学向勤中得，萤窗万卷书。这篇文章主要讲述JoinColumn vs mappedBy相关的知识，希望能为你提供帮助。

Stackoverflow有一道题JoinColumn vs mappedBy很有意思：

@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; ... }

上面两种级联有什么区别？

原题下面有两个高分答案。可惜对于我这种学渣，看完还是一头雾水。
看不懂别人写的，那就自己调调看。
准备环境磨刀不误砍柴工，先准备调试环境。
我用的是Spring Boot 2.0 + Spring Data JPA 2.0， Hibernate版本是5.2.14。
首先在Spring Boot的application.yml里开启Hibernate的调试选项。

spring: jpa: properties: hibernate: show_sql: true# 打印SQL语句 format_sql: true# 格式化SQL语句 use_sql_comments: true# 增加注释信息，就知道语句对应的Entity类型了 generate_statistics: true# 统计信息，给出了每一步的耗时信息

还要在log配置开启org.hibernate.type的debug级别的日志信息。我用的是log4j2，就需要在log4j2.yml文件中添加:

Configuration: Loggers Logger: - name: org.hibernate.type additivity: false level: trace# 这个最关键 AppenderRef: - ref: CONSOLE - ref: ROLLING_FILE

第一次测试原题里使用的是“公司-部门”模型，我这里本地已经有现成的“部门-雇员”模型，就直接复用了。道理是一样的。

@Entity @Table(name="department_demo") public class Department {@Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer id; private String name; @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.EAGER, mappedBy = "departmentId") private Set< Employee> employeeSet = new HashSet< > (); // setters & getters }

@Entity @Table(name="employee_demo") public class Employee {@Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer id; private String name; private Integer departmentId; // setters & getters }

在这里，Department使用主键id一对多的关联于Employee的departmentId属性。
然后就是保存一个对象看看。

@Service public class DepartmentService {@Autowired DepartmentRepository departmentRepository; @PostConstruct public void insertNewRecord() {Department department = new Department(); department.setName("Leader"); departmentRepository.save(department); Employee emily = new Employee(); emily.setName("David"); Employee alice = new Employee(); alice.setName("Wang Dali"); department.addEmployee(emily); department.addEmployee(alice); departmentRepository.save(department); } }

【JoinColumn vs mappedBy】启动之后发现log里是这样打印的。

2018-04-22 09:11:22,155:INFO restartedMain (StatisticalLoggingSessionEventListener.java:258) - Session Metrics { 0 nanoseconds spent acquiring 0 JDBC connections; 0 nanoseconds spent releasing 0 JDBC connections; 0 nanoseconds spent preparing 0 JDBC statements; 0 nanoseconds spent executing 0 JDBC statements; 0 nanoseconds spent executing 0 JDBC batches; 0 nanoseconds spent performing 0 L2C puts; 0 nanoseconds spent performing 0 L2C hits; 0 nanoseconds spent performing 0 L2C misses; 0 nanoseconds spent executing 0 flushes (flushing a total of 0 entities and 0 collections); 0 nanoseconds spent executing 0 partial-flushes (flushing a total of 0 entities and 0 collections) } Hibernate: /* insert com.example.demo.model.PO.Department */ insert into department_demo (name) values (?) 2018-04-22 09:11:22,314:TRACE restartedMain (BasicBinder.java:65) - binding parameter [1] as [VARCHAR] - [Leader] Hibernate: select currval(‘department_demo_id_seq‘) 2018-04-22 09:11:22,438:INFO restartedMain (StatisticalLoggingSessionEventListener.java:258) - Session Metrics { 733681 nanoseconds spent acquiring 1 JDBC connections; 0 nanoseconds spent releasing 0 JDBC connections; 6711770 nanoseconds spent preparing 2 JDBC statements; 75097150 nanoseconds spent executing 2 JDBC statements; 0 nanoseconds spent executing 0 JDBC batches; 0 nanoseconds spent performing 0 L2C puts; 0 nanoseconds spent performing 0 L2C hits; 0 nanoseconds spent performing 0 L2C misses; 14646304 nanoseconds spent executing 1 flushes (flushing a total of 1 entities and 1 collections); 0 nanoseconds spent executing 0 partial-flushes (flushing a total of 0 entities and 0 collections) } Hibernate: /* load com.example.demo.model.PO.Department */ select department0_.id as id1_1_1_, department0_.name as name2_1_1_, employeese1_.department_id as departme2_2_3_, employeese1_.id as id1_2_3_, employeese1_.id as id1_2_0_, employeese1_.department_id as departme2_2_0_, employeese1_.name as name3_2_0_ from department_demo department0_ left outer join employee_demo employeese1_ on department0_.id=employeese1_.department_id where department0_.id=? 2018-04-22 09:11:22,460:TRACE restartedMain (BasicBinder.java:65) - binding parameter [1] as [INTEGER] - [33] 2018-04-22 09:11:22,499:TRACE restartedMain (BasicExtractor.java:51) - extracted value ([id1_2_0_] : [INTEGER]) - [null] 2018-04-22 09:11:22,502:TRACE restartedMain (BasicExtractor.java:61) - extracted value ([name2_1_1_] : [VARCHAR]) - [Leader] 2018-04-22 09:11:22,503:TRACE restartedMain (BasicExtractor.java:51) - extracted value ([departme2_2_3_] : [INTEGER]) - [null] Hibernate: /* insert com.example.demo.model.PO.Employee */ insert into employee_demo (department_id, name) values (?, ?) 2018-04-22 09:11:22,516:TRACE restartedMain (BasicBinder.java:65) - binding parameter [1] as [INTEGER] - [33] 2018-04-22 09:11:22,516:TRACE restartedMain (BasicBinder.java:65) - binding parameter [2] as [VARCHAR] - [David] Hibernate: select currval(‘employee_demo_id_seq‘) Hibernate: /* insert com.example.demo.model.PO.Employee */ insert into employee_demo (department_id, name) values (?, ?) 2018-04-22 09:11:22,526:TRACE restartedMain (BasicBinder.java:65) - binding parameter [1] as [INTEGER] - [33] 2018-04-22 09:11:22,527:TRACE restartedMain (BasicBinder.java:65) - binding parameter [2] as [VARCHAR] - [Wang Dali] Hibernate: select currval(‘employee_demo_id_seq‘)

