diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/restclient/ApiClient.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/restclient/ApiClient.mustache
index 2eaf301646c9..b9c0720f5e46 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/restclient/ApiClient.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/restclient/ApiClient.mustache
@@ -359,10 +359,7 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
@@ -712,7 +709,7 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
      * @param requestBuilder The current request
      */
     protected void addHeadersToRequest(HttpHeaders headers, RestClient.RequestBodySpec requestBuilder) {
-        for (Entry<String, List<String>> entry : headers.entrySet()) {
+        for (Entry<String, List<String>> entry : headers.headerSet()) {
             List<String> values = entry.getValue();
             for(String value : values) {
                 if (value != null) {
diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/restclient/build.gradle.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/restclient/build.gradle.mustache
index 4aec430eb7c9..7addbad90401 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/restclient/build.gradle.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/restclient/build.gradle.mustache
@@ -108,7 +108,7 @@ ext {
     {{#openApiNullable}}
     jackson_databind_nullable_version = "0.2.6"
     {{/openApiNullable}}
-    spring_web_version = "6.1.6"
+    spring_web_version = "6.1.21"
     jakarta_annotation_version = "2.1.1"
     jodatime_version = "2.9.9"
     junit_version = "5.10.2"
diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/restclient/pom.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/restclient/pom.mustache
index f926d47b4b31..f1d08cf55dd0 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/restclient/pom.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/restclient/pom.mustache
@@ -346,7 +346,7 @@
         {{#swagger2AnnotationLibrary}}
         <swagger-annotations-version>2.2.15</swagger-annotations-version>
         {{/swagger2AnnotationLibrary}}
-        <spring-web-version>6.1.6</spring-web-version>
+        <spring-web-version>6.1.21</spring-web-version>
         <jackson-version>2.17.1</jackson-version>
         <jackson-databind-version>2.17.1</jackson-databind-version>
         {{#openApiNullable}}
diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/ApiClient.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/ApiClient.mustache
index e6d693e0b298..52b5b27e5e5a 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/ApiClient.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/ApiClient.mustache
@@ -373,10 +373,7 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
@@ -824,7 +821,12 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
      * @param requestBuilder The current request
      */
     protected void addHeadersToRequest(HttpHeaders headers, BodyBuilder requestBuilder) {
+        {{#useJakartaEe}}
+        for (Entry<String, List<String>> entry : headers.headerSet()) {
+        {{/useJakartaEe}}
+        {{^useJakartaEe}}
         for (Entry<String, List<String>> entry : headers.entrySet()) {
+        {{/useJakartaEe}}
             List<String> values = entry.getValue();
             for (String value : values) {
                 if (value != null) {
@@ -937,7 +939,12 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
                 return "";
             }
             StringBuilder builder = new StringBuilder();
+            {{#useJakartaEe}}
+            for (Entry<String, List<String>> entry : headers.headerSet()) {
+            {{/useJakartaEe}}
+            {{^useJakartaEe}}
             for (Entry<String, List<String>> entry : headers.entrySet()) {
+            {{/useJakartaEe}}
                 builder.append(entry.getKey()).append("=[");
                 for (String value : entry.getValue()) {
                     builder.append(value).append(",");
diff --git a/samples/client/echo_api/java/restclient/build.gradle b/samples/client/echo_api/java/restclient/build.gradle
index 5e8bce82ddba..1710716986be 100644
