MyBatis源码解读之延迟加载

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

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：

1. 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
2. lazyLoadingEnabled=true，将创建lazy1，lazy2，lazy3对应的Proxy延迟执行对象lazyLoader，并保存
3. 当逻辑触发lazyLoadTriggerMethods 对应的方法（equals,clone,hashCode,toString）则执行延迟加载
4. 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 {
/** aggressiveLazyLoading：
   * 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).
   * 默认为true
   * */
  protected boolean aggressiveLazyLoading;
  /**
   * 延迟加载触发方法
   */
  protected Set<String> lazyLoadTriggerMethods = new HashSet<String>(Arrays.asList(new String[] { "equals", "clone", "hashCode", "toString" }));
  /** 是否开启延迟加载 */
  protected boolean lazyLoadingEnabled = false;
  
  /**
   * 默认使用Javassist代理工厂
   * @param proxyFactory
   */
  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);

  /**
   * 创建代理
   * @param target 目标结果对象
   * @param lazyLoader 延迟加载对象
   * @param configuration 配置
   * @param objectFactory 对象工厂
   * @param constructorArgTypes 构造参数类型
   * @param constructorArgs 构造参数值
   * @return
   */
  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 {

    
    /**
   * 接口实现
   * @param target 目标结果对象
   * @param lazyLoader 延迟加载对象
   * @param configuration 配置
   * @param objectFactory 对象工厂
   * @param constructorArgTypes 构造参数类型
   * @param constructorArgs 构造参数值
   * @return
   */
  @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...
    
  /**
   * 代理对象实现，核心逻辑执行
   */
  private static class EnhancedResultObjectProxyImpl implements MethodHandler {
  
    /**
   * 创建代理对象
   * @param type
   * @param callback
   * @param constructorArgTypes
   * @param constructorArgs
   * @return
   */
  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;
  }
  
  
     /**
     * 代理对象执行
     * @param enhanced 原对象
     * @param method 原对象方法
     * @param methodProxy 代理方法
     * @param args 方法参数
     * @return
     * @throws Throwable
     */
    @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。

6. 参考资料

本博客MyBatis源码地址： - https://gitee.com/rainwen/mybatis
Java动态代理机制详解（JDK 和CGLIB，Javassist，ASM） - http://blog.csdn.net/luanlouis/article/details/24589193
从类加载到动态编译 - http://zhaoyw.cn/2017/07/classloader-dynamic
MyBatis 记录二: lazy loading - http://yoncise.com/2016/11/05/MyBatis-%E8%AE%B0%E5%BD%95%E4%BA%8C-lazy-loading/
MyBatis官方文档 - http://www.mybatis.org/mybatis-3/zh/index.html
MyBatis-Spring官方文档 - http://www.mybatis.org/spring/zh/index.html
MyBatis-Spring源码 - https://github.com/rainwen/spring

关于MyBatis源码解读之延迟加载就介绍到这里。如有疑问，欢迎留言，谢谢。