diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/native/api.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/native/api.mustache
index bc9a8833c12d..e9e12ce7bd2b 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/native/api.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/native/api.mustache
@@ -65,7 +65,7 @@ import java.util.concurrent.CompletableFuture;
 {{>generatedAnnotation}}
 
 {{#operations}}
-public class {{classname}} {
+public class {{classname}} implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -955,5 +955,20 @@ public class {{classname}} {
   {{/hasParams}}
   {{/vendorExtensions.x-group-parameters}}
   {{/operation}}
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
 {{/operations}}
\ No newline at end of file
diff --git a/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/AuthApi.java b/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/AuthApi.java
index bd330607f637..d995acd9923d 100644
--- a/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/AuthApi.java
+++ b/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/AuthApi.java
@@ -45,7 +45,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class AuthApi {
+public class AuthApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -367,4 +367,19 @@ private HttpRequest.Builder testAuthHttpBearerRequestBuilder(Map<String, String>
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/BodyApi.java b/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/BodyApi.java
index 4b3a88d46280..03c141a2a6d2 100644
--- a/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/BodyApi.java
+++ b/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/BodyApi.java
@@ -55,7 +55,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class BodyApi {
+public class BodyApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -1376,4 +1376,19 @@ private HttpRequest.Builder testEchoBodyTagResponseStringRequestBuilder(@javax.a
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/FormApi.java b/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/FormApi.java
index 29b40e5c6c7a..abe687cf3ca8 100644
--- a/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/FormApi.java
+++ b/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/FormApi.java
@@ -52,7 +52,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class FormApi {
+public class FormApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -607,4 +607,19 @@ private HttpRequest.Builder testFormOneofRequestBuilder(@javax.annotation.Nullab
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/HeaderApi.java b/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/HeaderApi.java
index 84bf5bc73ba3..2903d9a56877 100644
--- a/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/HeaderApi.java
+++ b/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/HeaderApi.java
@@ -52,7 +52,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class HeaderApi {
+public class HeaderApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -306,4 +306,19 @@ private HttpRequest.Builder testHeaderIntegerBooleanStringEnumsRequestBuilder(@j
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/PathApi.java b/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/PathApi.java
index bad5e1e95325..8e00c4c9504d 100644
--- a/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/PathApi.java
+++ b/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/PathApi.java
@@ -52,7 +52,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class PathApi {
+public class PathApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -307,4 +307,19 @@ private HttpRequest.Builder testsPathStringPathStringIntegerPathIntegerEnumNonre
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/QueryApi.java b/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/QueryApi.java
index e35b535834e9..401053070fbc 100644
--- a/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/QueryApi.java
+++ b/samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/QueryApi.java
@@ -58,7 +58,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class QueryApi {
+public class QueryApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -1567,4 +1567,19 @@ private HttpRequest.Builder testQueryStyleJsonSerializationObjectRequestBuilder(
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/AnotherFakeApi.java b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/AnotherFakeApi.java
index f0e67fd8b910..46d4445fa757 100644
--- a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/AnotherFakeApi.java
+++ b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/AnotherFakeApi.java
@@ -48,7 +48,7 @@
 import java.util.concurrent.CompletableFuture;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class AnotherFakeApi {
+public class AnotherFakeApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -297,4 +297,19 @@ private HttpRequest.Builder call123testSpecialTagsRequestBuilder(@javax.annotati
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/DefaultApi.java b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/DefaultApi.java
index cc59ce24aca9..7299f841b693 100644
--- a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/DefaultApi.java
+++ b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/DefaultApi.java
@@ -48,7 +48,7 @@
 import java.util.concurrent.CompletableFuture;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class DefaultApi {
+public class DefaultApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -283,4 +283,19 @@ private HttpRequest.Builder fooGetRequestBuilder(Map<String, String> headers) th
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/FakeApi.java b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/FakeApi.java
index 1362de4f52db..bb29de4e2bc9 100644
--- a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/FakeApi.java
+++ b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/FakeApi.java
@@ -65,7 +65,7 @@
 import java.util.concurrent.CompletableFuture;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class FakeApi {
+public class FakeApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -3051,4 +3051,19 @@ private HttpRequest.Builder testStringMapReferenceRequestBuilder(@javax.annotati
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/FakeClassnameTags123Api.java b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/FakeClassnameTags123Api.java
index 738f9fb75aa0..b19af910281f 100644
--- a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/FakeClassnameTags123Api.java
+++ b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/FakeClassnameTags123Api.java
@@ -54,7 +54,7 @@
 import java.util.concurrent.CompletableFuture;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class FakeClassnameTags123Api {
+public class FakeClassnameTags123Api implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -303,4 +303,19 @@ private HttpRequest.Builder testClassnameRequestBuilder(@javax.annotation.Nonnul
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/PetApi.java b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/PetApi.java
index 2069a0e69fee..7fd046a0c7c3 100644
--- a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/PetApi.java
+++ b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/PetApi.java
@@ -56,7 +56,7 @@
 import java.util.concurrent.CompletableFuture;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class PetApi {
+public class PetApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -1429,4 +1429,19 @@ private HttpRequest.Builder uploadFileWithRequiredFileRequestBuilder(@javax.anno
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/StoreApi.java b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/StoreApi.java
index 3ebe91accd79..a33900a1f7dc 100644
--- a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/StoreApi.java
+++ b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/StoreApi.java
@@ -54,7 +54,7 @@
 import java.util.concurrent.CompletableFuture;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class StoreApi {
