问题：JPA JoinColumn vs mappedBy

两者的区别是什么呢

@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注解按道理应该用于双向关系中的父类（company类）里面的。在这种情况下，它应该用在Company类指向它子类（Branch类）的成员变量’company’

JoinColumn注解通常被用于说明映射列（用于连接一个实体关系的），这个注解可以用在任何类里面（父类和子类），但是按道理来说只能用在一类（要么是父类要么是子类），这里的例子中，我是用在了双向关系里的子类那里，去显示出子类（Branch类）里面的一个外键

下面是一个例子:
父类，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;}}

子类，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指明了这个实体是关系的拥有者（对应着一个拥有外键的表和它的参照表）。但是mappedBy属性指明的是在这一边的实体是一个逆向的关系，而拥有者是在另一个实体里面的。这也意味着你可以在这个类里面使用到你标注了mappedBy其他表（完全双向关系）

另外，对于问题中的代码来说，正确的注解应该是下面这样的：

@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;
}

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