Implementation:Apache Druid Hjson Ace Mode

Knowledge Sources	Apache_Druid
Domains	Web Console, Syntax Highlighting
Last Updated	2026-02-10 10:00 GMT

Overview

Registers a custom HJSON syntax highlighting mode for the Ace editor used in the Apache Druid web console.

Description

This module defines and registers an HJSON (Human JSON) language mode with the Ace editor framework. It provides tokenization rules for HJSON syntax elements including comments (single-line, multi-line, and hash-style), strings (quoted and unquoted), multiline strings (triple-quoted), numbers, constants (true, false, null), key names, arrays, objects, and root objects. The module is a modified version of the brace library's HJSON mode, with folding functionality removed to avoid conflicts when loaded alongside the SQL mode.

Usage

Call initAceHjsonMode() during application initialization before any Ace editor instance needs to use HJSON syntax highlighting. This is typically invoked once at startup so that ace/mode/hjson becomes available as an editor mode.

Code Reference

Source Location

• Repository: Apache Druid
• File: web-console/src/ace-modes/hjson.ts
• Lines: 1-304

Signature

export function initAceHjsonMode(): void

Import

import { initAceHjsonMode } from './ace-modes/hjson';

I/O Contract

Inputs

Name	Type	Required	Description
(none)	-	-	This function takes no parameters. It uses the global ace instance from ace-builds to register the mode.

Outputs

Name	Type	Description
(void)	void	Registers ace/mode/hjson_highlight_rules and ace/mode/hjson as Ace modules as a side effect. No return value.

Usage Examples

Initializing HJSON Mode at Application Startup

import { initAceHjsonMode } from './ace-modes/hjson';

// Call once during app initialization
initAceHjsonMode();

// Then set the editor mode
editor.getSession().setMode('ace/mode/hjson');

Internals

Highlight Rules

The HjsonHighlightRules class defines tokenization rules for these HJSON constructs:

Rule Group	Token Type	Description
#comments	comment	Hash (#), single-line (//), and block (/* */) comments
#constant	constant	Boolean literals true, false, and null
#keyname	keyword	Object key names (quoted or unquoted) followed by a colon
#string	string	Double-quoted strings with escape sequence support
#mstring	string	Triple-single-quoted multiline strings
#ustring	string	Unquoted string values (HJSON extension)
#number	constant.numeric	Integer and decimal number literals
#object	paren	Curly-brace-delimited objects
#array	paren	Square-bracket-delimited arrays
#rootObject	paren	Root-level key-value pairs without braces

Mode Configuration

The HJSON mode supports:

• Line comments: //
• Block comments: /* */
• Mode ID: ace/mode/hjson
• File types: .hjson
• Scope name: source.hjson

Related Pages

• Environment:Apache_Druid_Web_Console_Development