[bug][maven] Fix custom windows classpaths in maven plugin (#7587)

* [maven] Fallback to templates using classpath rather than OS-specific paths

Previous checks would cause logic in Windows to return early, for
built-in templates only. This reorganizes and simplifies the ordering
behavior.

* Match classpath check in WorkflowSettings with that in TemplateManager

* [maven] Much needed unit/integration tests

This follows similar approach used in PMD and other plugins managed by
maven.

Unit tests simply verify we can load configuration as expected into the
Mojo.

Integration tests execute actual sample projects bound to the current
build's Maven plugin. This uses maven-invoker-plugin, which also allows
for specifying the maven options in invoker.properties to execute the test.
It also provides a verification framework using groovy files with the
required naming convention of "verify.groovy". This allows us to quickly
and easily check that certain files are outputted by generation, and we
may also spotcheck file contents.

templateResourcePath option is skipped on windows. I've tested back to
version 3.3.3 and this doesn't seem to have worked consistently with how
the property works on non-Windows.

* Set groovy 3.0.5 for test harness

* Print stacktrace on Maven error in Travis

* [maven] Set groovy version in tests to supported in Java 11+

* Puts maven integration tests in separate profile called 'integration'

.travis.yml

@@ -148,8 +148,8 @@ script:
   - /bin/bash ./bin/utils/detect_tab_in_java_class.sh
   # run integration tests defined in maven pom.xml
   # WARN: Travis will timeout after 10 minutes of no stdout/stderr activity, which is problematic with mvn --quiet.
-  - mvn --no-snapshot-updates --quiet --batch-mode --show-version clean install -Dorg.slf4j.simpleLogger.defaultLogLevel=error
+  - mvn -e --no-snapshot-updates --quiet --batch-mode --show-version clean install -Dorg.slf4j.simpleLogger.defaultLogLevel=error
-  - mvn --no-snapshot-updates --quiet --batch-mode --show-version verify -Psamples -Dorg.slf4j.simpleLogger.defaultLogLevel=error
+  - mvn -e --no-snapshot-updates --quiet --batch-mode --show-version verify -Psamples -Dorg.slf4j.simpleLogger.defaultLogLevel=error
 after_success:
   # push to maven repo
   - if [ $SONATYPE_USERNAME ] && [ "$TRAVIS_PULL_REQUEST" == "false" ]; then

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

@@ -29,6 +29,7 @@ import java.util.HashMap;
 import java.util.Map;
 import java.util.Objects;
 import java.util.concurrent.atomic.AtomicLong;
+import java.util.regex.Pattern;
 
 /**
  * Represents those settings applied to a generation workflow.
@@ -444,10 +445,20 @@ public class WorkflowSettings {
                     uri = f.toURI();
                     this.templateDir =  Paths.get(uri).toAbsolutePath().normalize().toString();
                 } else {
-                    URL url = this.getClass().getClassLoader().getResource(templateDir);
+                    String cpDir;
+                    // HACK: this duplicates TemplateManager.getCPResourcePath a bit. We should probably move that function to core.
+                    if (!"/".equals(File.separator)) {
+                        // Windows users may pass path specific to OS, but classpath must be "/" separators
+                        cpDir = templateDir.replaceAll(Pattern.quote(File.separator), "/");
+                    } else {
+                        cpDir = templateDir;
+                    }
+
+                    URL url = this.getClass().getClassLoader().getResource(cpDir);
                     if (url != null) {
                         try {
                             uri = url.toURI();
+                            // we can freely set to templateDir here and allow templating to manage template lookups
                             this.templateDir = templateDir;
                         } catch (URISyntaxException e) {
                             LOGGER.warn("The requested template was found on the classpath, but resulted in a syntax error.");

modules/openapi-generator-maven-plugin/examples/java-client.xml

@@ -53,8 +53,7 @@
                             <!-- target to generate java client code -->
                             <generatorName>java</generatorName>
 
-                            <!-- hint: if you want to generate java server code, e.g. based on Spring Boot,
+                            <templateDirectory>${project.basedir}/templates</templateDirectory>
-                                 you can use the following target: <generatorName>spring</generatorName> -->
 
                             <!-- pass any necessary config options -->
                             <configOptions>

modules/openapi-generator-maven-plugin/examples/templates/README.mustache

@@ -0,0 +1,21 @@
+# TEST TEST TEST
+
+# {{artifactId}}
+
+{{appName}}
+
+- API version: {{appVersion}}
+{{^hideGenerationTimestamp}}
+
+- Build date: {{generatedDate}}
+{{/hideGenerationTimestamp}}
+
+{{#appDescriptionWithNewLines}}{{{appDescriptionWithNewLines}}}{{/appDescriptionWithNewLines}}
+
+{{#infoUrl}}
+  For more information, please visit [{{{infoUrl}}}]({{{infoUrl}}})
+{{/infoUrl}}
+
+*Automatically generated by the [OpenAPI Generator](https://openapi-generator.tech)*
+
+… etc.

modules/openapi-generator-maven-plugin/pom.xml

@@ -16,6 +16,8 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <sonar.exclusions>**/src/main/java/org/openapitools/codegen/plugin/**/*</sonar.exclusions>
+        <!-- used for integration tests verification scripts, managed by the test harness plugin -->
+        <groovy.version>3.0.5</groovy.version>
     </properties>
     <dependencies>
         <dependency>
@@ -60,6 +62,24 @@
 <!--             <version>4.12</version> -->
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.apache.maven.shared</groupId>
+            <artifactId>maven-verifier</artifactId>
+            <version>1.7.2</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.maven.plugin-testing</groupId>
+            <artifactId>maven-plugin-testing-harness</artifactId>
+            <version>3.3.0</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.codehaus.plexus</groupId>
+            <artifactId>plexus-utils</artifactId>
+            <version>3.3.0</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
     <build>
         <pluginManagement>
@@ -108,7 +128,41 @@
         </plugins>
     </build>
     <profiles>
-
+        <profile>
+            <id>integration</id>
+            <build>
+                <plugins>
+                    <plugin>
+                        <groupId>org.apache.maven.plugins</groupId>
+                        <artifactId>maven-invoker-plugin</artifactId>
+                        <version>3.2.1</version>
+                        <configuration>
+                            <postBuildHookScript>verify</postBuildHookScript>
+                            <showVersion>true</showVersion>
+                            <streamLogs>true</streamLogs>
+                            <noLog>false</noLog>
+                            <showErrors>true</showErrors>
+                        </configuration>
+                        <dependencies>
+                            <dependency>
+                                <groupId>org.codehaus.groovy</groupId>
+                                <artifactId>groovy</artifactId>
+                                <version>${groovy.version}</version>
+                                <scope>runtime</scope>
+                            </dependency>
+                        </dependencies>
+                        <executions>
+                            <execution>
+                                <id>integration-test</id>
+                                <goals>
+                                    <goal>run</goal>
+                                </goals>
+                            </execution>
+                        </executions>
+                    </plugin>
+                </plugins>
+            </build>
+        </profile>
         <profile>
             <id>static-analysis</id>
             <build>