+public class StoreApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -660,4 +660,19 @@ private HttpRequest.Builder placeOrderRequestBuilder(@javax.annotation.Nonnull O
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/UserApi.java b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/UserApi.java
index 701e2d166b43..bcc60d50fe12 100644
--- a/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/UserApi.java
+++ b/samples/client/petstore/java/native-async/src/main/java/org/openapitools/client/api/UserApi.java
@@ -55,7 +55,7 @@
 import java.util.concurrent.CompletableFuture;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class UserApi {
+public class UserApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -1142,4 +1142,19 @@ private HttpRequest.Builder updateUserRequestBuilder(@javax.annotation.Nonnull S
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/FileApi.java b/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/FileApi.java
index 15e1d2bf65a1..38a893bf0898 100644
--- a/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/FileApi.java
+++ b/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/FileApi.java
@@ -47,7 +47,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class FileApi {
+public class FileApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -281,4 +281,19 @@ private HttpRequest.Builder fileIdGetRequestBuilder(String id, Map<String, Strin
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/FooApi.java b/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/FooApi.java
index cc8ff7d3a886..273b3a571025 100644
--- a/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/FooApi.java
+++ b/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/FooApi.java
@@ -49,7 +49,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class FooApi {
+public class FooApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -308,4 +308,19 @@ private HttpRequest.Builder fooDtParamGetRequestBuilder(java.time.@Nullable Inst
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/RequiredAndNullableApi.java b/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/RequiredAndNullableApi.java
index 0209382a5611..8abae28350ec 100644
--- a/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/RequiredAndNullableApi.java
+++ b/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/RequiredAndNullableApi.java
@@ -47,7 +47,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class RequiredAndNullableApi {
+public class RequiredAndNullableApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -286,4 +286,19 @@ private HttpRequest.Builder requiredAndNullablePostRequestBuilder(RequiredAndNul
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/UploadApi.java b/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/UploadApi.java
index 71362edfe174..5a8aaff1a26e 100644
--- a/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/UploadApi.java
+++ b/samples/client/petstore/java/native-jackson3-jspecify/src/main/java/org/openapitools/client/api/UploadApi.java
@@ -54,7 +54,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class UploadApi {
+public class UploadApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -438,4 +438,19 @@ private HttpRequest.Builder uploadPostRequestBuilder(@Nullable File _file, Map<S
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/AnotherFakeApi.java b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/AnotherFakeApi.java
index 5ed1f4e3cfde..a782d4bef1ee 100644
--- a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/AnotherFakeApi.java
+++ b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/AnotherFakeApi.java
@@ -47,7 +47,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class AnotherFakeApi {
+public class AnotherFakeApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -286,4 +286,19 @@ private HttpRequest.Builder call123testSpecialTagsRequestBuilder(@javax.annotati
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/DefaultApi.java b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/DefaultApi.java
index f6e4d43894ab..94c4defadd24 100644
--- a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/DefaultApi.java
+++ b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/DefaultApi.java
@@ -47,7 +47,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class DefaultApi {
+public class DefaultApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -272,4 +272,19 @@ private HttpRequest.Builder fooGetRequestBuilder(Map<String, String> headers) th
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/FakeApi.java b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/FakeApi.java
index 616320a9bb0a..42a451821a44 100644
--- a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/FakeApi.java
+++ b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/FakeApi.java
@@ -64,7 +64,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class FakeApi {
+public class FakeApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -2878,4 +2878,19 @@ private HttpRequest.Builder testStringMapReferenceRequestBuilder(@javax.annotati
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/FakeClassnameTags123Api.java b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/FakeClassnameTags123Api.java
index edf8566ddaf3..d2f3a5c3e2a1 100644
--- a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/FakeClassnameTags123Api.java
+++ b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/FakeClassnameTags123Api.java
@@ -53,7 +53,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class FakeClassnameTags123Api {
+public class FakeClassnameTags123Api implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -292,4 +292,19 @@ private HttpRequest.Builder testClassnameRequestBuilder(@javax.annotation.Nonnul
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/PetApi.java b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/PetApi.java
index a6a912dba95e..55b42525c3f4 100644
--- a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/PetApi.java
+++ b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/PetApi.java
@@ -55,7 +55,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class PetApi {
+public class PetApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -1350,4 +1350,19 @@ private HttpRequest.Builder uploadFileWithRequiredFileRequestBuilder(@javax.anno
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/StoreApi.java b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/StoreApi.java
index ae9774007059..d972395f349e 100644
--- a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/StoreApi.java
+++ b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/StoreApi.java
@@ -53,7 +53,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class StoreApi {
+public class StoreApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -623,4 +623,19 @@ private HttpRequest.Builder placeOrderRequestBuilder(@javax.annotation.Nonnull O
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/UserApi.java b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/UserApi.java
index 4661cd9bce3c..9eb44fd91c9e 100644
--- a/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/UserApi.java
+++ b/samples/client/petstore/java/native-jackson3/src/main/java/org/openapitools/client/api/UserApi.java
@@ -54,7 +54,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class UserApi {
+public class UserApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -1074,4 +1074,19 @@ private HttpRequest.Builder updateUserRequestBuilder(@javax.annotation.Nonnull S
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jakarta/src/main/java/org/openapitools/client/api/PetApi.java b/samples/client/petstore/java/native-jakarta/src/main/java/org/openapitools/client/api/PetApi.java
index f0f859337a3d..6bf82622c7fe 100644
--- a/samples/client/petstore/java/native-jakarta/src/main/java/org/openapitools/client/api/PetApi.java
+++ b/samples/client/petstore/java/native-jakarta/src/main/java/org/openapitools/client/api/PetApi.java
@@ -54,7 +54,7 @@
 import java.util.function.Consumer;
 
