mybatis 源码分析（八）ResultSetHandler 详解
- 2019 年 10 月 3 日
- 筆記
本篇博客就是 myabtis 系列的最后一篇了，还剩 ResultSetHandler 没有分析；作为整个 mybatis 最复杂最繁琐的部分，我不打算按步骤一次详解，因为里面的主要内容就是围绕 resultMap 按层次结构依次解析的，其中运用最多的就是反射，所以我这里将围绕延迟加载重点分析，另外本文使用的测试代码都是源码的测试案例；
一、ResultSetHandler 主体结构
public interface ResultSetHandler { // 负责结果集处理，完成映射返回结果对象 <E> List<E> handleResultSets(Statement stmt) throws SQLException; // 负责游标对象处理 <E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException; // 负责存储过程的输出参数 void handleOutputParameters(CallableStatement cs) throws SQLException; }以上就是 ResultSetHandler 的接口方法（mybatis 中只提供了唯一的实现类 DefaultResultSetHandler），在本篇博客中将主要以 handleResultSets 结果集处理作为主线分析；
在分析之前首先要清楚 handleResultSets 方法的处理流程就是围绕 resultMap 依次解析的，这里先看一个比较复杂的 resultMap 映射：
<resultMap id="detailedBlogResultMap" type="Blog"> <constructor> <idArg column="blog_id" javaType="int"/> </constructor> <result property="title" column="blog_title"/> <association property="author" javaType="Author"> <id property="id" column="author_id"/> <result property="username" column="author_username"/> <result property="password" column="author_password"/> </association> <collection property="posts" ofType="Post"> <id property="id" column="post_id"/> <result property="subject" column="post_subject"/> <association property="author" javaType="Author"/> <collection property="comments" ofType="Comment"> <id property="id" column="comment_id"/> </collection> <discriminator javaType="int" column="draft"> <case value="1" resultType="DraftPost"/> </discriminator> </collection> <association property="author" column="id" select="**.selectAuthorForBlog" fetchType="eager"/> <collection property="posts" javaType="ArrayList" column="id" ofType="Post" select="**.selectPostsForBlog" fetchType="lazy"/> </resultMap>当 mybatis 初始化完成后上面的配置都放到 MappedStatement.resultMaps 里面，在解析的时候就是通过 resultMap.id 取到对应的 resultMap 然后逐次解析；
1. 嵌套查询
这里之所以说 ResultSetHandler 是整个 mybatis 里面最复杂的，主要是嵌套查询的解析（association 一对一，collection 一对多），值得注意的是这里的嵌套查询是有两种方式的：
内部嵌套
<association property="author" javaType="Author"> <id property="id" column="author_id"/> <result property="username" column="author_username"/> <result property="password" column="author_password"/> </association> <collection property="posts" ofType="Post"> <id property="id" column="post_id"/> <result property="subject" column="post_subject"/> <association property="author" javaType="Author"/> <collection property="comments" ofType="Comment"> <id property="id" column="comment_id"/> </collection> <discriminator javaType="int" column="draft"> <case value="1" resultType="DraftPost"/> </discriminator> </collection>像这种嵌套查询是直接在同一个 resultMap 依次映射对应结果的，使用的是 SQL 多表连接，例如：
<select id="selectBlogDetails" resultMap="detailedBlogResultMap"> select B.id as blog_id, B.title as blog_title, B.author_id as blog_author_id, A.id as author_id, A.username as author_username, ... P.id as post_id, P.blog_id as post_blog_id, ... C.id as comment_id, C.post_id as comment_post_id, ... T.id as tag_id, T.name as tag_name from Blog B left outer join Author A on B.author_id = A.id left outer join Post P on B.id = P.blog_id left outer join Comment C on P.id = C.post_id left outer join Post_Tag PT on PT.post_id = P.id left outer join Tag T on PT.tag_id = T.id where B.id = #{id} </select>这里还有一种分离的内部嵌套：