modules/openapi-generator-maven-plugin/src/it/custom-template-resource/invoker.properties

@@ -0,0 +1,2 @@
+invoker.goals = -nsu generate-sources
+invoker.name = Test Custom Templates via Resource

modules/openapi-generator-maven-plugin/src/it/custom-template-resource/pom.xml

@@ -0,0 +1,57 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  ~ Copyright 2020 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+  ~
+  ~ Licensed under the Apache License, Version 2.0 (the "License");
+  ~ you may not use this file except in compliance with the License.
+  ~ You may obtain a copy of the License at
+  ~
+  ~     http://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+
+<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">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>org.openapitools.maven.its</groupId>
+    <artifactId>custom-template-resource</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>@project.groupId@</groupId>
+                <artifactId>@project.artifactId@</artifactId>
+                <version>@project.version@</version>
+                <configuration>
+                    <inputSpec>https://raw.githubusercontent.com/OpenAPITools/openapi-generator/master/modules/openapi-generator/src/test/resources/2_0/petstore.yaml</inputSpec>
+                    <generatorName>kotlin</generatorName>
+                    <output>${basedir}/out</output>
+                    <!--
+                        HACK: rather than compile our own resource, we'll pull from something that's already available: bash generator's templates
+                              This only works because bash shares only README.mustache template with kotlin
+                    -->
+                    <templateResourcePath>bash</templateResourcePath>
+                    <configOptions>
+                        <serializableModel>true</serializableModel>
+                    </configOptions>
+                </configuration>
+                <executions>
+                    <execution>
+                        <id>default</id>
+                        <phase>generate-sources</phase>
+                        <goals>
+                            <goal>generate</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+</project>

modules/openapi-generator-maven-plugin/src/it/custom-template-resource/verify.groovy

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2020 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+File readme = new File(basedir, "out/README.md")
+
+assert readme.isFile()
+if (File.separator == "/") {
+    // For whatever reason, resource path templates fail in this test in Windows
+    assert readme.text.contains("# OpenAPI Petstore Bash client")
+}
+
+File gradle = new File(basedir, "out/build.gradle")
+assert gradle.isFile()
+
+File api = new File(basedir, "out/src/main/kotlin/org/openapitools/client/apis/PetApi.kt")
+assert api.isFile()
+
+File model = new File(basedir, "out/src/main/kotlin/org/openapitools/client/models/Pet.kt")
+assert model.isFile()
+
+// note that in Java 11+, this anything matching this condition could fail due to
+// Illegal reflective access by org.codehaus.groovy.reflection.CachedClass
+// and cause tests to fail. This is more to document for engineers.
+if (GroovySystem.version.tokenize('.')[0].toInteger() < 3) {
+    throw new IllegalStateException("Found:" + GroovySystem.version + ", need Groovy 3.x or higher for Java 11+, so we require it for all versions")
+}

modules/openapi-generator-maven-plugin/src/it/custom-template/invoker.properties

@@ -0,0 +1,2 @@
+invoker.goals = -nsu generate-sources
+invoker.name = Test Custom Templates

modules/openapi-generator-maven-plugin/src/it/custom-template/pom.xml

@@ -0,0 +1,53 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  ~ Copyright 2020 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+  ~
+  ~ Licensed under the Apache License, Version 2.0 (the "License");
+  ~ you may not use this file except in compliance with the License.
+  ~ You may obtain a copy of the License at
+  ~
+  ~     http://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+
+<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">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>org.openapitools.maven.its</groupId>
+    <artifactId>custom-template</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>@project.groupId@</groupId>
+                <artifactId>@project.artifactId@</artifactId>
+                <version>@project.version@</version>
+                <configuration>
+                    <inputSpec>https://raw.githubusercontent.com/OpenAPITools/openapi-generator/master/modules/openapi-generator/src/test/resources/2_0/petstore.yaml</inputSpec>
+                    <generatorName>kotlin</generatorName>
+                    <output>${basedir}/out</output>
+                    <templateDirectory>${project.basedir}/templates</templateDirectory>
+                    <configOptions>
+                        <serializableModel>true</serializableModel>
+                    </configOptions>
+                </configuration>
+                <executions>
+                    <execution>
+                        <id>remote</id>
+                        <phase>generate-sources</phase>
+                        <goals>
+                            <goal>generate</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+</project>

modules/openapi-generator-maven-plugin/src/it/custom-template/templates/README.mustache

@@ -0,0 +1,21 @@
+# TEST TEST TEST
+
+# {{artifactId}}
+
+{{appName}}
+
+- API version: {{appVersion}}
+{{^hideGenerationTimestamp}}
+
+- Build date: {{generatedDate}}
+{{/hideGenerationTimestamp}}
+
+{{#appDescriptionWithNewLines}}{{{appDescriptionWithNewLines}}}{{/appDescriptionWithNewLines}}
+
+{{#infoUrl}}
+  For more information, please visit [{{{infoUrl}}}]({{{infoUrl}}})
+{{/infoUrl}}
+
+*Automatically generated by the [OpenAPI Generator](https://openapi-generator.tech)*
+
+… etc.

