Support generic target types in the RestTemplate

This change makes it possible to use the RestTemplate to read an HTTP
response into a target generic type object. The RestTemplate has three
new exchange(...) methods that accept ParameterizedTypeReference -- a
new class that enables capturing and passing generic type info.
See the Javadoc of the three new methods in RestOperations for a
short example.

To support this feature, the HttpMessageConverter is now extended by
GenericHttpMessageConverter, which adds a method for reading an
HttpInputMessage to a specific generic type. The new interface
is implemented by the MappingJacksonHttpMessageConverter and also by a
new Jaxb2CollectionHttpMessageConverter that can read read a generic
Collection where the generic type is a JAXB type annotated with
@XmlRootElement or @XmlType.

Issue: SPR-7023

spring-core/src/test/java/org/springframework/core/ParameterizedTypeReferenceTest.java

@@ -0,0 +1,61 @@
+/*
+ * 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.
+ * 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;
+
+import java.lang.reflect.Type;
+import java.util.List;
+import java.util.Map;
+
+import static org.junit.Assert.assertEquals;
+import org.junit.Test;
+
+/**
+ * Test fixture for {@link ParameterizedTypeReference}.
+ *
+ * @author Arjen Poutsma
+ * @author Rossen Stoyanchev
+ */
+public class ParameterizedTypeReferenceTest {
+
+	@Test
+	public void map() throws NoSuchMethodException {
+		Type mapType = getClass().getMethod("mapMethod").getGenericReturnType();
+		ParameterizedTypeReference<Map<Object,String>> mapTypeReference = new ParameterizedTypeReference<Map<Object,String>>() {};
+		assertEquals(mapType, mapTypeReference.getType());
+	}
+
+	@Test
+	public void list() throws NoSuchMethodException {
+		Type mapType = getClass().getMethod("listMethod").getGenericReturnType();
+		ParameterizedTypeReference<List<String>> mapTypeReference = new ParameterizedTypeReference<List<String>>() {};
+		assertEquals(mapType, mapTypeReference.getType());
+	}
+
+	@Test
+	public void string() {
+		ParameterizedTypeReference<String> typeReference = new ParameterizedTypeReference<String>() {};
+		assertEquals(String.class, typeReference.getType());
+	}
+
+	public static Map<Object, String> mapMethod() {
+		return null;
+	}
+
+	public static List<String> listMethod() {
+		return null;
+	}
+}