<resultMap id="blogResult" type="Blog"> <id property="id" column="blog_id" /> <result property="title" column="blog_title"/> <association property="author" column="blog_author_id" javaType="Author" resultMap="authorResult"/> </resultMap> <resultMap id="authorResult" type="Author"> <id property="id" column="author_id"/> <result property="username" column="author_username"/> <result property="password" column="author_password"/> <result property="email" column="author_email"/> <result property="bio" column="author_bio"/> </resultMap><select id="selectBlog" resultMap="blogResult"> select B.id as blog_id, B.title as blog_title, B.author_id as blog_author_id, A.id as author_id, A.username as author_username, A.password as author_password, A.email as author_email, A.bio as author_bio from Blog B left outer join Author A on B.author_id = A.id where B.id = #{id} </select>这中写法只是将 association、collection 部分分离出来，其实质都是一样的，都是一条多表连接的 SQL；
外部嵌套
还有另外一种是将多表连接的 SQL 拆分，每个属性单独发一条 SQL：
<resultMap id="blogResult" type="Blog"> <collection property="posts" javaType="ArrayList" column="id" ofType="Post" select="selectPostsForBlog"/> </resultMap> <select id="selectBlog" resultMap="blogResult"> SELECT * FROM BLOG WHERE ID = #{id} </select> <select id="selectPostsForBlog" resultType="Post"> SELECT * FROM POST WHERE BLOG_ID = #{id} </select>像这样在 association、collection 中使用 select 属性指定外部 SQL，其查询结果也是发两条 SQL，这里之所以没有详细写出每个属性的映射，是因为指定了 type 和 ofType，并开启的自动映射，mybatis 在执行的时候使用反射推断出来的；
这里的两种嵌套查询在初始化的时候就进行了单独的区分：
// org.apache.ibatis.builder.xml.XMLMapperBuilder String nestedResultMap = context.getStringAttribute("resultMap", processNestedResultMappings(context, Collections.emptyList(), resultType)); // org.apache.ibatis.submitted.nestedresulthandler.Mapper.mapper_resultMap[personResult]_collection[items] private String processNestedResultMappings(XNode context, List<ResultMapping> resultMappings, Class<?> enclosingType) throws Exception { if ("association".equals(context.getName()) || "collection".equals(context.getName()) || "case".equals(context.getName())) { if (context.getStringAttribute("select") == null) { validateCollection(context, enclosingType); ResultMap resultMap = resultMapElement(context, resultMappings, enclosingType); return resultMap.getId(); } } return null; } // org.apache.ibatis.mapping.ResultMap // resultMap.hasNestedResultMaps = resultMap.hasNestedResultMaps || (resultMapping.getNestedResultMapId() != null && resultMapping.getResultSet() == null);这里代码代码比较多所以只放了关键代码，其最终结果是内部嵌套由 ResultMap.hasNestedResultMaps 标识；
// org.apache.ibatis.builder.xml.XMLMapperBuilder private ResultMapping buildResultMappingFromContext(XNode context, Class<?> resultType, List<ResultFlag> flags) throws Exception { ... String nestedSelect = context.getStringAttribute("select"); String nestedResultMap = context.getStringAttribute("resultMap", processNestedResultMappings(context, Collections.emptyList(), resultType)); ... }外部查询的最终结果是由 ResultMapping.nestedQueryId 保存的，之所以这里强调这些是因为在 ResultSetHandler 解析的时候是分了内外部嵌套两种大的情况的；
2. 多结果集
此外分析之前首先还要知道 CallableStatement 调用存储过程的时候，会有多结果集的情况，例如：
create procedure sptest.getnamesanditemsbyid(in nameId integer) modifies sql data dynamic result sets 2 BEGIN ATOMIC declare cur1 cursor for select * from sptest.names where id = nameId; declare cur2 cursor for select * from sptest.items where name_id in (select id from sptest.names where id = nameId); open cur1; open cur2; END<resultMap type="org.apache.ibatis.submitted.sptests.Name" id="nameResultLinkedNoMatchingInfo"> <result column="ID" property="id"/> <result column="FIRST_NAME" property="firstName"/> <result column="LAST_NAME" property="lastName"/> <collection property="items" resultSet="items" resultMap="itemResult"/> </resultMap> <select id="getNamesAndItemsLinkedById" statementType="CALLABLE" resultSets="names,items" resultMap="nameResultLinkedNoMatchingInfo"> {call sptest.getnamesanditemsbyid(#{id,jdbcType=INTEGER,mode=IN})} </select>2. 整体流程
