Fix relative url as basePath

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

@@ -268,7 +268,11 @@ void configureGeneratorProperties() {
         URL url = URLPathUtils.getServerURL(openAPI, config.serverVariableOverrides());
         contextPath = removeTrailingSlash(config.escapeText(url.getPath())); // for backward compatibility
         basePathWithoutHost = contextPath;
-        basePath = removeTrailingSlash(config.escapeText(URLPathUtils.getHost(openAPI, config.serverVariableOverrides())));
+        if (URLPathUtils.isRelativeUrl(openAPI.getServers())) {
+            basePath = removeTrailingSlash(basePathWithoutHost);
+        } else {
+            basePath = removeTrailingSlash(config.escapeText(URLPathUtils.getHost(openAPI, config.serverVariableOverrides())));
+        }
     }
 
     private void configureOpenAPIInfo() {

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

@@ -232,4 +232,12 @@ private static URL getDefaultUrl() {
             return null;
         }
     }
+
+    public static boolean isRelativeUrl(List<Server> servers) {
+        if (servers != null && servers.size() > 0) {
+            final Server firstServer = servers.get(0);
+            return Pattern.matches("^(\\/[\\w\\d]+)+", firstServer.getUrl());
+        }
+        return false;
+    }
 }

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

@@ -662,6 +662,30 @@ public void testHandlesTrailingSlashInServers() {
         Assert.assertEquals(servers.get(1).url, "http://trailingshlash.io:80/v1");
         Assert.assertEquals(servers.get(2).url, "http://notrailingslash.io:80/v2");
     }
+
+    @Test
+    public void testHandlesRelativeUrlsInServers() {
+        OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/issue_10056.yaml");
+        ClientOptInput opts = new ClientOptInput();
+        opts.openAPI(openAPI);
+        DefaultCodegen config = new DefaultCodegen();
+        config.setStrictSpecBehavior(true);
+        opts.config(config);
+        final DefaultGenerator generator = new DefaultGenerator();
+        generator.opts(opts);
+        generator.configureGeneratorProperties();
+
+        List<File> files = new ArrayList<>();
+        List<String> filteredSchemas = ModelUtils.getSchemasUsedOnlyInFormParam(openAPI);
+        List<Object> allModels = new ArrayList<>();
+        generator.generateModels(files, allModels, filteredSchemas);
+        List<Object> allOperations = new ArrayList<>();
+        generator.generateApis(files, allOperations, allModels);
+
+        Map<String, Object> bundle = generator.buildSupportFileBundle(allOperations, allModels);
+        LinkedList<CodegenServer> servers = (LinkedList<CodegenServer>) bundle.get("servers");
+        Assert.assertEquals(servers.get(0).url, "/relative/url");
+    }
 
     @Test
     public void testProcessUserDefinedTemplatesWithConfig() throws IOException {

modules/openapi-generator/src/test/resources/3_0/issue_10056.yaml

@@ -0,0 +1,15 @@
+openapi: 3.0.1
+info:
+  title: OpenAPI Petstore
+  description: "sample spec"
+  license:
+    name: Apache-2.0
+    url: https://www.apache.org/licenses/LICENSE-2.0.html
+  version: 1.0.0
+servers:
+  - url: /relative/url
+tags: []
+paths: {}
+components:
+  schemas: {}
+  securitySchemes: {}