mybatis延迟加载原理（mybatis工作原理及流程）

1. 目的

本文主要解读MyBatis 延迟加载实现原理

2. 延迟加载如何使用

Setting 参数配置

设置参数描述有效值默认值lazyLoadingEnabled延迟加载的全局开关。当开启时，所有关联对象都会延迟加载。 特定关联关系中可通过设置fetchType属性来覆盖该项的开关状态。true、
falsefalseaggressiveLazyLoading当开启时，任何方法的调用都会加载该对象的所有属性。否则，每个属性会按需加载（参考lazyLoadTriggerMethods).true、falsefalse (true in ≤3.4.1)lazyLoadTriggerMethods指定哪个对象的方法触发一次延迟加载。用逗号分隔的方法列表。equals,clone,hashCode,toString

配置

<configuration>
 <settings>
 <!-- 开启延迟加载 -->
 <setting name="lazyLoadingEnabled" value="true" />
 <setting name="aggressiveLazyLoading" value="false" />
 <setting name="lazyLoadTriggerMethods" value="equals,clone,hashCode,toString" />
 </settings>
</configuration>

Mapper 配置

<mapper namespace="org.apache.ibatis.submitted.lazy_properties.Mapper">
 
 <resultMap type="org.apache.ibatis.submitted.lazy_properties.User"
 id="user">
 <id property="id" column="id" />
 <result property="name" column="name" />
 </resultMap>
 <!-- 结果对象 -->
 <resultMap type="org.apache.ibatis.submitted.lazy_properties.User" id="userWithLazyProperties" extends="user">
 <!-- 延迟加载对象lazy1 -->
 <association property="lazy1" column="id" select="getLazy1" fetchType="lazy" />
 <!-- 延迟加载对象lazy2 -->
 <association property="lazy2" column="id" select="getLazy2" fetchType="lazy" />
 <!-- 延迟加载集合lazy3 --> 
 <collection property="lazy3" column="id" select="getLazy3" fetchType="lazy" />
 </resultMap>
 <!-- 执行的查询 -->
 <select id="getUser" resultMap="userWithLazyProperties">
 select * from users where id = #{id}
 </select>
</mapper>

User 实体对象

public class User implements Cloneable {
 private Integer id;
 private String name;
 private User lazy1;
 private User lazy2;
 private List<User> lazy3;
 public int setterCounter;
 
 省略...
 }

执行解析：

• 调用getUser查询数据，从查询结果集解析数据到User对象，当数据解析到lazy1，lazy2，lazy3判断需要执行关联查询
• lazyLoadingEnabled=true，将创建lazy1，lazy2，lazy3对应的Proxy延迟执行对象lazyLoader，并保存
• 当逻辑触发lazyLoadTriggerMethods 对应的方法（equals,clone,hashCode,toString）则执行延迟加载
• 如果aggressiveLazyLoading=true，只要触发到对象任何的方法，就会立即加载所有属性的加载

3. 延迟加载原理实现

延迟加载主要是通过动态代理的形式实现，通过代理拦截到指定方法，执行数据加载。

MyBatis延迟加载主要使用：Javassist，Cglib实现，类图展示：

4. 延迟加载源码解析

Setting 配置加载：

public class Configuration {

 protected boolean aggressiveLazyLoading;
 
 protected Set<String> lazyLoadTriggerMethods = new HashSet<String>(Arrays.asList(new String[] { "equals", "clone", "hashCode", "toString" }));
 
 protected boolean lazyLoadingEnabled = false;
 
 
 public void setProxyFactory(ProxyFactory proxyFactory) {
 if (proxyFactory == null) {
 proxyFactory = new JavassistProxyFactory();
 }
 this.proxyFactory = proxyFactory;
 }
 
 //省略...
}

延迟加载代理对象创建

DefaultResultSetHandler

//#mark 创建结果对象
 private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, ResultLoaderMap lazyLoader, String columnPrefix) throws SQLException {
 this.useConstructorMappings = false; // reset previous mapping result
 final List<Class<?>> constructorArgTypes = new ArrayList<Class<?>>();
 final List<Object> constructorArgs = new ArrayList<Object>();
 //#mark 创建返回的结果映射的真实对象
 Object resultObject = createResultObject(rsw, resultMap, constructorArgTypes, constructorArgs, columnPrefix);
 if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
 final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
 for (ResultMapping propertyMapping : propertyMappings) {
 // issue gcode #109 && issue #149 判断属性有没配置嵌套查询，如果有就创建代理对象
 if (propertyMapping.getNestedQueryId() != null && propertyMapping.isLazy()) {
 //#mark 创建延迟加载代理对象
 resultObject = configuration.getProxyFactory().createProxy(resultObject, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
 break;
 }
 }
 }
 this.useConstructorMappings = resultObject != null && !constructorArgTypes.isEmpty(); // set current mapping result
 return resultObject;
 }

