Clarify SpEL usage on @EventListener

Issue: SPR-14812

Author: snicoll

spring-context/src/main/java/org/springframework/context/event/EventListener.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -87,7 +87,18 @@
 	/**
 	 * Spring Expression Language (SpEL) attribute used for making the
 	 * event handling conditional.
-	 * <p>Default is "", meaning the event is always handled.
+	 * <p>Default is {@code ""}, meaning the event is always handled.
+	 * <p>The SpEL expression evaluates against a dedicated context that
+	 * provides the following meta-data:
+	 * <ul>
+	 * <li>{@code #root.event}, {@code #root.args} for
+	 * references to the {@link ApplicationEvent} and method arguments
+	 * respectively.</li>
+	 * <li>Method arguments can be accessed by index. For instance the
+	 * first argument can be accessed via {@code #root.args[0]}, {@code #p0}
+	 * or {@code #a0}. Arguments can also be accessed by name if that
+	 * information is available.</li>
+	 * </ul>
 	 */
 	String condition() default "";
 

src/asciidoc/core-beans.adoc

@@ -8281,8 +8281,8 @@ event is equal to `foo`:
 [source,java,indent=0]
 [subs="verbatim,quotes"]
 ----
-	@EventListener(condition = "#event.test == 'foo'")
-	public void processBlackListEvent(BlackListEvent event) {
+	@EventListener(condition = "#blEvent.test == 'foo'")
+	public void processBlackListEvent(BlackListEvent blEvent) {
 		// notify appropriate parties via notificationAddress...
 	}
 ----
@@ -8295,22 +8295,22 @@ available to the context so one can use them for conditional event processing:
 |===
 | Name| Location| Description| Example
 
-| event
+| Event
 | root object
 | The actual `ApplicationEvent`
 | `#root.event`
 
-| args
+| Arguments array
 | root object
 | The arguments (as array) used for invoking the target
 | `#root.args[0]`
 
-| __argument name__
+| __Argument name__
 | evaluation context
 | Name of any of the method arguments. If for some reason the names are not available
   (e.g. no debug information), the argument names are also available under the `#a<#arg>`
   where __#arg__ stands for the argument index (starting from 0).
-| `#iban` or `#a0` (one can also use `#p0` or `#p<#arg>` notation as an alias).
+| `#blEvent` or `#a0` (one can also use `#p0` or `#p<#arg>` notation as an alias).
 |===
 
 Note that `#root.event` allows you to access to the underlying event, even if your method