第二次测试第二次将Department的级联注解做了修改。

@Entity @Table(name="department_demo") public class Department {@Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer id; private String name; @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.EAGER) @JoinColumn(name="departmentId", referencedColumnName = "id") private Set< Employee> employeeSet = new HashSet< > ();

打印的log是这样子。

2018-04-22 09:16:29,366:INFO restartedMain (StatisticalLoggingSessionEventListener.java:258) - Session Metrics { 0 nanoseconds spent acquiring 0 JDBC connections; 0 nanoseconds spent releasing 0 JDBC connections; 0 nanoseconds spent preparing 0 JDBC statements; 0 nanoseconds spent executing 0 JDBC statements; 0 nanoseconds spent executing 0 JDBC batches; 0 nanoseconds spent performing 0 L2C puts; 0 nanoseconds spent performing 0 L2C hits; 0 nanoseconds spent performing 0 L2C misses; 0 nanoseconds spent executing 0 flushes (flushing a total of 0 entities and 0 collections); 0 nanoseconds spent executing 0 partial-flushes (flushing a total of 0 entities and 0 collections) } Hibernate: /* insert com.example.demo.model.PO.Department */ insert into department_demo (name) values (?) 2018-04-22 09:16:29,478:TRACE restartedMain (BasicBinder.java:65) - binding parameter [1] as [VARCHAR] - [Leader] Hibernate: select currval(‘department_demo_id_seq‘) 2018-04-22 09:16:29,513:INFO restartedMain (StatisticalLoggingSessionEventListener.java:258) - Session Metrics { 743154 nanoseconds spent acquiring 1 JDBC connections; 0 nanoseconds spent releasing 0 JDBC connections; 4202462 nanoseconds spent preparing 2 JDBC statements; 9628594 nanoseconds spent executing 2 JDBC statements; 0 nanoseconds spent executing 0 JDBC batches; 0 nanoseconds spent performing 0 L2C puts; 0 nanoseconds spent performing 0 L2C hits; 0 nanoseconds spent performing 0 L2C misses; 11708469 nanoseconds spent executing 1 flushes (flushing a total of 1 entities and 1 collections); 0 nanoseconds spent executing 0 partial-flushes (flushing a total of 0 entities and 0 collections) } Hibernate: /* load com.example.demo.model.PO.Department */ select department0_.id as id1_1_1_, department0_.name as name2_1_1_, employeese1_.department_id as departme2_2_3_, employeese1_.id as id1_2_3_, employeese1_.id as id1_2_0_, employeese1_.department_id as departme2_2_0_, employeese1_.name as name3_2_0_ from department_demo department0_ left outer join employee_demo employeese1_ on department0_.id=employeese1_.department_id where department0_.id=? 2018-04-22 09:16:29,529:TRACE restartedMain (BasicBinder.java:65) - binding parameter [1] as [INTEGER] - [34] 2018-04-22 09:16:29,538:TRACE restartedMain (BasicExtractor.java:51) - extracted value ([id1_2_0_] : [INTEGER]) - [null] 2018-04-22 09:16:29,541:TRACE restartedMain (BasicExtractor.java:61) - extracted value ([name2_1_1_] : [VARCHAR]) - [Leader] 2018-04-22 09:16:29,542:TRACE restartedMain (BasicExtractor.java:51) - extracted value ([departme2_2_3_] : [INTEGER]) - [null] Hibernate: /* insert com.example.demo.model.PO.Employee */ insert into employee_demo (department_id, name) values (?, ?) 