How to Prevent Jackson from Serializing Lazy-Fetched Hibernate Objects?

Avoid Jackson Serialization on Lazy Fetched Objects

This article addresses the challenge of preventing Jackson from serializing unfetched, lazy-loaded objects, which can result in Hibernate-related exceptions.

Introduction

In the provided scenario, a User object with a lazy-fetched coordinates list results in a "could not initialize proxy - no Session" exception when Jackson attempts to serialize the object. This is because Jackson eagerly fetches the coordinates before serialization, violating the lazy loading mechanism.

Solution

The solution involves integrating a custom mapping converter that incorporates the Hibernate4Module. This module enhances Jackson's support for lazy-loaded objects.

Spring Java Configuration

1. Create a configuration class that extends WebMvcConfigurerAdapter and overrides configureMessageConverters:

@Configuration
@EnableWebMvc
public class MyConfigClass extends WebMvcConfigurerAdapter{

    public MappingJackson2HttpMessageConverter jacksonMessageConverter(){
        MappingJackson2HttpMessageConverter messageConverter = new MappingJackson2HttpMessageConverter();
        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(new Hibernate4Module());
        messageConverter.setObjectMapper(mapper);
        return messageConverter;
    }

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        converters.add(jacksonMessageConverter());
        super.configureMessageConverters(converters);
    }
}

2. Register the custom converter in the HttpMessageConverters list:

public @ResponseBody User getUser(@PathVariable String username) {
        
    User user = userService.getUser(username);
        
    return user;
        
}

Spring XML Configuration

1. Create a personalized mapper, as described in the linked tutorial:

<bean class="com.pastelstudios.json.HibernateAwareObjectMapper">

2. Configure the MappingJackson2HttpMessageConverter to use the personalized mapper:

<mvc:message-converters>
    <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
        <property name="objectMapper" ref="hibernateAwareObjectMapper" />
    </bean>
</mvc:message-converters>

By implementing this solution, Jackson is prevented from eagerly fetching unfetched lazy objects during serialization, resolving the exception and preserving the lazy loading behavior.

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