Skip to main content

Using Templates

It's easy to work with templates for codegen!

For maybe 90% of use cases, you will only need to modify the mustache template files to create your own custom generated code. If you need to include additional files in your generated output, manipulate the OpenAPI document inputs, or implement your own vendor extensions or other logic, you'll want to read customization after you read this document. Be sure to start here first, because templating is the easier concept and you'll need it for more advanced use cases.

The generator workflow has transforming logic as well as templates for each generation of code.

Each generator will create a data structure from the OpenAPI document; OpenAPI 2.0 and OpenAPI 3.x documents are normalized into the same API model within the generator. This model is then applied to the templates. While generators do not need to perform transformations, it's often necessary in order to add more advanced support for your language or framework. You may need to refer to the generator implementation to understand some of the logic while creating or customizing templates (see for an advanced example).

The transform logic needs to implement and is most easily done by extending Take a look at the various implementations as a guideline while the instructions get more complete.

Modifying Templates

OpenAPI Generator applies user-defined templates via options:

  • CLI: -t/--template CLI options
  • Maven Plugin: templateDirectory
  • Gradle Plugin: templateDir

Built-in templates are written in Mustache and processed by jmustache. Beginning with version 4.0.0, we support experimental Handlebars and user-defined template engines via plugins.

OpenAPI Generator supports user-defined templates. This approach is often the easiest when creating a custom template. Our generators implement a combination of language and framework features, and it's fully possible to use an existing generator to implement a custom template for a different framework. Suppose you have internal utilities which you'd like to incorporate into generated code (e.g. logging, monitoring, fault-handling)... this is easy to add via custom templates.

Note: You cannot use this approach to create new templates, only override existing ones. If you'd like to create a new generator to contribute back to the project, see in the repository root. If you'd like to create a private generator for more templating control, see the customization docs.

OpenAPI Generator not only supports local files for templating, but also templates defined on the classpath. This is a great option if you want to reuse templates across multiple projects. To load a template via classpath, you'll need to generate a little differently. For example, if you've created an artifact called template-classpath-example which contains extended templates for the htmlDocs generator with the following structure:

└── src
├── main
│   ├── java
│   └── resources
│   └── templates
│   └── htmlDocs
│   ├── index.mustache
│   └── style.css.mustache

You can define your classpath to contain your JAR and the openapi-generator-cli fat jar, then invoke main class org.openapitools.codegen.OpenAPIGenerator. For instance,

java -cp /path/totemplate-classpath-example-1.0-SNAPSHOT.jar:modules/openapi-generator-cli/target/openapi-generator-cli.jar \
org.openapitools.codegen.OpenAPIGenerator generate \
-i \
-g html -o template-example -t templates/htmlDocs

NOTE Running your custom generator in the example above requires adding it to the classpath. This differs on Windows slightly from unix.

Take note that our template directory is relative to the resource directory of the JAR defined on the classpath.

Retrieving Templates

You will need to find and retrieve the templates for your desired generator in order to redefine structures, documentation, or API logic. We cover template customization in the following sections.

In OpenAPI Generator 5.0 and later, you can use the CLI command author template to extract embedded templates for your target generator. For example:

openapi-generator author template -g java --library webclient

For OpenAPI Generator versions prior to 5.0, you will want to find the resources directory for the generator you want to extend. This is generally easy to find as directories commonly follow the convention of resources/<generator name>. In cases where you're unsure, you will need to find the embeddedTemplateDir assignment in your desired generator. This is almost always assigned in the constructor of the generator class. The C# generator assigns this as:

embeddedTemplateDir = templateDir = "csharp";

These templates are in our source repository at modules/openapi-generator/src/main/resources/csharp. Be sure to select the tag or branch for the version of OpenAPI Generator you're using before grabbing the templates.

NOTE If you have specific logic you'd like to modify such as modifying the generated README, you only need to pull and modify this individual template. OpenAPI Generator will lookup templates in this order:

  • User customized library path (e.g. custom_template/libraries/feign/model.mustache)
  • User customized generator top-level path (e.g. custom_template/model.mustache)
  • Embedded library path (e.g. resources/Java/libraries/feign/model.mustache)
  • Embedded top-level path (e.g. resources/Java/model.mustache)
  • Common embedded path (e.g. resources/_common/model.mustache)

Custom Logic

For this example, let's modify a Java client to use AOP via jcabi/jcabi-aspects. We'll log API method execution at the INFO level. The jcabi-aspects project could also be used to implement method retries on failures; this would be a great exercise to further play around with templating.