--- a/samples/client/echo_api/java/restclient/build.gradle
+++ b/samples/client/echo_api/java/restclient/build.gradle
@@ -100,7 +100,7 @@ ext {
     jackson_version = "2.17.1"
     jackson_databind_version = "2.17.1"
     jackson_databind_nullable_version = "0.2.6"
-    spring_web_version = "6.1.6"
+    spring_web_version = "6.1.21"
     jakarta_annotation_version = "2.1.1"
     jodatime_version = "2.9.9"
     junit_version = "5.10.2"
diff --git a/samples/client/echo_api/java/restclient/pom.xml b/samples/client/echo_api/java/restclient/pom.xml
index b075100afbb7..d26d11ae5d07 100644
--- a/samples/client/echo_api/java/restclient/pom.xml
+++ b/samples/client/echo_api/java/restclient/pom.xml
@@ -274,7 +274,7 @@
     </dependencies>
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <spring-web-version>6.1.6</spring-web-version>
+        <spring-web-version>6.1.21</spring-web-version>
         <jackson-version>2.17.1</jackson-version>
         <jackson-databind-version>2.17.1</jackson-databind-version>
         <jackson-databind-nullable-version>0.2.6</jackson-databind-nullable-version>
diff --git a/samples/client/echo_api/java/restclient/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/echo_api/java/restclient/src/main/java/org/openapitools/client/ApiClient.java
index f5c9f7608999..a6a859e61b31 100644
--- a/samples/client/echo_api/java/restclient/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/echo_api/java/restclient/src/main/java/org/openapitools/client/ApiClient.java
@@ -312,10 +312,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
@@ -665,7 +662,7 @@ protected RestClient.RequestBodySpec prepareRequest(String path, HttpMethod meth
      * @param requestBuilder The current request
      */
     protected void addHeadersToRequest(HttpHeaders headers, RestClient.RequestBodySpec requestBuilder) {
-        for (Entry<String, List<String>> entry : headers.entrySet()) {
+        for (Entry<String, List<String>> entry : headers.headerSet()) {
             List<String> values = entry.getValue();
             for(String value : values) {
                 if (value != null) {
diff --git a/samples/client/echo_api/java/resttemplate/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/echo_api/java/resttemplate/src/main/java/org/openapitools/client/ApiClient.java
index 7fa7402b198d..98e2f8282f48 100644
--- a/samples/client/echo_api/java/resttemplate/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/echo_api/java/resttemplate/src/main/java/org/openapitools/client/ApiClient.java
@@ -290,10 +290,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
diff --git a/samples/client/others/java/restclient-enum-in-multipart/build.gradle b/samples/client/others/java/restclient-enum-in-multipart/build.gradle
index 35b17b76d8f6..c43492ecbb13 100644
--- a/samples/client/others/java/restclient-enum-in-multipart/build.gradle
+++ b/samples/client/others/java/restclient-enum-in-multipart/build.gradle
@@ -100,7 +100,7 @@ ext {
     jackson_version = "2.17.1"
     jackson_databind_version = "2.17.1"
     jackson_databind_nullable_version = "0.2.6"
-    spring_web_version = "6.1.6"
+    spring_web_version = "6.1.21"
     jakarta_annotation_version = "2.1.1"
     jodatime_version = "2.9.9"
     junit_version = "5.10.2"
diff --git a/samples/client/others/java/restclient-enum-in-multipart/pom.xml b/samples/client/others/java/restclient-enum-in-multipart/pom.xml
index 1e6d72e1eb05..223d5e775e25 100644
--- a/samples/client/others/java/restclient-enum-in-multipart/pom.xml
+++ b/samples/client/others/java/restclient-enum-in-multipart/pom.xml
@@ -274,7 +274,7 @@
     </dependencies>
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <spring-web-version>6.1.6</spring-web-version>
+        <spring-web-version>6.1.21</spring-web-version>
         <jackson-version>2.17.1</jackson-version>
         <jackson-databind-version>2.17.1</jackson-databind-version>
         <jackson-databind-nullable-version>0.2.6</jackson-databind-nullable-version>
diff --git a/samples/client/others/java/restclient-enum-in-multipart/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/others/java/restclient-enum-in-multipart/src/main/java/org/openapitools/client/ApiClient.java
