Feature: Experimental Handlebars support (rienafairefr) (#2657)

* new module: openapi-generator-core
* templating engine adapters to support extension (currently only Handlebars)
* new `-e` templating engine CLI option
* adapt Generator to process Template with the passed TemplatingEngineAdpater
* add a MustacheEngineAdapter to the codegen in the unit tests
* force default MustacheEngineAdapter
* copy new core module in the root Dockerfile
* add processTemplatingEngine to CodegenConfig, to be overriden by Codegen classes if needed
* support multiple file extensions per templating engine adapter
* Extends handlebars experimental adapter with explicit contextual resolvers (e.g. map processing)
* Add new openapi-generator-core/pom.xml to release_version_update.sh
* A detailed message will be logged on missing handlebars helper
* Adds README documentation around template default and beta options
* Moves mustache package under new templating package
* Include built-in handlebars helpers which require explicit registration, and custom `startsWith` helper.

Dockerfile

@@ -18,6 +18,7 @@ COPY ./modules/openapi-generator-gradle-plugin ${GEN_DIR}/modules/openapi-genera
 COPY ./modules/openapi-generator-maven-plugin ${GEN_DIR}/modules/openapi-generator-maven-plugin
 COPY ./modules/openapi-generator-online ${GEN_DIR}/modules/openapi-generator-online
 COPY ./modules/openapi-generator-cli ${GEN_DIR}/modules/openapi-generator-cli
+COPY ./modules/openapi-generator-core ${GEN_DIR}/modules/openapi-generator-core
 COPY ./modules/openapi-generator ${GEN_DIR}/modules/openapi-generator
 COPY ./pom.xml ${GEN_DIR}
 

README.md

@@ -426,9 +426,12 @@ SYNOPSIS
                 [--artifact-version <artifact version>]
                 [(-c <configuration file> | --config <configuration file>)]
                 [-D <system properties>...]
+                [(-e <templating engine> | --engine <templating engine>)]
+                [--enable-post-process-file]
                 [(-g <generator name> | --generator-name <generator name>)]
-                [--git-repo-id <git repo id>] [--git-user-id <git user id>]
-                [--group-id <group id>] [--http-user-agent <http user agent>]
+                [--generate-alias-as-model] [--git-repo-id <git repo id>]
+                [--git-user-id <git user id>] [--group-id <group id>]
+                [--http-user-agent <http user agent>]
                 (-i <spec file> | --input-spec <spec file>)
                 [--ignore-file-override <ignore file override location>]
                 [--import-mappings <import mappings>...]
@@ -584,6 +587,9 @@ OpenAPI Generator core team members are contributors who have been making signif
 :heart: = Link to support the contributor directly 
 
 #### Template Creator
+
+**NOTE**: Embedded templates are only supported in _Mustache_ format. Support for all other formats is experimental and subject to change at any time.
+
 Here is a list of template creators:
  * API Clients:
    * Ada: @stcarrez

bin/utils/release_version_update.sh

@@ -36,6 +36,7 @@ echo "Release preparation: replacing $FROM with $TO in different files"
 declare -a files=("modules/openapi-generator-cli/pom.xml"
                   "modules/openapi-generator-gradle-plugin/gradle.properties"
                   "modules/openapi-generator-gradle-plugin/pom.xml"
+                  "modules/openapi-generator-core/pom.xml"
                   "modules/openapi-generator-maven-plugin/pom.xml"
                   "modules/openapi-generator-online/pom.xml"
                   "modules/openapi-generator/pom.xml"

modules/openapi-generator-cli/src/main/java/org/openapitools/codegen/cmd/Generate.java

@@ -70,6 +70,10 @@ public class Generate implements Runnable {
             description = "folder containing the template files")
     private String templateDir;
 
+    @Option(name = {"-e", "--engine"}, title = "templating engine",
+        description = "templating engine: \"mustache\" (default) or \"handlebars\" (beta)")
+    private String templatingEngine;
+
     @Option(
             name = {"-a", "--auth"},
             title = "authorization",
@@ -284,6 +288,10 @@ public class Generate implements Runnable {
             configurator.setTemplateDir(templateDir);
         }
 
+        if (isNotEmpty(templatingEngine)) {
+            configurator.setTemplatingEngineName(templatingEngine);
+        }
+
         if (isNotEmpty(apiPackage)) {
             configurator.setApiPackage(apiPackage);
         }

modules/openapi-generator-core/pom.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <parent>
+    <artifactId>openapi-generator-project</artifactId>
+    <groupId>org.openapitools</groupId>
+    <version>4.0.0-SNAPSHOT</version>
+    <relativePath>../..</relativePath>
+  </parent>
+  <modelVersion>4.0.0</modelVersion>
+
+  <artifactId>openapi-generator-core</artifactId>
+  <name>openapi-generator-core</name>
+  <url>https://github.com/openapitools/openapi-generator</url>
+</project>

modules/openapi-generator-core/src/main/java/org/openapitools/codegen/api/AbstractTemplatingEngineAdapter.java