The Java generator supports a library option. This option works by defining base templates, then applying library-specific template overrides. This allows for template reuse for libraries sharing the same programming language. Templates defined as a library need only modify or extend the templates concerning the library, and generation falls back to the root templates (the "defaults") when not extended by the library. Generators which support the library option will only support the libraries known by the generator at compile time, and will throw a runtime error if you try to provide a custom library name.

To get started, we will need to copy our target generator's directory in full.

The directory will be located under modules/openapi-generator/src/main/resources/{generator}. In general, the generator directory matches the generator name (what you would pass to the generator option), but this is not a requirement-- if you are having a hard time finding the template directory, look at the embeddedTemplateDir option in your target generator's implementation.

If you've already cloned openapi-generator, find and copy the modules/openapi-generator/src/main/resources/Java directory. If you have the Refined GitHub Chrome or Firefox Extension, you can navigate to this directory on GitHub and click the "Download" button. Or, to pull the directory from latest master:

mkdir -p ~/.openapi-generator/templates/ && cd $_
curl -L | tar xz
mv `ls`/modules/openapi-generator/src/main/resources/Java ./Java
\rm -rf OpenAPITools-openapi-generator-*
cd Java

Optional: Before modifying your templates, you may want to git init && git add . && git commit -am 'initial' so you can easily revert to the base templates.

At this point, you have every Java library's template locally. Let's delete all libraries except the resteasy library we'll be extending:

ls -d libraries/* | grep -v resteasy | xargs rm -rf

Execute tree in this Java directory and inspect the mustache files and directory structure. You'll notice there are quite a few templates in the directory root, but extending this root to support resteasy only requires modifying a handful of files:

tree libraries/resteasy/
├── ApiClient.mustache
├── JSON.mustache
├── api.mustache
├── build.gradle.mustache
├── build.sbt.mustache
└── pom.mustache

0 directories, 6 files

NOTE: Some generators may be sensitive to which files exist. If you're concerned with redundant files like pom.mustache and build.sbt.mustache, you can try deleting them. If the generator you're customizing fails at runtime, just touch these files to create an empty file.

First, let's add our new dependency to libraries/resteasy/build.gradle.mustache:

diff --git a/libraries/resteasy/build.gradle.mustache b/libraries/resteasy/build.gradle.mustache
index 3b40702..a6d12e0 100644
--- a/libraries/resteasy/build.gradle.mustache
+++ b/libraries/resteasy/build.gradle.mustache
@@ -134,6 +134,7 @@ ext {

dependencies {
+ compile "com.jcabi:jcabi-aspects:0.22.6"
compile "io.swagger:swagger-annotations:$swagger_annotations_version"
compile "org.jboss.resteasy:resteasy-client:$resteasy_version"
compile "org.jboss.resteasy:resteasy-multipart-provider:$resteasy_version"

Then, we'll add the necessary import to api.mustache. This file is the template which becomes the API invoking class (e.g. PetApi or StoreApi).

diff --git a/libraries/resteasy/api.mustache b/libraries/resteasy/api.mustache
index a4d0f9f..49b17c7 100644
--- a/libraries/resteasy/api.mustache
+++ b/libraries/resteasy/api.mustache
@@ -1,5 +1,6 @@
package {{package}};

+import com.jcabi.aspects.Loggable;
import {{invokerPackage}}.ApiException;
import {{invokerPackage}}.ApiClient;
import {{invokerPackage}}.Configuration;

Next, we'll find the code which generates API methods. You'll see {{#operations}}{{#operation}} which is a mustache "loop" which executes the template logic if the model applied to the template has an operations array, and a non-null operation instance in that array. You can pass --global-property debugOpenAPI=true when generating via CLI to inspect the full object model.

