Another approach

org.eclipse.lsp4j.jsonrpc/src/main/java/org/eclipse/lsp4j/jsonrpc/Launcher.java

@@ -235,7 +235,12 @@ public static class Builder<T> {
 		protected boolean validateMessages;
 		protected Consumer<GsonBuilder> configureGson;
 		protected ClassLoader classLoader;
+		/**
+		 * @deprecated use {@link #tracer} instead
+		 */
+		@Deprecated
 		protected MessageTracer messageTracer;
+		protected PrintWriter tracer;
 
 		public Builder<T> setLocalService(Object localService) {
 			this.localServices = Collections.singletonList(localService);
@@ -294,7 +299,7 @@ public Builder<T> validateMessages(boolean validate) {
 
 		public Builder<T> traceMessages(PrintWriter tracer) {
 			if (tracer != null) {
-				this.messageTracer = new MessageTracer(tracer);
+				this.tracer = tracer;
 			}
 			return this;
 		}
@@ -318,8 +323,8 @@ public Launcher<T> create() {
 			// Create the JSON handler, remote endpoint and remote proxy
 			MessageJsonHandler jsonHandler = createJsonHandler();
 
-			if (messageTracer != null) {
-				messageTracer.setJsonHandler(jsonHandler);
+			if (tracer != null) {
+				messageTracer = new MessageTracer(tracer, jsonHandler);
 			}
 
 			RemoteEndpoint remoteEndpoint = createRemoteEndpoint(jsonHandler);

org.eclipse.lsp4j.jsonrpc/src/main/java/org/eclipse/lsp4j/jsonrpc/MessageTracer.java

@@ -27,16 +27,13 @@
  */
 public class MessageTracer implements Function<MessageConsumer, MessageConsumer> {
 	private final PrintWriter printWriter;
-	private MessageJsonHandler jsonHandler;
+	private final MessageJsonHandler jsonHandler;
 	private final Map<String, RequestMetadata> sentRequests = new HashMap<>();
 	private final Map<String, RequestMetadata> receivedRequests = new HashMap<>();
 
-	MessageTracer(PrintWriter printWriter) {
+	MessageTracer(PrintWriter printWriter, MessageJsonHandler jsonHandler) {
 		this.printWriter = Objects.requireNonNull(printWriter);
-	}
-
-	public void setJsonHandler(MessageJsonHandler jsonHandler) {
-		this.jsonHandler = jsonHandler;
+		this.jsonHandler = Objects.requireNonNull(jsonHandler);
 	}
 
 	@Override

org.eclipse.lsp4j.jsonrpc/src/main/java/org/eclipse/lsp4j/jsonrpc/TracingMessageConsumer.java

@@ -15,6 +15,7 @@
 import java.time.Clock;
 import java.time.Instant;
 import java.time.format.DateTimeFormatter;
+import java.util.Collections;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Objects;
@@ -49,6 +50,7 @@ public class TracingMessageConsumer implements MessageConsumer {
 	 * @param sentRequests A map that keeps track of pending sent request data.
 	 * @param receivedRequests A map that keeps track of pending received request data.
 	 * @param printWriter Where to write the log to.
+	 * @param jsonHandler The handler to be used to serialize messages.
 	 * @param clock The clock that is used to calculate timestamps and durations.
 	 */
 	public TracingMessageConsumer(
@@ -67,6 +69,54 @@ public TracingMessageConsumer(
 	 * @param receivedRequests A map that keeps track of pending received request data.
 	 * @param printWriter Where to write the log to.
 	 * @param clock The clock that is used to calculate timestamps and durations.
+	 * @deprecated use {@link TracingMessageConsumer#TracingMessageConsumer(MessageConsumer, Map, Map, PrintWriter, MessageJsonHandler, Clock)}
+	 */
+	@Deprecated
+	public TracingMessageConsumer(
+			MessageConsumer messageConsumer,
+			Map<String, RequestMetadata> sentRequests,
+			Map<String, RequestMetadata> receivedRequests,
+			PrintWriter printWriter,
+			Clock clock) {
+		this(messageConsumer, sentRequests, receivedRequests, printWriter, defaultHandlerWithoutUserAdapters(), clock, null);
+	}
+
+	/**
+	 * For backward compatibility, we are using a default handler that does not contain any user adapters.
+	 */
+	private static MessageJsonHandler defaultHandlerWithoutUserAdapters() {
+		return new MessageJsonHandler(Collections.emptyMap(), gsonBuilder -> {
+			gsonBuilder.setPrettyPrinting();
+		});
+	}
+
+	/**
+	 * @param messageConsumer The {@link MessageConsumer} to wrap.
+	 * @param sentRequests A map that keeps track of pending sent request data.
+	 * @param receivedRequests A map that keeps track of pending received request data.
+	 * @param printWriter Where to write the log to.
+	 * @param clock The clock that is used to calculate timestamps and durations.
+	 * @param locale THe Locale to format the timestamps and durations, or <code>null</code> to use default locale.
+	 * @deprecated use {@link TracingMessageConsumer#TracingMessageConsumer(MessageConsumer, Map, Map, PrintWriter, MessageJsonHandler, Clock, Locale)}
+	 */
+	@Deprecated
+	public TracingMessageConsumer(
+			MessageConsumer messageConsumer,
+			Map<String, RequestMetadata> sentRequests,
+			Map<String, RequestMetadata> receivedRequests,
+			PrintWriter printWriter,
+			Clock clock,
+			Locale locale) {
+		this(messageConsumer, sentRequests, receivedRequests, printWriter, defaultHandlerWithoutUserAdapters(), clock, locale);
+	}
+
+	/**
+	 * @param messageConsumer The {@link MessageConsumer} to wrap.
+	 * @param sentRequests A map that keeps track of pending sent request data.
+	 * @param receivedRequests A map that keeps track of pending received request data.
+	 * @param printWriter Where to write the log to.
+	 * @param jsonHandler The handler to be used to serialize messages.
+	 * @param clock The clock that is used to calculate timestamps and durations.
 	 * @param locale THe Locale to format the timestamps and durations, or <code>null</code> to use default locale.
 	 */
 	public TracingMessageConsumer(