modules/openapi-generator-maven-plugin/src/it/custom-template/verify.groovy

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2020 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+File readme = new File(basedir, "out/README.md")
+
+assert readme.isFile()
+assert readme.text.contains("# TEST TEST TEST")
+assert readme.text.contains("# kotlin-client")
+assert readme.text.contains("OpenAPI Petstore")
+
+File gradle = new File(basedir, "out/build.gradle")
+assert gradle.isFile()
+
+File api = new File(basedir, "out/src/main/kotlin/org/openapitools/client/apis/PetApi.kt")
+assert api.isFile()
+
+File model = new File(basedir, "out/src/main/kotlin/org/openapitools/client/models/Pet.kt")
+assert model.isFile()
+
+// note that in Java 11+, this anything matching this condition could fail due to
+// Illegal reflective access by org.codehaus.groovy.reflection.CachedClass
+// and cause tests to fail. This is more to document for engineers.
+if (GroovySystem.version.tokenize('.')[0].toInteger() < 3) {
+    throw new IllegalStateException("Found:" + GroovySystem.version + ", need Groovy 3.x or higher for Java 11+, so we require it for all versions")
+}

modules/openapi-generator-maven-plugin/src/test/java/org/openapitools/codegen/plugin/BaseTestCase.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2020 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.openapitools.codegen.plugin;
+
+import org.apache.maven.plugin.testing.AbstractMojoTestCase;
+
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+/**
+ * A base test class where we can add helper methods and whatnot
+ */
+public abstract class BaseTestCase extends AbstractMojoTestCase {
+    protected Path getUnitTestDir() {
+        return Paths.get(getBasedir(), "src", "test", "resources", "unit");
+    }
+}

modules/openapi-generator-maven-plugin/src/test/java/org/openapitools/codegen/plugin/CodeGenMojoTest.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2020 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.openapitools.codegen.plugin;
+
+import org.junit.Test;
+import org.openapitools.codegen.plugin.stubs.StubUtility;
+
+import java.io.File;
+import java.util.Map;
+
+public class CodeGenMojoTest extends BaseTestCase {
+    @Override
+    protected void setUp() throws Exception {
+        super.setUp();
+    }
+
+    @SuppressWarnings("unchecked")
+    public void testCommonConfiguration() throws Exception {
+        File testPom = StubUtility.basedPath(getUnitTestDir().toFile(), "common-maven", "common-maven.xml").toFile();
+        final CodeGenMojo mojo = (CodeGenMojo) lookupMojo("generate", testPom);
+        mojo.execute();
+        assertEquals("java", getVariableValueFromObject(mojo, "generatorName"));
+        assertEquals("jersey2", getVariableValueFromObject(mojo, "library"));
+        assertEquals("remote.org.openapitools.client.api", getVariableValueFromObject(mojo, "apiPackage"));
+        assertEquals("remote.org.openapitools.client.model", getVariableValueFromObject(mojo, "modelPackage"));
+        assertEquals("remote.org.openapitools.client", getVariableValueFromObject(mojo, "invokerPackage"));
+
+        Map<String, Object> configOptions = (Map<String, Object>) getVariableValueFromObject(mojo, "configOptions");
+        assertNotNull(configOptions);
+        assertEquals("joda", configOptions.get("dateLibrary"));
+    }
+}

modules/openapi-generator-maven-plugin/src/test/java/org/openapitools/codegen/plugin/stubs/CommonMavenProjectStub.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2020 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.openapitools.codegen.plugin.stubs;
+
+import org.apache.maven.plugin.testing.stubs.MavenProjectStub;
+
+public class CommonMavenProjectStub extends MavenProjectStub {
+    public CommonMavenProjectStub() {
+        StubUtility.configureStub(this,"common-maven", "common-maven.xml");
+    }
+}

modules/openapi-generator-maven-plugin/src/test/java/org/openapitools/codegen/plugin/stubs/StubUtility.java

@@ -0,0 +1,94 @@
+/*
+ * Copyright 2020 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.openapitools.codegen.plugin.stubs;
+
+import org.apache.maven.model.Build;
+import org.apache.maven.model.Model;
+import org.apache.maven.model.io.xpp3.MavenXpp3Reader;
+import org.apache.maven.plugin.testing.stubs.ArtifactStub;
+import org.apache.maven.plugin.testing.stubs.DefaultArtifactHandlerStub;
+import org.apache.maven.plugin.testing.stubs.MavenProjectStub;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.ArrayList;
+import java.util.List;
+
+public abstract class StubUtility {
+    private StubUtility() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+
+    /**
+     * Configures a stub to conventional directories based on test name and pom file name.
+     * <p>
+     * Taken largely from PMD plugin:
+     * https://github.com/apache/maven-pmd-plugin/blob/d766fdb0c93a6630ad7a788f260746b05c804a71/src/test/java/org/apache/maven/plugins/pmd/stubs/CustomConfigurationMavenProjectStub.java
+     * License: Apache 2.0
+     *
+     * @param configure   The stub to configure
+     * @param testName    A name used to identify this stub, used in resource lookup
+     * @param pomFileName The filename and extension, e.g. "my-test-pom.xml"
+     */
+    public static void configureStub(MavenProjectStub configure, String testName, String pomFileName) {
+        MavenXpp3Reader pomReader = new MavenXpp3Reader();
+        Model model = null;
+        try {
+            File pomFile = basedPath(
+                    configure.getBasedir(), "src", "test", "resources", "unit", testName, pomFileName
+            ).toFile();
+            model = pomReader.read(new InputStreamReader(new FileInputStream(pomFile), StandardCharsets.UTF_8));
+            configure.setModel(model);
+        } catch (Exception ignored) {
+
+        }
+
+        configure.setGroupId(model.getGroupId());
+        configure.setArtifactId(model.getArtifactId());
+        configure.setVersion(model.getVersion());
+        configure.setName(model.getName());
+        configure.setUrl(model.getUrl());
+        configure.setPackaging(model.getPackaging());
+
+        Build build = new Build();
+        build.setFinalName(model.getBuild().getFinalName());
+        build.setDirectory(basedPath(configure.getBasedir(), "target", "test", "unit", testName, "target").toString());
+        build.setSourceDirectory(basedPath(configure.getBasedir(), "src", "test", "resources", "unit", testName).toString());
+        configure.setBuild(build);
+
+        List<String> compileSourceRoots = new ArrayList<>();
+        compileSourceRoots.add(basedPath(configure.getBasedir(), "src", "test", "resources", "unit", testName, "src").toString());
+        configure.setCompileSourceRoots(compileSourceRoots);
+
+        ArtifactStub artifactStub = new ArtifactStub();
+        artifactStub.setGroupId(configure.getGroupId());
+        artifactStub.setArtifactId(configure.getArtifactId());
+        artifactStub.setVersion(configure.getVersion());
+        artifactStub.setArtifactHandler(new DefaultArtifactHandlerStub("jar"));
+        configure.setArtifact(artifactStub);
+
+        configure.setFile(configure.getBasedir().toPath().resolve(pomFileName).toFile());
+    }
+
+    public static Path basedPath(File baseDir, String first, String... more) {
+        return baseDir.toPath().resolve(Paths.get(first, more)).normalize().toAbsolutePath();
+    }
+}

