Google App Engine 中尝试获取实体中的字段值时出现 NullPointerException 异常

Question

我有一个学生实体，其中有一个字段是对这样的部门实体的引用。 @Load private @Index Ref<Department> department;
然后 Department 字段的 getter 和 setter 如下所示

 public Department getDepartment() {
    return department.get();
}

public void setDepartment(Key<Department> department) {
    this.department = Ref.create(department);
}

学生实体中还有一个字段，其中包含对 SchoolFaculty 实体的引用

@Load
private @Index Ref<SchoolFaculty> schoolFaculty;

有以下getter和setter

  public SchoolFaculty getSchoolFaculty() {
      if (schoolFaculty != null) 
           return schoolFaculty.get();
 return null;     
}

public void setSchoolFaculty(Key<SchoolFaculty> schoolFaculty) { 
    this.schoolFaculty = Ref.create(schoolFaculty); 
}

Department 实体中有一个字段，其中包含 Faculty 实体的键，如下所示

@Index
private Key<SchoolFaculty> schoolFaculty;

字段的 getter 和 setter 如下所示

 public String getSchoolFaculty() {
    return schoolFaculty.toWebSafeString();
}


public void setSchoolFaculty(Key<SchoolFaculty> schoolFaculty) {
    this.schoolFaculty = schoolFaculty;
}

我使用 google Api Explorer 在本地数据存储中创建了部门和 SchoolFaculty 实体，部门将其 SchoolFaculty 引用设置为有效值，该值也存在于 SchoolFaculty table，那是该部门有教员，但是当我尝试使用以下代码设置学生的 SchoolFaculty 时，我得到了 NullPointerException。这是代码

 student.setDepartment(Key.create(Department.class,student_programme.getDepartmentRef()));// student_programme is a reference to an Entity which I use to get the department and it works fine and the student gets its Department set to the Department Value.
        if(student.getDepartment().getSchoolFaculty()) != null){     student.setSchoolFaculty(Key.create(SchoolFaculty.class,getDepartment().getSchoolFaculty()));}// this  is the line throws a null pointer Exception

我正在为数据存储事务使用 objectify，并且已添加对 objectify 依赖项的所有引用。

Answer 1

因为 student.getDepartment().getSchoolFaculty() 在触发条件语句之前抛出空指针，并且 google appengine 键无法为空实体或 websafestring 创建键。因此，请仔细调试并检查 getDepartment() 是否首先返回 null 或它 returns 的内容，如果要使用 Key 创建带有数据存储的键的空字段，则使用 默认值 例如

Key.create(SchoolFaculty.class, "default"); // when it is null

希望这有用

Google App Engine 中尝试获取实体中的字段值时出现 NullPointerException 异常

NullPointerException Exception when trying to get the value of a field in an Entity in Google App Engine

java

google-app-engine

google-cloud-platform

google-cloud-datastore

objectify