使用Set而不是List时的异常

Question

我有一个spring boot项目和一些实体，具体来说，我有一个学生班，有一个DesiredCourses列表，应该是一个集合

当我使用：

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

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

一切正常但当我使用

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

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

我明白了

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])

我有什么遗漏或需要做的额外工作吗？

根据请求，equals和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;
}

Answer 1

Jackson无法将json数组转换为hashSet。为了做到这一点，您需要创建一个自定义杰克逊转换器。这里有一个例子http://kdubblabs.com/java/retrofit-by-square/retrofit-using-jackson-json-conversion/

Answer 2

正如alexwen在评论中提到的，这种方法不起作用的原因是没有在hashcode/equals方法中处理null