少年辛苦终身事，莫向光阴惰寸功。这篇文章主要讲述mybatis如何根据mapper接口生成其实现类相关的知识，希望能为你提供帮助。
序mybatis里头给sqlSession指定执行哪条sql的时候，有两种方式，一种是写mapper的xml的namespace+statementId，如下：

public Student findStudentById(Integer studId) { logger.debug("Select Student By ID :{}", studId); SqlSession sqlSession = MyBatisSqlSessionFactory.getSqlSession(); try { return sqlSession.selectOne("com.mybatis3.StudentMapper.findStudentById", studId); } finally { sqlSession.close(); } }

另外一种方法是指定mapper的接口：

public Student findStudentById(Integer studId) { logger.debug("Select Student By ID :{}", studId); SqlSession sqlSession = MyBatisSqlSessionFactory.getSqlSession(); try { StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class); return studentMapper.findStudentById(studId); } finally { sqlSession.close(); } }

一般的话，比较推荐第二种方法，因为手工写namespace和statementId极大增加了犯错误的概率，而且也降低了开发的效率。
问题 mapper的实现类如何生成
如果使用mapper接口的方式，问题来了，这个是个接口，通过sqlSession对象get出来的一定是个实现类，问题是，我们并没有手工去写 实现类，那么谁去干了这件事情呢？答案是mybatis通过JDK的动态代理方式，在启动加载配置文件时，根据配置mapper的xml去生成。
mybatis-spring帮忙做了什么
自动open和close session
一、mapper代理类是如何生成的 启动时加载解析mapper的xml如果不是集成spring的，会去读取< mappers> 节点，去加载mapper的xml配置

< ?xml version="1.0" encoding="UTF-8" ?> < !DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd"> < configuration> < settings> < setting name="cacheEnabled" value="https://www.songbingjia.com/android/true"/> < setting name="lazyLoadingEnabled" value="https://www.songbingjia.com/android/true"/> < setting name="multipleResultSetsEnabled" value="https://www.songbingjia.com/android/true"/> < setting name="useColumnLabel" value="https://www.songbingjia.com/android/true"/> < setting name="useGeneratedKeys" value="https://www.songbingjia.com/android/false"/> < setting name="defaultExecutorType" value="https://www.songbingjia.com/android/SIMPLE"/> < setting name="defaultStatementTimeout" value="https://www.songbingjia.com/android/2"/> < /settings> < typeAliases> < typeAlias alias="CommentInfo" type="com.xixicat.domain.CommentInfo"/> < /typeAliases> < environments default="development"> < environment id="development"> < transactionManager type="JDBC"/> < dataSource type="POOLED"> < property name="driver" value="https://www.songbingjia.com/android/com.mysql.jdbc.Driver"/> < property name="url" value="https://www.songbingjia.com/android/jdbc:mysql://localhost:3306/demo"/> < property name="username" value="https://www.songbingjia.com/android/root"/> < property name="password" value=""/> < /dataSource> < /environment> < /environments> < mappers> < mapper resource="com/xixicat/dao/CommentMapper.xml"/> < /mappers> < /configuration>

如果是集成spring的，会去读spring的sqlSessionFactory的xml配置中的mapperLocations，然后去解析mapper的xml

< bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean"> < property name="dataSource" ref="dataSource"/> < !-- 配置mybatis配置文件的位置 --> < property name="configLocation" value="https://www.songbingjia.com/android/classpath:mybatis-config.xml"/> < property name="typeAliasesPackage" value="https://www.songbingjia.com/android/com.xixicat.domain"/> < !-- 配置扫描Mapper XML的位置 --> < property name="mapperLocations" value="https://www.songbingjia.com/android/classpath:com/xixicat/dao/*.xml"/> < /bean>

然后绑定namespace(XMLMapperBuilder)

private void bindMapperForNamespace() { String namespace = builderAssistant.getCurrentNamespace(); if (namespace != null) { Class< ?> boundType = null; try { boundType = Resources.classForName(namespace); } catch (ClassNotFoundException e) { //ignore, bound type is not required } if (boundType != null) { if (!configuration.hasMapper(boundType)) { // Spring may not know the real resource name so we set a flag // to prevent loading again this resource from the mapper interface // look at MapperAnnotationBuilder#loadXmlResource configuration.addLoadedResource("namespace:" + namespace); configuration.addMapper(boundType); } } } }

