問題：JPA JoinColumn vs mappedBy

兩者的區別是什么呢

@Entity
public class Company {@OneToMany(cascade = CascadeType.ALL , fetch = FetchType.LAZY)@JoinColumn(name = "companyIdRef", referencedColumnName = "companyId")private List<Branch> branches;...
}

@Entity
public class Company {@OneToMany(cascade = CascadeType.ALL , fetch = FetchType.LAZY, mappedBy = "companyIdRef")private List<Branch> branches;...
}

回答一

mappedBy注解按道理應該用于雙向關系中的父類（company類）里面的。在這種情況下，它應該用在Company類指向它子類（Branch類）的成員變量’company’

JoinColumn注解通常被用于說明映射列（用于連接一個實體關系的），這個注解可以用在任何類里面（父類和子類），但是按道理來說只能用在一類（要么是父類要么是子類），這里的例子中，我是用在了雙向關系里的子類那里，去顯示出子類（Branch類）里面的一個外鍵

下面是一個例子:
父類，Company

@Entity
public class Company {private int companyId;private String companyName;private List<Branch> branches;@Id@GeneratedValue@Column(name="COMPANY_ID")public int getCompanyId() {return companyId;}public void setCompanyId(int companyId) {this.companyId = companyId;}@Column(name="COMPANY_NAME")public String getCompanyName() {return companyName;}public void setCompanyName(String companyName) {this.companyName = companyName;}@OneToMany(fetch=FetchType.LAZY,cascade=CascadeType.ALL,mappedBy="company")public List<Branch> getBranches() {return branches;}public void setBranches(List<Branch> branches) {this.branches = branches;}}

子類，Branch

@Entity
public class Branch {private int branchId;private String branchName;private Company company;@Id@GeneratedValue@Column(name="BRANCH_ID")public int getBranchId() {return branchId;}public void setBranchId(int branchId) {this.branchId = branchId;}@Column(name="BRANCH_NAME")public String getBranchName() {return branchName;}public void setBranchName(String branchName) {this.branchName = branchName;}@ManyToOne(fetch=FetchType.LAZY)@JoinColumn(name="COMPANY_ID")public Company getCompany() {return company;}public void setCompany(Company company) {this.company = company;}}

回答二

@JoinColumn指明了這個實體是關系的擁有者（對應著一個擁有外鍵的表和它的參照表）。但是mappedBy屬性指明的是在這一邊的實體是一個逆向的關系，而擁有者是在另一個實體里面的。這也意味著你可以在這個類里面使用到你標注了mappedBy其他表（完全雙向關系）

另外，對于問題中的代碼來說，正確的注解應該是下面這樣的：

@Entity
public class Company {@OneToMany(mappedBy = "company",orphanRemoval = true,fetch = FetchType.LAZY,cascade = CascadeType.ALL)private List<Branch> branches;
}@Entity
public class Branch {@ManyToOne(fetch = FetchType.LAZY)@JoinColumn(name = "companyId")private Company company;
}

文章翻譯自Stack Overflow：https://stackoverflow.com/questions/11938253/jpa-joincolumn-vs-mappedby