Delayed loading source code analysis:

• Delay loading
  Is in need of ⽤ Only when the data is available ⾏ load , Unwanted ⽤ When the data is received, the data is not loaded . Lazy loading is also called lazy loading .

• advantage ：
  First query from a single table , When necessary, the query is associated from the associated table ,⼤⼤ carry ⾼ Database performance , Because the query table needs to ⽐ It is faster to query multiple tables by association .

• shortcoming ：
  Because only when needed ⽤ When it comes to data , It's going to be ⾏ Database query , In this way ⼤ When querying batch data , Because of the inquiry ⼯ It also takes time to do , So
  Can cause ⽤ The waiting time of the customer changes ⻓, cause ⽤ Decline in user experience .

• In multiple tables ：
  ⼀ For more than , Many to many ： Usually ⽤ Delay loading
  ⼀ Yes ⼀（ How to ⼀）： Usually ⽤⽴ The loading

• Be careful ：
  Deferred loading is based on nested queries

• Realize local delayed loading
  stay association and collection There are labels in ⼀ individual fetchType attribute , By modifying its value , You can modify the local loading strategy . fetchType=“lazy”

<!--  Turn on ⼀ For more than   Delay loading  -->
<resultMap id="userMap" type="user">
<id column="id" property="id"></id>
<result column="username" property="username"></result>
<result column="password" property="password"></result>
<result column="birthday" property="birthday"></result>
<!--
fetchType="lazy"  Lazy loading strategy 
fetchType="eager" ⽴ Load policy 
-->
<collection property="orderList" ofType="order" column="id"
select="com.lagou.dao.OrderMapper.findByUid" fetchType="lazy">
</collection>
</resultMap>
<select id="findAll" resultMap="userMap">
SELECT * FROM `user`
</select>

• Global delay loading
  stay Mybatis The core of ⼼ To configure ⽂ Can make ⽤setting Tag modify global loading policy .

<settings>
<!-- Turn on the global delay loading function -->
	<setting name="lazyLoadingEnabled" value="true"/>
</settings>

• The principle of delayed loading is realized
  Its principle is , send ⽤ CGLIB or Javassist( Default ) establish ⽬ The proxy object of the target object . Tune up ⽤ The delay loading property of the proxy object
  getting ⽅ Legal time , Into the ⼊ Interceptor ⽅ Law .⽐ If tune ⽤ a.getB().getName() ⽅ Law , Into the ⼊ The interceptor invoke(…) ⽅ Law ,
  Find out a.getB() When delayed loading is required , Then the saved query Association will be sent separately B Object's SQL , hold B Look up , then
  transfer ⽤a.setB(b) ⽅ Law , therefore a object b The property has a value , Then finish a.getB().getName() ⽅ The tone of law ⽤. This is the delay
  The basic principle of loading
  summary ： Delayed loading is mainly realized in the form of dynamic agent , Intercept the specified through the proxy ⽅ Law , Of board ⾏ Data loading .

Delay loading principle （ Source analysis )
MyBatis Delayed loading mainly makes ⽤：Javassist,Cglib Realization , Class diagram display ：

Setting Configuration is loaded ：
Delay load proxy object creation
Mybatis The query result of is by ResultSetHandler Pick up ⼝ Of handleResultSets()⽅ Can't handle .ResultSetHandler Pick up ⼝ Only ⼀
Implementation ,DefaultResultSetHandler, Next, let's look at the delay loading related ⼀ A nuclear ⼼ Of ⽅ Law
By default ⽤javassistProxy Into the ⾏ Proxy object creation
//aggressive ⼀ The secondary loading property requires delayed loading properties or contains trigger delayed loading ⽅ Law
if

adopt invoke Method , Judge whether the attribute data contained in the called method is configured to enable lazy loaded fields , If it is through the proxy object again load Query the database , Otherwise go straight back to .