JsonMappingException: could not initialize proxy - no Session

Question

I am building a RESTful Web Service that consumes and returns JSON. I am encountering the following stack trace when I try to fetch an ESRBRating object from the database th

Answer 1

@JsonIgnore annotation might solve your problem, but it causes that particular field that you initially set as FetchType.LAZY to be completely ignored. A better alternative is given here https://stackoverflow.com/a/21760361/3609067

Answer 2

This typically happens when you are returning an object via @Responsebody (or in your case response body by way of @RestController) and an object is being serialized but has children in a LAZY collection that have not been referenced. By the time you are in your controller there is no longer a transaction active that will facilitate them being fetched (like the one your started in your @Service). You can either make your fetch strategy EAGER, bring in your collections by reference while still in your transaction or make your LAZY collections JSON Transient.

Answer 3

First download jackson-datatype-hibernate4-2.2.3.jar or higher version according to jackson-core version. Then use this code in config file and you can use fetch strategy LAZY without any error.

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.datatype.hibernate4.Hibernate4Module;


public class ApplicationConfig extends WebMvcConfigurerAdapter
{
public MappingJackson2HttpMessageConverter jacksonMessageConverter(){
    MappingJackson2HttpMessageConverter messageConverter = new  MappingJackson2HttpMessageConverter();

    ObjectMapper mapper = new ObjectMapper();
    //Registering Hibernate4Module to support lazy objects
    mapper.registerModule(new Hibernate4Module());

    messageConverter.setObjectMapper(mapper);
    return messageConverter;

}

@Override
public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
    //Here we add our custom-configured HttpMessageConverter
    converters.add(jacksonMessageConverter());
    super.configureMessageConverters(converters);
}

Answer 4

Solved for Spring Boot 2.3.0 by adding to gradle.build:

implementation 'com.fasterxml.jackson.datatype:jackson-datatype-hibernate5:2.11.3'

And declaring one more bean:

@Bean
public Module datatypeHibernateModule() {
    return new Hibernate5Module();
}