modules/openapi-generator-maven-plugin/src/test/resources/unit/common-maven/common-maven.xml

@@ -0,0 +1,56 @@
+<!--
+  ~ Copyright 2020 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+  ~
+  ~ Licensed under the Apache License, Version 2.0 (the "License");
+  ~ you may not use this file except in compliance with the License.
+  ~ You may obtain a copy of the License at
+  ~
+  ~     http://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+
+<project>
+    <modelVersion>4.0.0</modelVersion>
+    <groupId>common.maven</groupId>
+    <artifactId>common-maven</artifactId>
+    <packaging>jar</packaging>
+    <version>1.0.0-SNAPSHOT</version>
+    <name>OpenAPI Generator Configuration Test</name>
+    <url>https://openapi-generator.tech/</url>
+    <build>
+        <finalName>common-maven</finalName>
+        <plugins>
+            <plugin>
+                <groupId>org.openapitools</groupId>
+                <artifactId>openapi-generator-maven-plugin</artifactId>
+                <configuration>
+                    <project implementation="org.openapitools.codegen.plugin.stubs.CommonMavenProjectStub"/>
+                    <inputSpec>${basedir}/src/test/resources/unit/common-maven/petstore.yaml</inputSpec>
+                    <generatorName>java</generatorName>
+                    <configOptions>
+                        <dateLibrary>joda</dateLibrary>
+                    </configOptions>
+                    <library>jersey2</library>
+                    <output>${basedir}/target/generated-sources/common-maven/remote-openapi</output>
+                    <apiPackage>remote.org.openapitools.client.api</apiPackage>
+                    <modelPackage>remote.org.openapitools.client.model</modelPackage>
+                    <invokerPackage>remote.org.openapitools.client</invokerPackage>
+                </configuration>
+                <executions>
+                    <execution>
+                        <id>default</id>
+                        <phase>generate-sources</phase>
+                        <goals>
+                            <goal>generate</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+</project>

modules/openapi-generator-maven-plugin/src/test/resources/unit/common-maven/petstore.yaml

