JsonMappingException when using a Set instead of a List

Question

I have a spring boot project with some entities, in specific, I have a Student Class with a DesiredCourses list which should be a Set<>.

When I use:

@OneToMany(mappedBy = "student", cascade = CascadeType.ALL)
public List<StudentDesiredCourseEntity> getStudentDesiredCourses() {
    return studentDesiredCourses;
}

public void setStudentDesiredCourses(List<StudentDesiredCourseEntity> studentDesiredCourses) {
    this.studentDesiredCourses = studentDesiredCourses;
}

Everything works fine but when I use

@OneToMany(mappedBy = "student", cascade = CascadeType.ALL)
public Set<StudentDesiredCourseEntity> getStudentDesiredCourses() {
    return studentDesiredCourses;
}

public void setStudentDesiredCourses(Set<StudentDesiredCourseEntity> studentDesiredCourses) {
    this.studentDesiredCourses = studentDesiredCourses;
}

I get

org.springframework.http.converter.HttpMessageNotReadableException",
"message":"Could not read JSON: (was java.lang.NullPointerException) (through reference chain: edu.cs6310.project4.entities.StudentEntity[\"studentDesiredCourses\"]->java.util.HashSet[0]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: edu.cs6310.project4.entities.StudentEntity[\"studentDesiredCourses\"]->java.util.HashSet[0])

Is there something I am missing or something extra that needs to be done?

As requested, the equals and hashcode

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof StudentDesiredCourseEntity)) return false;

    StudentDesiredCourseEntity that = (StudentDesiredCourseEntity) o;

    if (!course.equals(that.course)) return false;
    if (!priority.equals(that.priority)) return false;
    if (!student.equals(that.student)) return false;

    return true;
}

@Override
public int hashCode() {
    int result = priority.hashCode();
    result = 31 * result + course.hashCode();
    result = 31 * result + student.hashCode();
    return result;
}

I am wondering if the HashCode or Equals method may be the culprit do you have the HashCode and Equals for the StudentDesiredCourseEntity? — alexwen
– alexwen, Commented Apr 19, 2015 at 4:26
Ahhhh you know what, I never considered that my Student or Course could come back as null.... I'd bet that is the problem — Daniel Esponda
– Daniel Esponda, Commented Apr 19, 2015 at 15:05

Daniel Esponda · Accepted Answer · 2015-04-20 13:27:55Z

4

As mentioned by alexwen in comments, the reason this didn't work was due to not handling nulls in the hashcode/equals methods

answered Apr 20, 2015 at 13:27

Daniel Esponda

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Comments

Mostafa EL BARRAK · Accepted Answer · 2015-04-18 19:58:20Z

2

Jackson can't convert a json array into a hashSet. In order to do that, you need to create a custom Jackson Converter. Here is an example http://kdubblabs.com/java/retrofit-by-square/retrofit-using-jackson-json-conversion/

answered Apr 18, 2015 at 19:58

Mostafa EL BARRAK

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2 Comments

Daniel Esponda Over a year ago

I would think that is is a common enough request that it would be auto handled....

alexwen Over a year ago

Jackson supports Sets, I think something else may be going on here.

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