Further down in api.mustache, find implementation of the method call, and add the @Loggable annotation. This template is easy because it has a single method implementation.

diff --git a/libraries/resteasy/api.mustache b/libraries/resteasy/api.mustache
index 49b17c7..16ee191 100644
--- a/libraries/resteasy/api.mustache
+++ b/libraries/resteasy/api.mustache
@@ -57,6 +57,7 @@ public class {{classname}} {
+ @Loggable(Loggable.INFO)
public {{#returnType}}{{{returnType}}} {{/returnType}}{{^returnType}}void {{/returnType}}{{operationId}}({{#allParams}}{{{dataType}}} {{paramName}}{{^-last}}, {{/-last}}{{/allParams}}) throws ApiException {
Object {{localVariablePrefix}}localVarPostBody = {{#bodyParam}}{{paramName}}{{/bodyParam}}{{^bodyParam}}new Object(){{/bodyParam}};

Finally, because our new dependency relies on AspectJ and code weaving, let's modify the build.gradle.mustache again to set this up.

diff --git a/build.gradle.mustache b/build.gradle.mustache
index 04a9d55..7a93c50 100644
--- a/build.gradle.mustache
+++ b/build.gradle.mustache
@@ -1,5 +1,6 @@
apply plugin: 'idea'
apply plugin: 'eclipse'
+apply plugin: 'aspectj'

group = '{{groupId}}'
version = '{{artifactVersion}}'
@@ -12,6 +13,7 @@ buildscript {
dependencies {
classpath ''
classpath 'com.github.dcendents:android-maven-gradle-plugin:1.5'
+ classpath "net.uberfoo.gradle:gradle-aspectj:2.2"

@@ -140,9 +142,18 @@ ext {
jersey_version = "1.19.4"
jodatime_version = "2.9.9"
junit_version = "4.13"
+ aspectjVersion = '1.9.0'

+sourceCompatibility = '1.8'
+targetCompatibility = '1.8'
dependencies {
+ compile "com.jcabi:jcabi-aspects:0.22.6"
+ aspectpath "com.jcabi:jcabi-aspects:0.22.6"
+ // usually, client code leaves logging implementation to the consumer code
+ compile "org.apache.logging.log4j:log4j-slf4j-impl:2.8.2"
+ compile "org.apache.logging.log4j:log4j-core:2.8.2"
compile "io.swagger:swagger-annotations:$swagger_annotations_version"
compile "com.sun.jersey:jersey-client:$jersey_version"
compile "com.sun.jersey.contribs:jersey-multipart:$jersey_version"

NOTE: This example includes log4j-slf4j-impl to demonstrate that our new code is working. Generally you'll want to leave logging implementations up to your consumers.

And because the java client generates with an outdated Gradle 2.6, let's update the gradle version in the default template (Java/

diff --git a/ b/
index b7a3647..3d9d088 100644
--- a/
+++ b/
@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME

Now we're ready to generate the client with our simple changes. When we pass the template directory option to our toolset, we must pass the generator's root directory and not the library-only directory.

openapi-generator generate -g java --library resteasy \
-t ~/.openapi-generator/templates/Java \
-o ~/.openapi-generator/example \

Make sure your custom template compiles:

cd ~/.openapi-generator/example
gradle assemble
# or, regenerate the wrapper
gradle wrapper --gradle-version 4.8 --distribution-type all
./gradlew assemble

You should see a log message showing our added dependency being downloaded:


And for the sake of verifying our AOP modifications work, let's create a src/main/resources/ file in our new client code:

status = error
dest = err
name = PropertiesConfig

property.filename = target/rolling/rollingtest.log

filter.threshold.type = ThresholdFilter
filter.threshold.level = debug

appender.console.type = Console = STDOUT
appender.console.layout.type = PatternLayout
appender.console.layout.pattern = %m%n
appender.console.filter.threshold.type = ThresholdFilter
appender.console.filter.threshold.level = error

appender.rolling.type = RollingFile = RollingFile
appender.rolling.fileName = ${filename}
appender.rolling.filePattern = target/rolling2/test1-%d{MM-dd-yy-HH-mm-ss}-%i.log.gz
appender.rolling.layout.type = PatternLayout
appender.rolling.layout.pattern = %d %p %C{1.} [%t] %m%n
appender.rolling.policies.type = Policies
appender.rolling.policies.time.type = TimeBasedTriggeringPolicy
appender.rolling.policies.time.interval = 2
appender.rolling.policies.time.modulate = true
appender.rolling.policies.size.type = SizeBasedTriggeringPolicy
appender.rolling.strategy.type = DefaultRolloverStrategy
appender.rolling.strategy.max = 5 = org.openapitools.client.api.PetApi
logger.rolling.level = debug
logger.rolling.additivity = false
logger.rolling.appenderRef.rolling.ref = RollingFile

rootLogger.level = info
rootLogger.appenderRef.stdout.ref = STDOUT

Execute ./gradlew build and then cat target/rolling/rollingtest.log. You should see messages logged for every call in PetApi with a stubbed unit test.