上图就是 ResultSetHandler.handleResultSet 的主要流程，这里只保留了重要的部分：
- 内外部嵌套查询的分支；
- 外部嵌套查询与一级缓存；
- 外部嵌套查询的延迟加载，主要是代理对象、ResultLoader、ResultLoaderMap三个对象；
其余的部分这里就不再详细分析了，一下将主要讲解外部嵌套查询的延迟加载；
二、cglib 和 javassisit 动态代理
在讲解延迟加载之前，需要首先简单了解一下动态代理，因为普通的 JavaBean 对象一般都没有实现接口，所以不能使用 java.lang.reflect.Proxy，在 mybatis 中提供了另外两种动态代理 cglib 和 javassisit；
1. cglib
public class Car { String name; public String getName() { return name; } public void setName(String name) { this.name = name; } } @Test public void test() { Enhancer enhancer = new Enhancer(); enhancer.setSuperclass(Car.class); enhancer.setCallback((MethodInterceptor) (obj, method, args, proxy) -> { System.out.println("代理开始"); Object object = proxy.invokeSuper(obj, args); System.out.println("result: " + object); System.out.println("代理结束"); return object; }); Car car = (Car) enhancer.create(); car.setName("Test"); car.getName(); }打印：
代理开始
result: null
代理结束
代理开始
result: Test
代理结束
2. javassist
public class Car { String name; public String getName() { return name; } public void setName(String name) { this.name = name; } } @Test public void test() throws IllegalAccessException, InstantiationException { ProxyFactory proxyFactory = new ProxyFactory(); proxyFactory.setSuperclass(Car.class); // 设置拦截目标 proxyFactory.setFilter(m -> m.getName().startsWith("get") || m.getName().startsWith("set")); proxyFactory.setHandler((self, thisMethod, proceed, arg) -> { System.out.println("代理开始"); Object object = proceed.invoke(self, arg); System.out.println("result: " + object); System.out.println("代理结束"); return object; }); Class clazz = proxyFactory.createClass(); Car car = (Car) clazz.newInstance(); car.setName("Test"); car.getName(); }打印：
代理开始
result: null
代理结束
代理开始
result: Test
代理结束
三、延迟加载
通过上面的讲解大家应该清楚只有外部嵌套查询才有延迟加载功能；此外和延迟加载相关的配置：
- proxyFactory：（CGLIB | JAVASSIST-默认）指定 mybatis 延迟加载的代理工具；
- lazyLoadingEnabled：（true | false-默认）延迟加载的全局开关。可使用 association、collection 的 fetchType （lazy|eager）属性覆盖；
- aggressiveLazyLoading：（false| true-默认）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；
1. demo
<setting name="proxyFactory" value="JAVASSIST"/> <setting name="lazyLoadingEnabled" value="true"/> <setting name="aggressiveLazyLoading" value="false"/><resultMap id="user" type="org.apache.ibatis.submitted.lazy_properties.User"> <id property="id" column="id"/> <result property="name" column="name"/> </resultMap> <resultMap id="userWithLazyProperties" type="org.apache.ibatis.submitted.lazy_properties.User" extends="user"> <association property="lazy1" column="id" select="getLazy1" fetchType="lazy"/> <association property="lazy2" column="id" select="getLazy2" fetchType="eager"/> <collection property="lazy3" column="id" select="getLazy3" fetchType="lazy"/> </resultMap> <select id="getUser" resultMap="userWithLazyProperties"> select * from users where id = #{id} </select>public class User { private Integer id; private String name; private User lazy1; private User lazy2; private List<User> lazy3; ... } @Test void test() { try (SqlSession sqlSession = sqlSessionFactory.openSession()) { Mapper mapper = sqlSession.getMapper(Mapper.class); User user = mapper.getUser(1); System.out.println("----getLazy1: " + user.getLazy1()); System.out.println("----getLazy2: " + user.getLazy2()); System.out.println("----getLazy3: " + user.getLazy3()); } }打印：
DEBUG [main] – ==> Preparing: select * from users where id = ?