@@ -0,0 +1,40 @@
+package org.openapitools.codegen.api;
+
+import java.util.Locale;
+
+/**
+ * Provides abstractions around the template engine adapter interface, for reuse by implementers.
+ */
+public abstract class AbstractTemplatingEngineAdapter implements TemplatingEngineAdapter {
+
+    /**
+     * Gets all possible template paths for a given location.
+     *
+     * @param location The full location of the template.
+     *
+     * @return A new array of locations, modified according to the extensions or other adapter rules.
+     */
+    protected String[] getModifiedFileLocation(String location) {
+        String[] extensions = getFileExtensions();
+        String[] result = new String[extensions.length];
+        for (int i = 0; i < extensions.length; i++) {
+            String extension = extensions[i];
+            result[i] = String.format(Locale.ROOT, "%s.%s", getPathWithoutExtension(location), extension);
+        }
+        return result;
+    }
+
+    /**
+     * Returns the path without an extension for an input location.
+     *
+     * @param location The location of the file, with original file extension intact.
+     *
+     * @return The full path, without extension (e.g. /path/to/file.txt => /path/to/file)
+     */
+    private String getPathWithoutExtension(String location) {
+        if (location == null) return null;
+        int idx = location.lastIndexOf('.');
+        if (idx == -1) return location;
+        return location.substring(0, idx);
+    }
+}

modules/openapi-generator-core/src/main/java/org/openapitools/codegen/api/TemplatingEngineAdapter.java

@@ -0,0 +1,30 @@
+package org.openapitools.codegen.api;
+
+import java.io.IOException;
+import java.util.Map;
+
+/**
+ * Each templating engine is called by an Adapter, selected at runtime
+ */
+public interface TemplatingEngineAdapter{
+
+  /**
+   * Compiles a template into a string
+   *
+   * @param generator From where we can fetch the templates content (e.g. an instance of DefaultGenerator)
+   * @param bundle The map of values to pass to the template
+   * @param templateFile The name of the template (e.g. model.mustache )
+   * @return the processed template result
+   * @throws IOException an error ocurred in the template processing
+   */
+  String compileTemplate(TemplatingGenerator generator, Map<String, Object> bundle,
+                         String templateFile) throws IOException;
+
+  /**
+   * During generation, if a supporting file has a file extension that is
+   * inside that array, then it is considered a templated supporting file
+   * and we use the templating engine adapter to generate it
+   * @return string array of the valid file extensions for this templating engine
+   */
+  String[] getFileExtensions();
+}

modules/openapi-generator-core/src/main/java/org/openapitools/codegen/api/TemplatingGenerator.java

@@ -0,0 +1,17 @@
+package org.openapitools.codegen.api;
+
+/**
+ * interface to the full template content
+ *   implementers might take into account the -t cli option,
+ *   look in the resources for a language specific template, etc
+ */
+public interface TemplatingGenerator {
+
+  /**
+   * returns the template content by name
+   * @param name the template name (e.g. model.mustache)
+   * @return the contents of that template
+   */
+  String getFullTemplateContents(String name);
+
+}

modules/openapi-generator/pom.xml

@@ -215,6 +215,16 @@
             <artifactId>jmustache</artifactId>
             <version>${jmustache-version}</version>
         </dependency>
+        <dependency>
+            <groupId>com.github.jknack</groupId>
+            <artifactId>handlebars</artifactId>
+            <version>${handlebars.java-version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.github.jknack</groupId>
+            <artifactId>handlebars-jackson2</artifactId>
+            <version>${handlebars.java-version}</version>
+        </dependency>
         <dependency>
             <groupId>commons-io</groupId>
             <artifactId>commons-io</artifactId>
@@ -300,6 +310,11 @@
             <artifactId>jackson-datatype-threetenbp</artifactId>
             <version>${jackson-threetenbp-version}</version>
         </dependency>
+        <dependency>
+            <groupId>org.openapitools</groupId>
+            <artifactId>openapi-generator-core</artifactId>
+            <version>4.0.0-SNAPSHOT</version>
+        </dependency>
     </dependencies>
     <repositories>
         <repository>

modules/openapi-generator/src/main/java/org/openapitools/codegen/AbstractGenerator.java

@@ -22,6 +22,7 @@ import java.nio.file.Files;
 import java.nio.file.StandardCopyOption;
 import java.util.Arrays;
 import org.apache.commons.lang3.StringUtils;
+import org.openapitools.codegen.api.TemplatingGenerator;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -30,7 +31,7 @@ import java.nio.file.Paths;
 import java.util.Scanner;
 import java.util.regex.Pattern;
 
-public abstract class AbstractGenerator {
+public abstract class AbstractGenerator implements TemplatingGenerator {
     private static final Logger LOGGER = LoggerFactory.getLogger(AbstractGenerator.class);
     
     /**

modules/openapi-generator/src/main/java/org/openapitools/codegen/CodegenConfig.java

@@ -24,6 +24,7 @@ import io.swagger.v3.oas.models.media.Schema;
 import io.swagger.v3.oas.models.security.SecurityScheme;
 import io.swagger.v3.oas.models.servers.Server;
 import io.swagger.v3.oas.models.servers.ServerVariable;
+import org.openapitools.codegen.api.TemplatingEngineAdapter;
 
 import java.io.File;
 import java.util.List;
@@ -149,6 +150,8 @@ public interface CodegenConfig {
 
     Compiler processCompiler(Compiler compiler);
 
+    TemplatingEngineAdapter processTemplatingEngine(TemplatingEngineAdapter templatingEngine);
+
     String sanitizeTag(String tag);
 