Congratulations! You've now modified one of the built-in templates to meet your client code's needs.

Adding/modifying template logic simply requires a little bit of mustache, for which you can use existing templates as a guide.

Custom Engines

Custom template engine support is experimental

If Mustache or the experimental Handlebars engines don't suit your needs, you can define an adapter to your templating engine of choice. To do this, you'll need to define a new project which consumes the openapi-generator-core artifact, and at a minimum implement TemplatingEngineAdapter.

This example:

  • creates an adapter providing the fundamental logic to compile Pebble Templates
  • will be implemented in Kotlin to demonstrate ServiceLoader configuration specific to Kotlin (Java will be similar)
  • requires Gradle 5.0+
  • provides project setup instructions for IntelliJ

To begin, create a new Gradle project with Kotlin support. To do this, go to FileNewProject, choose "Gradle" and "Kotlin". Specify groupId org.openapitools.examples and artifactId pebble-template-adapter.

Ensure the new project uses Gradle 5.0. Navigate to the newly created directory and execute:

gradle wrapper --gradle-version 5.0

In build.gradle, we'll add a dependency for OpenAPI Tools core which defines the interface and an abstract helper type for implementing the adapter. We'll also pull in the Pebble artifact. We'll be evaluating this new artifact locally, so we'll also add the Maven plugin for installing to the local maven repository. We'll also create a fatjar using the shadow plugin to simplify our classpath.

Modifications to the new project's build.gradle should be made in the plugins and dependencies nodes:

 plugins {
id 'org.jetbrains.kotlin.jvm' version '1.3.11'
id "com.github.johnrengelman.shadow" version "5.0.0"

dependencies {
compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8"
compile "org.openapitools:openapi-generator-core:4.0.0-SNAPSHOT"
compile "io.pebbletemplates:pebble:3.0.8"

The above configuration for the shadow plugin is strictly optional. It is not needed, for instance, if you plan to publish your adapter and consume it via the Maven or Gradle plugins.

Next, create a new class file called PebbleTemplateEngineAdapter under src/kotlin. We'll define the template adapter's name as pebble and we'll also list this as the only supported file extension. We'll implement the adapter by extending AbstractTemplatingEngineAdapter, which includes reusable logic, such as retrieving a list of all possible template names for our provided template extensions(s).

The class in its simplest form looks like this (with inline comments):

// Allows specifying engine by class name
// e.g. -e org.openapitools.examples.templating.PebbleTemplateAdapter
package org.openapitools.examples.templating

// imports

class PebbleTemplateAdapter : AbstractTemplatingEngineAdapter() {
// initialize the template compilation engine
private val engine: PebbleEngine = PebbleEngine.Builder()
.loader(DelegatingLoader(listOf(FileLoader(), ClasspathLoader())))

// allows targeting engine by id/name: -e pebble
override fun getIdentifier(): String = "pebble"

override fun compileTemplate(
generator: TemplatingGenerator?,
bundle: MutableMap<String, Any>?,
templateFile: String?
): String {
// This will convert, for example, model.mustache to model.pebble
val modifiedTemplate = this.getModifiedFileLocation(templateFile).first()

// Uses generator built-in template resolution strategy to find the full template file
val filePath = generator?.getFullTemplatePath(modifiedTemplate)

val writer = StringWriter()
// Conditionally writes out the template if found.
if (filePath != null) {
engine.getTemplate(filePath.toAbsolutePath().toString())?.evaluate(writer, bundle)
return writer.toString()

override fun getFileExtensions(): Array<String> = arrayOf("pebble")

Lastly, create a file resources/META-INF/services/org.openapitools.codegen.api.TemplatingEngineAdapter, containing the full class path to the above class:


This allows the adapter to load via ServiceLoader, and to be referenced via the identifier pebble. This is optional; if you don't provide the above file and contents, you'll only be able to load the engine via full class name (explained in a bit).