DEBUG [main] – ==> Parameters: 1(Integer)
DEBUG [main] – ====> Preparing: select 12 id, ‘lazy2’ name from (values(0))
DEBUG [main] – ====> Parameters:
DEBUG [main] – <==== Total: 1
DEBUG [main] – <== Total: 1
DEBUG [main] – ==> Preparing: select 11 id, ‘lazy1’ name from (values(0))
DEBUG [main] – ==> Parameters:
DEBUG [main] – <== Total: 1
—-getLazy1: User{id=11, name=’lazy1′}
—-getLazy2: User{id=12, name=’lazy2′}
DEBUG [main] – ==> Preparing: select 13 id, ‘lazy3’ name from (values(0))
DEBUG [main] – ==> Parameters:
DEBUG [main] – <== Total: 1
—-getLazy3: [User{id=13, name=’lazy3′}]
从打印的顺序可以看出当 mapper.getUser(1) 的时候，就已经获取了 user 和 lazy2，而 lazy1 和 lazy3 则是在 get 的时候才加载；这里在看一下 aggressiveLazyLoading = true 的效果：
打印：
DEBUG [main] – ==> Preparing: select * from users where id = ?
DEBUG [main] – ==> Parameters: 1(Integer)
DEBUG [main] – ====> Preparing: select 12 id, ‘lazy2’ name from (values(0))
DEBUG [main] – ====> Parameters:
DEBUG [main] – <==== Total: 1
DEBUG [main] – ====> Preparing: select 11 id, ‘lazy1’ name from (values(0))
DEBUG [main] – ====> Parameters:
DEBUG [main] – <==== Total: 1
DEBUG [main] – ====> Preparing: select 13 id, ‘lazy3’ name from (values(0))
DEBUG [main] – ====> Parameters:
DEBUG [main] – <==== Total: 1
DEBUG [main] – <== Total: 1
—-getLazy1: User{id=11, name=’lazy1′}
—-getLazy2: User{id=12, name=’lazy2′}
—-getLazy3: [User{id=13, name=’lazy3′}]
这里也能看到首先是获取 user 和 lazy2，然后在 user.getLazy1() 的时候同时加载了 lazy1 和 lazy3；
2. 创建代理
在上面已经讲过了，在使用延迟加载的时候：
- 首先判断是否有延迟加载属性，有就使用代理包装结果集对象；
- 然后判断一级缓存中时候有对应的外部嵌套，有就取缓存；如果没有就将外部嵌套包装为 ResultLoader 对象；
- 然后判断外部嵌套是否需要延迟加载，如果是就将 ResultLoader 加入到 ResultLoaderMap 中，如果不需要就直接加载 resultLoader.loadResult();
创建代理：首先获取代理工厂，然后创建代理类；
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) { if (propertyMapping.getNestedQueryId() != null && propertyMapping.isLazy()) { resultObject = configuration.getProxyFactory().createProxy(resultObject, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs); break; } } } this.useConstructorMappings = resultObject != null && !constructorArgTypes.isEmpty(); return resultObject; }3. 代理工厂
这里 CglibProxyFactory 和 JavassistProxyFactory 的流程都是一样的，所以我们就以 CglibProxyFactory 为例进行简单分析：
crateProxy：
static Object crateProxy(Class<?> type, Callback callback, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) { Enhancer enhancer = new Enhancer(); enhancer.setCallback(callback); enhancer.setSuperclass(type); try { type.getDeclaredMethod(WRITE_REPLACE_METHOD); // ObjectOutputStream will call writeReplace of objects returned by writeReplace if (LogHolder.log.isDebugEnabled()) { LogHolder.log.debug(WRITE_REPLACE_METHOD + " method was found on bean " + type + ", make sure it returns this"); } } catch (NoSuchMethodException e) { enhancer.setInterfaces(new Class[]{WriteReplaceInterface.class}); } catch (SecurityException e) { // nothing to do here } Object enhanced; if (constructorArgTypes.isEmpty()) { enhanced = enhancer.create(); } else { Class<?>[] typesArray = constructorArgTypes.toArray(new Class[constructorArgTypes.size()]); Object[] valuesArray = constructorArgs.toArray(new Object[constructorArgs.size()]); enhanced = enhancer.create(typesArray, valuesArray); } return enhanced; }这里创建大致和上面给出的 demo 差不多，都是指定父类，设置回调；接下来我们继续看拦截的具体内容：