     String toApiFilename(String name);
@@ -260,8 +263,11 @@ public interface CodegenConfig {
      */
     void setOpenAPI(OpenAPI openAPI);
 
+    void setTemplatingEngine(TemplatingEngineAdapter s);
+
+    TemplatingEngineAdapter getTemplatingEngine();
+
     public boolean isEnableMinimalUpdate();
 
     public void setEnableMinimalUpdate(boolean isEnableMinimalUpdate);
-
 }

modules/openapi-generator/src/main/java/org/openapitools/codegen/CodegenConstants.java

@@ -184,6 +184,9 @@ public class CodegenConstants {
     public static final String DOTNET_FRAMEWORK = "targetFramework";
     public static final String DOTNET_FRAMEWORK_DESC = "The target .NET framework version.";
 
+    public static final String TEMPLATING_ENGINE = "templatingEngine";
+    public static final String TEMPLATING_ENGINE_DESC = "The templating engine plugin to use: \"mustache\" (default) or \"handlebars\" (beta)";
+
     public static enum MODEL_PROPERTY_NAMING_TYPE {camelCase, PascalCase, snake_case, original}
 
     public static enum ENUM_PROPERTY_NAMING_TYPE {camelCase, PascalCase, snake_case, original, UPPERCASE}

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

@@ -41,9 +41,11 @@ import org.apache.commons.lang3.StringEscapeUtils;
 import org.apache.commons.lang3.StringUtils;
 import org.apache.commons.lang3.tuple.Pair;
 import org.openapitools.codegen.CodegenDiscriminator.MappedModel;
+import org.openapitools.codegen.api.TemplatingEngineAdapter;
 import org.openapitools.codegen.config.GeneratorProperties;
 import org.openapitools.codegen.examples.ExampleGenerator;
 import org.openapitools.codegen.serializer.SerializerUtils;
+import org.openapitools.codegen.templating.MustacheEngineAdapter;
 import org.openapitools.codegen.utils.ModelUtils;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -111,6 +113,8 @@ public class DefaultCodegen implements CodegenConfig {
     protected String ignoreFilePathOverride;
     // flag to indicate whether to use environment variable to post process file
     protected boolean enablePostProcessFile = false;
+    private TemplatingEngineAdapter templatingEngine = new MustacheEngineAdapter();
+
     // flag to indicate whether to only update files whose contents have changed
     protected boolean enableMinimalUpdate = false;
 
@@ -462,6 +466,12 @@ public class DefaultCodegen implements CodegenConfig {
         return compiler;
     }
 
+    // override with any special handling for the templating engine
+    @SuppressWarnings("unused")
+    public TemplatingEngineAdapter processTemplatingEngine(TemplatingEngineAdapter templatingEngine) {
+        return templatingEngine;
+    }
+
     // override with any special text escaping logic
     @SuppressWarnings("static-method")
     public String escapeText(String input) {
@@ -3828,6 +3838,16 @@ public class DefaultCodegen implements CodegenConfig {
         return sanitizeName(name, "\\W");
     }
 
+    @Override
+    public void setTemplatingEngine(TemplatingEngineAdapter templatingEngine) {
+        this.templatingEngine = templatingEngine;
+    }
+
+    @Override
+    public TemplatingEngineAdapter getTemplatingEngine() {
+        return this.templatingEngine;
+    }
+
     /**
      * Sanitize name (parameter, property, method, etc)
      *

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultGenerator.java

@@ -17,8 +17,6 @@
 
 package org.openapitools.codegen;
 
-import com.samskivert.mustache.Mustache;
-import com.samskivert.mustache.Template;
 import io.swagger.v3.core.util.Json;
 import io.swagger.v3.oas.models.OpenAPI;
 import io.swagger.v3.oas.models.Operation;
@@ -35,7 +33,10 @@ import org.apache.commons.io.IOUtils;
 import org.apache.commons.lang3.ObjectUtils;
 import org.apache.commons.lang3.StringUtils;
 import org.openapitools.codegen.config.GeneratorProperties;
+import org.openapitools.codegen.api.TemplatingEngineAdapter;
 import org.openapitools.codegen.ignore.CodegenIgnoreProcessor;
+import org.openapitools.codegen.templating.MustacheEngineAdapter;
+import org.openapitools.codegen.config.GeneratorProperties;
 import org.openapitools.codegen.utils.ImplementationVersion;
 import org.openapitools.codegen.utils.ModelUtils;
 import org.openapitools.codegen.utils.URLPathUtils;
@@ -53,6 +54,7 @@ public class DefaultGenerator extends AbstractGenerator implements Generator {
     protected ClientOptInput opts;
     protected OpenAPI openAPI;
     protected CodegenIgnoreProcessor ignoreProcessor;
+    protected TemplatingEngineAdapter templatingEngine;
     private Boolean generateApis = null;
     private Boolean generateModels = null;
     private Boolean generateSupportingFiles = null;
@@ -77,6 +79,7 @@ public class DefaultGenerator extends AbstractGenerator implements Generator {
         this.openAPI = opts.getOpenAPI();
         this.config = opts.getConfig();
         this.config.additionalProperties().putAll(opts.getOpts().getProperties());
+        this.templatingEngine = this.config.getTemplatingEngine();
 
