Introduce alias for 'value' attribute in @Payload

This commit introduces 'expression' as an alias for 'value' in @Payload.

Issue: SPR-11393

spring-messaging/src/main/java/org/springframework/messaging/handler/annotation/Payload.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -22,6 +22,7 @@ import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
+import org.springframework.core.annotation.AliasFor;
 import org.springframework.messaging.converter.MessageConverter;
 
 /**
@@ -31,6 +32,7 @@ import org.springframework.messaging.converter.MessageConverter;
  * specific MIME type to an Object matching the target method parameter.
  *
  * @author Rossen Stoyanchev
+ * @author Sam Brannen
  * @since 4.0
  */
 @Target({ElementType.PARAMETER, ElementType.METHOD})
@@ -38,14 +40,23 @@ import org.springframework.messaging.converter.MessageConverter;
 @Documented
 public @interface Payload {
 
+	/**
+	 * Alias for {@link #expression}.
+	 */
+	@AliasFor(attribute="expression")
+	String value() default "";
+
 	/**
 	 * A SpEL expression to be evaluated against the payload object as the root context.
+	 * <p>
 	 * This attribute may or may not be supported depending on whether the message being
-	 * handled contains a non-primitive Object as its payload or is in serialized form
-	 * and requires message conversion.
-	 * <p>When processing STOMP over WebSocket messages this attribute is not supported.
+	 * handled contains a non-primitive Object as its payload or is in serialized form and
+	 * requires message conversion.
+	 * <p>
+	 * When processing STOMP over WebSocket messages this attribute is not supported.
 	 */
-	String value() default "";
+	@AliasFor(attribute="value")
+	String expression() default "";
 
 	/**
 	 * Whether payload content is required.

spring-messaging/src/main/java/org/springframework/messaging/handler/annotation/support/PayloadArgumentResolver.java

@@ -87,7 +87,7 @@ public class PayloadArgumentResolver implements HandlerMethodArgumentResolver {
 	@Override
 	public Object resolveArgument(MethodParameter param, Message<?> message) throws Exception {
 		Payload ann = param.getParameterAnnotation(Payload.class);
-		if (ann != null && StringUtils.hasText(ann.value())) {
+		if (ann != null && StringUtils.hasText(ann.expression())) {
 			throw new IllegalStateException("@Payload SpEL expressions not supported by this resolver");
 		}