SimpUser exposes Principal

Closes gh-25191

spring-messaging/src/test/java/org/springframework/messaging/simp/user/TestSimpUser.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,11 +16,14 @@
 
 package org.springframework.messaging.simp.user;
 
+import java.security.Principal;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
 
+import org.springframework.lang.Nullable;
+
 /**
  * @author Rossen Stoyanchev
  */
@@ -41,6 +44,12 @@ public class TestSimpUser implements SimpUser {
 		return name;
 	}
 
+	@Nullable
+	@Override
+	public Principal getPrincipal() {
+		return null;
+	}
+
 	@Override
 	public Set<SimpSession> getSessions() {
 		return new HashSet<>(this.sessions.values());