private final ResultLoaderMap lazyLoader; public Object intercept(Object enhanced, Method method, Object[] args, MethodProxy methodProxy) throws Throwable { final String methodName = method.getName(); try { // 锁定 ResultLoaderMap 对象 synchronized (lazyLoader) { // 创建代理的时候设置的 WriteReplaceInterface 接口 if (WRITE_REPLACE_METHOD.equals(methodName)) { 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 CglibSerialStateHolder(original, lazyLoader.getProperties(), objectFactory, constructorArgTypes, constructorArgs); } else { return original; } // 真正延迟加载的逻辑处理 } else { // ResultLoaderMap 数量大于 0，就表示还有待加载的属性 if (lazyLoader.size() > 0 && !FINALIZE_METHOD.equals(methodName)) { // aggressive = true，或者 equals、clone、hashCode、toString 之一，就加载全部方法 if (aggressive || lazyLoadTriggerMethods.contains(methodName)) { lazyLoader.loadAll(); // 调用某属性的 set 方法时，表示不需要从数据库再加载了，所以将其移除 } else if (PropertyNamer.isSetter(methodName)) { final String property = PropertyNamer.methodToProperty(methodName); lazyLoader.remove(property); // 加载单个属性 } else if (PropertyNamer.isGetter(methodName)) { final String property = PropertyNamer.methodToProperty(methodName); if (lazyLoader.hasLoader(property)) { lazyLoader.load(property); } } } } } return methodProxy.invokeSuper(enhanced, args); } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } }4. 延迟加载属性加载
// org.apache.ibatis.executor.loader.ResultLoaderMap public boolean load(String property) throws SQLException { // 先删除 key，防止第二次查询数据库 LoadPair pair = loaderMap.remove(property.toUpperCase(Locale.ENGLISH)); if (pair != null) { // 查数据库 pair.load(); return true; } return false; } public void load(final Object userObject) throws SQLException { if (this.metaResultObject == null || this.resultLoader == null) { ... this.metaResultObject = config.newMetaObject(userObject); this.resultLoader = new ResultLoader(config, new ClosedExecutor(), ms, this.mappedParameter, metaResultObject.getSetterType(this.property), null, null); } /* We are using a new executor because we may be (and likely are) on a new thread * and executors aren't thread safe. (Is this sufficient?) * * A better approach would be making executors thread safe. */ if (this.serializationCheck == null) { final ResultLoader old = this.resultLoader; this.resultLoader = new ResultLoader(old.configuration, new ClosedExecutor(), old.mappedStatement, old.parameterObject, old.targetType, old.cacheKey, old.boundSql); } // 查询数据库，并反射设置属性 this.metaResultObject.setValue(property, this.resultLoader.loadResult()); }// org.apache.ibatis.executor.loader.ResultLoader public Object loadResult() throws SQLException { // 查询结果 List<Object> list = selectList(); // 转换结果类型 resultObject = resultExtractor.extractObjectFromList(list, targetType); return resultObject; } // 这里又是从 Executor 出发，再查数据库了 private <E> List<E> selectList() throws SQLException { Executor localExecutor = executor; if (Thread.currentThread().getId() != this.creatorThreadId || localExecutor.isClosed()) { localExecutor = newExecutor(); } try { return localExecutor.<E> query(mappedStatement, parameterObject, RowBounds.DEFAULT, Executor.NO_RESULT_HANDLER, cacheKey, boundSql); } finally { if (localExecutor != executor) { localExecutor.close(false); } } }以上就是延迟加载的全部流程了，