2018-04-22 09:16:29,552:TRACE restartedMain (BasicBinder.java:65) - binding parameter [1] as [INTEGER] - [34] 2018-04-22 09:16:29,552:TRACE restartedMain (BasicBinder.java:65) - binding parameter [2] as [VARCHAR] - [Wang Dali] Hibernate: select currval(‘employee_demo_id_seq‘) Hibernate: /* insert com.example.demo.model.PO.Employee */ insert into employee_demo (department_id, name) values (?, ?) 2018-04-22 09:16:29,555:TRACE restartedMain (BasicBinder.java:65) - binding parameter [1] as [INTEGER] - [34] 2018-04-22 09:16:29,556:TRACE restartedMain (BasicBinder.java:65) - binding parameter [2] as [VARCHAR] - [David] Hibernate: select currval(‘employee_demo_id_seq‘) Hibernate: /* create one-to-many row com.example.demo.model.PO.Department.employeeSet */ update employee_demo set department_id=? where id=? 2018-04-22 09:16:29,576:TRACE restartedMain (BasicBinder.java:65) - binding parameter [1] as [INTEGER] - [34] 2018-04-22 09:16:29,577:TRACE restartedMain (BasicBinder.java:65) - binding parameter [2] as [INTEGER] - [30] Hibernate: /* create one-to-many row com.example.demo.model.PO.Department.employeeSet */ update employee_demo set department_id=? where id=? 2018-04-22 09:16:29,595:TRACE restartedMain (BasicBinder.java:65) - binding parameter [1] as [INTEGER] - [34] 2018-04-22 09:16:29,596:TRACE restartedMain (BasicBinder.java:65) - binding parameter [2] as [INTEGER] - [29] 2018-04-22 09:16:29,600:INFO restartedMain (StatisticalLoggingSessionEventListener.java:258) - Session Metrics { 225339 nanoseconds spent acquiring 1 JDBC connections; 0 nanoseconds spent releasing 0 JDBC connections; 731494 nanoseconds spent preparing 7 JDBC statements; 24985288 nanoseconds spent executing 7 JDBC statements; 0 nanoseconds spent executing 0 JDBC batches; 0 nanoseconds spent performing 0 L2C puts; 0 nanoseconds spent performing 0 L2C hits; 0 nanoseconds spent performing 0 L2C misses; 35806114 nanoseconds spent executing 1 flushes (flushing a total of 3 entities and 1 collections); 0 nanoseconds spent executing 0 partial-flushes (flushing a total of 0 entities and 0 collections) }

简单分析对比log，我们可以看出，两次持久化新的employee对象时，都会：

1. 执行一次left join查询语句，查询department关联的所有employee对象
2. 每一个新的employee对象，都会执行一次插入操作

不同之处在于，第二次使用JoinColumn注解的时候，log显示：每一个新增employee对象都执行了一次update语句，更新了外键。
原因就在于，mappedBy将外键的赋值操作委托给了Employee对象。而JoinColumn则选择由Department对象自己来约束外键的关联。
两个注解只有少许区别，但是最终的执行结果差异却很大。多出来的写操作，在生产环境下很容易对数据库构成很大的压力。在代码中完成对Employee对象departmentId属性的赋值，显然是一个更为合适的方案。
实际上，上面两个方案是JPA API文档里OneToMany注解的示例2、示例3.
示例1的双向注解，也可以避免多余的写操作。
参考资料

1. [JavaEE - JPA] 性能优化: 如何定位性能问题
2. The best way to map a @OneToMany relationship with JPA and Hibernate

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