@@ -0,0 +1,736 @@
+openapi: 3.0.0
+servers:
+  - url: 'http://petstore.swagger.io/v2'
+info:
+  description: >-
+    This is a sample server Petstore server. For this sample, you can use the api key
+    `special-key` to test the authorization filters.
+  version: 1.0.0
+  title: OpenAPI Petstore
+  license:
+    name: Apache-2.0
+    url: 'https://www.apache.org/licenses/LICENSE-2.0.html'
+tags:
+  - name: pet
+    description: Everything about your Pets
+  - name: store
+    description: Access to Petstore orders
+  - name: user
+    description: Operations about user
+paths:
+  /pet:
+    post:
+      tags:
+        - pet
+      summary: Add a new pet to the store
+      description: ''
+      operationId: addPet
+      responses:
+        '200':
+          description: successful operation
+          content:
+            application/xml:
+              schema:
+                $ref: '#/components/schemas/Pet'
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Pet'
+        '405':
+          description: Invalid input
+      security:
+        - petstore_auth:
+            - 'write:pets'
+            - 'read:pets'
+      requestBody:
+        $ref: '#/components/requestBodies/Pet'
+    put:
+      tags:
+        - pet
+      summary: Update an existing pet
+      description: ''
+      operationId: updatePet
+      responses:
+        '200':
+          description: successful operation
+          content:
+            application/xml:
+              schema:
+                $ref: '#/components/schemas/Pet'
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Pet'
+        '400':
+          description: Invalid ID supplied
+        '404':
+          description: Pet not found
+        '405':
+          description: Validation exception
+      security:
+        - petstore_auth:
+            - 'write:pets'
+            - 'read:pets'
+      requestBody:
+        $ref: '#/components/requestBodies/Pet'
+  /pet/findByStatus:
+    get:
+      tags:
+        - pet
+      summary: Finds Pets by status
+      description: Multiple status values can be provided with comma separated strings
+      operationId: findPetsByStatus
+      parameters:
+        - name: status
+          in: query
+          description: Status values that need to be considered for filter
+          required: true
+          style: form
+          explode: false
+          schema:
+            type: array
+            items:
+              type: string
+              enum:
+                - available
+                - pending
+                - sold
+              default: available
+      responses:
+        '200':
+          description: successful operation
+          content:
+            application/xml:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Pet'
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Pet'
+        '400':
+          description: Invalid status value
+      security:
+        - petstore_auth:
+            - 'read:pets'
+  /pet/findByTags:
+    get:
+      tags:
+        - pet
+      summary: Finds Pets by tags
+      description: >-
+        Multiple tags can be provided with comma separated strings. Use tag1,
+        tag2, tag3 for testing.
+      operationId: findPetsByTags
+      parameters:
+        - name: tags
+          in: query
+          description: Tags to filter by
+          required: true
+          style: form
+          explode: false
+          schema:
+            type: array
+            items:
+              type: string
+      responses:
+        '200':
+          description: successful operation
+          content:
+            application/xml:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Pet'
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Pet'
+        '400':
+          description: Invalid tag value
+      security:
+        - petstore_auth:
+            - 'read:pets'
+      deprecated: true
+  '/pet/{petId}':
+    get:
+      tags:
+        - pet
+      summary: Find pet by ID
+      description: Returns a single pet
+      operationId: getPetById
+      parameters:
+        - name: petId
+          in: path
+          description: ID of pet to return
+          required: true
+          schema:
+            type: integer
+            format: int64
+      responses:
+        '200':
+          description: successful operation
+          content:
+            application/xml:
+              schema:
+                $ref: '#/components/schemas/Pet'
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Pet'
+        '400':
+          description: Invalid ID supplied
+        '404':
+          description: Pet not found
+      security:
+        - api_key: []
+    post:
+      tags:
+        - pet
+      summary: Updates a pet in the store with form data
+      description: ''
+      operationId: updatePetWithForm
+      parameters:
+        - name: petId
+          in: path
+          description: ID of pet that needs to be updated
+          required: true
+          schema:
+            type: integer
+            format: int64
+      responses:
+        '405':
+          description: Invalid input
+      security:
+        - petstore_auth:
+            - 'write:pets'
+            - 'read:pets'
+      requestBody:
+        content:
+          application/x-www-form-urlencoded:
+            schema:
+              type: object
+              properties:
+                name:
+                  description: Updated name of the pet
+                  type: string
+                status:
+                  description: Updated status of the pet
+                  type: string
+    delete:
+      tags:
+        - pet
+      summary: Deletes a pet
+      description: ''
+      operationId: deletePet
+      parameters:
+        - name: api_key
+          in: header
+          required: false
+          schema:
+            type: string
+        - name: petId
+          in: path
+          description: Pet id to delete
+          required: true
+          schema:
+            type: integer
+            format: int64
+      responses:
+        '400':
+          description: Invalid pet value
+      security:
+        - petstore_auth:
+            - 'write:pets'
+            - 'read:pets'
+  '/pet/{petId}/uploadImage':
+    post:
+      tags:
+        - pet
+      summary: uploads an image
+      description: ''
+      operationId: uploadFile
+      parameters:
+        - name: petId
+          in: path
+          description: ID of pet to update
+          required: true
+          schema:
+            type: integer
+            format: int64
+      responses:
+        '200':
+          description: successful operation
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ApiResponse'
+      security:
+        - petstore_auth:
+            - 'write:pets'
+            - 'read:pets'
+      requestBody:
+        content:
+          multipart/form-data:
+            schema:
+              type: object
+              properties:
+                additionalMetadata:
+                  description: Additional data to pass to server
+                  type: string
+                file:
+                  description: file to upload
+                  type: string
+                  format: binary
+  /store/inventory:
+    get:
+      tags:
+        - store
+      summary: Returns pet inventories by status
+      description: Returns a map of status codes to quantities
+      operationId: getInventory
+      responses:
+        '200':
+          description: successful operation
+          content:
+            application/json:
+              schema:
+                type: object
+                additionalProperties:
+                  type: integer
+                  format: int32
+      security:
+        - api_key: []
+  /store/order:
+    post:
+      tags:
+        - store
+      summary: Place an order for a pet
+      description: ''
+      operationId: placeOrder
+      responses:
+        '200':
+          description: successful operation
+          content:
+            application/xml:
+              schema:
+                $ref: '#/components/schemas/Order'
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Order'
+        '400':
+          description: Invalid Order
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/Order'
+        description: order placed for purchasing the pet
+        required: true
+  '/store/order/{orderId}':
+    get:
+      tags:
+        - store
+      summary: Find purchase order by ID
+      description: >-
+        For valid response try integer IDs with value <= 5 or > 10. Other values
+        will generated exceptions
+      operationId: getOrderById
+      parameters:
+        - name: orderId
+          in: path
+          description: ID of pet that needs to be fetched
+          required: true
+          schema:
+            type: integer
+            format: int64
+            minimum: 1
+            maximum: 5
+      responses:
+        '200':
+          description: successful operation
+          content:
+            application/xml:
+              schema:
+                $ref: '#/components/schemas/Order'
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Order'
+        '400':
+          description: Invalid ID supplied
+        '404':
+          description: Order not found
+    delete:
+      tags:
+        - store
+      summary: Delete purchase order by ID
+      description: >-
+        For valid response try integer IDs with value < 1000. Anything above
