From 8f927df6421943d8bd612a78ed8efe9ab166f1ca Mon Sep 17 00:00:00 2001
From: Jorge <jorgerma@ext.inditex.com>
Date: Thu, 9 Apr 2026 16:21:05 +0200
Subject: [PATCH] fix: handle null query parameters in REST client

---
 .../Java/libraries/restclient/api.mustache    |  4 ++-
 .../codegen/java/JavaClientCodegenTest.java   | 16 ++++++++++
 .../org/openapitools/client/api/QueryApi.java | 32 +++++++++++--------
 3 files changed, 38 insertions(+), 14 deletions(-)

diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/restclient/api.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/restclient/api.mustache
index 91c106b48d6b..48f7beb1ea4b 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/restclient/api.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/restclient/api.mustache
@@ -115,9 +115,11 @@ public class {{classname}} {
             {{^queryIsJsonMimeType}}
             {{#isExplode}}
                 {{#hasVars}}
+        if ({{paramName}} != null) {
                     {{#vars}}
-        queryParams.putAll(apiClient.parameterToMultiValueMap({{#collectionFormat}}ApiClient.CollectionFormat.valueOf("{{{.}}}".toUpperCase(Locale.ROOT)){{/collectionFormat}}{{^collectionFormat}}null{{/collectionFormat}}, "{{baseName}}", {{paramName}}.{{getter}}()));
+            queryParams.putAll(apiClient.parameterToMultiValueMap({{#collectionFormat}}ApiClient.CollectionFormat.valueOf("{{{.}}}".toUpperCase(Locale.ROOT)){{/collectionFormat}}{{^collectionFormat}}null{{/collectionFormat}}, "{{baseName}}", {{paramName}}.{{getter}}()));
                     {{/vars}}
+        }
                 {{/hasVars}}
                 {{^hasVars}}
         queryParams.putAll(apiClient.parameterToMultiValueMap({{#collectionFormat}}ApiClient.CollectionFormat.valueOf("{{{.}}}".toUpperCase(Locale.ROOT)){{/collectionFormat}}{{^collectionFormat}}null{{/collectionFormat}}, "{{baseName}}", {{paramName}}));
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
index 5935b2e61aa0..8b8a497d4074 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
@@ -2642,6 +2642,22 @@ public void testQueryParamsExploded_whenQueryParamIsNull() {
         assertFileContains(output.resolve("src/main/java/xyz/abcdef/api/DepartmentApi.java"), "if (filter != null) {");
     }
 
+    @Test
+    public void testQueryParamsExploded_whenQueryParamIsNull_restclient() {
+        final Path output = newTempFolder();
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName(JAVA_GENERATOR)
+                .setLibrary(JavaClientCodegen.RESTCLIENT)
+                .setAdditionalProperties(Map.of(CodegenConstants.API_PACKAGE, "xyz.abcdef.api"))
+                .setInputSpec("src/test/resources/3_0/issue_17555.yaml")
+                .setOutputDir(output.toString().replace("\\", "/"));
+
+        List<File> files = new DefaultGenerator().opts(configurator.toClientOptInput()).generate();
+
+        validateJavaSourceFiles(files);
+        assertFileContains(output.resolve("src/main/java/xyz/abcdef/api/DepartmentApi.java"), "if (filter != null) {");
+    }
+
     @Test
     public void generateAllArgsConstructor() {
         Map<String, File> files = generateFromContract("src/test/resources/3_0/java/all_args_constructor.yaml", JavaClientCodegen.RESTTEMPLATE,
diff --git a/samples/client/echo_api/java/restclient/src/main/java/org/openapitools/client/api/QueryApi.java b/samples/client/echo_api/java/restclient/src/main/java/org/openapitools/client/api/QueryApi.java
index ff8744f47a76..149d9e7ab45c 100644
--- a/samples/client/echo_api/java/restclient/src/main/java/org/openapitools/client/api/QueryApi.java
+++ b/samples/client/echo_api/java/restclient/src/main/java/org/openapitools/client/api/QueryApi.java
@@ -306,12 +306,14 @@ private ResponseSpec testQueryStyleDeepObjectExplodeTrueObjectRequestCreation(@j
         final MultiValueMap<String, String> cookieParams = new LinkedMultiValueMap<>();
         final MultiValueMap<String, Object> formParams = new LinkedMultiValueMap<>();
 
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "id", queryObject.getId()));
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "name", queryObject.getName()));
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "category", queryObject.getCategory()));
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "photoUrls", queryObject.getPhotoUrls()));
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "tags", queryObject.getTags()));
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "status", queryObject.getStatus()));
+        if (queryObject != null) {
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "id", queryObject.getId()));
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "name", queryObject.getName()));
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "category", queryObject.getCategory()));
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "photoUrls", queryObject.getPhotoUrls()));
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "tags", queryObject.getTags()));
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "status", queryObject.getStatus()));
+        }
 
         final String[] localVarAccepts = { 
             "text/plain"
@@ -595,7 +597,9 @@ private ResponseSpec testQueryStyleFormExplodeTrueArrayStringRequestCreation(@ja
         final MultiValueMap<String, String> cookieParams = new LinkedMultiValueMap<>();
         final MultiValueMap<String, Object> formParams = new LinkedMultiValueMap<>();
 
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "values", queryObject.getValues()));
+        if (queryObject != null) {
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "values", queryObject.getValues()));
+        }
 
         final String[] localVarAccepts = { 
             "text/plain"
@@ -666,12 +670,14 @@ private ResponseSpec testQueryStyleFormExplodeTrueObjectRequestCreation(@jakarta
         final MultiValueMap<String, String> cookieParams = new LinkedMultiValueMap<>();
         final MultiValueMap<String, Object> formParams = new LinkedMultiValueMap<>();
 
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "id", queryObject.getId()));
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "name", queryObject.getName()));
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "category", queryObject.getCategory()));
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "photoUrls", queryObject.getPhotoUrls()));
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "tags", queryObject.getTags()));
-        queryParams.putAll(apiClient.parameterToMultiValueMap(null, "status", queryObject.getStatus()));
+        if (queryObject != null) {
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "id", queryObject.getId()));
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "name", queryObject.getName()));
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "category", queryObject.getCategory()));
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "photoUrls", queryObject.getPhotoUrls()));
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "tags", queryObject.getTags()));
+            queryParams.putAll(apiClient.parameterToMultiValueMap(null, "status", queryObject.getStatus()));
+        }
 
         final String[] localVarAccepts = { 
             "text/plain"