mustache

Mustache Template-Based Generation Example

Java code generation using Mustache templates and the FHIR R4 specification.

Overview

This example demonstrates how to generate Java classes from FHIR R4 using template-based code generation. It includes:

• Template-driven Java class generation with Mustache
• Automatic resource and complex type modeling
• Utility class generation for common operations
• Post-generation hooks for code formatting and testing
• Custom name transformations for Java conventions

Setup

Generate Java Types

From the project root:

cd codegen
bun install
bun run examples/mustache/mustache-java-r4-gen.ts

This will output to ./examples/mustache/mustache-java-r4-output/

Build Java Project

cd examples/mustache/mustache-java-r4-output
mvn clean package

Configuration

The generation is configured via java/config.json. Key settings include:

Type Mapping: Maps FHIR primitive types to Java types:

• boolean → Boolean
• date → String
• dateTime → OffsetDateTime
• decimal → BigDecimal
• integer → Integer

Name Transformations: Applies Java naming conventions:

• Enum values: <= → LESS_OR_EQUAL
• Types: Add DTO suffix
• Fields: Rename reserved words

Post-Generation Hooks:

• Run Spotless for code formatting
• Execute Maven tests

Template Structure

Templates

Located in java/templates/:

• model/resource_or_complex_type.mustache - Main template for resources and complex types
• model/utils/*.mustache - Utility class templates
• annotated_type.mustache - Type with annotations
• plain_type.mustache - Simple type definition
• primitive_wrapped_plain_type.mustache - Wrapped primitive types

Static Files

Located in java/static/:

• pom.xml - Maven project configuration
• model/src/ - Base Java project structure

Using Generated Types

Create a Resource

Patient patient = new Patient()
    .setResourceType("Patient")
    .setId("patient-1")
    .setGender("male");

Add Extensions

patient.getExtension().add(new Extension()
    .setUrl("http://example.com/extension")
    .setValue(new StringType("value")));

Serialization

ObjectMapper mapper = new ObjectMapper();
String json = mapper.writeValueAsString(patient);
Patient deserialized = mapper.readValue(json, Patient.class);

Customization

Change Output Package

Edit java/config.json and update the package property in renderings:

"properties": {
  "package": "com.mycompany.fhir.models"
}

Filter Specific Resources

Customize which resources to generate via whitelist/blacklist in config:

"filters": {
  "resource": {
    "whitelist": ["Patient", "Observation"]
  }
}

Add Post-Generation Steps

Add hooks to run additional tools after generation:

"hooks": {
  "afterGenerate": [
    {"cmd": "mvn", "args": ["clean", "compile"]}
  ]
}

Template Variables

Templates have access to the following context:

TypeViewModel:

• name - Resource or type name
• saveName - Name safe for use as Java class name
• description - FHIR description
• baseType - Parent type if any
• fields - Array of fields

FieldViewModel:

• name - Field name
• saveName - Safe field name
• type - Field type
• isArray - Whether field is a list
• required - Whether field is required
• description - Field description

Special Variables:

• meta.timestamp - Generation timestamp
• meta.generator - Generator identification
• properties - Custom properties from config

Next Steps

• See examples/ overview for other language examples
• Check ../../docs/guides/mustache-generator.md for detailed Mustache template documentation
• Review java/templates/ for template examples