Use static INSTANCE singleton for stateless HttpServerResponseMutators#18326
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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.
laurit
reviewed
Apr 27, 2026
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| public class GrizzlyHttpResponseMutator implements HttpServerResponseMutator<HttpResponsePacket> { | ||
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| public static final GrizzlyHttpResponseMutator INSTANCE = new GrizzlyHttpResponseMutator(); |
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Could consider using the enum singleton as it is more compact, a bunch of HttpServerResponseMutators already use it
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yeah, makes sense
it would be nice to have a consistent story for when / whether we use enum singleton pattern
I did a quick check and looks like we have pretty even split of enum singleton INSTANCE to class singleton INSTANCE.
I'm good with any range of never to always or something in the middle
maybe just embrace them? (except maybe for public library classes in order to preserve flexibility)
laurit
approved these changes
Apr 27, 2026
# Conflicts: # instrumentation/servlet/servlet-3.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/servlet/v3_0/Servlet3HttpServerResponseMutator.java
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Since they're currently being unnecessarily allocated on the hotpath.