Configuring auto-serialization of Guava Multimap with Spring Boot when the `SpringBootApplication` impelments `WebMvcConfigurer`

Question

I have the following SpringBootApplication:

import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

@SpringBootApplication
@Configuration
public class RestServer implements WebMvcConfigurer {
  public static void main(String[] args) {
    SpringApplication.run(RestServer.class, args);
  }
}

And I would like Guava's Multimap get automatically serialized to JSON.

So I have this code in one of my classes, where all other fields are successfully serialized to JSON:

@JsonProperty("multimap_test")
public Multimap<Instrument, AdjustedLimitOrder> getMultimap() {
  return someMultiMap;
}

But the returned JSON field looks like this:

"multimap_test": {
  "empty": false
}

Do I have to specify a deserializer on the field?

Using this doesn't work because there is no default constructor:

@JsonProperty("multimap_test")
public Multimap<Instrument, AdjustedLimitOrder> getMultimap() {
  return someMultimap;
}

Note that my example is implementing WebMvcConfigurer.

Here is a list of links of instructions that didn't seem conclusive on how to do this, because they discuss the case when the SpringBootApplication implements SpringBootServletInitializer or WebMvcConfigurerAdapter (which is deprecated).

• http://gdpotter.com/2017/05/24/custom-spring-mvc-jackson/
• Enable json serialization of Multimap in Spring Boot Project

How can Guava's Multimap get automatically serialized?

Answer 1

The spring boot MVC auto-configuration already configures to use Jackson to input/output JSON if Jackson is found in classpath. But the configured Jackson by default will not support Guava Multimap, you have to register GuavaModule to the Jackson ObjectMapper in order to support it (see docs).

From the JacksonAutoConfiguration docs , it will auto-register GuavaModule to the ObjectMapper if we declare it as a bean. So in order to serialize/desearialize Multimap , you have to

1. Add Jackson 's Guava module to pom.xml :

    <dependency>
      <groupId>com.fasterxml.jackson.datatype</groupId>
      <artifactId>jackson-datatype-guava</artifactId>
    </dependency>
   

2. Declare a GuavaModule bean in @SpringBootApplication :

     @Bean
     public Module guavaModule() {
         return new GuavaModule();
     }
   

And it is done . WebMvcConfigurer / WebMvcConfigurerAdapter is intended for customizing spring boot MVC default settings . You don't need to customize it for your case as Spring MVC already work with Jackson to input/output JSON out of the box . What you need to do is to configure Jackson but not configure spring boot MVC . So, it does not matter whether SpringBootApplication implement or not implement WebMvcConfigurer/WebMvcConfigurerAdapter if you just want to auto-serialise Multimap.