MyBatis 如何支持延迟加载？现实原理是什么？

缺乏、安全感 2022-12-14 13:38 106阅读 0赞

支持延迟加载的配置：  
**在配置文件的****标签内设置参数**

*  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
 *  aggressiveLazyLoading：开启时，任一方法的调用都会加载该对象的所有延迟加载属性。否则，每个延迟加载属性会按需加载
 *  lazyLoadTriggerMethods：指定对象的哪些方法触发一次延迟加载

**resultMap 中配置 <association> 或 <collection>**

配置与测试示例

//配置文件
    < settings>
        < setting name="lazyLoadingEnabled" value="true"/>
        < setting name="aggressiveLazyLoading" value="false"/>
        < setting name="lazyLoadTriggerMethods" value=""/>
    < /settings>
    
    
    //Mapper xml
    < select id="selectUserWithLazyInfo" resultMap="UserWithLazyInfo">
        select * from user where id = 1
    < /select>
    
    < resultMap id="UserWithLazyInfo" type="constxiong.po.User">
        < id property="id" column="id"/>
        < result property="name" column="name"/>
        < association property="info" javaType="constxiong.po.Info" select="constxiong.mapper.InfoMapper.selectInfoByUserId" column="id"/>
    < /resultMap>
    
    
    //InfoMapper
    public interface InfoMapper {
        @Select("select * from info where user_id = #{userId}")
        @Results(value = {@Result(column="user_id", property = "userId")})
        Info selectInfoByUserId(int userId);
    }
    
    
    //测试代码
    System.out.println("------ selectUserWithLazyInfo ------");
    User user = userMapper.selectUserWithLazyInfo();
    System.out.println(user);
    System.out.println(user.getInfo());
    
    
    //打印 User 对象里的 Info 为空，使用 getInfo 能够查询对应的值
    ------ selectUserWithLazyInfo ------
    User{id=1, name='ConstXiong1', mc='null', info=null, articles=null}
    Info{userId=1, name=大熊}

实现原理：

支持延迟加载是通过字节码增强实现的，MyBatis 3.3 及以上默认使用了 javassist，3.3 以前使用 cglib 实现。

我本地用的 MyBatis 3.5.5，使用了 javassist 增强，核心源码如下

//DefaultResultSetHandler getRowValue 获取每条的查询数据，resultMap 中如果包含懒加载 rowValue 在 createResultObject 方法通过 javassist 代理增强
    private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, String columnPrefix) throws SQLException {
        final ResultLoaderMap lazyLoader = new ResultLoaderMap();
        //对象数据，通过 javassist 代理增强
        Object rowValue = createResultObject(rsw, resultMap, lazyLoader, columnPrefix);
        if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
            final MetaObject metaObject = configuration.newMetaObject(rowValue);
            boolean foundValues = this.useConstructorMappings;
            if (shouldApplyAutomaticMappings(resultMap, false)) {
                foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;
            }
            //根据从数据库查询到的 resultSet，根据 resultMap 通过反射设置 rowValue 的值
            foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;
            foundValues = lazyLoader.size() > 0 || foundValues;
            rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
        }
        return rowValue;
    }
    
    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<>();
        final List<Object> constructorArgs = new ArrayList<>();
        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) {
                //如果返回对象的属性中包含懒加载，使用 javassist 代理增强，当设置属性值时被代理到 JavassistProxyFactory 的 invoke 方法
                if (propertyMapping.getNestedQueryId() != null && propertyMapping.isLazy()) {
                    resultObject = configuration.getProxyFactory().createProxy(resultObject, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
                    break;
                }
            }
        }
        this.useConstructorMappings = resultObject != null && !constructorArgTypes.isEmpty(); // set current mapping result
        return resultObject;
    }

//JavassistProxyFactory 的 invoke 方法
    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)) {
                    ...
                } else {
                    if (lazyLoader.size() > 0 && !FINALIZE_METHOD.equals(methodName)) {
                        if (aggressive || lazyLoadTriggerMethods.contains(methodName)) {
                            lazyLoader.loadAll();
                        } else if (PropertyNamer.isSetter(methodName)) {
                            final String property = PropertyNamer.methodToProperty(methodName);
                            lazyLoader.remove(property);
                        } else if (PropertyNamer.isGetter(methodName)) {
                            //测试代码中 user.getInfo() 方法的调用，在此执行懒加载查询关联 SQL 设置 info 属性
                            final String property = PropertyNamer.methodToProperty(methodName);
                            if (lazyLoader.hasLoader(property)) {
                                lazyLoader.load(property);
                            }
                        }
                    }
                }
            }
            return methodProxy.invoke(enhanced, args);
        } catch (Throwable t) {
            throw ExceptionUtil.unwrapThrowable(t);
        }
    }

完整 Demo：

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