         String ignoreFileLocation = this.config.getIgnoreFilePathOverride();
         if (ignoreFileLocation != null) {
@@ -95,6 +98,13 @@ public class DefaultGenerator extends AbstractGenerator implements Generator {
         return this;
     }
 
+    private void configPostProcessMustacheCompiler() {
+        if (this.templatingEngine instanceof MustacheEngineAdapter) {
+            MustacheEngineAdapter mustacheEngineAdapter = (MustacheEngineAdapter) this.templatingEngine;
+            mustacheEngineAdapter.setCompiler(this.config.processCompiler(mustacheEngineAdapter.getCompiler()));
+        }
+    }
+
     /**
      * Programmatically disable the output of .openapi-generator/VERSION, .openapi-generator-ignore,
      * or other metadata files used by OpenAPI Generator.
@@ -702,21 +712,9 @@ public class DefaultGenerator extends AbstractGenerator implements Generator {
                 }
 
                 if (ignoreProcessor.allowsFile(new File(outputFilename))) {
-                    if (templateFile.endsWith("mustache")) {
-                        String template = readTemplate(templateFile);
-                        Mustache.Compiler compiler = Mustache.compiler();
-                        compiler = config.processCompiler(compiler);
-                        Template tmpl = compiler
-                                .withLoader(new Mustache.TemplateLoader() {
-                                    @Override
-                                    public Reader getTemplate(String name) {
-                                        return getTemplateReader(getFullTemplateFile(config, name + ".mustache"));
-                                    }
-                                })
-                                .defaultValue("")
-                                .compile(template);
-
-                        writeToFile(outputFilename, tmpl.execute(bundle));
+                    if (Arrays.stream(templatingEngine.getFileExtensions()).anyMatch(templateFile::endsWith)) {
+                        String templateContent = templatingEngine.compileTemplate(this, bundle, support.templateFile);
+                        writeToFile(outputFilename, templateContent);
                         File written = new File(outputFilename);
                         files.add(written);
                         if (config.isEnablePostProcessFile()) {
@@ -891,6 +889,9 @@ public class DefaultGenerator extends AbstractGenerator implements Generator {
         configureGeneratorProperties();
         configureOpenAPIInfo();
 
+        // If the template adapter is mustache, we'll set the config-modified Compiler.
+        configPostProcessMustacheCompiler();
+
         List<File> files = new ArrayList<File>();
         // models
         List<String> filteredSchemas = ModelUtils.getSchemasUsedOnlyInFormParam(openAPI);
@@ -911,24 +912,16 @@ public class DefaultGenerator extends AbstractGenerator implements Generator {
         return files;
     }
 
+    @Override
+    public String getFullTemplateContents(String templateName) {
+        return readTemplate(getFullTemplateFile(config, templateName));
+    }
+
     protected File processTemplateToFile(Map<String, Object> templateData, String templateName, String outputFilename) throws IOException {
         String adjustedOutputFilename = outputFilename.replaceAll("//", "/").replace('/', File.separatorChar);
         if (ignoreProcessor.allowsFile(new File(adjustedOutputFilename))) {
-            String templateFile = getFullTemplateFile(config, templateName);
-            String template = readTemplate(templateFile);
-            Mustache.Compiler compiler = Mustache.compiler();
-            compiler = config.processCompiler(compiler);
-            Template tmpl = compiler
-                    .withLoader(new Mustache.TemplateLoader() {
-                        @Override
-                        public Reader getTemplate(String name) {
-                            return getTemplateReader(getFullTemplateFile(config, name + ".mustache"));
-                        }
-                    })
-                    .defaultValue("")
-                    .compile(template);
-
-            writeToFile(adjustedOutputFilename, tmpl.execute(templateData));
+            String templateContent = templatingEngine.compileTemplate(this, templateData, templateName);
+            writeToFile(adjustedOutputFilename, templateContent);
             return new File(adjustedOutputFilename);
         }
 

modules/openapi-generator/src/main/java/org/openapitools/codegen/config/CodegenConfigurator.java

@@ -17,8 +17,6 @@
 
 package org.openapitools.codegen.config;
 