 @jakarta.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class PetApi {
+public class PetApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -1218,4 +1218,19 @@ private HttpRequest.Builder uploadFileRequestBuilder(@jakarta.annotation.Nonnull
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jakarta/src/main/java/org/openapitools/client/api/StoreApi.java b/samples/client/petstore/java/native-jakarta/src/main/java/org/openapitools/client/api/StoreApi.java
index 6bad55659cc9..9470d0da7652 100644
--- a/samples/client/petstore/java/native-jakarta/src/main/java/org/openapitools/client/api/StoreApi.java
+++ b/samples/client/petstore/java/native-jakarta/src/main/java/org/openapitools/client/api/StoreApi.java
@@ -52,7 +52,7 @@
 import java.util.function.Consumer;
 
 @jakarta.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class StoreApi {
+public class StoreApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -622,4 +622,19 @@ private HttpRequest.Builder placeOrderRequestBuilder(@jakarta.annotation.Nonnull
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-jakarta/src/main/java/org/openapitools/client/api/UserApi.java b/samples/client/petstore/java/native-jakarta/src/main/java/org/openapitools/client/api/UserApi.java
index 6ca1975fb1d3..9b63c5fe522c 100644
--- a/samples/client/petstore/java/native-jakarta/src/main/java/org/openapitools/client/api/UserApi.java
+++ b/samples/client/petstore/java/native-jakarta/src/main/java/org/openapitools/client/api/UserApi.java
@@ -53,7 +53,7 @@
 import java.util.function.Consumer;
 
