Deserializing single-attribute JSON payload in Spring MVC controller - json

Question

I want to create controller methods that semantically look like the following
public HttpEntity<?> deleteUser(String userId){
...
}
The client is going to pass the user ID as part of the JSON payload. If I try to annotate #RequestBody the string parameter and issue a {"userId":"foo"} payload, then I get an exception
Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_OBJECT token
at [Source: java.io.PushbackInputStream#7311a203; line: 1, column: 1]
at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:148) ~[jackson-databind-2.6.1.jar:2.6.1]
at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:854) ~[jackson-databind-2.6.1.jar:2.6.1]
at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:62) ~[jackson-databind-2.6.1.jar:2.6.1]
at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:11) ~[jackson-databind-2.6.1.jar:2.6.1]
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3702) ~[jackson-databind-2.6.1.jar:2.6.1]
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2798) ~[jackson-databind-2.6.1.jar:2.6.1]
at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:221) ~[spring-web-4.2.0.RELEASE.jar:4.2.0.RELEASE]
And that is reasonable because JSON wants to deserialize a complex object (with namely one attribute) into a String.
I also know that "foo" is not valid JSON. And I know that I can use a Map<String,Object> or even better a ModelMap, and as a last resort I could use query string and #RequestParam, but today I have been clearly asked by my boss to find a way to use a plain string instead of an object, in order for code to look more readable.
How can I force Jackson/MVC to deserialize only the "username" property into a plain old String?

Answer 1

You will usually see this type of error when Spring MVC finds a request mapping that matches the URL path but the parameters (or headers or something) don't match what the handler method is expecting.
If you use the #RequestBody annotation then Spring MVC is expecting to map the entire body of the POST request to an Object,it dont work with String by default.
There are different way to do this as listed below:
1) Change method type of deleteUser() method type to GET instead of Post and use userId as String.
2) You could simply inject the HttpServletRequest into your method and read the body:
public void deleteUser(HttpServletRequest request) {
String userID = IOUtils.toString( request.getInputStream());
// do stuff
}
3) Use a wrapper (java model of the JSON object) that could replace the String parameter,and also this will work fine with the json coming in your post.
public class UserWrapper {
private String userId;
//getter setters
and then use in your controller as:
public void deleteUser(#RequestBody UserWrapper user) {
//do your stuff
}
4) Spring provides a way to configure multiple message converters as shown below:
Note: Then, requests to the various methods must specify the "content-type" header with an appropriate value. For those methods where the request body is mapped to a JAXB bean, specify "application/xml". And for those where the request body is a String, use "text/plain".
<bean class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter">
<property name="messageConverters">
<list>
<ref bean="jsonConverter" />
<ref bean="marshallingConverter" />
<ref bean="stringHttpMessageConverter" />
</list>
</property>
</bean>
<bean id="jsonConverter"
class="org.springframework.http.converter.json.MappingJacksonHttpMessageConverter">
<property name="supportedMediaTypes" value="application/json" />
</bean>
<bean id="marshallingConverter"
class="org.springframework.http.converter.xml.MarshallingHttpMessageConverter">
<constructor-arg ref="jaxb2Marshaller" />
<property name="supportedMediaTypes" value="application/xml"/>
</bean>
<bean id="stringHttpMessageConverter"
class="org.springframework.http.converter.StringHttpMessageConverter">
<property name="supportedMediaTypes" value="text/plain"/>
</bean>
Hope this help you!