-import static org.apache.commons.lang3.StringUtils.isNotEmpty;
-
 import com.fasterxml.jackson.annotation.JsonAnyGetter;
 import com.fasterxml.jackson.annotation.JsonAnySetter;
 import com.fasterxml.jackson.databind.ObjectMapper;
@@ -29,41 +27,25 @@ import io.swagger.v3.oas.models.OpenAPI;
 import io.swagger.v3.parser.core.models.AuthorizationValue;
 import io.swagger.v3.parser.core.models.ParseOptions;
 import io.swagger.v3.parser.core.models.SwaggerParseResult;
+import org.apache.commons.io.FilenameUtils;
+import org.apache.commons.lang3.Validate;
+import org.openapitools.codegen.*;
+import org.openapitools.codegen.auth.AuthParser;
+import org.openapitools.codegen.languages.*;
+import org.openapitools.codegen.templating.HandlebarsEngineAdapter;
+import org.openapitools.codegen.templating.MustacheEngineAdapter;
+import org.openapitools.codegen.utils.ModelUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.io.File;
 import java.io.IOException;
 import java.io.Serializable;
 import java.nio.file.Paths;
-import java.util.HashMap;
-import java.util.HashSet;
-import java.util.List;
-import java.util.Locale;
-import java.util.Map;
-import java.util.Set;
-import org.apache.commons.io.FilenameUtils;
-import org.apache.commons.lang3.Validate;
-import org.openapitools.codegen.CliOption;
-import org.openapitools.codegen.ClientOptInput;
-import org.openapitools.codegen.ClientOpts;
-import org.openapitools.codegen.CodegenConfig;
-import org.openapitools.codegen.CodegenConfigLoader;
-import org.openapitools.codegen.CodegenConstants;
-import org.openapitools.codegen.SpecValidationException;
-import org.openapitools.codegen.auth.AuthParser;
-import org.openapitools.codegen.languages.CSharpNancyFXServerCodegen;
-import org.openapitools.codegen.languages.CppQt5ClientCodegen;
-import org.openapitools.codegen.languages.CppRestSdkClientCodegen;
-import org.openapitools.codegen.languages.CppTizenClientCodegen;
-import org.openapitools.codegen.languages.JavaJerseyServerCodegen;
-import org.openapitools.codegen.languages.PhpLumenServerCodegen;
-import org.openapitools.codegen.languages.PhpSlimServerCodegen;
-import org.openapitools.codegen.languages.PhpZendExpressivePathHandlerServerCodegen;
-import org.openapitools.codegen.languages.RubySinatraServerCodegen;
-import org.openapitools.codegen.languages.ScalaAkkaClientCodegen;
-import org.openapitools.codegen.languages.ScalaHttpClientCodegen;
-import org.openapitools.codegen.languages.SwiftClientCodegen;
-import org.openapitools.codegen.utils.ModelUtils;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
+import java.util.*;
+
+import static org.apache.commons.lang3.StringUtils.isEmpty;
+import static org.apache.commons.lang3.StringUtils.isNotEmpty;
 
 /**
  * A class that contains all codegen configuration properties a user would want to manipulate. An
@@ -103,6 +85,7 @@ public class CodegenConfigurator implements Serializable {
     private boolean enablePostProcessFile;
     private boolean enableMinimalUpdate;
     private String templateDir;
+    private String templatingEngineName;
     private String auth;
     private String apiPackage;
     private String modelPackage;
@@ -582,6 +565,7 @@ public class CodegenConfigurator implements Serializable {
         checkAndSetAdditionalProperty(artifactVersion, CodegenConstants.ARTIFACT_VERSION);
         checkAndSetAdditionalProperty(templateDir, toAbsolutePathStr(templateDir),
                 CodegenConstants.TEMPLATE_DIR);
+        checkAndSetAdditionalProperty(templatingEngineName, CodegenConstants.TEMPLATING_ENGINE);
         checkAndSetAdditionalProperty(modelNamePrefix, CodegenConstants.MODEL_NAME_PREFIX);
         checkAndSetAdditionalProperty(modelNameSuffix, CodegenConstants.MODEL_NAME_SUFFIX);
         checkAndSetAdditionalProperty(gitUserId, CodegenConstants.GIT_USER_ID);
@@ -595,6 +579,13 @@ public class CodegenConfigurator implements Serializable {
             config.setLibrary(library);
         }
 
+        // Built-in templates are mustache, but allow users to use a simplified handlebars engine for their custom templates.
+        if (isEmpty(templatingEngineName) || templatingEngineName.equals("mustache")) {
+            config.setTemplatingEngine(new MustacheEngineAdapter());
+        } else if (templatingEngineName.equals("handlebars")) {
+            config.setTemplatingEngine(new HandlebarsEngineAdapter());
+        }
+
         config.additionalProperties().putAll(additionalProperties);
 
         ClientOptInput input = new ClientOptInput().config(config);
@@ -602,8 +593,7 @@ public class CodegenConfigurator implements Serializable {
         final List<AuthorizationValue> authorizationValues = AuthParser.parse(auth);
         ParseOptions options = new ParseOptions();
         options.setResolve(true);
-        SwaggerParseResult result =
-                new OpenAPIParser().readLocation(inputSpec, authorizationValues, options);
+        SwaggerParseResult result = new OpenAPIParser().readLocation(inputSpec, authorizationValues, options);
 
         Set<String> validationMessages = new HashSet<>(result.getMessages());
         OpenAPI specification = result.getOpenAPI();
@@ -732,4 +722,9 @@ public class CodegenConfigurator implements Serializable {
         }
         return null;
     }
+
+    public CodegenConfigurator setTemplatingEngineName(String templatingEngineName) {
+        this.templatingEngineName = templatingEngineName;
+        return this;
+    }
 }

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java

@@ -25,7 +25,7 @@ import io.swagger.v3.oas.models.media.Schema;
 import org.apache.commons.io.FilenameUtils;
 import org.apache.commons.lang3.StringUtils;
 import org.openapitools.codegen.*;
-import org.openapitools.codegen.mustache.*;
+import org.openapitools.codegen.templating.mustache.*;
 import org.openapitools.codegen.utils.ModelUtils;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCppCodegen.java