Now, build the fatjar for this new adapter:

./gradlew shadowJar

To test compilation of some templates, we'll need to first create one or more template files. Create a temp directory at /tmp/pebble-example/templates and add the following files.


package {{packageName}}

import (
{% for item in imports %}
{% endfor %}

type Generated{{classname}}Servicer

// etc


package {{packageName}}

{% for item in models %}
{% if item.isEnum %}
// TODO: enum
{% else %}
{% if item.description is not empty %}// {{item.description}}{% endif %}
type {{item.classname}} struct {
{% for var in item.model.vars %}
{% if var.description is not empty %}// {{var.description}}{% endif %}
{{}} {% if var.isNullable %}*{% endif %}{{var.dataType}} `json:"{{var.baseName}}{% if var.required == false %},omitempty{% endif %}"{% if var.withXml == true %} xml:"{{var.baseName}}{% if var.isXmlAttribute %},attr{% endif %}"{% endif %}`
{% endfor %}
{% endif %}
{% endfor %}

Find object structures passed to templates later in this document's Structures section.

Finally, we can compile some code by explicitly defining our classpath and jar entrypoint for CLI (be sure to modify /your/path below)

java $JAVA_OPTS -cp /your/path/build/libs/pebble-template-adapter-1.0-SNAPSHOT-all.jar:modules/openapi-generator-cli/target/openapi-generator-cli.jar \
org.openapitools.codegen.OpenAPIGenerator \
generate \
-g go \
-i \
-e pebble \
-o /tmp/pebble-example/out \
-t /tmp/pebble-example/templates \
--global-property models,modelDocs,modelTests,apis,apiTests,apiDocs

NOTE Running your custom generator requires adding it to the classpath. This differs on Windows slightly from unix.

In the above example, we've targeted our custom template engine adapter via -e pebble. If you don't include the SPI file under META-INF/services, you'll need to specify the exact classpath: org.openapitools.examples.templating.PebbleTemplateAdapter. Notice that the target class here matches the Kotlin class name. This is because of the @file:JvmName annotation.

Congratulations on creating a custom templating engine adapter!


Aside from transforming an API document, the implementing class gets to decide how to apply the data structure to templates. We can decide which data structure to apply to which template files. You have the following structures at your disposal.

Examples for the following structures will be presented using the following spec document:

  swagger: "2.0"
version: "1.0.0"
title: "Swagger Petstore"
description: "A sample API that uses a petstore as an example to demonstrate features in the swagger-2.0 specification"
termsOfService: ""
name: "Swagger API Team"
name: "MIT"
host: ""
basePath: "/api"
- "http"
- "application/json"
- "application/json"
description: "Returns all pets from the system that the user has access to"
- "application/json"
description: "A list of pets."
type: "array"
$ref: "#/definitions/Pet"
type: "object"
- "id"
- "name"
type: "integer"
format: "int64"
type: "string"
type: "string"


Inspect operation structures passed to templates with system property --global-property debugOpenAPI=true


openapi-generator generate -g go \
-o out \
-i petstore-minimal.yaml \
--global-property debugOpenAPI=true

There is a data structure which represents all the operations that are defined in the OpenAPI specification. A single API file is created for each OperationGroup, which is essentially a grouping of different operations. See the addOperationToGroup in for details on this operation.

You can have many files created for each OperationGroup by processing multiple templates and assigning a different file naming pattern to them. So for a single file per operation:

// process the `api.mustache` template and output a single file with suffix `.java`:
apiTemplateFiles.put("api.mustache", ".java");

For C-like languages which also require header files, you may create two files per operation.

// create a header and implementation for each operation group:
apiTemplateFiles.put("api-header.mustache", ".h");
apiTemplateFiles.put("api-body.mustache", ".m");

Here, an Operation with tag Pet will generate two files: SWGPetApi.h and SWGPetApi.m. The SWG prefix and Api suffix are options specific to the Objective-C generator.


Inspect models passed to templates with system property --global-property debugModels=true


openapi-generator generate -g go \
-o out \
-i petstore-minimal.yaml \
--global-property debugModels=true

Each model identified inside the generator will be passed into the Models data structure and will generate a new model file (or files) for each model.