index 7551ed6a2627..37363f787ca2 100644
--- a/samples/client/others/java/restclient-enum-in-multipart/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/others/java/restclient-enum-in-multipart/src/main/java/org/openapitools/client/ApiClient.java
@@ -311,10 +311,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
@@ -664,7 +661,7 @@ protected RestClient.RequestBodySpec prepareRequest(String path, HttpMethod meth
      * @param requestBuilder The current request
      */
     protected void addHeadersToRequest(HttpHeaders headers, RestClient.RequestBodySpec requestBuilder) {
-        for (Entry<String, List<String>> entry : headers.entrySet()) {
+        for (Entry<String, List<String>> entry : headers.headerSet()) {
             List<String> values = entry.getValue();
             for(String value : values) {
                 if (value != null) {
diff --git a/samples/client/others/java/restclient-useAbstractionForFiles/build.gradle b/samples/client/others/java/restclient-useAbstractionForFiles/build.gradle
index 89044977e9c2..041c588113e2 100644
--- a/samples/client/others/java/restclient-useAbstractionForFiles/build.gradle
+++ b/samples/client/others/java/restclient-useAbstractionForFiles/build.gradle
@@ -100,7 +100,7 @@ ext {
     jackson_version = "2.17.1"
     jackson_databind_version = "2.17.1"
     jackson_databind_nullable_version = "0.2.6"
-    spring_web_version = "6.1.6"
+    spring_web_version = "6.1.21"
     jakarta_annotation_version = "2.1.1"
     jodatime_version = "2.9.9"
     junit_version = "5.10.2"
diff --git a/samples/client/others/java/restclient-useAbstractionForFiles/pom.xml b/samples/client/others/java/restclient-useAbstractionForFiles/pom.xml
index 9d271da9bb3c..f9b5f1a8d679 100644
--- a/samples/client/others/java/restclient-useAbstractionForFiles/pom.xml
+++ b/samples/client/others/java/restclient-useAbstractionForFiles/pom.xml
@@ -274,7 +274,7 @@
     </dependencies>
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <spring-web-version>6.1.6</spring-web-version>
+        <spring-web-version>6.1.21</spring-web-version>
         <jackson-version>2.17.1</jackson-version>
         <jackson-databind-version>2.17.1</jackson-databind-version>
         <jackson-databind-nullable-version>0.2.6</jackson-databind-nullable-version>
diff --git a/samples/client/others/java/restclient-useAbstractionForFiles/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/others/java/restclient-useAbstractionForFiles/src/main/java/org/openapitools/client/ApiClient.java
index d8854fc4c5d1..789fcce382f7 100644
--- a/samples/client/others/java/restclient-useAbstractionForFiles/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/others/java/restclient-useAbstractionForFiles/src/main/java/org/openapitools/client/ApiClient.java
@@ -310,10 +310,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
@@ -663,7 +660,7 @@ protected RestClient.RequestBodySpec prepareRequest(String path, HttpMethod meth
      * @param requestBuilder The current request
      */
     protected void addHeadersToRequest(HttpHeaders headers, RestClient.RequestBodySpec requestBuilder) {
-        for (Entry<String, List<String>> entry : headers.entrySet()) {
+        for (Entry<String, List<String>> entry : headers.headerSet()) {
             List<String> values = entry.getValue();
             for(String value : values) {
                 if (value != null) {
diff --git a/samples/client/others/java/resttemplate-list-schema-validation/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/others/java/resttemplate-list-schema-validation/src/main/java/org/openapitools/client/ApiClient.java
index 529661682ae1..bef11d2220b2 100644
--- a/samples/client/others/java/resttemplate-list-schema-validation/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/others/java/resttemplate-list-schema-validation/src/main/java/org/openapitools/client/ApiClient.java
@@ -233,10 +233,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
