JToon – TOON Format for Java

⚠️ Beta Status (v1.x.x): This library is in active development and working towards spec compliance. Beta published to Maven Central. API may change before 2.0.0 release.

Compact, human-readable serialization format for LLM contexts with 30-60% token reduction vs JSON. Combines YAML-like indentation with CSV-like tabular arrays. Working towards full compatibility with the official TOON specification.

Key Features: Minimal syntax • TOON Encoding and Decoding • Tabular arrays for uniform data • Array length validation • Java 17 • full Jackson Annotation Support • Comprehensive test coverage.

Installation

Maven Central

JToon is available on Maven Central. Add it to your project using your preferred build tool:

Gradle (Groovy DSL):

dependencies {
    implementation 'dev.toonformat:jtoon:1.0.9'
}

Gradle (Kotlin DSL):

dependencies {
    implementation("dev.toonformat:jtoon:1.0.9")
}

Maven:

<dependency>
    <groupId>dev.toonformat</groupId>
    <artifactId>jtoon</artifactId>
    <version>1.0.9</version>
</dependency>

Note: See the latest version on Maven Central (also shown in the badge above).

Alternative: Manual Installation

You can also download the JAR directly from the GitHub Releases page and add it to your project's classpath.

Quick Start

import dev.toonformat.jtoon.JToon;
import java.util.*;

record User(int id, String name, List<String> tags, boolean active, List<?> preferences) {}
record Data(User user) {}

User user = new User(123, "Ada", List.of("reading", "gaming"), true, List.of());
Data data = new Data(user);

System.out.println(JToon.encode(data));

Output:

user:
  id: 123
  name: Ada
  tags[2]: reading,gaming
  active: true
  preferences[0]:

Type Conversions

Some Java-specific types are automatically normalized for LLM-safe output:

API

JToon.encode(Object value): String

JToon.encode(Object value, EncodeOptions options): String

JToon.encodeJson(String json): String

JToon.encodeJson(String json, EncodeOptions options): String

Converts any Java object or JSON-string to TOON format.

Parameters:

• value – Any Java object (Map, List, primitive, or nested structure). Non-serializable values are converted to null. Java temporal types are converted to ISO strings, Optional is unwrapped, and Stream is materialized.
• options – Optional encoding options (EncodeOptions record):
  • indent – Number of spaces per indentation level (default: 2)
  • delimiter – Delimiter enum for array values and tabular rows: Delimiter.COMMA (default), Delimiter.TAB, or Delimiter.PIPE
  • lengthMarker – Boolean to prefix array lengths with # (default: false)
  • flatten – Boolean to key folding to collapse single-key wrapper chains (default: OFF).
  • flattenDepth – maximum number of segments to fold (default: Infinity)

For encodeJson overloads:

• json – A valid JSON string to be parsed and encoded. Invalid or blank JSON throws IllegalArgumentException.

Returns:

A TOON-formatted string with no trailing newline or spaces.

Example:

import dev.toonformat.jtoon.JToon;
import com.fasterxml.jackson.annotation.JsonIgnore;
import java.util.*;

record Item(String sku, int qty, double price,  @JsonIgnore double internPrice) {}
record Data(List<Item> items) {}

Item item1 = new Item("A1", 2, 9.99, 8.50);
Item item2 = new Item("B2", 1, 14.5, 14.0);
Data data = new Data(List.of(item1, item2));

System.out.println(JToon.encode(data));

Output:

items[2]{sku,qty,price}:
  A1,2,9.99
  B2,1,14.5

The Jackson Annotation @JsonIgnore will help to keep fields from exposing.

Encode a plain JSON string

String json = """
{
  "user": {
    "id": 123,
    "name": "Ada",
    "tags": ["reading", "gaming"]
  }
}
""";
System.out.println(JToon.encodeJson(json));

Output:

user:
  id: 123
  name: Ada
  tags[2]: reading,gaming

Delimiter Options

The delimiter option allows you to choose between comma (default), tab, or pipe delimiters for array values and tabular rows. Alternative delimiters can provide additional token savings in specific contexts.

Tab Delimiter (\t)

Using tab delimiters instead of commas can reduce token count further, especially for tabular data:

import dev.toonformat.jtoon.*;
import java.util.*;

record Item(String sku, String name, int qty, double price) {}

record Data(List<Item> items) {}

Item item1 = new Item("A1", "Widget", 2, 9.99);
Item item2 = new Item("B2", "Gadget", 1, 14.5);
Data data = new Data(List.of(item1, item2));

EncodeOptions options = new EncodeOptions(2, Delimiter.TAB, false, KeyFolding.OFF, 3);
System.out.println(JToon.encode(data, options));

Output:

items[2 ]{sku name qty price}:
  A1 Widget 2 9.99
  B2 Gadget 1 14.5

Benefits:

• Tabs are single characters and often tokenize more efficiently than commas.
• Tabs rarely appear in natural text, reducing the need for quote-escaping.
• The delimiter is explicitly encoded in the array header, making it self-descriptive.

