java - 在 Java 中使用 MyBatis 3 进行延迟加载

Question

我正在使用 Mybatis(3.2.7 版本)作为我的 JAVA 项目的 ORM 框架。因为我有 JPA 背景，所以我很想探索 Mybatis 支持的 LAZYLOADING。但我无法理解任何实质性内容。
(我正在使用 JAVA API 和注释配置 MYBATIS，仅用于查询目的)

根据 Mybatis 文档:<我>1。 lazyLoadingEnabled:默认值=TRUE

全局启用或禁用延迟加载。启用后，所有关系都将是惰性的加载。可以使用 fetchType 属性为特定关系取代此值在上面。

2。 aggressiveLazyLoading :默认值=TRUE

启用后，具有延迟加载属性的对象将在调用任何延迟属性时完全加载。否则，每个属性都按需加载。

使用以下属性，我尝试了以下代码:

一个。 JAVA 类:

反馈.java

public class Feedback implements Serializable {
private static final long serialVersionUID = 1L;

private int id;
private String message;

   /**
   * while loading Feedback, I want sender object to be lazily loaded
   */
private User sender;
private boolean seen;

// getters and setters
}

用户.java

public class User implements Serializable, {
private static final long serialVersionUID = 1L;
private int id;
private String email;

// getters and setters
}

数据库架构:

反馈表

                Table "public.feedback"

  Column | Type      |    Modifiers                       
-------------+-----------+-------------------------------------------------------
 id          | integer   | PRIMARY KEY
 seen        | boolean   | not null
 sender_id   | integer   | FOREIGN KEY (sender_id) REFERENCES users(id)
 message     | text      | 

用户表:

                Table "public.users"

Column   | Type     |     Modifiers                      
-------------+----------+----------------------------------------------------
id          | integer  | PRIMARY KEY
email       | text     | 

通过 JAVA API 配置 MyBatis:

DataSource dataSource = new PGSimpleDataSource();
        ((PGSimpleDataSource) dataSource).setServerName("localhost");
        ((PGSimpleDataSource) dataSource).setDatabaseName(dbName);
        ((PGSimpleDataSource) dataSource).setPortNumber(5432);
        ((PGSimpleDataSource) dataSource).setUser(new UnixSystem().getUsername());
        ((PGSimpleDataSource) dataSource).setPassword("");

        TransactionFactory transactionFactory = new JdbcTransactionFactory();
        Environment environment = new Environment(dbName, transactionFactory, dataSource);
        Configuration configuration = new Configuration(environment);
             configuration.addMapper(FeedbackMapper.class);

            //
             configuration.setAggressiveLazyLoading(false);
             sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);

在 Feedbackmapper 中查询数据库和数据库查询:

d.1 Feedbackmapper 中的代码:

@Select("SELECT f.id, f.message, f.seen, f.sender_id FROM feedback f WHERE f.id= #{feedbackId}")
@Results(value = { 
        @Result(property = "id", column = "id"),
        @Result(property = "sender", column = "sender_id", javaType = User.class, one = @One(select = "getUser", fetchType=FetchType.DEFAULT))
})
public Feedback getFeedback(@Param("feedbackId") int feedbackId);

@Select("SELECT id, email FROM users WHERE id=#{id}")
public User getUser(int id);

d.2:在feedbackMapper中调用查询的代码

    // setup Mybatis session factory and config
    Feedback feedback =feedbackMapper.getFeedback(70000);
    System.out.println(feedback);

但是在查询 getFeedback(id) 时仍然会填充“发件人”对象。我希望发件人对象不应该立即填充，但只有在我对获取的反馈对象调用 getSender() 时才填充。请帮忙。

我最近的观察:

Mybatis 团队确实在他们的文档中弄错了，即在文档中:

1. lazyLoadingEnabled:默认值=TRUE

2. aggressiveLazyLoading : 默认值=TRUE

   但是看看他们的源代码:

    protected boolean lazyLoadingEnabled = false;
    protected boolean aggressiveLazyLoading = true;
   

   **然而，被更正后，结果不受影响，延迟加载不起作用:( **

Answer 1

我想我找到了一种启用延迟加载的方法(虽然不确定百分比):

• MyBatis 文档在配置中有如下设置:

设置:lazyLoadTriggerMethods

描述:指定哪个对象的方法触发延迟加载

有效值:以逗号分隔的方法名称列表

默认值:equals,clone,hashCode,toString

• 根据源代码，这个东西正确映射到文档中给出的内容:

  public class Configuration {
  // other attributes + methods
  protected Set<String> lazyLoadTriggerMethods = new HashSet<String>(Arrays.asList(new         
  String[] { "equals", "clone", "hashCode", "toString" }));
  }
  

  • Mybatis 配置如下:

    TransactionFactory transactionFactory = new JdbcTransactionFactory();
    Environment environment = new Environment(dbName, transactionFactory,  dataSource);
    Configuration configuration = new Configuration(environment);
    
    /**
    *This is the cause why LAZY LOADING is working now
    */
      configuration.getLazyLoadTriggerMethods().clear();
    
      ///////////////////////////////////////////////////
      configuration.setLazyLoadingEnabled(true);
      configuration.setAggressiveLazyLoading(false);
    

  • Mapper 中的查询(大部分不变):

    @Select("SELECT id, message, seen, sender_id 
    FROM feedback WHERE f.id= #{feedbackId}")
    @Results(value = { 
       @Result(property = "id", column = "id"),
       @Result(property = "sender", column = "sender_id", javaType = User.class, one = @One(select = "getUser"))
    
    // Set fetchType as DEFAULT or LAZY or don't set at all-- lazy loading takes place
    // Set fetchType as EAGER --sender Object is loaded immediately
    
    })
    public Feedback getFeedback(@Param("feedbackId") int feedbackId);
    
    @Select("SELECT id, email FROM users WHERE id=#{id}")
    public User getUser(int id);
    

-调用mapper的JAVA代码

        FeedbackMapper mapper = sqlSession.getMapper(FeedbackMapper.class);
        Feedback feedback =mapper.getFeedback(69999);                   
        System.out.println("1. Feedback object before sender lazily load: \n"+ feedback);
        System.out.println("2. Sender loaded explicitly \n" +feedback.getSender());
        System.out.println("3. Feedback object after sender loading \n" + feedback);

• 代码的输出

<我> 1. sender懒加载前的反馈对象:

{id : 69999,  message : message123, sender : null, seen : false}

2. Sender loaded explicitly 

{id : 65538 , email: hemant@gmail.com}

3. Feedback object after sender loading:

{id : 69999, message : message123, sender : {id : 65538, email : hemant@gmail.com},
 seen : false}

• 虽然这工作令人满意，但在做

configuration.getLazyLoadTriggerMethods().clear();

但是由于 Mybatis 中缺少文档，我不确定这是否与任何缺点相关。