代理功能实现

由于Javasisst和Cglib的代理实现基本相同，这里主要介绍Javasisst

ProxyFactory接口定义

public interface ProxyFactory {

void setProperties(Properties properties);

Object createProxy(Object target, ResultLoaderMap lazyLoader, Configuration configuration, ObjectFactory objectFactory, List<Class<?>> constructorArgTypes, List<Object> constructorArgs);

}

JavasisstProxyFactory实现

public class JavassistProxyFactory implements org.apache.ibatis.executor.loader.ProxyFactory {
 
 
 @Override
 public Object createProxy(Object target, ResultLoaderMap lazyLoader, Configuration configuration, ObjectFactory objectFactory, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
 return EnhancedResultObjectProxyImpl.createProxy(target, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
 }
 //省略...
 
 
 private static class EnhancedResultObjectProxyImpl implements MethodHandler {
 
 
 static Object crateProxy(Class<?> type, MethodHandler callback, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
 ProxyFactory enhancer = new ProxyFactory();
 enhancer.setSuperclass(type);
 try {
 //通过获取对象方法，判断是否存在该方法
 type.getDeclaredMethod(WRITE_REPLACE_METHOD);
 // ObjectOutputStream will call writeReplace of objects returned by writeReplace
 if (log.isDebugEnabled()) {
 log.debug(WRITE_REPLACE_METHOD + " method was found on bean " + type + ", make sure it returns this");
 }
 } catch (NoSuchMethodException e) {
 //没找到该方法，实现接口
 enhancer.setInterfaces(new Class[]{WriteReplaceInterface.class});
 } catch (SecurityException e) {
 // nothing to do here
 }
 Object enhanced;
 Class<?>[] typesArray = constructorArgTypes.toArray(new Class[constructorArgTypes.size()]);
 Object[] valuesArray = constructorArgs.toArray(new Object[constructorArgs.size()]);
 try {
 //创建新的代理对象
 enhanced = enhancer.create(typesArray, valuesArray);
 } catch (Exception e) {
 throw new ExecutorException("Error creating lazy proxy. Cause: " + e, e);
 }
 //设置代理执行器
 ((Proxy) enhanced).setHandler(callback);
 return enhanced;
 }
 
 
 
 @Override
 public Object invoke(Object enhanced, Method method, Method methodProxy, Object[] args) throws Throwable {
 final String methodName = method.getName();
 try {
 synchronized (lazyLoader) {
 if (WRITE_REPLACE_METHOD.equals(methodName)) { 
 //忽略暂未找到具体作用
 Object original;
 if (constructorArgTypes.isEmpty()) {
 original = objectFactory.create(type);
 } else {
 original = objectFactory.create(type, constructorArgTypes, constructorArgs);
 }
 PropertyCopier.copyBeanProperties(type, enhanced, original);
 if (lazyLoader.size() > 0) {
 return new JavassistSerialStateHolder(original, lazyLoader.getProperties(), objectFactory, constructorArgTypes, constructorArgs);
 } else {
 return original;
 }
 } else {
 //延迟加载数量大于0
 if (lazyLoader.size() > 0 && !FINALIZE_METHOD.equals(methodName)) {
 //aggressive 一次加载性所有需要要延迟加载属性或者包含触发延迟加载方法
 if (aggressive || lazyLoadTriggerMethods.contains(methodName)) {
 log.debug("==> laze lod trigger method:" + methodName + ",proxy method:" + methodProxy.getName() + " class:" + enhanced.getClass());
 //一次全部加载
 lazyLoader.loadAll();
 } else if (PropertyNamer.isSetter(methodName)) {
 //判断是否为set方法，set方法不需要延迟加载
 final String property = PropertyNamer.methodToProperty(methodName);
 lazyLoader.remove(property);
 } else if (PropertyNamer.isGetter(methodName)) {
 final String property = PropertyNamer.methodToProperty(methodName);
 if (lazyLoader.hasLoader(property)) {
 //延迟加载单个属性
 lazyLoader.load(property);
 log.debug("load one :" + methodName);
 }
 }
 }
 }
 }
 return methodProxy.invoke(enhanced, args);
 } catch (Throwable t) {
 throw ExceptionUtil.unwrapThrowable(t);
 }
 }
 }

5. 注意事项

1. IDEA调试问题 当配置aggressiveLazyLoading=true，在使用IDEA进行调试的时候，如果断点打到代理执行逻辑当中，你会发现延迟加载的代码永远都不能进入，总是会被提前执行。 主要产生的原因在aggressiveLazyLoading，因为在调试的时候，IDEA的Debuger窗体中已经触发了延迟加载对象的方法。

如图：调试还未进入lazyLoader.loadAll(); 实际日志已经显示延迟加载已经完成，代码与日志通过颜色区分。