5. 延迟加载与一级缓存
上面我们将了当一级缓存中有外部嵌套查询缓存的时候，会直接取缓存，而不是延迟加载：
private Object getNestedQueryMappingValue(ResultSet rs, MetaObject metaResultObject, ResultMapping propertyMapping, ResultLoaderMap lazyLoader, String columnPrefix) throws SQLException { final String nestedQueryId = propertyMapping.getNestedQueryId(); final String property = propertyMapping.getProperty(); final MappedStatement nestedQuery = configuration.getMappedStatement(nestedQueryId); final Class<?> nestedQueryParameterType = nestedQuery.getParameterMap().getType(); final Object nestedQueryParameterObject = prepareParameterForNestedQuery(rs, propertyMapping, nestedQueryParameterType, columnPrefix); Object value = null; if (nestedQueryParameterObject != null) { final BoundSql nestedBoundSql = nestedQuery.getBoundSql(nestedQueryParameterObject); final CacheKey key = executor.createCacheKey(nestedQuery, nestedQueryParameterObject, RowBounds.DEFAULT, nestedBoundSql); final Class<?> targetType = propertyMapping.getJavaType(); // 判断一级缓存 if (executor.isCached(nestedQuery, key)) { executor.deferLoad(nestedQuery, metaResultObject, property, key, targetType); value = DEFERRED; } else { final ResultLoader resultLoader = new ResultLoader(configuration, executor, nestedQuery, nestedQueryParameterObject, targetType, key, nestedBoundSql); if (propertyMapping.isLazy()) { lazyLoader.addLoader(property, metaResultObject, resultLoader); value = DEFERRED; } else { value = resultLoader.loadResult(); } } } return value; }下面我们就实验一下：
<setting name="proxyFactory" value="JAVASSIST"/> <setting name="lazyLoadingEnabled" value="true"/> <setting name="aggressiveLazyLoading" value="false"/> <setting name="localCacheScope" value="SESSION"/><resultMap id="FatherMap" type="Father"> <id property="id" column="id"/> <result property="name" column="name"/> <association property="grandFather" column="grand_father_id" select="org.apache.ibatis.submitted.lazyload_common_property.GrandFatherMapper.selectById" fetchType="lazy"/> </resultMap> <select id="selectById" resultMap="FatherMap" parameterType="int"> SELECT id, name, grand_father_id FROM Father WHERE id = #{id} </select><resultMap id="GrandFatherMap" type="GrandFather"> <id property="id" column="id"/> <result property="name" column="name"/> </resultMap> <select id="selectById" resultMap="GrandFatherMap" parameterType="int"> SELECT id, name FROM GrandFather WHERE id = #{id} </select>@Test void test1() { try (SqlSession sqlSession = sqlSessionFactory.openSession()) { GrandFatherMapper grandFatherMapper = sqlSession.getMapper(GrandFatherMapper.class); FatherMapper fatherMapper = sqlSession.getMapper(FatherMapper.class); GrandFather grandFather = grandFatherMapper.selectById(1); System.out.println("----- get grandFather: " + grandFather); Father father = fatherMapper.selectById(1); System.out.println("----- get father: " + father.getName()); System.out.println("----- get father.grandFather: " + father.getGrandFather()); } }打印：
DEBUG [main] – ==> Preparing: SELECT id, name FROM GrandFather WHERE id = ?