A Pet model with three properties will provide a lot of information about the type and properties. The output from --global-property debugModels=true is presented in truncated format here.

[ {
"importPath" : "openapi.Pet",
"model" : {
"name" : "Pet",
"classname" : "Pet",
"classVarName" : "Pet",
"modelJson" : "{\n \"required\" : [ \"id\", \"name\" ],\n \"type\" : \"object\",\n \"properties\" : {\n \"id\" : {\n \"type\" : \"integer\",\n \"format\" : \"int64\"\n },\n \"name\" : {\n \"type\" : \"string\"\n },\n \"tag\" : {\n \"type\" : \"string\"\n }\n }\n}",
"dataType" : "map[string]interface{}",
"classFilename" : "model_pet",
"isAlias" : false,
"isString" : false,
"isInteger" : false,
"vars" : [ {
"baseName" : "id",
"getter" : "getId",
"setter" : "setId",
"dataType" : "int64",
"datatypeWithEnum" : "int64",
"dataFormat" : "int64",
"name" : "Id",
"defaultValueWithParam" : " =;",
"baseType" : "int64",
"example" : "null",
"jsonSchema" : "{\n \"type\" : \"integer\",\n \"format\" : \"int64\"\n}",
"exclusiveMinimum" : false,
"exclusiveMaximum" : false,
"required" : true,
"hasMoreNonReadOnly" : true,
"isPrimitiveType" : true,
"isModel" : false,
"isContainer" : false,
"isNotContainer" : true,
"isString" : false,
"isNumeric" : true,
"isInteger" : false,
"isLong" : true,
"isNumber" : false,
"isFloat" : false,
"isDouble" : false,
"isByteArray" : false,
"isBinary" : false,
"isFile" : false,
"isBoolean" : false,
"isDate" : false,
"isDateTime" : false,
"isUuid" : false,
"isEmail" : false,
"isFreeFormObject" : false,
"isArray" : false,
"isMap" : false,
"isEnum" : false,
"isReadOnly" : false,
"isWriteOnly" : false,
"isNullable" : false,
"vendorExtensions" : { },
"hasValidation" : false,
"isInherited" : false,
"nameInCamelCase" : "Id",
"nameInSnakeCase" : "ID",
"isXmlAttribute" : false,
"isXmlWrapped" : false,
"datatype" : "int64",
"iexclusiveMaximum" : false
}, {
"baseName" : "name",
"getter" : "getName",
"setter" : "setName",
"dataType" : "string",
"datatypeWithEnum" : "string",
"name" : "Name",
"defaultValueWithParam" : " =;",
"baseType" : "string",
"example" : "null",
"jsonSchema" : "{\n \"type\" : \"string\"\n}",
"exclusiveMinimum" : false,
"exclusiveMaximum" : false,
"required" : true,
"hasMoreNonReadOnly" : true,
"isPrimitiveType" : true,
"isModel" : false,
"isContainer" : false,
"isNotContainer" : true,
"isString" : true,
"isNumeric" : false,
"isInteger" : false,
"isLong" : false,
"isNumber" : false,
"isFloat" : false,
"isDouble" : false,
"isByteArray" : false,
"isBinary" : false,
"isFile" : false,
"isBoolean" : false,
"isDate" : false,
"isDateTime" : false,
"isUuid" : false,
"isEmail" : false,
"isFreeFormObject" : false,
"isArray" : false,
"isMap" : false,
"isEnum" : false,
"isReadOnly" : false,
"isWriteOnly" : false,
"isNullable" : false,
"vendorExtensions" : { },
"hasValidation" : false,
"isInherited" : false,
"nameInCamelCase" : "Name",
"nameInSnakeCase" : "NAME",
"isXmlAttribute" : false,
"isXmlWrapped" : false,
"datatype" : "string",
"iexclusiveMaximum" : false
}, {
"baseName" : "tag",
"getter" : "getTag",
"setter" : "setTag",
"dataType" : "string",
"datatypeWithEnum" : "string",
"name" : "Tag",
"defaultValueWithParam" : " = data.tag;",
"baseType" : "string",
"example" : "null",
"jsonSchema" : "{\n \"type\" : \"string\"\n}",
"exclusiveMinimum" : false,
"exclusiveMaximum" : false,
"required" : false,
"hasMoreNonReadOnly" : false,
"isPrimitiveType" : true,
"isModel" : false,
"isContainer" : false,
"isNotContainer" : true,
"isString" : true,
"isNumeric" : false,
"isInteger" : false,
"isLong" : false,
"isNumber" : false,
"isFloat" : false,
"isDouble" : false,
"isByteArray" : false,
"isBinary" : false,
"isFile" : false,
"isBoolean" : false,
"isDate" : false,
"isDateTime" : false,
"isUuid" : false,
"isEmail" : false,
"isFreeFormObject" : false,
"isArray" : false,
"isMap" : false,
"isEnum" : false,
"isReadOnly" : false,
"isWriteOnly" : false,
"isNullable" : false,
"vendorExtensions" : { },
"hasValidation" : false,
"isInherited" : false,
"nameInCamelCase" : "Tag",
"nameInSnakeCase" : "TAG",
"isXmlAttribute" : false,
"isXmlWrapped" : false,
"datatype" : "string",
"iexclusiveMaximum" : false
} ],
"requiredVars" : [ /* id, name */ ],
"optionalVars" : [ /* tag */ ],
"readOnlyVars" : [ ],
"readWriteVars" : [ /* lists metadata for all three properties */ ],
"allVars" : [ /* lists all properties */],
"parentVars" : [ ],
"mandatory" : [ "id", "name" ],
"allMandatory" : [ "id", "name" ],
"imports" : [ ],
"hasVars" : true,
"emptyVars" : false,
"hasMoreModels" : false,
"hasEnums" : false,
"isEnum" : false,
"hasRequired" : true,
"hasOptional" : true,
"isArray" : false,
"hasChildren" : false,
"isMap" : false,
"hasOnlyReadOnly" : false,
"vendorExtensions" : { }
} ]

