Add ResolvableTypeProvider

Provide a mean to detect the actual ResolvableType based on a instance as
a counter measure to type erasure.

Upgrade the event infrastructure to detect if the event (or the payload)
implements such interface. When this is the case, the return value of
`getResolvableType` is used to validate its generic type against the
method signature of the listener.

Issue: SPR-13069

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

@@ -67,6 +67,7 @@ import org.springframework.util.StringUtils;
  *
  * @author Phillip Webb
  * @author Juergen Hoeller
+ * @author Stephane Nicoll
  * @since 4.0
  * @see #forField(Field)
  * @see #forMethodParameter(Method, int)
@@ -74,6 +75,8 @@ import org.springframework.util.StringUtils;
  * @see #forConstructorParameter(Constructor, int)
  * @see #forClass(Class)
  * @see #forType(Type)
+ * @see #forInstance(Object)
+ * @see ResolvableTypeProvider
  */
 @SuppressWarnings("serial")
 public class ResolvableType implements Serializable {
@@ -984,6 +987,26 @@ public class ResolvableType implements Serializable {
 		return forType(syntheticType, new TypeVariablesVariableResolver(variables, generics));
 	}
 
+	/**
+	 * Return a {@link ResolvableType} for the specified instance. The instance does not
+	 * convey generic information but if it implements {@link ResolvableTypeProvider} a
+	 * more precise {@link ResolvableType} can be used than the simple one based on
+	 * the {@link #forClass(Class) Class instance}.
+	 * @param instance the instance
+	 * @return a {@link ResolvableType} for the specified instance
+	 * @see ResolvableTypeProvider
+	 */
+	public static ResolvableType forInstance(Object instance) {
+		Assert.notNull(instance, "Instance must not be null");
+		if (instance instanceof ResolvableTypeProvider) {
+			ResolvableType type = ((ResolvableTypeProvider) instance).getResolvableType();
+			if (type != null) {
+				return type;
+			}
+		}
+		return ResolvableType.forClass(instance.getClass());
+	}
+
 	/**
 	 * Return a {@link ResolvableType} for the specified {@link Field}.
 	 * @param field the source field

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

@@ -0,0 +1,40 @@
+/*
+ * 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.core;
+
+/**
+ * Any object can implement this interface to provide its actual {@link ResolvableType}.
+ * <p>
+ * Such information is very useful when figuring out if the instance matches a generic
+ * signature as Java does not convey the signature at runtime.
+ * <p>
+ * Users of this interface should be careful in complex hierarchy scenarios, especially
+ * when the generic type signature of the class changes in sub-classes. It is always
+ * possible to return {@code null} to fallback on a default behaviour.
+ *
+ * @author Stephane Nicoll
+ * @since 4.2
+ */
+public interface ResolvableTypeProvider {
+
+	/**
+	 * Return the {@link ResolvableType} describing this instance or {@code null} if some
+	 * sort of default should be applied instead.
+	 */
+	ResolvableType getResolvableType();
+
+}