DEBUG [main] – ==> Parameters: 1(Integer)
DEBUG [main] – <== Total: 1
—– get grandFather: GrandFather{id=1, name=’John Smith sen’}
DEBUG [main] – ==> Preparing: SELECT id, name, grand_father_id FROM Father WHERE id = ?
DEBUG [main] – ==> Parameters: 1(Integer)
DEBUG [main] – <== Total: 1
—– get father: John Smith
—– get father.grandFather: GrandFather{id=1, name=’John Smith sen’}
这里我们首先获取了一次 GrandFather，保证一级缓存中有，然后获取 Father，延迟加载 GrandFather；从上面的结果可以看到，确实延迟加载是从一级缓存中取的；
6. 延迟加载与二级缓存
上面我们讲过了外部嵌套查询的时候是从 Executor 开始的，那么必然有一级缓存和二级缓存；这里先说结论嵌套查询使用二级缓存一定要在同一个 namespace 里面，否则会出现脏读现象；下面举例说明：
<setting name="proxyFactory" value="JAVASSIST"/> <setting name="lazyLoadingEnabled" value="true"/> <setting name="aggressiveLazyLoading" value="false"/> <setting name="localCacheScope" value="STATEMENT"/> <setting name="cacheEnabled" value="true"/>// org/apache/ibatis/submitted/lazyload_common_property/FatherMapper.xml <mapper namespace="org.apache.ibatis.submitted.lazyload_common_property.FatherMapper"> <cache/> <resultMap id="FatherMap" type="Father"> <id property="id" column="id"/> <result property="name" column="name"/> <association property="grandFather" column="grand_father_id" select="org.apache.ibatis.submitted.lazyload_common_property.GrandFatherMapper.selectById" fetchType="lazy"/> </resultMap> <select id="selectById" resultMap="FatherMap" parameterType="int"> SELECT id, name, grand_father_id FROM Father WHERE id = #{id} </select> <update id="updateById" flushCache="true"> update Father set name = #{name} where id = #{id} </update> </mapper>// org/apache/ibatis/submitted/lazyload_common_property/GrandFatherMapper.xml <mapper namespace="org.apache.ibatis.submitted.lazyload_common_property.GrandFatherMapper"> <cache/> <resultMap id="GrandFatherMap" type="GrandFather"> <id property="id" column="id"/> <result property="name" column="name"/> </resultMap> <select id="selectById" resultMap="GrandFatherMap" parameterType="int"> SELECT id, name FROM GrandFather WHERE id = #{id} </select> <update id="updateById" flushCache="true"> update GrandFather set name = #{name} where id = #{id} </update> </mapper>@Test void test2() { try (SqlSession sqlSession1 = sqlSessionFactory.openSession(); SqlSession sqlSession2 = sqlSessionFactory.openSession(); ) { GrandFatherMapper grandFatherMapper1 = sqlSession1.getMapper(GrandFatherMapper.class); GrandFatherMapper grandFatherMapper2 = sqlSession2.getMapper(GrandFatherMapper.class); FatherMapper fatherMapper1 = sqlSession1.getMapper(FatherMapper.class); FatherMapper fatherMapper2 = sqlSession2.getMapper(FatherMapper.class); Father father1 = fatherMapper1.selectById(1); System.out.println("----- session1 get father(put cache): " + father1); sqlSession1.commit(); Father father2 = fatherMapper2.selectById(1); System.out.println("----- session2 get father(get cache): " + father2); // 测试重点 // fatherMapper1.updateById(1, "TestName"); grandFatherMapper1.updateById(1, "TestName"); sqlSession1.commit(); System.out.println("----- session1 update(put cache)"); Father father3 = fatherMapper2.selectById(1); System.out.println("----- session2 get father(get cache): " + father3); } }测试流程：
- 首先 session1 查询并提交二级缓存
- 然后 session2 查询检查二级缓存是否生效
- 然后 session1 修改缓存，并提交
- 最后 session2 再查查询，看是否检查到缓存的修改
打印：
DEBUG [main] – ==> Preparing: SELECT id, name, grand_father_id FROM Father WHERE id = ?
