錯誤發生的情境如下，一個部門Department有多個員工Employee的一對多/多對一關聯。
Employee.java
@Entity
@Table(name="Department")
public class Department {
@id
@Column(name="DepartmentId")
private int departmentId;
@OneToMany(mappedBy="employeeId", cascade={CascadeType.PERSIST, CascadeType.MERGE}) // 串聯新增和更新
private List<Employee> employeeList;
// getter and setter ommitted
}
Department.java
@Entity
@Table(name="Employee")
public class Employee {
@id
@Column(name="EmployeeId")
private int employeeId;
@ManyToOne
@JoinColumn(name="departmentId")
private Department department;
// getter and setter ommitted
}
以上設定在串聯新增Department及所屬的Employee時沒有問題，但串聯更新(update)時出現了未知的StackOverflowError。
最後是透過在@ManyToOne設定fetch=FetchType.LAZY解決。
Employee.java
@Entity
@Table(name="Department")
public class Department {
@id
@Column(name="DepartmentId")
private int departmentId;
@OneToMany(mappedBy="employeeId", cascade={CascadeType.PERSIST, CascadeType.MERGE}) // 串聯新增和更新
private List<Employee> employeeList;
// getter and setter ommitted
}
Department.java
@Entity
@Table(name="Employee")
public class Employee {
@id
@Column(name="EmployeeId")
private int employeeId;
@ManyToOne(fetch=FetchType.LAZY) // @ManyToOne JPA預設的fetch=FetchType.EAGER，改為FetchType.LAZY
@JoinColumn(name="departmentId")
private Department department;
// getter and setter ommitted
}
查了一下錯誤原因脫不了無限迴圈的問題，我猜是因為@ManyToOne預設為FetchType.EAGER，我的程式在執行時會先查詢Department物件，因此所屬的Employee與關聯的Department（即被查詢的Department本身）會同時一併取出，而在更新查出的Department及關聯的Employee時，Employee所屬的Department物件一併被影響，形成無限迴圈而導致StackOverflowError。
而改為@ManyToOne(fetch=FetchType.LAZY)，則查詢時Employee關聯的Department並未一起取出（只有在存取時才會查詢），所以之後更新查詢出的實體時不會導致無窮迴圈。
另外在網路上找解法時，發現StackOverflowError錯誤也會出現在當覆寫toString()方法中彼此呼叫了關聯物件的方法，在查詢時會引發。
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