diff --git a/samples/client/others/java/resttemplate-useAbstractionForFiles/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/others/java/resttemplate-useAbstractionForFiles/src/main/java/org/openapitools/client/ApiClient.java
index e9fa6599350d..08e58d65f18d 100644
--- a/samples/client/others/java/resttemplate-useAbstractionForFiles/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/others/java/resttemplate-useAbstractionForFiles/src/main/java/org/openapitools/client/ApiClient.java
@@ -233,10 +233,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
diff --git a/samples/client/petstore/java/restclient-nullable-arrays/build.gradle b/samples/client/petstore/java/restclient-nullable-arrays/build.gradle
index 4fc07a99b554..5390d126476d 100644
--- a/samples/client/petstore/java/restclient-nullable-arrays/build.gradle
+++ b/samples/client/petstore/java/restclient-nullable-arrays/build.gradle
@@ -100,7 +100,7 @@ ext {
     jackson_version = "2.17.1"
     jackson_databind_version = "2.17.1"
     jackson_databind_nullable_version = "0.2.6"
-    spring_web_version = "6.1.6"
+    spring_web_version = "6.1.21"
     jakarta_annotation_version = "2.1.1"
     jodatime_version = "2.9.9"
     junit_version = "5.10.2"
diff --git a/samples/client/petstore/java/restclient-nullable-arrays/pom.xml b/samples/client/petstore/java/restclient-nullable-arrays/pom.xml
index e3cbc88733eb..0a06567b5a2e 100644
--- a/samples/client/petstore/java/restclient-nullable-arrays/pom.xml
+++ b/samples/client/petstore/java/restclient-nullable-arrays/pom.xml
@@ -274,7 +274,7 @@
     </dependencies>
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <spring-web-version>6.1.6</spring-web-version>
+        <spring-web-version>6.1.21</spring-web-version>
         <jackson-version>2.17.1</jackson-version>
         <jackson-databind-version>2.17.1</jackson-databind-version>
         <jackson-databind-nullable-version>0.2.6</jackson-databind-nullable-version>
diff --git a/samples/client/petstore/java/restclient-nullable-arrays/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/restclient-nullable-arrays/src/main/java/org/openapitools/client/ApiClient.java
index 67e82c5369c8..eddb1c87bcb1 100644
--- a/samples/client/petstore/java/restclient-nullable-arrays/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/restclient-nullable-arrays/src/main/java/org/openapitools/client/ApiClient.java
@@ -310,10 +310,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
@@ -663,7 +660,7 @@ protected RestClient.RequestBodySpec prepareRequest(String path, HttpMethod meth
      * @param requestBuilder The current request
      */
     protected void addHeadersToRequest(HttpHeaders headers, RestClient.RequestBodySpec requestBuilder) {
-        for (Entry<String, List<String>> entry : headers.entrySet()) {
+        for (Entry<String, List<String>> entry : headers.headerSet()) {
             List<String> values = entry.getValue();
             for(String value : values) {
                 if (value != null) {
diff --git a/samples/client/petstore/java/restclient-swagger2/build.gradle b/samples/client/petstore/java/restclient-swagger2/build.gradle
index 620565aa93f1..9e04e393a93d 100644
--- a/samples/client/petstore/java/restclient-swagger2/build.gradle
+++ b/samples/client/petstore/java/restclient-swagger2/build.gradle
@@ -101,7 +101,7 @@ ext {
     jackson_version = "2.17.1"
     jackson_databind_version = "2.17.1"
     jackson_databind_nullable_version = "0.2.6"
-    spring_web_version = "6.1.6"
+    spring_web_version = "6.1.21"
     jakarta_annotation_version = "2.1.1"
     jodatime_version = "2.9.9"
     junit_version = "5.10.2"
diff --git a/samples/client/petstore/java/restclient-swagger2/pom.xml b/samples/client/petstore/java/restclient-swagger2/pom.xml
index 5ec46d119590..0290b96a03c5 100644
--- a/samples/client/petstore/java/restclient-swagger2/pom.xml
+++ b/samples/client/petstore/java/restclient-swagger2/pom.xml
@@ -280,7 +280,7 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <swagger-annotations-version>2.2.15</swagger-annotations-version>
-        <spring-web-version>6.1.6</spring-web-version>
+        <spring-web-version>6.1.21</spring-web-version>
         <jackson-version>2.17.1</jackson-version>