DEBUG [main] – ==> Parameters: 1(Integer)
DEBUG [main] – <== Total: 1
DEBUG [main] – Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.GrandFatherMapper]: 0.0
DEBUG [main] – ==> Preparing: SELECT id, name FROM GrandFather WHERE id = ?
DEBUG [main] – ==> Parameters: 1(Integer)
DEBUG [main] – <== Total: 1
—– session1 get father(put cache): Father{id=1, name=’John Smith’, grandFather=GrandFather{id=1, name=’John Smith sen’}}
DEBUG [main] – Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.FatherMapper]: 0.5
—– session2 get father(get cache): Father{id=1, name=’John Smith’, grandFather=GrandFather{id=1, name=’John Smith sen’}}
DEBUG [main] – ==> Preparing: update GrandFather set name = ? where id = ?
DEBUG [main] – ==> Parameters: TestName(String), 1(Integer)
DEBUG [main] – <== Updates: 1
DEBUG [main] – Committing JDBC Connection [org.hsqldb.jdbc.JDBCConnection@2f01783a]
—– session1 update(put cache)
DEBUG [main] – Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.FatherMapper]: 0.6666666666666666
—– session2 get father(get cache): Father{id=1, name=’John Smith’, grandFather=GrandFather{id=1, name=’John Smith sen’}}
注意看这里二级缓存生效了，但是出现了脏读：
然后我们将上面的注释打开：
DEBUG [main] – ==> Preparing: SELECT id, name, grand_father_id FROM Father WHERE id = ?
DEBUG [main] – ==> Parameters: 1(Integer)
DEBUG [main] – <== Total: 1
DEBUG [main] – Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.GrandFatherMapper]: 0.0
DEBUG [main] – ==> Preparing: SELECT id, name FROM GrandFather WHERE id = ?
DEBUG [main] – ==> Parameters: 1(Integer)
DEBUG [main] – <== Total: 1
—– session1 get father(put cache): Father{id=1, name=’John Smith’, grandFather=GrandFather{id=1, name=’John Smith sen’}}
DEBUG [main] – Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.FatherMapper]: 0.5
—– session2 get father(get cache): Father{id=1, name=’John Smith’, grandFather=GrandFather{id=1, name=’John Smith sen’}}
DEBUG [main] – ==> Preparing: update Father set name = ? where id = ?
DEBUG [main] – ==> Parameters: TestName(String), 1(Integer)
DEBUG [main] – <== Updates: 1
DEBUG [main] – ==> Preparing: update GrandFather set name = ? where id = ?
DEBUG [main] – ==> Parameters: TestName(String), 1(Integer)
DEBUG [main] – <== Updates: 1
DEBUG [main] – Committing JDBC Connection [org.hsqldb.jdbc.JDBCConnection@2f01783a]
—– session1 update(put cache)
DEBUG [main] – Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.FatherMapper]: 0.3333333333333333
DEBUG [main] – ==> Preparing: SELECT id, name, grand_father_id FROM Father WHERE id = ?
DEBUG [main] – ==> Parameters: 1(Integer)
DEBUG [main] – <== Total: 1
DEBUG [main] – Cache Hit Ratio [org.apache.ibatis.submitted.lazyload_common_property.GrandFatherMapper]: 0.0
DEBUG [main] – ==> Preparing: SELECT id, name FROM GrandFather WHERE id = ?
DEBUG [main] – ==> Parameters: 1(Integer)
DEBUG [main] – <== Total: 1
—– session2 get father(get cache): Father{id=1, name=’TestName’, grandFather=GrandFather{id=1, name=’TestName’}}
这次发现脏读消失了？？其原因就是第一次之修改了 GrandFather，虽然 Father 中有 GrandFather 属性，但是刷新缓存的时候并不会刷新 Father，所以出现的脏读；其解决办法就是使用
这里再提醒一下本文中使用的测试案例都能在 mybatis 源码的单元测试用找到；
