Avoid NPE in AutowiredAnnotationBeanPostProcessor

Prior to this change, AABPP#determineRequiredStatus never checked the
return value of ReflectionUtils#findMethod when searching for a
'#required' attribute. This call returns null for annotations such as
@Inject, @value and @resource, and subsequently causes a
NullPointerException to be thrown when ReflectionUtils#invokeMethod is
called. The NPE is caught immediately and #determineRequiredStatus
returns defaulting to true, but this this approach is inefficient. It
is also problematic for users who have set breakpoints on NPE -- they
end up debugging into Spring internals, which is a false positive.

This commit checks the return value of of ReflectionUtils#findMethod,
and in the case of null, eagerly returns true.  There is no change to
external behavior, simply a more efficient and debugging-friendly
implementation.

Existing test cases already cover this change, given that it is purely
a refactoring.

Issue: SPR-9316

Author: marschall

spring-beans/src/main/java/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2011 the original author or authors.
+ * Copyright 2002-2012 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.
@@ -405,10 +405,16 @@ protected <T> Map<String, T> findAutowireCandidates(Class<T> type) throws BeansE
 	protected boolean determineRequiredStatus(Annotation annotation) {
 		try {
 			Method method = ReflectionUtils.findMethod(annotation.annotationType(), this.requiredParameterName);
+			if (method == null) {
+				// annotations like @Inject, @Value and @Resource don't have a method
+				// (attribute) named "required" -> default to required status
+				return true;
+			}
 			return (this.requiredParameterValue == (Boolean) ReflectionUtils.invokeMethod(method, annotation));
 		}
 		catch (Exception ex) {
-			// required by default
+			// an exception was thrown during reflective invocation of the required
+			// attribute -> default to required status
 			return true;
 		}
 	}