Refactor WebSession#getAttribute options

Issue: SPR-15718

spring-web/src/main/java/org/springframework/web/server/WebSession.java

@@ -19,10 +19,12 @@ package org.springframework.web.server;
 import java.time.Duration;
 import java.time.Instant;
 import java.util.Map;
-import java.util.Optional;
 
 import reactor.core.publisher.Mono;
 
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+
 /**
  * Main contract for using a server-side session that provides access to session
  * attributes across HTTP requests.
@@ -48,12 +50,42 @@ public interface WebSession {
 	Map<String, Object> getAttributes();
 
 	/**
-	 * Return the attribute value if present.
+	 * Return the session attribute value if present.
 	 * @param name the attribute name
 	 * @param <T> the attribute type
 	 * @return the attribute value
 	 */
-	<T> Optional<T> getAttribute(String name);
+	@SuppressWarnings("unchecked")
+	@Nullable
+	default <T> T getAttribute(String name) {
+		return (T) getAttributes().get(name);
+	}
+
+	/**
+	 * Return the session attribute value or if not present raise an
+	 * {@link IllegalArgumentException}.
+	 * @param name the attribute name
+	 * @param <T> the attribute type
+	 * @return the attribute value
+	 */
+	@SuppressWarnings("unchecked")
+	default <T> T getRequiredAttribute(String name) {
+		T value = getAttribute(name);
+		Assert.notNull(value, "Required attribute '" + name + "' is missing.");
+		return value;
+	}
+
+	/**
+	 * Return the session attribute value, or a default, fallback value.
+	 * @param name the attribute name
+	 * @param defaultValue a default value to return instead
+	 * @param <T> the attribute type
+	 * @return the attribute value
+	 */
+	@SuppressWarnings("unchecked")
+	default <T> T getAttributeOrDefault(String name, T defaultValue) {
+		return (T) getAttributes().getOrDefault(name, defaultValue);
+	}
 
 	/**
 	 * Force the creation of a session causing the session id to be sent when

spring-web/src/main/java/org/springframework/web/server/session/DefaultWebSession.java

@@ -20,7 +20,6 @@ import java.time.Clock;
 import java.time.Duration;
 import java.time.Instant;
 import java.util.Map;
-import java.util.Optional;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Supplier;
@@ -108,11 +107,6 @@ public class DefaultWebSession implements ConfigurableWebSession, Serializable {
 		return this.attributes;
 	}
 
-	@Override @SuppressWarnings("unchecked")
-	public <T> Optional<T> getAttribute(String name) {
-		return Optional.ofNullable((T) this.attributes.get(name));
-	}
-
 	@Override
 	public Instant getCreationTime() {
 		return this.creationTime;