Malformed JSON: Unexpected '<' in Spring

Question

I want to produce json from JPA @Entity, I have

@Entity
@JsonAutoDetect
public class Bar implements Serializable { 
 @Id
 @GeneratedValue
 private Integer id;
 private String title;
 //omitting other stuff
}

my controller is

@RestController
public class BarController {
 @Autowired
 private BarService barService;
 @RequestMapping(value = "/", method = RequestMethod.GET, headers = "Accept=application/json", produces={"application/json"})
 public List<Bar> list() { 
 return barService.findAllBars());
 }
}

I'm having this error in browser

HTTP 406

and in Postman Postman Malformed JSON

what is wrong with it.

• legendofawesomeness: Are you serializing your list to JSON before returning it?
• chrylis -cautiouslyoptimistic-: I don't know how you're configuring Spring, but this often means you don't have JSON converters installed. I use Spring Boot, which does that for me.
• Arshad Ali: @chrylis Spring Boot is not the choice then??
• chrylis -cautiouslyoptimistic-: Spring Boot is the choice. A plain Boot setup wouldn't have this issue.

Answer 1

The "Malformed JSON" message is from the "Pretty" printing. Click "Raw" to see the actual response.

The actual response is a 406 Not Acceptable error (says so on your screen) with a payload of HTML (hence the unexpected < from "Pretty") that says the request has been rejected by the server.

Remove the headers = "Accept=application/json" from the @RequestMapping. The produces={"application/json"} is already telling Spring to only call this method if application/json is an acceptable response, which it likely is, but the header might say *.*, or something more complex like text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8, both of which will allow application/json.

Of course, since this is likely an AJAX call that is expecting JSON, it should have listed only application/json in the accept value. Check the code executing the AJAX call if that is not the case.

Answer 2

I caught up one fatal mistake which is you are getting list of Bars barService.findAllBars(), you may need to convert that list to json adding the method as

public static String toJSON(Object object) 
 { 
 if ( object == null ){
 return "{}"; 
 } 
 try { 
 ObjectMapper mapper = new ObjectMapper(); 
 return mapper.writeValueAsString(object); 
 } 
 catch (Exception e) { 
 e.printStackTrace(); 
 } 
 return "{}"; 
 } 

Now make change as

@RequestMapping(value = "/", method = RequestMethod.GET, produces={"application/json"})
 public String list() {
 return toJSON(barService.findAllBars());
 }

hope this works, if any issues feel free to query in comments session.

• chrylis -cautiouslyoptimistic-: Spring has built-in support for handling that conversion in a more flexible and reliable way.
• Logical Error: It helped, but when I have some join and have a field like @ManyToMany(cascade = {CascadeType.MERGE}) @JoinTable(name = "foos_bars", joinColumns = @JoinColumn(name = "foos_id"), inverseJoinColumns = @JoinColumn(name = "bars_id")) private List<Foos> foos; i get >org.codehaus.jackson.map.JsonMappingException: failed to lazily initialize a collection of role: pkj.Bar.foos, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]->pkj.entity.Bar["foos"]), how to solve this one
• Arshad Ali: @chrylis any example please in that case??
• BoltClock: Can you stop harassing chrylis? Asking once is enough. You might want to take a good look at your own comments before flagging others as rude.
• kryger: @ArshadAli It all started here: spring.io/blog/2010/01/25/ajax-simplifications-in-spring-3-0. You're essentially suggesting reverting to using pre-2010 Spring instead of addressing the real problem.

Answer 3

I suspect there is something amiss with your accept header in your get request. Try setting the header to

"Accept=*/*"

and see what you get back.

Answer 4

Use JsonFormatter https://jsonformatter.curiousconcept.com/ to test the JSON before proceeding further. Its a very good tool that validates JSON and shows you possible errors with line number.