enum singletons

Author: trask

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

@@ -10,11 +10,8 @@
 import org.glassfish.grizzly.http.util.DataChunk;
 import org.glassfish.grizzly.http.util.MimeHeaders;
 
-public class GrizzlyHttpResponseMutator implements HttpServerResponseMutator<HttpResponsePacket> {
-
-  public static final GrizzlyHttpResponseMutator INSTANCE = new GrizzlyHttpResponseMutator();
-
-  private GrizzlyHttpResponseMutator() {}
+public enum GrizzlyHttpResponseMutator implements HttpServerResponseMutator<HttpResponsePacket> {
+  INSTANCE;
 
   @Override
   public void appendHeader(HttpResponsePacket response, String name, String value) {

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

@@ -8,11 +8,8 @@
 import io.opentelemetry.javaagent.bootstrap.http.HttpServerResponseMutator;
 import org.eclipse.jetty.server.Response;
 
-class Jetty12ResponseMutator implements HttpServerResponseMutator<Response> {
-
-  public static final Jetty12ResponseMutator INSTANCE = new Jetty12ResponseMutator();
-
-  private Jetty12ResponseMutator() {}
+enum Jetty12ResponseMutator implements HttpServerResponseMutator<Response> {
+  INSTANCE;
 
   @Override
   public void appendHeader(Response response, String name, String value) {

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

@@ -8,11 +8,8 @@
 import io.opentelemetry.javaagent.bootstrap.http.HttpServerResponseMutator;
 import javax.servlet.http.HttpServletResponse;
 
-final class Jetty8ResponseMutator implements HttpServerResponseMutator<HttpServletResponse> {
-
-  static final Jetty8ResponseMutator INSTANCE = new Jetty8ResponseMutator();
-
-  private Jetty8ResponseMutator() {}
+enum Jetty8ResponseMutator implements HttpServerResponseMutator<HttpServletResponse> {
+  INSTANCE;
 
   @Override
   public void appendHeader(HttpServletResponse response, String name, String value) {

instrumentation/servlet/servlet-3.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/servlet/v3_0/Servlet3HttpServerResponseMutator.java

@@ -8,13 +8,9 @@
 import io.opentelemetry.javaagent.bootstrap.http.HttpServerResponseMutator;
 import javax.servlet.http.HttpServletResponse;
 
-public class Servlet3HttpServerResponseMutator
+public enum Servlet3HttpServerResponseMutator
     implements HttpServerResponseMutator<HttpServletResponse> {
-
-  public static final Servlet3HttpServerResponseMutator INSTANCE =
-      new Servlet3HttpServerResponseMutator();
-
-  private Servlet3HttpServerResponseMutator() {}
+  INSTANCE;
 
   @Override
   public void appendHeader(HttpServletResponse response, String name, String value) {