ParameterNameDiscoverer may return individual null entries in an array

Issue: SPR-17565

(cherry picked from commit c48672c4c7)

spring-context/src/main/java/org/springframework/context/expression/MethodBasedEvaluationContext.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -101,7 +101,7 @@ public class MethodBasedEvaluationContext extends StandardEvaluationContext {
 			}
 			setVariable("a" + i, value);
 			setVariable("p" + i, value);
-			if (paramNames != null) {
+			if (paramNames != null && paramNames[i] != null) {
 				setVariable(paramNames[i], value);
 			}
 		}

spring-core/src/main/java/org/springframework/core/ParameterNameDiscoverer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -36,9 +36,11 @@ import org.springframework.lang.Nullable;
 public interface ParameterNameDiscoverer {
 
 	/**
-	 * Return parameter names for this method,
-	 * or {@code null} if they cannot be determined.
-	 * @param method method to find parameter names for
+	 * Return parameter names for a method, or {@code null} if they cannot be determined.
+	 * <p>Individual entries in the array may be {@code null} if parameter names are only
+	 * available for some parameters of the given method but not for others. However,
+	 * it is recommended to use stub parameter names instead wherever feasible.
+	 * @param method the method to find parameter names for
 	 * @return an array of parameter names if the names can be resolved,
 	 * or {@code null} if they cannot
 	 */
@@ -46,9 +48,11 @@ public interface ParameterNameDiscoverer {
 	String[] getParameterNames(Method method);
 
 	/**
-	 * Return parameter names for this constructor,
-	 * or {@code null} if they cannot be determined.
-	 * @param ctor constructor to find parameter names for
+	 * Return parameter names for a constructor, or {@code null} if they cannot be determined.
+	 * <p>Individual entries in the array may be {@code null} if parameter names are only
+	 * available for some parameters of the given constructor but not for others. However,
+	 * it is recommended to use stub parameter names instead wherever feasible.
+	 * @param ctor the constructor to find parameter names for
 	 * @return an array of parameter names if the names can be resolved,
 	 * or {@code null} if they cannot
 	 */