Proper way to convert Spring HATEOAS Link to Object - spring

Question

I have a very simple controller the makes an HTTP request and receives some resources in HATEOAS format.
package com.provider.spring.controller;
import java.util.List;
import org.springframework.hateoas.Link;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.client.RestTemplate;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.provider.employee.EmployeeDTO;
import com.provider.http.HttpComponentsClientHttpRequestFactoryBasicAuth;
import com.provider.spring.rest.Resource;
#Controller
public class EmployeeController {
private static final String REL_SELF = "self";
private static final String REL_SEARCH = "search";
private static final String REL_EMPLOYEE = "employee";
private static final String RESOURCE_URI = "http://localhost:8080/employees";
private RestTemplate restTemplate = new RestTemplate(new HttpComponentsClientHttpRequestFactoryBasicAuth("user", "password"));
private List<EmployeeDTO> employees;
#RequestMapping("/employees")
public String getAllEmployees() {
String result = null;
try {
String resultBody = restTemplate.getForObject(RESOURCE_URI, String.class);
ObjectMapper objectMapper = new ObjectMapper();
Resource<EmployeeDTO> resource = objectMapper.readValue(resultBody, Resource.class);
// Get objects with relation "employee"
for(Link l : resource.getLinks()) {
if(l.getRel().equals(REL_EMPLOYEE)) {
// TODO: Construct EmployeeDTO from Link.
// TODO: Add EmployeeDTO to list.
}
}
}
catch(Exception e) {
e.printStackTrace();
result = "error";
return result;
}
return result;
}
}
Is there an existing prefered or standard way of converting Links to Objects?
See here for details on the EmployeeDTO: https://gist.github.com/Xerosigma/64469a30355f5de0228a

Answer 1

A couple ways to do this with Jackson:
Without custom converter:
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.registerModule(new Jackson2HalModule());
RestTemplate restTemplate= new RestTemplate();
String resultBody = restTemplate.getForObject(link.getHref(), String.class);
EmployeeDTO resource = objectMapper.readValue(resultBody, EmployeeDTO.class);
With custom converter:
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.registerModule(new Jackson2HalModule());
RestTemplate restTemplate= new RestTemplate();
MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
converter.setObjectMapper(objectMapper);
restTemplate.setMessageConverters(Collections.<HttpMessageConverter<?>> singletonList(converter));
EmployeeDTO resource = restTemplate.getForObject(link.getHref(), EmployeeDTO.class);

Answer 2

You can register some MessageConverters in the constructor of the RestTemplate or use the setMessageConverters. Both take a List of HttpMessageConverters as a parameter.
If you have it configured like that, you can use the exchange or the getForObject-method with your expected resource-type. The RestTemplate will run through all the converters and check if it can convert the json/xml/... to your object.
Here is an example:
List<HttpMessageConverter<?>> converters = new LinkedList<>();
converters.add(new MappingJacksonHttpMessageConverter());
RestTemplate template = new RestTemplate(converters);
// or use the set method
template.setMessageConverters(converters);
// make url call
Resource<EmployeeDTO> resource = template.getForObject("url", Resource.class);