Templates are passed redundant properties, depending on the semantics of the array. For example:

  • vars lists all defined model properties
  • requiredVars lists all model properties marked with required in the spec document
  • optionalVars lists all model properties not marked with required in the spec document
  • readWriteVars lists all model properties not marked with readonly in the spec document
  • readOnlyVars lists all model properties marked with readonly in the spec document
  • allVars lists all model properties. This may include the same set as vars, but may also include generator-defined properties

We expose the same properties in multiple sets because this allows us to conditionally iterate over properties based on some condition ("is it required" or "is it readonly"). This is driven by the use of the logic-less Mustache templates. It is possible that models passed to the templating engine may be cleaned up as we support more template engines, but such an effort will go through a deprecation phase and would be communicated at runtime through log messages.


Inspect supportingFiles passed to templates with system property --global-property debugSupportingFiles=true


openapi-generator generate -g go \
-o out \
-i petstore-minimal.yaml \
--global-property debugSupportingFiles=true

This is a "catch-all" which gives you the entire structure--operations, model, etc--so you can create "single-file" code from them.

Supporting files can either be processed through the templating engine or copied as-is. When creating your own templates, you're limited to the files and extensions expected by the generator implementation. For more control over the supporting files produced by a generator, see our customization documentation.


This is a very limited list of variable name explanations. Feel free to open a pull request to add to this documentation!

  • complexType: stores the name of the model (e.g. Pet)
  • isContainer: true if the parameter or property is an array or a map.
  • isPrimitiveType: true if the parameter or property type is a primitive type (e.g. string, integer, etc) as defined in the spec.

More variables can be found here.

Mustache Lambdas

Many generators (those extending DefaultCodegen) come with a small set of lambda functions available under the key lambda:

  • lowercase - Converts all of the characters in this fragment to lower case using the rules of the ROOT locale.
  • uppercase - Converts all of the characters in this fragment to upper case using the rules of the ROOT locale.
  • titlecase - Converts text in a fragment to title case. For example once upon a time to Once Upon A Time.
  • camelcase - Converts text in a fragment to camelCase. For example Input-text to inputText.
  • uncamelize - Converts text in a fragment from camelCase or PascalCase to a string of words separated by whitespaces. For example inputText to Input Text.
  • indented - Prepends 4 spaces indention from second line of a fragment on. First line will be indented by Mustache.
  • indented_8 - Prepends 8 spaces indention from second line of a fragment on. First line will be indented by Mustache.
  • indented_12 - Prepends 12 spaces indention from second line of a fragment on. First line will be indented by Mustache.
  • indented_16 -Prepends 16 spaces indention from second line of a fragment on. First line will be indented by Mustache.

