Custom Spring HttpMessageConverter not being used by the framework - spring

Question

I have a Spring mvc #RestController class where the method parameter is annotated with #RequestBody. Something like this:
#RestController
#RequestMapping("/features")
public class FeatureController {
#PostMapping
public Feature createFeature(#RequestBody Feature feature) {
......
}
}
My Feature class has a private constructor built using a Builder pattern. So I created HttpMessageConverter called FeatureConverter and registered it properly using extendMessageConverters. The converter uses Jackson to parse the JSON and then uses Feature.Builder to create an instance of Feature.
My problem is that Spring registers an instance of MappingJackson2HttpMessageConverter before my custom FeatureConverter. As a result the parsing is attempted by MappingJackson2HttpMessageConverter even before it can reach FeatureConverter. Since the constructor is private so MappingJackson2HttpMessageConverter fails.
My question is how do I change the order so that FeatureConverter is asked before MappingJackson2HttpMessageConverter. Is there a proper way of doing it?

Answer 1

Spring MVC WebMvcConfigurerAdapter initialize HttpMessageConverters like that:
protected final List<HttpMessageConverter<?>> getMessageConverters() {
if (this.messageConverters == null) {
this.messageConverters = new ArrayList<HttpMessageConverter<?>>();
configureMessageConverters(this.messageConverters);
if (this.messageConverters.isEmpty()) {
addDefaultHttpMessageConverters(this.messageConverters);
}
extendMessageConverters(this.messageConverters);
}
return this.messageConverters;
}
See: Github - WebMvcConfigurationSupport
As you can see there is 3 method:
configureMessageConverters, empty by default it's where you are suppose to add converters
addDefaultHttpMessageConverters, where as the name said will create all default converters, the default MappingJackson2HttpMessageConverter is created there.
extendMessageConverters which as the documentation:
Override this method to extend or modify the list of converters after
it has been configured. This may be useful for example to allow
default converters to be registered and then insert a custom
converter through this method.
Is to use when you want to modify the list after having all the defaults converter added.
To answer your question, if you don't need defaults converters you just have to Override configureMessageConverters and add it:
#Override
public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
converters.add(new FeatureConverter());
}
If you need them you can either copy-paste the addDefaultHttpMessageConverters content and add it into configureMessageConverters and play with the order of initialisation there (ugly solution).
Otherwise you can use extendMessageConverters to add it after the default initialization and play with the order:
#Override
public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
converters.add(the_index_you_want, new FeatureConverter());
}

Answer 2

If I understand you right you want to use your Builder for deserialization of Feature object.
Maybe you can just use Jackson annotations (without FeatureConverter):
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import com.fasterxml.jackson.databind.annotation.JsonPOJOBuilder;
#JsonDeserialize(builder = Feature.FeatureBuilder.class)
public class Feature {
private final String name;
public Feature(String name) {
this.name = name;
}
public String getName() {
return name;
}
/**
* Builder
*/
#JsonPOJOBuilder(withPrefix = "")
public static final class FeatureBuilder {
private String name = "";
private FeatureBuilder() {
}
public static FeatureBuilder create() {
return new FeatureBuilder();
}
public FeatureBuilder name(String name) {
this.name = name;
return this;
}
/**
* Build Feature object
*/
public Feature build() {
Feature feature = new Feature(this.name);
return feature;
}
}
}
If you use Lombok:
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import com.fasterxml.jackson.databind.annotation.JsonPOJOBuilder;
import lombok.Builder;
import lombok.Getter;
#Getter
#Builder
#JsonDeserialize(builder = Feature.FeatureBuilder.class)
public class Feature {
private final String name;
/**
* Builder
*/
#JsonPOJOBuilder(withPrefix = "")
public static final class FeatureBuilder {
}
}