Expose Principal in ServerRequest

This commit exposes the Principal in ServerRequest.

Issue: SPR-15552

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/DefaultServerRequest.java

@@ -19,6 +19,7 @@ package org.springframework.web.reactive.function.server;
 import java.net.InetSocketAddress;
 import java.net.URI;
 import java.nio.charset.Charset;
+import java.security.Principal;
 import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
@@ -157,6 +158,11 @@ class DefaultServerRequest implements ServerRequest {
 		return this.exchange.getSession();
 	}
 
+	@Override
+	public Mono<? extends Principal> principal() {
+		return this.exchange.getPrincipal();
+	}
+
 	private ServerHttpRequest request() {
 		return this.exchange.getRequest();
 	}

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/RequestPredicates.java

@@ -17,6 +17,7 @@
 package org.springframework.web.reactive.function.server;
 
 import java.net.URI;
+import java.security.Principal;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashSet;
@@ -543,6 +544,11 @@ public abstract class RequestPredicates {
 			return this.request.session();
 		}
 
+		@Override
+		public Mono<? extends Principal> principal() {
+			return this.request.principal();
+		}
+
 		@Override
 		public String toString() {
 			return method() + " " +  path();

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/ServerRequest.java

@@ -19,6 +19,7 @@ package org.springframework.web.reactive.function.server;
 import java.net.InetSocketAddress;
 import java.net.URI;
 import java.nio.charset.Charset;
+import java.security.Principal;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
@@ -168,6 +169,11 @@ public interface ServerRequest {
 	 */
 	Mono<WebSession> session();
 
+	/**
+	 * Return the authenticated user for the request, if any.
+	 */
+	Mono<? extends Principal> principal();
+
 
 	/**
 	 * Represents the headers of the HTTP request.

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/support/ServerRequestWrapper.java

@@ -19,6 +19,7 @@ package org.springframework.web.reactive.function.server.support;
 import java.net.InetSocketAddress;
 import java.net.URI;
 import java.nio.charset.Charset;
+import java.security.Principal;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
@@ -144,6 +145,10 @@ public class ServerRequestWrapper implements ServerRequest {
 		return this.delegate.session();
 	}
 
+	@Override
+	public Mono<? extends Principal> principal() {
+		return this.delegate.principal();
+	}
 
 	/**
 	 * Implementation of the {@code Headers} interface that can be subclassed