+        1000 or nonintegers will generate API errors
+      operationId: deleteOrder
+      parameters:
+        - name: orderId
+          in: path
+          description: ID of the order that needs to be deleted
+          required: true
+          schema:
+            type: string
+      responses:
+        '400':
+          description: Invalid ID supplied
+        '404':
+          description: Order not found
+  /user:
+    post:
+      tags:
+        - user
+      summary: Create user
+      description: This can only be done by the logged in user.
+      operationId: createUser
+      responses:
+        default:
+          description: successful operation
+      security:
+        - api_key: []
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/User'
+        description: Created user object
+        required: true
+  /user/createWithArray:
+    post:
+      tags:
+        - user
+      summary: Creates list of users with given input array
+      description: ''
+      operationId: createUsersWithArrayInput
+      responses:
+        default:
+          description: successful operation
+      security:
+        - api_key: []
+      requestBody:
+        $ref: '#/components/requestBodies/UserArray'
+  /user/createWithList:
+    post:
+      tags:
+        - user
+      summary: Creates list of users with given input array
+      description: ''
+      operationId: createUsersWithListInput
+      responses:
+        default:
+          description: successful operation
+      security:
+        - api_key: []
+      requestBody:
+        $ref: '#/components/requestBodies/UserArray'
+  /user/login:
+    get:
+      tags:
+        - user
+      summary: Logs user into the system
+      description: ''
+      operationId: loginUser
+      parameters:
+        - name: username
+          in: query
+          description: The user name for login
+          required: true
+          schema:
+            type: string
+            pattern: '^[a-zA-Z0-9]+[a-zA-Z0-9\.\-_]*[a-zA-Z0-9]+$'
+        - name: password
+          in: query
+          description: The password for login in clear text
+          required: true
+          schema:
+            type: string
+      responses:
+        '200':
+          description: successful operation
+          headers:
+            Set-Cookie:
+              description: >-
+                Cookie authentication key for use with the `api_key`
+                apiKey authentication.
+              schema:
+                type: string
+                example: AUTH_KEY=abcde12345; Path=/; HttpOnly
+            X-Rate-Limit:
+              description: calls per hour allowed by the user
+              schema:
+                type: integer
+                format: int32
+            X-Expires-After:
+              description: date in UTC when toekn expires
+              schema:
+                type: string
+                format: date-time
+          content:
+            application/xml:
+              schema:
+                type: string
+            application/json:
+              schema:
+                type: string
+        '400':
+          description: Invalid username/password supplied
+  /user/logout:
+    get:
+      tags:
+        - user
+      summary: Logs out current logged in user session
+      description: ''
+      operationId: logoutUser
+      responses:
+        default:
+          description: successful operation
+      security:
+        - api_key: []
+  '/user/{username}':
+    get:
+      tags:
+        - user
+      summary: Get user by user name
+      description: ''
+      operationId: getUserByName
+      parameters:
+        - name: username
+          in: path
+          description: The name that needs to be fetched. Use user1 for testing.
+          required: true
+          schema:
+            type: string
+      responses:
+        '200':
+          description: successful operation
+          content:
+            application/xml:
+              schema:
+                $ref: '#/components/schemas/User'
+            application/json:
+              schema:
+                $ref: '#/components/schemas/User'
+        '400':
+          description: Invalid username supplied
+        '404':
+          description: User not found
+    put:
+      tags:
+        - user
+      summary: Updated user
+      description: This can only be done by the logged in user.
+      operationId: updateUser
+      parameters:
+        - name: username
+          in: path
+          description: name that need to be deleted
+          required: true
+          schema:
+            type: string
+      responses:
+        '400':
+          description: Invalid user supplied
+        '404':
+          description: User not found
+      security:
+        - api_key: []
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/User'
+        description: Updated user object
+        required: true
+    delete:
+      tags:
+        - user
+      summary: Delete user
+      description: This can only be done by the logged in user.
+      operationId: deleteUser
+      parameters:
+        - name: username
+          in: path
+          description: The name that needs to be deleted
+          required: true
+          schema:
+            type: string
+      responses:
+        '400':
+          description: Invalid username supplied
+        '404':
+          description: User not found
+      security:
+        - api_key: []
+externalDocs:
+  description: Find out more about Swagger
+  url: 'http://swagger.io'
+components:
+  requestBodies:
+    UserArray:
+      content:
+        application/json:
+          schema:
+            type: array
+            items:
+              $ref: '#/components/schemas/User'
+      description: List of user object
+      required: true
+    Pet:
+      content:
+        application/json:
+          schema:
+            $ref: '#/components/schemas/Pet'
+        application/xml:
+          schema:
+            $ref: '#/components/schemas/Pet'
+      description: Pet object that needs to be added to the store
+      required: true
+  securitySchemes:
+    petstore_auth:
+      type: oauth2
+      flows:
+        implicit:
+          authorizationUrl: 'http://petstore.swagger.io/api/oauth/dialog'
+          scopes:
+            'write:pets': modify pets in your account
+            'read:pets': read your pets
+    api_key:
+      type: apiKey
+      name: api_key
+      in: header
+  schemas:
+    Order:
+      title: Pet Order
+      description: An order for a pets from the pet store
+      type: object
+      properties:
+        id:
+          type: integer
+          format: int64
+        petId:
+          type: integer
+          format: int64
+        quantity:
+          type: integer
+          format: int32
+        shipDate:
+          type: string
+          format: date-time
+        status:
+          type: string
+          description: Order Status
+          enum:
+            - placed
+            - approved
+            - delivered
+        complete:
+          type: boolean
+          default: false
+      xml:
+        name: Order
+    Category:
+      title: Pet category
+      description: A category for a pet
+      type: object
+      properties:
+        id:
+          type: integer
+          format: int64
+        name:
+          type: string
+          pattern: '^[a-zA-Z0-9]+[a-zA-Z0-9\.\-_]*[a-zA-Z0-9]+$'
+      xml:
+        name: Category
+    User:
+      title: a User
+      description: A User who is purchasing from the pet store
+      type: object
+      properties:
+        id:
+          type: integer
+          format: int64
+        username:
+          type: string
+        firstName:
+          type: string
+        lastName:
+          type: string
+        email:
+          type: string
+        password:
+          type: string
+        phone:
+          type: string
+        userStatus:
+          type: integer
+          format: int32
+          description: User Status
+      xml:
+        name: User
+    Tag:
+      title: Pet Tag
+      description: A tag for a pet
+      type: object
+      properties:
+        id:
+          type: integer
+          format: int64
+        name:
+          type: string
+      xml:
+        name: Tag
+    Pet:
+      title: a Pet
+      description: A pet for sale in the pet store
+      type: object
+      required:
+        - name
+        - photoUrls
+      properties:
+        id:
+          type: integer
+          format: int64
+        category:
+          $ref: '#/components/schemas/Category'
+        name:
+          type: string
+          example: doggie
+        photoUrls:
+          type: array
+          xml:
+            name: photoUrl
+            wrapped: true
+          items:
+            type: string
+        tags:
+          type: array
+          xml:
+            name: tag
+            wrapped: true
+          items:
+            $ref: '#/components/schemas/Tag'
+        status:
+          type: string
+          description: pet status in the store
+          enum:
+            - available
+            - pending
+            - sold
+      xml:
+        name: Pet
+    ApiResponse:
+      title: An uploaded response
+      description: Describes the result of uploading an image resource
+      type: object
+      properties:
+        code:
+          type: integer
+          format: int32
+        type:
+          type: string
+        message:
+          type: string

