In mybatis framework, lazy loading and even differences table query

1. primer     

mybatis lazy loading, mainly used in a complex entity class data types of properties, including one and one-to-many relationship. This kind of property is often also correspond to another data tables, and demand do not necessarily need the actual query data table, then the time delay in loading table query with respect to even have a great advantage, to achieve the purpose loaded on demand . This cost has great significance in improving the access speed and reduce system resources.

2. Double-table query

BACKGROUND: user entity class contains a character User Role entity class attributes

User entity class attributes

    private Integer id; // id 
    private String userCode; // user code 
    private String the userName; // User name 
    private String the userPassword; // user password 
    private Integer userRole; // associated user id field role, and the roles table smbms_role corresponding 
    private Role Role; // one to one correspondence between the 
    Private Integer gender; // sex 
    Private a date Birthday; // date of birth 
    Private String phone; // phone 
    Private String address; // address
    Private String userRoleName; // user role name 
    Private Integer createdBy; // creator of 
    Private a Date creationDate; // create time 
    Private Integer modifyBy; // updated by 
    Private a Date ModifyDate; // update time

Role entity class attributes,

  Private Integer the above mentioned id; // the above mentioned id 
    
    Private String roleCode; // character encoding 
    
    Private String roleName; // role names 
    
    Private Integer createdBy; // creator of 
    
    Private a Date creationDate; // create time 
    
    Private Integer modifyBy; // updated by 
    
    Private a Date ModifyDate ; // update

 

xml sql statement mapping file

<!-- 演示 一对一 association 的连表查询 start -->

    <resultMap type="Role" id="roleMap">
        <id property="id" column="r_id" />
        <result property="roleCode" column="roleCode" />
        <result property="roleName" column="roleName" />
    </resultMap>
    <resultMap type="User" id="userRoleResult">
        <id property="id" column="id" />
        <result property="userCode" column="userCode" />
        <result property="userName" column="userName" />
        <association property="role" javaType="Role" resultMap="roleMap" />
    </resultMap>
    <!-- 根据用户表中角色id 和角色表做联表查询 -->
    <select id="getUserListByRoleId" parameterType="int" resultMap="userRoleResult">
        select u.* ,r.id as r_id,r.roleCode,r.roleName from
        smbms_user
        u,smbms_role r
        where u.userRole = #{userRoleId} and u.userRole = r.id
    </select>


<!-- 演示 一对一 association 的连表查询 end -->

 Test Methods

  @Test
    public void selectUserByIdsAndRoleId()
    {
        List<User> users = sqlSession.getMapper(UserMapper.class).selectUserByIdsAndRoleId(3, Arrays.asList(4, 3, 7,12));
        for (User userElement : users)
        {
            System.out.println(userElement);
        }
    }

 

 3. Delayed loading

Lazy loading can be configured in the main configuration file mybatis-config.xml, the configuration here is global configuration, the default delay loading process all queries SQL.

< Settings > 
<-! Enable lazy loading -> 
< Setting name = "lazyLoadingEnabled" value = "to true"  /> 
<-! Close actively loaded -> 
< Setting name = "aggressiveLazyLoading" value = "false"  / > 
</ Settings >

 

Also, if the configuration of the global satisfied, may be added to collect or add attributes assocation tag fetchType = "lazy" (delay loading) or fetchType = "eager" (immediate loading) In a specific SQL statement mapping,

Loading the SQL herein cover SQL loading of global configuration, pseudocode

<collection property="addressList" 
ofType="Address" column="id"
    fetchType="lazy|eager"/>

 

 

The mapping code xml sql

    
<!--演示 一对多 collection  -->
    <resultMap type="User" id="userAddressResult">
        <id property="id" column="id" />
        <result property="userCode" column="userCode" />
        <result property="phone" column="phone" />
        <result property="userName" column="userName" />
        <collection property="addressList" ofType= "Address" column = "the above mentioned id" 
            the SELECT = "lazyFindAddressListByUserId" fetchType = "lazy" /> 
    </ resultMap > 
    <-! Delayed demand loading sql statement (ie may not use this sql) -> 
    < the SELECT the above mentioned id = "lazyFindAddressListByUserId" the parameterType = "int" the resultType = "the address" > 
        SELECT * WHERE smbms_address from the userId ID = # {} 
    </ SELECT > 
    <-! ID to obtain a plurality of user addresses according to a user -> 
    < SELECT ID = "getAddressListByUserId" parameterType="Integer"
        resultMap="userAddressResult">
    select  * from smbms_user where id = #{userId}
    </select>

 

 Not using the test method addressList address information

 @Test
    public void getAddressListByUserId()
    {
        List<User> users = sqlSession.getMapper(UserMapper.class).getAddressListByUserId(5);
        for (User userElement : users)
        {
          //不使用addressList属性
             System.out.println(userElement.getPhone() +" "+userElement.getUserCode());
           //   System.out.println(userElement);
        }
    }

 Console displays only one sql statement, in fact, only a user queries the table

 

 Using the test method addressList address information

 @Test
    public void getAddressListByUserId()
    {
        List<User> users = sqlSession.getMapper(UserMapper.class).getAddressListByUserId(5);
        for (User userElement : users)
        {
            // 使用addressList属性
            // System.out.println(userElement.getPhone() +" "+userElement.getUserCode());
            System.out.println(userElement);
        }
    }

 The console displays only two sql statement, in fact, only the first query the user table, and then the user id to query the address table