JSON - Jackson to the rescue

Sometimes you have to fetch some data from the server in JavaScript, JSON is pretty good choice for this task.

Let's play with the Employer - Employee - Benefit example from the post JPA Demystified (episode 1) - @OneToMany and @ManyToOne mappings. We will use it inside the web application based on Spring Framework. Our first controller will return the employees list as the response body, in our case MappingJacksonHttpMessageConverter will be used automagically for converting the value returned by handleGet method to the response send to client.

@Controller
@RequestMapping("/employee-list.json")
public class EmployeeListController {
    @Autowired
    private EmployerDAO employerDAO;

    @RequestMapping(method = RequestMethod.GET)
    @ResponseBody
    public List<Employee> handleGet(@RequestParam("employerId") Long employerId) {
        return employerDAO.getEmployees(employerId);
    }
}

When we try to fetch the data for the first time, we encounter beautiful exception: JsonMappingException: Infinite recursion (StackOverflowError) - caused by bi-directional references between the Employer - Employee - Benefit.

Looking for the possible solution, I've found a note Handle bi-directional references using declarative method(s), and after reading it, I've corrected the domain entities in following way:

@Entity
@Table(name = "EMPLOYERS")
public class Employer implements Serializable {
...
    @JsonManagedReference("employer-employee")
    @OneToMany(mappedBy = "employer", cascade = CascadeType.PERSIST)
    public List getEmployees() {
        return employees;
    }
...
}

@Entity
@Table(name = "EMPLOYEES")
public class Employee implements Serializable {
...
    @JsonManagedReference("employee-benefit")
    @OneToMany(mappedBy = "employee", cascade = CascadeType.PERSIST)
    public List getBenefits() {
        return benefits;
    }

    @JsonBackReference("employer-employee")
    @ManyToOne(optional = false)
    @JoinColumn(name = "EMPLOYER_ID")
    public Employer getEmployer() {
        return employer;
    }
...
}

@Entity
@Table(name = "BENEFITS")
public class Benefit implements Serializable {
...
    @JsonBackReference("employee-benefit")
    @ManyToOne(optional = false)
    @JoinColumn(name = "EMPLOYEE_ID")
    public Employee getEmployee() {
        return employee;
    }
...
}

After performing the above changes, I could finally enjoy the JSON response returned by my code:

[{"id":1, "benefits":[{"name":"Healthy Employees", "id":1, "type":"HEALTH_COVERAGE", "startDate":1104534000000, "endDate":null}, {"name":"Gold Autumn","id":2,"type":"RETIREMENT_PLAN","startDate":1104534000000,"endDate":null},{"name":"Always Secured","id":3,"type":"GROUP_TERM_LIFE","startDate":1104534000000,"endDate":null}],"firstName":"John"},{"id":2,"benefits":[],"firstName":"Mary"},{"id":3,"benefits":[],"firstName":"Eugene"}]

And as usual some links for the dessert:

• JSON - JavaScript Object Notation
• Jackson - High-performance JSON processor