Lambda is invoked by lambda.[lambda name] expression. For example: {{#lambda.lowercase}}FRAGMENT TO LOWERCASE{{/lambda.lowercase}} to lower case text between lambda.lowercase.


OpenAPI supports a concept called "Extensions". These are called "Specification Extensions" in 3.x and "Vendor Extensions" in 2.0. You'll see them referred to as "Vendor Extensions" in most places in this project.

Vendor extensions allow you to provide vendor-specific configurations to your specification document.

For example, suppose you use your specification document for code generation with a (hypothetical) C# OpenAPI generator supporting a desired operationId prefix where the extension is x-csharp-operationid, you can define this property alongside the object you'd like to extend (which would be a Path Object in this case). You could then apply additional extensions alongside this property, whether they're for another language or other tooling.

Well-defined vendor extensions don't cause conflicts with other tooling.

The following are vendor extensions supported by OpenAPI Generator. The list may not be up-to-date, the best way is to look for "x-" in the built-in mustache templates.

All generators (core)


x-enum-varnames can be used to have another enum name for the corresponding value. This is used to define names of the enum items.

x-enum-descriptions can be used to provide an individual description for each value. This is used for comments in the code (like javadoc if the target language is java).

x-enum-descriptions and x-enum-varnames are each expected to be list of items containing the same number of items as enum. The order of the items in the list matters: their position is used to group them together.


type: integer
format: int32
- 42
- 18
- 56
- 'Blue sky'
- 'Slightly overcast'
- 'Take an umbrella with you'
- Sunny
- Cloudy
- Rainy

In the example for the integer value 42, the description will be Blue sky and the name of the enum item will be Sunny (some generators changes it to SUNNY to respect some coding convention).



To customize the method name, you can provide a different name in x-objc-operationId, e.g.

summary: Add a new pet to the store
description: ''
operationId: addPet
x-objc-operationId: CreateNewPet

Java (Feign)


A single Accepts value as the Feign API client needs a single value for Accepts header, e.g.

- application/json
- application/xml
x-accepts: application/json


A single "Content-Type" value as the Feign API client needs a single value for Content-Type header, e.g.

- application/xml
- application/json
x-content-type: application/json



Each response may specify a unique x-response-id. rust-server will use this to name the corresponding enum variant in the code. e.g.

description: OK
x-response-id: Pong

MySQL Schema


MySQL schema generator creates vendor extensions based on openapi dataType and dataFormat. When user defined extensions with same key already exists codegen accepts those as is. It means it won't validate properties or correct it for you. Every model in definitions can contain table related and column related extensions like in example below:

description: This should be most common InnoDB table
type: object
description: >-
This column should be unsigned BIGINT with AUTO_INCREMENT
type: integer
format: int64
colName: id
colDataType: DECIMAL
- argumentValue: 16
isString: false
- argumentValue: 4
isString: false
colUnsigned: true
colNotNull: true
defaultValue: AUTO_INCREMENT
isString: false
isNumeric: false
isKeyword: true
colComment: >-
Column comment. This column should be unsigned BIGINT with AUTO_INCREMENT
tblName: orders
tblStorageEngine: InnoDB
tblComment: >-
Table comment. This should be most common InnoDB table

There are properties that are not implemented by now(tblStorageEngine), but you can see how generator can be enhanced in future.

Mustache Tips

Here are a few tips we've found useful for new template authors. For more details on Mustache see mustache.5. See also samskivert/jmustache for implementation-specific details.


To access the first or last element in a list using Mustache:

{{#vars}}{{#-first}} this is the first element {{.}} {{/-first}}{{/vars}}
{{#vars}}{{#-last}} this is the last element {{.}} {{/-last}}{{/vars}}


Mustache evaluates template variables contextually. If the variable isn't found in the immediate object, mustache will search the parent. This is similar to JavaScript's prototype object (if you're familiar with the concept).

You can inspect this entire context by outputting {{this}}. For example:



If you'd like a 1-based index in your array traversal, you can use {{-index}}:

{{#enums}}{{-index}} {{enum}}{{/enums}}