modules/openapi-generator-maven-plugin/src/test/resources/unit/common-maven/src/main/java/com/example/Example.java

@@ -0,0 +1,7 @@
+package com.example;
+
+public class Example {
+    public static void main(String[] args) {
+        System.out.println("You did it!");
+    }
+}

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

@@ -6,6 +6,7 @@ import org.openapitools.codegen.TemplateManager;
 import org.openapitools.codegen.api.TemplatePathLocator;
 
 import java.io.File;
+import java.nio.file.Paths;
 
 /**
  * Locates templates according to {@link CodegenConfig} settings.
@@ -23,7 +24,7 @@ public class GeneratorTemplateContentLocator implements TemplatePathLocator {
     }
 
     private String buildLibraryFilePath(String dir, String library, String file) {
-        return dir + File.separator + "libraries" + File.separator + library + File.separator + file;
+        return Paths.get(dir, "libraries", library, file).normalize().toString();
     }
 
     /**
@@ -34,6 +35,10 @@ public class GeneratorTemplateContentLocator implements TemplatePathLocator {
      * @return true if file is an embedded resource, false if it does not exist
      */
     public boolean embeddedTemplateExists(String name) {
+        return classpathTemplateExists(name);
+    }
+
+    private boolean classpathTemplateExists(String name) {
         return this.getClass().getClassLoader().getResource(TemplateManager.getCPResourcePath(name)) != null;
     }
 
@@ -60,20 +65,26 @@ public class GeneratorTemplateContentLocator implements TemplatePathLocator {
         if (StringUtils.isNotEmpty(library)) {
             //look for the file in the library subfolder of the supplied template
             final String libTemplateFile = buildLibraryFilePath(config.templateDir(), library, relativeTemplateFile);
-            if (new File(libTemplateFile).exists() || this.getClass().getClassLoader().getResource(libTemplateFile) != null) {
+            // looks for user-defined file or classpath
+            // supports template dir which refers to local file system or custom path in classpath as defined by templateDir
+            if (new File(libTemplateFile).exists() || classpathTemplateExists(libTemplateFile)) {
                 return libTemplateFile;
             }
         }
 
-        //check the supplied template main folder for the file
+        // check the supplied template main folder for the file
+        // File.separator is necessary here as the file load is OS-specific
         final String template = config.templateDir() + File.separator + relativeTemplateFile;
-        if (new File(template).exists() || this.getClass().getClassLoader().getResource(template) != null) {
+        // looks for user-defined file or classpath
+        // supports template dir which refers to local file system or custom path in classpath as defined by templateDir
+        if (new File(template).exists() || classpathTemplateExists(template)) {
             return template;
         }
 
         //try the embedded template library folder next
         if (StringUtils.isNotEmpty(library)) {
             final String embeddedLibTemplateFile = buildLibraryFilePath(config.embeddedTemplateDir(), library, relativeTemplateFile);
+            // *only* looks for those files in classpath as defined by embeddedTemplateDir
             if (embeddedTemplateExists(embeddedLibTemplateFile)) {
                 // Fall back to the template file embedded/packaged in the JAR file library folder...
                 return embeddedLibTemplateFile;
@@ -82,6 +93,7 @@ public class GeneratorTemplateContentLocator implements TemplatePathLocator {
 
         // Fall back to the template file for generator root directory embedded/packaged in the JAR file...
         String loc = config.embeddedTemplateDir() + File.separator + relativeTemplateFile;
+        // *only* looks for those files in classpath as defined by embeddedTemplateDir
         if (embeddedTemplateExists(loc)) {
             return loc;
         }

