如何將實體對象綁定到本地@Transient屬性？

Question

EmployeeInfo實體類

@Entity
@Table(name = "employeeinfo")
public class EmployeeInfo{

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "id")
private Long id;
@Column(name = "employeeId")
private String employeeId;
@Column(name = "firstName")
private String firstName;
@Column(name = "middleName")
private String middleName;
@Column(name = "lastName")
private String lastName;

......

}

另一個Entity類ProjectTaskComments

@Entity
@Table(name = "projecttaskcomments")
public class ProjectTaskComments{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "id")
    private Long id;
    @Basic(optional = false)
    @Column(name = "comments")
    private String comments;

    @Basic(optional = false)
    @Column(name = "commentTime")
    @Temporal(TemporalType.TIMESTAMP)
    private Date commentTime;
    @Column(name = "fkCommentedBy")
    private Long fkCommentedBy;


    @Transient
    @JsonIgnoreProperties
    private EmployeeInfo commentedEmployee;
    @Transient
    @Autowired
    EmployeeInfoService employeeInfoService; 


   public EmployeeInfo getCommentedEmployee() {
        EmployeeInfo employeeInfo;
        employeeInfo = employeeInfoService.getSingleEmployeeInfoByFkUserId(this.fkCommentedBy);
        if(employeeInfo != null) {
            this.commentedEmployee.setEmployeeId(employeeInfo.getEmployeeId());
            this.commentedEmployee.setFirstName(employeeInfo.getFirstName());
            this.commentedEmployee.setMiddleName(employeeInfo.getMiddleName());
            this.commentedEmployee.setLastName(employeeInfo.getLastName());
            this.commentedEmployee.setPhoto(employeeInfo.getPhoto());
            return commentedEmployee;
        } else {
            return null;
        }
    }

}

我試圖通過fkCommentedBy屬性在getCommentedEmployee（）方法中找到一個EmployeeInfo對象，並設置為@Transient屬性commentedEmployee。

我發現以下錯誤：

2018-10-11 13:07:56.834  WARN 16756 --- [nio-8081-exec-2] .w.s.m.s.DefaultHandlerExceptionResolver : Failed to write HTTP message: org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: (was java.lang.NullPointerException); nested exception is com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: java.util.ArrayList[1]->com.activeboss.model.pm.ProjectTasks["projecttaskcommentsCollection"]->org.hibernate.collection.internal.PersistentBag[0]->com.activeboss.model.pm.ProjectTaskComments["commentedEmployee"])
2018-10-11 13:07:56.853  WARN 16756 --- [nio-8081-exec-2] .w.s.m.s.DefaultHandlerExceptionResolver : Resolved exception caused by Handler execution: org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: (was java.lang.NullPointerException); nested exception is com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: java.util.ArrayList[1]->com.activeboss.model.pm.ProjectTasks["projecttaskcommentsCollection"]->org.hibernate.collection.internal.PersistentBag[0]->com.activeboss.model.pm.ProjectTaskComments["commentedEmployee"])

我該如何解決？

Answer 1

@Transient的目的是為非持久屬性建模，因此我不清楚為什么要在通過“ fkCommentedBy”屬性持久化commentedByEmployee屬性時對@transient進行建模。 IMO，@ ManyToOne在這種情況下更合適。

@Entity
@Table(name = "projecttaskcomments")
public class ProjectTaskComments {

// .... other declarations 
@ManyToOne
@JoinColumn(name="fkCommentedBy")
private EmployeeInfo commentedEmployee;
// ..... other code
}

現在，如果仍然要使用@Transient，則需要在getter方法中確保對EmployeeInfoService對象具有有效的引用。 @Autowired在這里不起作用，因為ProjectTaskComments不是spring托管的bean。

Answer 2

同意@KishoreKirdat，需要檢查null並進行一些初始化：

public EmployeeInfo getCommentedEmployee() {
  // check here
  if (employeeInfoService == null) return null;

  EmployeeInfo employeeInfo = employeeInfoService.getSingle...;
  if (employeeInfo != null) {
    // init here
    commentedEmployee = new EmployeeInfo();

    commentedEmployee.set...;
    return commentedEmployee;
  } else {
    return null;
  }
}

private void setCommentedEmployee(EmployeeInfo employeeInfo) {
  // do nothing
}

Answer 3

是的，終於可以解決了。 我剛剛做了以下工作：

1. 在ProjectTaskComments類中添加了@Component：

    @Entity @Component @Table(name = "projecttaskcomments") public class ProjectTaskComments{ ........ 

2. 將EmployeeInfoService聲明為靜態，並為該服務添加了一個seter方法，並對其進行@Autowired。

    @Transient private static EmployeeInfoService employeeInfoService; @Autowired public void setEmployeeInfoService(EmployeeInfoService employeeInfoService) { this.employeeInfoService = employeeInfoService; }