only appears in an empty array becomes the language’s “any” type (any, Any, serde_json::Value, Object, object, JsonElement).
• Integers are 64-bit (int64, long, Long, i64) and everything else numeric is a 64-bit float. Date strings stay strings.
• Python dataclasses can’t rename keys, so a renamed field gets a comment with its JSON key; use Pydantic for aliases.
• Optional Go fields use omitempty, so a zero value (0, false, empty string) is left out when encoding. Nullable items inside an array are not marked nullable.
• Java output has one public type per file: only the root is public, so save the file under the root’s name (the download does).
• Nesting deeper than 200 levels becomes the “any” type.

Privacy

Code is generated entirely in your browser; your JSON is never uploaded or stored. Share copies a link with your input and options in the URL’s # fragment, which browsers don’t send to servers, though anyone with the link can read it. If you use Send to… or receive JSON from another tool, it is handed over through this tab’s session storage and removed as soon as the receiving tool reads it.

Frequently asked questions

Why is a Go field a pointer?

A key whose value is sometimes null becomes a pointer (*string) so nil can mean null. Slices and any can already be nil, so they stay as they are. Keys missing from some objects get omitempty instead.

Should I use a Python dataclass or Pydantic?

Pydantic v2 validates and converts JSON for you and supports aliases for keys like site-url. Dataclasses are plain and have no dependencies, but you must map the keys to fields yourself.

Why do some fields have a rename annotation?

Languages prefer their own naming (snake_case in Python and Rust, camelCase in Java and Kotlin, PascalCase in Go and C#), so the JSON key is recorded in an annotation. Choose Keep JSON names to use the keys as they are where the language allows it.

What happens when the JSON root is an array?

The items are merged into one type named after the root with an Item suffix, and languages with aliases add one for the array (type Root []RootItem in Go, Root = list[RootItem] in Python). Java and C# add a comment instead.