@@ -25,7 +25,7 @@ import org.apache.commons.lang3.StringUtils;
 import org.openapitools.codegen.CodegenConfig;
 import org.openapitools.codegen.CodegenProperty;
 import org.openapitools.codegen.DefaultCodegen;
-import org.openapitools.codegen.mustache.IndentedLambda;
+import org.openapitools.codegen.templating.mustache.IndentedLambda;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ErlangClientCodegen.java

@@ -21,7 +21,7 @@ import com.samskivert.mustache.Mustache;
 import com.samskivert.mustache.Template;
 import io.swagger.v3.oas.models.media.Schema;
 import org.openapitools.codegen.*;
-import org.openapitools.codegen.mustache.JoinWithCommaLambda;
+import org.openapitools.codegen.templating.mustache.JoinWithCommaLambda;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaClientCodegen.java

@@ -23,7 +23,7 @@ import org.openapitools.codegen.*;
 import org.openapitools.codegen.languages.features.BeanValidationFeatures;
 import org.openapitools.codegen.languages.features.GzipFeatures;
 import org.openapitools.codegen.languages.features.PerformBeanValidationFeatures;
-import org.openapitools.codegen.mustache.CaseFormatLambda;
+import org.openapitools.codegen.templating.mustache.CaseFormatLambda;
 import org.openapitools.codegen.utils.ProcessUtils;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinServerCodegen.java

@@ -24,7 +24,7 @@ import org.openapitools.codegen.CliOption;
 import org.openapitools.codegen.CodegenConstants;
 import org.openapitools.codegen.CodegenType;
 import org.openapitools.codegen.SupportingFile;
-import org.openapitools.codegen.mustache.*;
+import org.openapitools.codegen.templating.mustache.*;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ScalaPlayFrameworkServerCodegen.java

@@ -21,7 +21,7 @@ import com.samskivert.mustache.Mustache;
 import io.swagger.v3.oas.models.media.ArraySchema;
 import io.swagger.v3.oas.models.media.Schema;
 import org.openapitools.codegen.*;
-import org.openapitools.codegen.mustache.*;
+import org.openapitools.codegen.templating.mustache.IndentedLambda;
 import org.openapitools.codegen.utils.ModelUtils;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;

modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/HandlebarsEngineAdapter.java

@@ -0,0 +1,75 @@
+package org.openapitools.codegen.templating;
+
+import com.github.jknack.handlebars.Context;
+import com.github.jknack.handlebars.Handlebars;
+import com.github.jknack.handlebars.Jackson2Helper;
+import com.github.jknack.handlebars.Template;
+import com.github.jknack.handlebars.context.FieldValueResolver;
+import com.github.jknack.handlebars.context.JavaBeanValueResolver;
+import com.github.jknack.handlebars.context.MapValueResolver;
+import com.github.jknack.handlebars.helper.ConditionalHelpers;
+import com.github.jknack.handlebars.helper.StringHelpers;
+import com.github.jknack.handlebars.io.AbstractTemplateLoader;
+import com.github.jknack.handlebars.io.StringTemplateSource;
+import com.github.jknack.handlebars.io.TemplateLoader;
+import com.github.jknack.handlebars.io.TemplateSource;
+import org.openapitools.codegen.api.AbstractTemplatingEngineAdapter;
+import org.openapitools.codegen.api.TemplatingGenerator;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+import java.util.Locale;
+import java.util.Map;
+
+
+public class HandlebarsEngineAdapter extends AbstractTemplatingEngineAdapter {
+    static Logger LOGGER = LoggerFactory.getLogger(HandlebarsEngineAdapter.class);
+    private final String[] extensions = new String[]{"handlebars", "hbs"};
+
+    public String compileTemplate(TemplatingGenerator generator,
+                                  Map<String, Object> bundle, String templateFile) throws IOException {
+        TemplateLoader loader = new AbstractTemplateLoader() {
+            @Override
+            public TemplateSource sourceAt(String location) {
+                return findTemplate(generator, location);
+            }
+        };
+
+        Context context = Context
+                .newBuilder(bundle)
+                .resolver(
+                        MapValueResolver.INSTANCE,
+                        JavaBeanValueResolver.INSTANCE,
+                        FieldValueResolver.INSTANCE)
+                .build();
+
+        Handlebars handlebars = new Handlebars(loader);
+        handlebars.registerHelperMissing((obj, options) -> {
+            LOGGER.warn(String.format(Locale.ROOT, "Unregistered helper name '%s', processing template:\n%s", options.helperName, options.fn.text()));
+            return "";
+        });
+        handlebars.registerHelper("json", Jackson2Helper.INSTANCE);
+        StringHelpers.register(handlebars);
+        handlebars.registerHelpers(ConditionalHelpers.class);
+        handlebars.registerHelpers(org.openapitools.codegen.templating.handlebars.StringHelpers.class);
+        Template tmpl = handlebars.compile(templateFile);
+        return tmpl.apply(context);
+    }
+
+    public TemplateSource findTemplate(TemplatingGenerator generator, String templateFile) {
+        for (String file : getModifiedFileLocation(templateFile)) {
+            try {
+                return new StringTemplateSource(file, generator.getFullTemplateContents(file));
+            } catch (Exception ignored) {
+            }
+        }
+        throw new RuntimeException("couldnt find a subtemplate " + templateFile);
+    }
+
+    @Override
+    public String[] getFileExtensions() {
+        return extensions;
+    }
+}
+

modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/MustacheEngineAdapter.java