         <jackson-databind-version>2.17.1</jackson-databind-version>
         <jackson-databind-nullable-version>0.2.6</jackson-databind-nullable-version>
diff --git a/samples/client/petstore/java/restclient-swagger2/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/restclient-swagger2/src/main/java/org/openapitools/client/ApiClient.java
index e785d06609f0..66803f7358ff 100644
--- a/samples/client/petstore/java/restclient-swagger2/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/restclient-swagger2/src/main/java/org/openapitools/client/ApiClient.java
@@ -330,10 +330,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
@@ -683,7 +680,7 @@ protected RestClient.RequestBodySpec prepareRequest(String path, HttpMethod meth
      * @param requestBuilder The current request
      */
     protected void addHeadersToRequest(HttpHeaders headers, RestClient.RequestBodySpec requestBuilder) {
-        for (Entry<String, List<String>> entry : headers.entrySet()) {
+        for (Entry<String, List<String>> entry : headers.headerSet()) {
             List<String> values = entry.getValue();
             for(String value : values) {
                 if (value != null) {
diff --git a/samples/client/petstore/java/restclient-useSingleRequestParameter-static/build.gradle b/samples/client/petstore/java/restclient-useSingleRequestParameter-static/build.gradle
index 7bca4d48a8db..746855acc1a4 100644
--- a/samples/client/petstore/java/restclient-useSingleRequestParameter-static/build.gradle
+++ b/samples/client/petstore/java/restclient-useSingleRequestParameter-static/build.gradle
@@ -100,7 +100,7 @@ ext {
     jackson_version = "2.17.1"
     jackson_databind_version = "2.17.1"
     jackson_databind_nullable_version = "0.2.6"
-    spring_web_version = "6.1.6"
+    spring_web_version = "6.1.21"
     jakarta_annotation_version = "2.1.1"
     jodatime_version = "2.9.9"
     junit_version = "5.10.2"
diff --git a/samples/client/petstore/java/restclient-useSingleRequestParameter-static/pom.xml b/samples/client/petstore/java/restclient-useSingleRequestParameter-static/pom.xml
index 04bfd8849d6b..bbdda1589158 100644
--- a/samples/client/petstore/java/restclient-useSingleRequestParameter-static/pom.xml
+++ b/samples/client/petstore/java/restclient-useSingleRequestParameter-static/pom.xml
@@ -274,7 +274,7 @@
     </dependencies>
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <spring-web-version>6.1.6</spring-web-version>
+        <spring-web-version>6.1.21</spring-web-version>
         <jackson-version>2.17.1</jackson-version>
         <jackson-databind-version>2.17.1</jackson-databind-version>
         <jackson-databind-nullable-version>0.2.6</jackson-databind-nullable-version>
diff --git a/samples/client/petstore/java/restclient-useSingleRequestParameter-static/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/restclient-useSingleRequestParameter-static/src/main/java/org/openapitools/client/ApiClient.java
index e785d06609f0..66803f7358ff 100644
--- a/samples/client/petstore/java/restclient-useSingleRequestParameter-static/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/restclient-useSingleRequestParameter-static/src/main/java/org/openapitools/client/ApiClient.java
@@ -330,10 +330,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
@@ -683,7 +680,7 @@ protected RestClient.RequestBodySpec prepareRequest(String path, HttpMethod meth
      * @param requestBuilder The current request
      */
     protected void addHeadersToRequest(HttpHeaders headers, RestClient.RequestBodySpec requestBuilder) {
-        for (Entry<String, List<String>> entry : headers.entrySet()) {
+        for (Entry<String, List<String>> entry : headers.headerSet()) {
             List<String> values = entry.getValue();
             for(String value : values) {
                 if (value != null) {
diff --git a/samples/client/petstore/java/restclient-useSingleRequestParameter/build.gradle b/samples/client/petstore/java/restclient-useSingleRequestParameter/build.gradle
index 15075161d76e..8a9945a9a629 100644
--- a/samples/client/petstore/java/restclient-useSingleRequestParameter/build.gradle
