Improve Jackson 2.7 compatibility

This commit introduces a conditional call in
AbstractJackson2HttpMessageConverter#getJavaType() in order to avoid
calling TypeFactory#constructType() with a null contextClass parameter,
since this is not supported by Jackson 2.7 anymore.

It allows to use Spring Framework 4.2.x with Jackson 2.7.1+ for most use
cases, but with some limitations. For example, with Jackson 2.7
TypeVariable resolution from method parameters does not work.

As a consequence, after this commit Spring Framework 4.2.x still fully
supports Jackson up to version 2.6, but improves Jackson 2.7 compatibility.
Full support for Jackson 2.7 is provided as of Spring Framework 4.3.x.

Issue: SPR-13853

spring-web/src/main/java/org/springframework/http/converter/json/AbstractJackson2HttpMessageConverter.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.
@@ -31,6 +31,7 @@ import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.ObjectWriter;
 import com.fasterxml.jackson.databind.SerializationFeature;
 import com.fasterxml.jackson.databind.ser.FilterProvider;
+import com.fasterxml.jackson.databind.type.TypeFactory;
 
 import org.springframework.http.HttpInputMessage;
 import org.springframework.http.HttpOutputMessage;
@@ -48,7 +49,7 @@ import org.springframework.util.TypeUtils;
  * Abstract base class for Jackson based and content type independent
  * {@link HttpMessageConverter} implementations.
  *
- * <p>Compatible with Jackson 2.1 and higher.
+ * <p>Compatible with Jackson 2.1 to 2.6.
  *
  * @author Arjen Poutsma
  * @author Keith Donald
@@ -308,7 +309,10 @@ public abstract class AbstractJackson2HttpMessageConverter extends AbstractGener
 	 * @return the Jackson JavaType
 	 */
 	protected JavaType getJavaType(Type type, Class<?> contextClass) {
-		return this.objectMapper.getTypeFactory().constructType(type, contextClass);
+		TypeFactory tf = this.objectMapper.getTypeFactory();
+		// Conditional call because Jackson 2.7 does not support null contextClass anymore
+		// TypeVariable resolution will not work with Jackson 2.7, see SPR-13853 for more details
+		return (contextClass != null ? tf.constructType(type, contextClass) : tf.constructType(type));
 	}
 
 	/**

spring-web/src/main/java/org/springframework/http/converter/json/MappingJackson2HttpMessageConverter.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.
@@ -35,7 +35,7 @@ import org.springframework.http.MediaType;
  *
  * <p>The default constructor uses the default configuration provided by {@link Jackson2ObjectMapperBuilder}.
  *
- * <p>Compatible with Jackson 2.1 and higher.
+ * <p>Compatible with Jackson 2.1 to 2.6.
  *
  * @author Arjen Poutsma
  * @author Keith Donald

spring-web/src/main/java/org/springframework/http/converter/xml/MappingJackson2XmlHttpMessageConverter.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.
@@ -35,7 +35,7 @@ import org.springframework.util.Assert;
  *
  * <p>The default constructor uses the default configuration provided by {@link Jackson2ObjectMapperBuilder}.
  *
- * <p>Compatible with Jackson 2.1 and higher.
+ * <p>Compatible with Jackson 2.1 to 2.6.
  *
  * @author Sebastien Deleuze
  * @since 4.1