@@ -0,0 +1,52 @@
+package org.openapitools.codegen.templating;
+
+import com.samskivert.mustache.Mustache;
+import com.samskivert.mustache.Template;
+import org.openapitools.codegen.api.TemplatingEngineAdapter;
+import org.openapitools.codegen.api.TemplatingGenerator;
+
+import java.io.IOException;
+import java.io.Reader;
+import java.io.StringReader;
+import java.util.Map;
+
+
+public class MustacheEngineAdapter implements TemplatingEngineAdapter {
+
+    public String[] extensions = new String[]{"mustache"};
+    Mustache.Compiler compiler = Mustache.compiler();
+
+    @Override
+    public String compileTemplate(TemplatingGenerator generator, Map<String, Object> bundle,
+                                  String templateFile) throws IOException {
+        Template tmpl = compiler
+                .withLoader(name -> findTemplate(generator, name))
+                .defaultValue("")
+                .compile(generator.getFullTemplateContents(templateFile));
+
+        return tmpl.execute(bundle);
+    }
+
+    public Reader findTemplate(TemplatingGenerator generator, String name) {
+        for (String extension : extensions) {
+            try {
+                return new StringReader(generator.getFullTemplateContents(name + "." + extension));
+            } catch (Exception ignored) {
+            }
+        }
+        throw new RuntimeException("couldnt find a subtemplate " + name);
+    }
+
+    public Mustache.Compiler getCompiler() {
+        return compiler;
+    }
+
+    public void setCompiler(Mustache.Compiler compiler) {
+        this.compiler = compiler;
+    }
+
+    @Override
+    public String[] getFileExtensions() {
+        return extensions;
+    }
+}

modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/handlebars/StringHelpers.java

@@ -0,0 +1,65 @@
+package org.openapitools.codegen.templating.handlebars;
+
+import com.github.jknack.handlebars.Helper;
+import com.github.jknack.handlebars.Options;
+import com.github.jknack.handlebars.TagType;
+
+import java.io.IOException;
+import java.util.Locale;
+
+public enum StringHelpers implements Helper<Object> {
+
+    /**
+     * Indicates the string starts with the defined value.
+     * For example:
+     *
+     * <pre>
+     * {{startsWith a b ["insensitive"]}}
+     * </pre>
+     *
+     * <pre>
+     * {{startsWith a text='b' [insensitive=true]}}
+     * </pre>
+     *
+     * Render 'yes' or 'no':
+     * <pre>
+     *   {{#startsWith a b}}
+     *     yes
+     *   {{else}}
+     *     no
+     *   {{/startsWith}}
+     * </pre>
+     *
+     * Render 'true' or 'false':
+     * <pre>
+     *   {{startsWith a b}}
+     * </pre>
+     *
+     * Render 'y' or 'n':
+     * <pre>
+     *   {{startsWith a b yes='y' no='n'}}
+     * </pre>
+     *
+     * If value is "handlebars.java", the output will be "Handlebars.java".
+     */
+    startsWith {
+        @Override
+        public Object apply(Object value, Options options) throws IOException {
+            String match = options.param(0, options.hash("text", ""));
+            if (match.length() < 1) {
+                return false;
+            }
+
+            boolean caseInsensitive = options.hash("insensitive", false);
+            boolean result = caseInsensitive ? value.toString().toLowerCase(Locale.ROOT).startsWith(match.toLowerCase(Locale.ROOT)) : value.toString().startsWith(match);
+
+            if (options.tagType == TagType.SECTION) {
+                return result ? options.fn() : options.inverse();
+            }
+
+            return result
+                    ? options.hash("yes", true)
+                    : options.hash("no", false);
+        }
+    }
+}

modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/mustache/CamelCaseLambda.java

@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-package org.openapitools.codegen.mustache;
+package org.openapitools.codegen.templating.mustache;
 
 import com.samskivert.mustache.Mustache;
 import com.samskivert.mustache.Template;

modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/mustache/CaseFormatLambda.java

@@ -1,4 +1,4 @@
-package org.openapitools.codegen.mustache;
+package org.openapitools.codegen.templating.mustache;
 
 import com.google.common.base.CaseFormat;
 import com.samskivert.mustache.Mustache;

modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/mustache/IndentedLambda.java

@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-package org.openapitools.codegen.mustache;
+package org.openapitools.codegen.templating.mustache;
 
 import com.samskivert.mustache.Mustache;
 import com.samskivert.mustache.Template;

modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/mustache/JoinWithCommaLambda.java

@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-package org.openapitools.codegen.mustache;
+package org.openapitools.codegen.templating.mustache;
 
 import com.samskivert.mustache.Mustache;
 import com.samskivert.mustache.Template;

modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/mustache/LowercaseLambda.java

@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-package org.openapitools.codegen.mustache;
+package org.openapitools.codegen.templating.mustache;
 
 import com.samskivert.mustache.Mustache;
 import com.samskivert.mustache.Template;

modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/mustache/TitlecaseLambda.java

@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-package org.openapitools.codegen.mustache;
+package org.openapitools.codegen.templating.mustache;
 
 import com.samskivert.mustache.Mustache;
 import com.samskivert.mustache.Template;

modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/mustache/UppercaseLambda.java