+++ b/samples/client/petstore/java/restclient-useSingleRequestParameter/build.gradle
@@ -100,7 +100,7 @@ ext {
     jackson_version = "2.17.1"
     jackson_databind_version = "2.17.1"
     jackson_databind_nullable_version = "0.2.6"
-    spring_web_version = "6.1.6"
+    spring_web_version = "6.1.21"
     jakarta_annotation_version = "2.1.1"
     jodatime_version = "2.9.9"
     junit_version = "5.10.2"
diff --git a/samples/client/petstore/java/restclient-useSingleRequestParameter/pom.xml b/samples/client/petstore/java/restclient-useSingleRequestParameter/pom.xml
index 05566ac5820e..5949c228b99d 100644
--- a/samples/client/petstore/java/restclient-useSingleRequestParameter/pom.xml
+++ b/samples/client/petstore/java/restclient-useSingleRequestParameter/pom.xml
@@ -274,7 +274,7 @@
     </dependencies>
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <spring-web-version>6.1.6</spring-web-version>
+        <spring-web-version>6.1.21</spring-web-version>
         <jackson-version>2.17.1</jackson-version>
         <jackson-databind-version>2.17.1</jackson-databind-version>
         <jackson-databind-nullable-version>0.2.6</jackson-databind-nullable-version>
diff --git a/samples/client/petstore/java/restclient-useSingleRequestParameter/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/restclient-useSingleRequestParameter/src/main/java/org/openapitools/client/ApiClient.java
index e785d06609f0..66803f7358ff 100644
--- a/samples/client/petstore/java/restclient-useSingleRequestParameter/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/restclient-useSingleRequestParameter/src/main/java/org/openapitools/client/ApiClient.java
@@ -330,10 +330,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
@@ -683,7 +680,7 @@ protected RestClient.RequestBodySpec prepareRequest(String path, HttpMethod meth
      * @param requestBuilder The current request
      */
     protected void addHeadersToRequest(HttpHeaders headers, RestClient.RequestBodySpec requestBuilder) {
-        for (Entry<String, List<String>> entry : headers.entrySet()) {
+        for (Entry<String, List<String>> entry : headers.headerSet()) {
             List<String> values = entry.getValue();
             for(String value : values) {
                 if (value != null) {
diff --git a/samples/client/petstore/java/restclient/build.gradle b/samples/client/petstore/java/restclient/build.gradle
index b0abe35f7c28..acd8556e73b7 100644
--- a/samples/client/petstore/java/restclient/build.gradle
+++ b/samples/client/petstore/java/restclient/build.gradle
@@ -100,7 +100,7 @@ ext {
     jackson_version = "2.17.1"
     jackson_databind_version = "2.17.1"
     jackson_databind_nullable_version = "0.2.6"
-    spring_web_version = "6.1.6"
+    spring_web_version = "6.1.21"
     jakarta_annotation_version = "2.1.1"
     jodatime_version = "2.9.9"
     junit_version = "5.10.2"
diff --git a/samples/client/petstore/java/restclient/pom.xml b/samples/client/petstore/java/restclient/pom.xml
index 9d5369c46fb4..9f8889ec1443 100644
--- a/samples/client/petstore/java/restclient/pom.xml
+++ b/samples/client/petstore/java/restclient/pom.xml
@@ -274,7 +274,7 @@
     </dependencies>
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <spring-web-version>6.1.6</spring-web-version>
+        <spring-web-version>6.1.21</spring-web-version>
         <jackson-version>2.17.1</jackson-version>
         <jackson-databind-version>2.17.1</jackson-databind-version>
         <jackson-databind-nullable-version>0.2.6</jackson-databind-nullable-version>
diff --git a/samples/client/petstore/java/restclient/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/restclient/src/main/java/org/openapitools/client/ApiClient.java
index e785d06609f0..66803f7358ff 100644
--- a/samples/client/petstore/java/restclient/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/restclient/src/main/java/org/openapitools/client/ApiClient.java
@@ -330,10 +330,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
@@ -683,7 +680,7 @@ protected RestClient.RequestBodySpec prepareRequest(String path, HttpMethod meth
      * @param requestBuilder The current request
      */
