JoinColumn vs mappedBy

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Stackoverflow有一道題JoinColumn vs mappedBy很有意思：

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

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

上面兩種級聯有什麼區別？

原題下面有兩個高分答案。可惜對於我這種學渣，看完還是一頭霧水。

看不懂別人寫的，那就自己調調看。

準備環境

磨刀不誤砍柴工，先準備調試環境。

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裡開啟Hibernate的調試選項。

spring:  jpa:    properties:      hibernate:        show_sql: true  # 列印SQL語句        format_sql: true  # 格式化SQL語句        use_sql_comments: true  # 增加註釋資訊，就知道語句對應的Entity類型了        generate_statistics: true  # 統計資訊，給出了每一步的耗時資訊

還要在log配置開啟org.hibernate.type的debug層級的日誌資訊。我用的是log4j2，就需要在log4j2.yml檔案中添加:

Configuration:  Loggers    Logger:      - name: org.hibernate.type        additivity: false        level: trace  # 這個最關鍵        AppenderRef:          - ref: CONSOLE          - ref: ROLLING_FILE

第一次測試

原題裡使用的是“公司-部門”模型，我這裡本地已經有現成的“部門-僱員”模型，就直接複用了。道理是一樣的。

@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}

在這裡，Department使用主鍵id一對多的關聯於Employee的departmentId屬性。

然後就是儲存一個對象看看。

@Servicepublic 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);    }}

啟動之後發現log裡是這樣列印的。

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‘)

第二次測試

第二次將Department的級聯註解做了修改。

@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<>();

列印的log是這樣子。

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)}

簡單分析

對比log，我們可以看出，兩次持久化新的employee對象時，都會：

1. 執行一次left join查詢語句，查詢department關聯的所有employee對象
2. 每一個新的employee對象，都會執行一次插入操作

不同之處在於，第二次使用JoinColumn註解的時候，log顯示：每一個新增employee對象都執行了一次update語句，更新了外鍵。

原因就在於，mappedBy將外鍵的賦值操作委託給了Employee對象。而JoinColumn則選擇由Department對象自己來約束外鍵的關聯。

兩個註解只有少許區別，但是最終的執行結果差異卻很大。多出來的寫操作，在生產環境下很容易對資料庫構成很大的壓力。在代碼中完成對Employee對象departmentId屬性的賦值，顯然是一個更為合適的方案。

實際上，上面兩個方案是JPA API文檔裡OneToMany註解的樣本2、樣本3.

樣本1的雙向註解，也可以避免多餘的寫操作。

參考資料

1. [JavaEE - JPA] 效能最佳化: 如何定位效能問題
2. The best way to map a @OneToMany relationship with JPA and Hibernate

JoinColumn vs mappedBy