@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-package org.openapitools.codegen.mustache;
+package org.openapitools.codegen.templating.mustache;
 
 import com.samskivert.mustache.Mustache;
 import com.samskivert.mustache.Template;

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/jaxrs/JavaJerseyServerCodegenTest.java

@@ -10,6 +10,7 @@ import org.openapitools.codegen.MockDefaultGenerator;
 import org.openapitools.codegen.MockDefaultGenerator.WrittenTemplateBasedFile;
 import org.openapitools.codegen.TestUtils;
 import org.openapitools.codegen.languages.JavaJerseyServerCodegen;
+import org.openapitools.codegen.templating.MustacheEngineAdapter;
 import org.testng.Assert;
 import org.testng.annotations.BeforeMethod;
 import org.testng.annotations.Test;

modules/openapi-generator/src/test/java/org/openapitools/codegen/templating/handlebars/StringHelpersTest.java

@@ -0,0 +1,86 @@
+package org.openapitools.codegen.templating.handlebars;
+
+import com.github.jknack.handlebars.Context;
+import com.github.jknack.handlebars.Handlebars;
+import com.github.jknack.handlebars.Template;
+import com.github.jknack.handlebars.context.FieldValueResolver;
+import org.testng.annotations.BeforeMethod;
+import org.testng.annotations.Test;
+
+import java.io.IOException;
+import java.util.HashMap;
+
+import static org.testng.Assert.assertEquals;
+
+public class StringHelpersTest {
+
+    private Handlebars handlebars = null;
+
+    private void evaluate(HashMap<String, Object> data, String template, String expect) throws IOException {
+
+        Context context = Context
+                .newBuilder(data)
+                .resolver(
+                        FieldValueResolver.INSTANCE)
+                .build();
+
+        Template tmpl = handlebars.compileInline(template);
+        String actual = tmpl.apply(context);
+        assertEquals(actual, expect);
+    }
+
+    @BeforeMethod
+    public void setup() {
+        handlebars = new Handlebars();
+        handlebars.registerHelpers(StringHelpers.class);
+    }
+
+    @Test(description = "Handlebars StringHelpers.startsWith, section")
+    public void startsWithSectionalTest() throws IOException {
+        HashMap<String, Object> data = new HashMap<String, Object>() {{
+            put("asdf", "asdf");
+            put("a", "a");
+            put("b", "b");
+        }};
+
+        String template = "{{~#startsWith asdf a ~}}yes{{~else~}}no{{~/startsWith~}}";
+        evaluate(data, template, "yes");
+
+        template = "{{~#startsWith asdf b ~}}yes{{~else~}}no{{~/startsWith~}}";
+        evaluate(data, template, "no");
+    }
+
+    @Test(description = "Handlebars StringHelpers.startsWith")
+    public void startsWithTest() throws IOException {
+        HashMap<String, Object> data = new HashMap<String, Object>() {{
+            put("asdf", "asdf");
+            put("ASDF", "ASDF");
+            put("a", "a");
+            put("b", "b");
+        }};
+        evaluate(data, "{{startsWith asdf a}}", "true");
+        evaluate(data, "{{startsWith asdf b}}", "false");
+        evaluate(data, "{{startsWith ASDF a insensitive=true }}", "true");
+        evaluate(data, "{{startsWith ASDF a insensitive=false }}", "false");
+        evaluate(data, "{{startsWith ASDF 'a' insensitive=true }}", "true");
+        evaluate(data, "{{startsWith ASDF b insensitive=true }}", "false");
+        evaluate(data, "{{startsWith ASDF 'b' insensitive=true }}", "false");
+        evaluate(data, "{{startsWith ASDF insensitive=true text='a'}}", "true");
+        evaluate(data, "{{startsWith ASDF insensitive=true text='a' yes='✓' no='✘'}}", "✓");
+        evaluate(data, "{{startsWith ASDF insensitive=false text='a' yes='✓' no='✘'}}", "✘");
+    }
+
+    @Test(description = "Handlebars StringHelpers.startsWith, yes/no override")
+    public void startsWithYesOverrideTest() throws IOException {
+        HashMap<String, Object> data = new HashMap<String, Object>() {{
+            put("asdf", "asdf");
+            put("a", "a");
+            put("b", "b");
+        }};
+        String template = "{{startsWith asdf a yes='y' no='n'}}";
+        evaluate(data, template, "y");
+
+        template = "{{startsWith asdf b yes='y' no='n'}}";
+        evaluate(data, template, "n");
+    }
+}

pom.xml

@@ -1293,6 +1293,7 @@
         <module>modules/openapi-generator-maven-plugin</module>
         <module>modules/openapi-generator-gradle-plugin</module>
         <module>modules/openapi-generator-online</module>
+        <module>modules/openapi-generator-core</module>
     </modules>
     <reporting>
         <outputDirectory>target/site</outputDirectory>
@@ -1372,6 +1373,7 @@
         <slf4j-version>1.7.12</slf4j-version>
         <scala-maven-plugin-version>3.2.1</scala-maven-plugin-version>
         <jmustache-version>1.14</jmustache-version>
+        <handlebars.java-version>4.1.2</handlebars.java-version>
         <testng-version>6.14.3</testng-version>
         <surefire-version>2.22.1</surefire-version>
         <jmockit-version>1.43</jmockit-version>