How to Avoid Jackson Serialization Errors with Lazy-Loaded Hibernate Objects?

Avoiding Jackson Serialization of Non-Fetched Lazy Objects

Jackson, a popular Java library for data binding, can encounter issues when serializing objects with lazy-fetched properties. In this case, the unfetched property, which is not loaded into the session, causes a "could not initialize proxy - no Session" exception during serialization.

Solution:

To resolve this issue, we need to inform Jackson to ignore lazy-fetched properties during serialization. In older versions of Spring (3.1 and below), this was achieved by creating a custom HibernateAwareObjectMapper and adding it to a custom MappingJackson2HttpMessageConverter.

However, in Spring versions 3.1.2 and above, Spring provides its own MappingJackson2HttpMessageConverter that includes the necessary functionality. We can configure this converter and add it to the application's HttpMessageConverters.

Implementation:

Java Configuration:

@Configuration
@EnableWebMvc
public class MyConfigClass extends WebMvcConfigurerAdapter {

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

    public MappingJackson2HttpMessageConverter jacksonMessageConverter() {
        MappingJackson2HttpMessageConverter messageConverter = new MappingJackson2HttpMessageConverter();

        ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(new Hibernate4Module());

        messageConverter.setObjectMapper(mapper);
        return messageConverter;
    }

}

XML Configuration:

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

By configuring the MappingJackson2HttpMessageConverter to use Hibernate4Module, Jackson will be instructed to ignore lazy-fetched properties during serialization. This resolves the issue of uninitialized proxies and allows your controller to return User objects without encountering serialization errors.

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