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

@@ -433,4 +433,208 @@ public class DefaultGeneratorTest {
         Assert.assertEquals(((Schema) codegenResponse.schema).getPattern(), expectedPattern);
         Assert.assertEquals(codegenResponse.pattern, escapedPattern);
     }
+
+    @Test
+    public void testBuiltinLibraryTemplates() throws IOException {
+        Path target = Files.createTempDirectory("test");
+        File output = target.toFile();
+        try {
+            final CodegenConfigurator configurator = new CodegenConfigurator()
+                    .setGeneratorName("kotlin")
+                    .setLibrary("jvm-okhttp4")
+                    .setInputSpec("src/test/resources/3_0/petstore.yaml")
+                    .setSkipOverwrite(false)
+                    .setOutputDir(target.toAbsolutePath().toString());
+
+            final ClientOptInput clientOptInput = configurator.toClientOptInput();
+            DefaultGenerator generator = new DefaultGenerator(false);
+
+            generator.setGeneratorPropertyDefault(CodegenConstants.MODELS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_DOCS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.APIS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.SUPPORTING_FILES, "true");
+            generator.setGeneratorPropertyDefault(CodegenConstants.API_DOCS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_TESTS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.API_TESTS, "false");
+
+            List<File> files = generator.opts(clientOptInput).generate();
+
+            Assert.assertEquals(files.size(), 20);
+
+            // Generator should report a library templated file as a generated file
+            TestUtils.ensureContainsFile(files, output, "src/main/kotlin/org/openapitools/client/infrastructure/Errors.kt");
+
+            // Generated file should exist on the filesystem after generation
+            File generatedFile = new File(output, "src/main/kotlin/org/openapitools/client/infrastructure/Errors.kt");
+            Assert.assertTrue(generatedFile.exists());
+
+            // Generated file should contain some expected text
+            TestUtils.assertFileContains(generatedFile.toPath(), "package org.openapitools.client.infrastructure",
+                    "open class ClientException",
+                    "open class ServerException");
+        } finally {
+            output.delete();
+        }
+    }
+
+    @Test
+    public void testBuiltinNonLibraryTemplates() throws IOException {
+        Path target = Files.createTempDirectory("test");
+        File output = target.toFile();
+        try {
+            final CodegenConfigurator configurator = new CodegenConfigurator()
+                    .setGeneratorName("kotlin")
+                    .setInputSpec("src/test/resources/3_0/petstore.yaml")
+                    .setSkipOverwrite(false)
+                    .setOutputDir(target.toAbsolutePath().toString());
+
+            final ClientOptInput clientOptInput = configurator.toClientOptInput();
+            DefaultGenerator generator = new DefaultGenerator(false);
+
+            generator.setGeneratorPropertyDefault(CodegenConstants.MODELS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_DOCS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.APIS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.SUPPORTING_FILES, "true");
+            generator.setGeneratorPropertyDefault(CodegenConstants.API_DOCS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_TESTS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.API_TESTS, "false");
+
+            List<File> files = generator.opts(clientOptInput).generate();
+
+            Assert.assertEquals(files.size(), 20);
+
+            // Generator should report README.md as a generated file
+            TestUtils.ensureContainsFile(files, output, "README.md");
+
+            // Generated file should exist on the filesystem after generation
+            File readme = new File(output, "README.md");
+            Assert.assertTrue(readme.exists());
+
+            // README.md should contain some expected text
+            TestUtils.assertFileContains(readme.toPath(), "# org.openapitools.client - Kotlin client library for OpenAPI Petstore",
+                    "## Requires",
+                    "## Build",
+                    "## Features/Implementation Notes");
+        } finally {
+            output.delete();
+        }
+    }
+
+    @Test
+    public void testCustomLibraryTemplates() throws IOException {
+        Path target = Files.createTempDirectory("test");
+        Path templates = Files.createTempDirectory("templates");
+        File output = target.toFile();
+        try {
+            // Create custom template
+            File customTemplate = new File(templates.toFile(), "libraries/jvm-okhttp/infrastructure/Errors.kt.mustache");
+            new File(customTemplate.getParent()).mkdirs();
+            StringBuilder sb = new StringBuilder();
+            sb.append("// {{someKey}}").append("\n");
+            sb.append("@file:Suppress(\"unused\")").append("\n");
+            sb.append("package org.openapitools.client.infrastructure").append("\n");
+            sb.append("import java.lang.RuntimeException").append("\n");
+            sb.append("open class CustomException(").append("\n");
+            sb.append("  message: kotlin.String? = null, val statusCode: Int = -1, val response: Response? = null) : RuntimeException(message) {").append("\n");
+            sb.append("    companion object {").append("\n");
+            sb.append("      private const val serialVersionUID: Long = 789L").append("\n");
+            sb.append("    }").append("\n");
+            sb.append("}").append("\n");
+            Files.write(customTemplate.toPath(),
+                    sb.toString().getBytes(StandardCharsets.UTF_8),
+                    StandardOpenOption.CREATE);
+
+            final CodegenConfigurator configurator = new CodegenConfigurator()
+                    .setGeneratorName("kotlin")
+                    .addAdditionalProperty("someKey", "testCustomLibraryTemplates")
+                    .setTemplateDir(templates.toAbsolutePath().toString())
+                    .setLibrary("jvm-okhttp4")
+                    .setInputSpec("src/test/resources/3_0/petstore.yaml")
+                    .setSkipOverwrite(false)
+                    .setOutputDir(target.toAbsolutePath().toString());
+
+            final ClientOptInput clientOptInput = configurator.toClientOptInput();
+            DefaultGenerator generator = new DefaultGenerator(false);
+
+            generator.setGeneratorPropertyDefault(CodegenConstants.MODELS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_DOCS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.APIS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.SUPPORTING_FILES, "true");
+            generator.setGeneratorPropertyDefault(CodegenConstants.API_DOCS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_TESTS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.API_TESTS, "false");
+
+            List<File> files = generator.opts(clientOptInput).generate();
+
+            Assert.assertEquals(files.size(), 20);
+
+            // Generator should report a library templated file as a generated file
+            TestUtils.ensureContainsFile(files, output, "src/main/kotlin/org/openapitools/client/infrastructure/Errors.kt");
+
+            // Generated file should exist on the filesystem after generation
+            File readme = new File(output, "src/main/kotlin/org/openapitools/client/infrastructure/Errors.kt");
+            Assert.assertTrue(readme.exists());
+
+            // Generated file should contain our custom templated text
+            TestUtils.assertFileContains(readme.toPath(), "// testCustomLibraryTemplates",
+                    "package org.openapitools.client.infrastructure",
+                    "open class CustomException(",
+                    "private const val serialVersionUID: Long = 789L");
+        } finally {
+            output.delete();
+            templates.toFile().delete();
+        }
+    }
+
+    @Test
+    public void testCustomNonLibraryTemplates() throws IOException {
+        Path target = Files.createTempDirectory("test");
+        Path templates = Files.createTempDirectory("templates");
+        File output = target.toFile();
+        try {
+            // Create custom template
+            File customTemplate = new File(templates.toFile(), "README.mustache");
+            new File(customTemplate.getParent()).mkdirs();
+            Files.write(customTemplate.toPath(),
+                    "# {{someKey}}".getBytes(StandardCharsets.UTF_8),
+                    StandardOpenOption.CREATE);
+
+            final CodegenConfigurator configurator = new CodegenConfigurator()
+                    .setGeneratorName("kotlin")
+                    .addAdditionalProperty("someKey", "testCustomNonLibraryTemplates")
+                    .setTemplateDir(templates.toAbsolutePath().toString())
+                    .setInputSpec("src/test/resources/3_0/petstore.yaml")
+                    .setSkipOverwrite(false)
+                    .setOutputDir(target.toAbsolutePath().toString());
+
+            final ClientOptInput clientOptInput = configurator.toClientOptInput();
+            DefaultGenerator generator = new DefaultGenerator(false);
+
+            generator.setGeneratorPropertyDefault(CodegenConstants.MODELS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_DOCS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.APIS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.SUPPORTING_FILES, "true");
+            generator.setGeneratorPropertyDefault(CodegenConstants.API_DOCS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.MODEL_TESTS, "false");
+            generator.setGeneratorPropertyDefault(CodegenConstants.API_TESTS, "false");
+
+            List<File> files = generator.opts(clientOptInput).generate();
+
+            Assert.assertEquals(files.size(), 20);
+
+            // Generator should report README.md as a generated file
+            TestUtils.ensureContainsFile(files, output, "README.md");
+
+            // Generated file should exist on the filesystem after generation
+            File readme = new File(output, "README.md");
+            Assert.assertTrue(readme.exists());
+
+            // README.md should contain our custom templated text
+            TestUtils.assertFileContains(readme.toPath(), "# testCustomNonLibraryTemplates");
+        } finally {
+            output.delete();
+            templates.toFile().delete();
+        }
+    }
 }
+