     protected void addHeadersToRequest(HttpHeaders headers, RestClient.RequestBodySpec requestBuilder) {
-        for (Entry<String, List<String>> entry : headers.entrySet()) {
+        for (Entry<String, List<String>> entry : headers.headerSet()) {
             List<String> values = entry.getValue();
             for(String value : values) {
                 if (value != null) {
diff --git a/samples/client/petstore/java/resttemplate-jakarta/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/resttemplate-jakarta/src/main/java/org/openapitools/client/ApiClient.java
index 2e825efb8f68..7ac20122dfb4 100644
--- a/samples/client/petstore/java/resttemplate-jakarta/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/resttemplate-jakarta/src/main/java/org/openapitools/client/ApiClient.java
@@ -291,10 +291,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
@@ -742,7 +739,7 @@ public <T> ResponseEntity<T> invokeAPI(String path, HttpMethod method, Map<Strin
      * @param requestBuilder The current request
      */
     protected void addHeadersToRequest(HttpHeaders headers, BodyBuilder requestBuilder) {
-        for (Entry<String, List<String>> entry : headers.entrySet()) {
+        for (Entry<String, List<String>> entry : headers.headerSet()) {
             List<String> values = entry.getValue();
             for (String value : values) {
                 if (value != null) {
@@ -845,7 +842,7 @@ protected String headersToString(HttpHeaders headers) {
                 return "";
             }
             StringBuilder builder = new StringBuilder();
-            for (Entry<String, List<String>> entry : headers.entrySet()) {
+            for (Entry<String, List<String>> entry : headers.headerSet()) {
                 builder.append(entry.getKey()).append("=[");
                 for (String value : entry.getValue()) {
                     builder.append(value).append(",");
diff --git a/samples/client/petstore/java/resttemplate-swagger1/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/resttemplate-swagger1/src/main/java/org/openapitools/client/ApiClient.java
index 73b70a2c7950..58ea23dc114c 100644
--- a/samples/client/petstore/java/resttemplate-swagger1/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/resttemplate-swagger1/src/main/java/org/openapitools/client/ApiClient.java
@@ -291,10 +291,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
diff --git a/samples/client/petstore/java/resttemplate-swagger2/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/resttemplate-swagger2/src/main/java/org/openapitools/client/ApiClient.java
index 73b70a2c7950..58ea23dc114c 100644
--- a/samples/client/petstore/java/resttemplate-swagger2/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/resttemplate-swagger2/src/main/java/org/openapitools/client/ApiClient.java
@@ -291,10 +291,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
diff --git a/samples/client/petstore/java/resttemplate-withXml/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/resttemplate-withXml/src/main/java/org/openapitools/client/ApiClient.java
index e97f7ad6449b..5cbefb77c42c 100644
--- a/samples/client/petstore/java/resttemplate-withXml/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/resttemplate-withXml/src/main/java/org/openapitools/client/ApiClient.java
@@ -354,10 +354,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }
 
diff --git a/samples/client/petstore/java/resttemplate/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/resttemplate/src/main/java/org/openapitools/client/ApiClient.java
index c8a2498a4e1a..c07a058a0a19 100644
--- a/samples/client/petstore/java/resttemplate/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/resttemplate/src/main/java/org/openapitools/client/ApiClient.java
@@ -349,10 +349,7 @@ public ApiClient setUserAgent(String userAgent) {
      * @return ApiClient this client
      */
     public ApiClient addDefaultHeader(String name, String value) {
-        if (defaultHeaders.containsKey(name)) {
-            defaultHeaders.remove(name);
-        }
-        defaultHeaders.add(name, value);
+        defaultHeaders.set(name, value);
         return this;
     }