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

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。