Considerations:

• Some terminals and editors may collapse or expand tabs visually.
• String values containing tabs will still require quoting.

Pipe Delimiter (|)

Pipe delimiters offer a middle ground between commas and tabs:

// Using the same Item and Data records from above
EncodeOptions options = new EncodeOptions(2, Delimiter.PIPE, false, KeyFolding.OFF, 3);
System.out.println(JToon.encode(data, options));

Output:

items[2|]{sku|name|qty|price}:
  A1|Widget|2|9.99
  B2|Gadget|1|14.5

Length Marker Option

The lengthMarker option adds an optional hash (#) prefix to array lengths to emphasize that the bracketed value represents a count, not an index:

import dev.toonformat.jtoon.*;
import java.util.*;

record Item(String sku, int qty, double price) {}

record Data(List<String> tags, List<Item> items) {}

Item item1 = new Item("A1", 2, 9.99);
Item item2 = new Item("B2", 1, 14.5);
Data data = new Data(List.of("reading", "gaming", "coding"), List.of(item1, item2));

System.out.println(JToon.encode(data, new EncodeOptions(2, Delimiter.COMMA, true, KeyFolding.OFF, 3)));
// tags[#3]: reading,gaming,coding
// items[#2]{sku,qty,price}:
//   A1,2,9.99
//   B2,1,14.5

// Works with custom delimiters
System.out.println(JToon.encode(data, new EncodeOptions(2, Delimiter.PIPE, true, KeyFolding.OFF, 3)));
// tags[#3|]: reading|gaming|coding
// items[#2|]{sku|qty|price}:
//   A1|2|9.99
//   B2|1|14.5

JToon.decode(String toon): Object

JToon.decode(String toon, DecodeOptions options): Object

JToon.decodeToJson(String toon): String

JToon.decodeToJson(String toon, DecodeOptions options): String

Converts TOON-formatted strings back to Java objects or JSON.

Parameters:

• toon – TOON-formatted input string
• options – Optional decoding options (DecodeOptions record):
  • indent – Number of spaces per indentation level (default: 2)
  • delimiter – Expected delimiter: Delimiter.COMMA (default), Delimiter.TAB, or Delimiter.PIPE
  • strict – Boolean for validation mode. When true (default), throws IllegalArgumentException on invalid input. When false, returns null on errors.
  • expandPaths – Boolean Path expansion mode for dotted keys (default: OFF).

Returns:

For decode: A Java object (Map for objects, List for arrays, primitives for scalars, or null)

For decodeToJson: A JSON string representation

Example:

import dev.toonformat.jtoon.JToon;

String toon = """
    users[2]{id,name,role}:
      1,Alice,admin
      2,Bob,user
    """;

// Decode to Java objects
Object result = JToon.decode(toon);

// Decode directly to JSON string
String json = JToon.decodeToJson(toon);

Round-Trip Conversion

import dev.toonformat.jtoon.*;
import java.util.*;

// Original data
Map<String, Object> data = new LinkedHashMap<>();
data.put("id", 123);
data.put("name", "Ada");
data.put("tags", Arrays.asList("dev", "admin"));

 // Encode to TOON
String toon = JToon.encode(data);

// Decode back to objects
 Object decoded = JToon.decode(toon);

// Values are preserved (note: integers decode as Long)

Custom Decode Options

import dev.toonformat.jtoon.*;

String toon = "tags[3|]: a|b|c";

// Decode with pipe delimiter
DecodeOptions options = new DecodeOptions(2, Delimiter.PIPE, true);
Object result = JToon.decode(toon, options);

// Lenient mode (returns null on errors instead of throwing)
DecodeOptions lenient = DecodeOptions.withStrict(false);
Object result2 = JToon.decode(invalidToon, lenient);

CI/CD: GitHub Actions • Java 17 • Coverage enforcement • PR coverage comments

Project Status

This project is 100% compliant with TOON specification. Release conformance enforced on CI/CD.

See CONTRIBUTING.md for detailed guidelines.

Documentation

• 📘 Full Documentation - Extended guides and references
• 🔧 API Reference - Detailed Javadoc
• 📋 Format Specification - TOON syntax and rules
• 📜 TOON Spec - Official specification
• 🐛 Issues - Bug reports and features
• 🤝 Contributing - Contribution guidelines

License

MIT License – see LICENSE for details