Use static INSTANCE singleton for stateless HttpServerResponseMutators

Adds 'static final INSTANCE' field and private constructor to the stateless final-class HttpServerResponseMutator implementations (Grizzly, Helidon, Jetty 8/11/12, Servlet 3) and updates their call sites to use INSTANCE instead of 'new XyzMutator()'. These mutators are invoked from per-response advice / filters / handlers, matching the hot-path exception in general-rules.md ('Prefer Instance Creation Over Singletons'). This also brings them in line with the existing enum-singleton mutators (Armeria, JavaHttpServer, Netty 3.8/4.0/4.1, Tomcat 10, Restlet 1/2, Undertow) and the static-INSTANCE mutators (Tomcat 7, Servlet 2.2). Akka and Pekko mutators are intentionally left unchanged because they hold per-request header state.

Author: trask

instrumentation/grizzly-2.3/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/grizzly/v2_3/GrizzlyHttpResponseMutator.java

@@ -12,6 +12,10 @@
 
 public class GrizzlyHttpResponseMutator implements HttpServerResponseMutator<HttpResponsePacket> {
 
+  public static final GrizzlyHttpResponseMutator INSTANCE = new GrizzlyHttpResponseMutator();
+
+  private GrizzlyHttpResponseMutator() {}
+
   @Override
   public void appendHeader(HttpResponsePacket response, String name, String value) {
     MimeHeaders headers = response.getHeaders();

instrumentation/grizzly-2.3/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/grizzly/v2_3/HttpServerFilterInstrumentation.java

@@ -50,7 +50,7 @@ public static void onEnter(
         @Advice.Argument(2) HttpResponsePacket response) {
       Context context = GrizzlyStateStorage.getContext(ctx);
       HttpServerResponseCustomizerHolder.getCustomizer()
-          .customize(context, response, new GrizzlyHttpResponseMutator());
+          .customize(context, response, GrizzlyHttpResponseMutator.INSTANCE);
     }
 
     @Advice.OnMethodExit(onThrowable = Throwable.class, suppress = Throwable.class, inline = false)

instrumentation/helidon-4.3/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/helidon/v4_3/HelidonServerResponseMutator.java

@@ -10,6 +10,10 @@
 
 final class HelidonServerResponseMutator implements HttpServerResponseMutator<ServerResponse> {
 
+  static final HelidonServerResponseMutator INSTANCE = new HelidonServerResponseMutator();
+
+  private HelidonServerResponseMutator() {}
+
   @Override
   public void appendHeader(ServerResponse res, String name, String value) {
     res.header(name, value);

instrumentation/helidon-4.3/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/helidon/v4_3/ResponseCustomizingFilter.java

@@ -21,7 +21,7 @@ public void filter(FilterChain chain, RoutingRequest req, RoutingResponse res) {
 
     var context = Context.current();
     HttpServerResponseCustomizerHolder.getCustomizer()
-        .customize(context, res, new HelidonServerResponseMutator());
+        .customize(context, res, HelidonServerResponseMutator.INSTANCE);
     chain.proceed();
   }
 }

instrumentation/jetty/jetty-11.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/jetty/v11_0/Jetty11InstrumentationModule.java

@@ -77,7 +77,7 @@ public static AdviceScope start(HttpServletRequest request, HttpServletResponse
         // Must be set here since Jetty handlers can use startAsync outside of servlet scope.
         helper().setAsyncListenerResponse(context, response);
         HttpServerResponseCustomizerHolder.getCustomizer()
-            .customize(context, response, new Jetty11ResponseMutator());
+            .customize(context, response, Jetty11ResponseMutator.INSTANCE);
         return new AdviceScope(requestContext, context, scope);
       }
 

instrumentation/jetty/jetty-11.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/jetty/v11_0/Jetty11ResponseMutator.java

@@ -10,6 +10,10 @@
 
 final class Jetty11ResponseMutator implements HttpServerResponseMutator<HttpServletResponse> {
 
+  static final Jetty11ResponseMutator INSTANCE = new Jetty11ResponseMutator();
+
+  private Jetty11ResponseMutator() {}
+
   @Override
   public void appendHeader(HttpServletResponse response, String name, String value) {
     response.addHeader(name, value);

instrumentation/jetty/jetty-12.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/jetty/v12_0/Jetty12ResponseMutator.java

@@ -10,6 +10,10 @@
 
 public class Jetty12ResponseMutator implements HttpServerResponseMutator<Response> {
 
+  public static final Jetty12ResponseMutator INSTANCE = new Jetty12ResponseMutator();
+
+  private Jetty12ResponseMutator() {}
+
   @Override
   public void appendHeader(Response response, String name, String value) {
     response.getHeaders().add(name, value);

instrumentation/jetty/jetty-12.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/jetty/v12_0/Jetty12ServerInstrumentation.java

@@ -61,7 +61,7 @@ public static AdviceScope start(Request request, Response response) {
         Context context = helper().start(parentContext, request, response);
         Scope scope = context.makeCurrent();
         HttpServerResponseCustomizerHolder.getCustomizer()
-            .customize(context, response, new Jetty12ResponseMutator());
+            .customize(context, response, Jetty12ResponseMutator.INSTANCE);
         return new AdviceScope(context, scope);
       }
 

instrumentation/jetty/jetty-8.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/jetty/v8_0/Jetty8InstrumentationModule.java

@@ -78,7 +78,7 @@ public static AdviceScope start(HttpServletRequest request, HttpServletResponse
         // Must be set here since Jetty handlers can use startAsync outside of servlet scope.
         helper().setAsyncListenerResponse(context, response);
         HttpServerResponseCustomizerHolder.getCustomizer()
-            .customize(context, response, new Jetty8ResponseMutator());
+            .customize(context, response, Jetty8ResponseMutator.INSTANCE);
         return new AdviceScope(requestContext, context, scope);
       }
 

instrumentation/jetty/jetty-8.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/jetty/v8_0/Jetty8ResponseMutator.java

@@ -9,6 +9,11 @@
 import javax.servlet.http.HttpServletResponse;
 
 final class Jetty8ResponseMutator implements HttpServerResponseMutator<HttpServletResponse> {
+
+  static final Jetty8ResponseMutator INSTANCE = new Jetty8ResponseMutator();
+
+  private Jetty8ResponseMutator() {}
+
   @Override
   public void appendHeader(HttpServletResponse response, String name, String value) {
     response.addHeader(name, value);