 @jakarta.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class UserApi {
+public class UserApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -1073,4 +1073,19 @@ private HttpRequest.Builder updateUserRequestBuilder(@jakarta.annotation.Nonnull
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native-useGzipFeature/src/main/java/org/openapitools/client/api/PetApi.java b/samples/client/petstore/java/native-useGzipFeature/src/main/java/org/openapitools/client/api/PetApi.java
index 7bb13aefb65e..fef3e5fdf84c 100644
--- a/samples/client/petstore/java/native-useGzipFeature/src/main/java/org/openapitools/client/api/PetApi.java
+++ b/samples/client/petstore/java/native-useGzipFeature/src/main/java/org/openapitools/client/api/PetApi.java
@@ -47,7 +47,7 @@
 import java.util.function.Supplier;
 
 @jakarta.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class PetApi {
+public class PetApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -289,4 +289,19 @@ private HttpRequest.Builder addPetRequestBuilder(@jakarta.annotation.Nonnull Pet
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/AnotherFakeApi.java b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/AnotherFakeApi.java
index c4a50c649bf7..4b78f076304d 100644
--- a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/AnotherFakeApi.java
+++ b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/AnotherFakeApi.java
@@ -46,7 +46,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class AnotherFakeApi {
+public class AnotherFakeApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -285,4 +285,19 @@ private HttpRequest.Builder call123testSpecialTagsRequestBuilder(@javax.annotati
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/DefaultApi.java b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/DefaultApi.java
index c992098e1f09..1f0d27c7c054 100644
--- a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/DefaultApi.java
+++ b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/DefaultApi.java
@@ -46,7 +46,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class DefaultApi {
+public class DefaultApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -271,4 +271,19 @@ private HttpRequest.Builder fooGetRequestBuilder(Map<String, String> headers) th
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/FakeApi.java b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/FakeApi.java
index 1e62b57aaaf7..6465d568c4bb 100644
--- a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/FakeApi.java
+++ b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/FakeApi.java
@@ -63,7 +63,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class FakeApi {
+public class FakeApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -2877,4 +2877,19 @@ private HttpRequest.Builder testStringMapReferenceRequestBuilder(@javax.annotati
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/FakeClassnameTags123Api.java b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/FakeClassnameTags123Api.java
index f8d7893fc79c..34de6aac1dbb 100644
--- a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/FakeClassnameTags123Api.java
+++ b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/FakeClassnameTags123Api.java
@@ -52,7 +52,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class FakeClassnameTags123Api {
+public class FakeClassnameTags123Api implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -291,4 +291,19 @@ private HttpRequest.Builder testClassnameRequestBuilder(@javax.annotation.Nonnul
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/PetApi.java b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/PetApi.java
index 315e3cf68a95..2471bc42da95 100644
--- a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/PetApi.java
+++ b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/PetApi.java
@@ -54,7 +54,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class PetApi {
+public class PetApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -1349,4 +1349,19 @@ private HttpRequest.Builder uploadFileWithRequiredFileRequestBuilder(@javax.anno
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/StoreApi.java b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/StoreApi.java
index ec93ee23484a..01eadd8e3407 100644
--- a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/StoreApi.java
+++ b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/StoreApi.java
@@ -52,7 +52,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class StoreApi {
+public class StoreApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -622,4 +622,19 @@ private HttpRequest.Builder placeOrderRequestBuilder(@javax.annotation.Nonnull O
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }
diff --git a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/UserApi.java b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/UserApi.java
index 63704487316a..d68182bb7c93 100644
--- a/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/UserApi.java
+++ b/samples/client/petstore/java/native/src/main/java/org/openapitools/client/api/UserApi.java
@@ -53,7 +53,7 @@
 import java.util.function.Consumer;
 
 @javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.26.0-SNAPSHOT")
-public class UserApi {
+public class UserApi implements AutoCloseable {
   /**
    * Utility class for extending HttpRequest.Builder functionality.
    */
@@ -1073,4 +1073,19 @@ private HttpRequest.Builder updateUserRequestBuilder(@javax.annotation.Nonnull S
     return localVarRequestBuilder;
   }
 
+  /**
+   * Closes the {@link HttpClient} that this instance created from the {@link ApiClient}
+   * it was constructed with. {@code ApiClient.getHttpClient()} builds a new client on
+   * every call, so the client held here is owned exclusively by this instance and is not
+   * shared with any other API instance.
+   *
+   * <p>{@link HttpClient} only became {@link AutoCloseable} in Java 21, hence the guard;
+   * on earlier runtimes there is no way to release the client's threads early.
+   */
+  @Override
+  public void close() throws Exception {
+    if (memberVarHttpClient instanceof AutoCloseable) {
+      ((AutoCloseable) memberVarHttpClient).close();
+    }
+  }
 }