这里先去判断该namespace能不能找到对应的class，若可以则调用

configuration.addMapper(boundType);

configuration委托给MapperRegistry:

public < T> void addMapper(Class< T> type) { mapperRegistry.addMapper(type); }

生成该mapper的代理工厂(MapperRegistry)

public < T> void addMapper(Class< T> type) { if (type.isInterface()) { if (hasMapper(type)) { throw new BindingException("Type " + type + " is already known to the MapperRegistry."); } boolean loadCompleted = false; try { knownMappers.put(type, new MapperProxyFactory< T> (type)); // It‘s important that the type is added before the parser is run // otherwise the binding may automatically be attempted by the // mapper parser. If the type is already known, it won‘t try. MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type); parser.parse(); loadCompleted = true; } finally { if (!loadCompleted) { knownMappers.remove(type); } } } }

这里的重点就是MapperProxyFactory类：

public class MapperProxyFactory< T> {private final Class< T> mapperInterface; private final Map< Method, MapperMethod> methodCache = new ConcurrentHashMap< Method, MapperMethod> (); public MapperProxyFactory(Class< T> mapperInterface) { this.mapperInterface = mapperInterface; }public Class< T> getMapperInterface() { return mapperInterface; }public Map< Method, MapperMethod> getMethodCache() { return methodCache; }@SuppressWarnings("unchecked") protected T newInstance(MapperProxy< T> mapperProxy) { return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy); }public T newInstance(SqlSession sqlSession) { final MapperProxy< T> mapperProxy = new MapperProxy< T> (sqlSession, mapperInterface, methodCache); return newInstance(mapperProxy); }}

getMapper的时候生成mapper代理类

@SuppressWarnings("unchecked") public < T> T getMapper(Class< T> type, SqlSession sqlSession) { final MapperProxyFactory< T> mapperProxyFactory = (MapperProxyFactory< T> ) knownMappers.get(type); if (mapperProxyFactory == null) { throw new BindingException("Type " + type + " is not known to the MapperRegistry."); } try { return mapperProxyFactory.newInstance(sqlSession); } catch (Exception e) { throw new BindingException("Error getting mapper instance. Cause: " + e, e); } }

new出来MapperProxy

public T newInstance(SqlSession sqlSession) { final MapperProxy< T> mapperProxy = new MapperProxy< T> (sqlSession, mapperInterface, methodCache); return newInstance(mapperProxy); } @SuppressWarnings("unchecked") protected T newInstance(MapperProxy< T> mapperProxy) { return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy); }

这里给代理类注入了sqlSession
MapperProxy实现InvocationHandler接口进行拦截代理

public class MapperProxy< T> implements InvocationHandler, Serializable {private static final long serialVersionUID = -6424540398559729838L; private final SqlSession sqlSession; private final Class< T> mapperInterface; private final Map< Method, MapperMethod> methodCache; public MapperProxy(SqlSession sqlSession, Class< T> mapperInterface, Map< Method, MapperMethod> methodCache) { this.sqlSession = sqlSession; this.mapperInterface = mapperInterface; this.methodCache = methodCache; }@Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { if (Object.class.equals(method.getDeclaringClass())) { try { return method.invoke(this, args); } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } } final MapperMethod mapperMethod = cachedMapperMethod(method); return mapperMethod.execute(sqlSession, args); }private MapperMethod cachedMapperMethod(Method method) { MapperMethod mapperMethod = methodCache.get(method); if (mapperMethod == null) { mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()); methodCache.put(method, mapperMethod); } return mapperMethod; } }

这里的代理拦截，主要是寻找到MapperMethod，通过它去执行SQL。
MapperMethod委托给SqlSession去执行sql

public Object execute(SqlSession sqlSession, Object[] args) { Object result; if (SqlCommandType.INSERT == command.getType()) { Object param = method.convertArgsToSqlCommandParam(args); result = rowCountResult(sqlSession.insert(command.getName(), param)); } else if (SqlCommandType.UPDATE == command.getType()) { Object param = method.convertArgsToSqlCommandParam(args); result = rowCountResult(sqlSession.update(command.getName(), param)); } else if (SqlCommandType.DELETE == command.getType()) { Object param = method.convertArgsToSqlCommandParam(args); result = rowCountResult(sqlSession.delete(command.getName(), param)); } else if (SqlCommandType.SELECT == command.getType()) { if (method.returnsVoid() & & method.hasResultHandler()) { executeWithResultHandler(sqlSession, args); result = null; } else if (method.returnsMany()) { result = executeForMany(sqlSession, args); } else if (method.returnsMap()) { result = executeForMap(sqlSession, args); } else { Object param = method.convertArgsToSqlCommandParam(args); result = sqlSession.selectOne(command.getName(), param); } } else if (SqlCommandType.FLUSH == command.getType()) { result = sqlSession.flushStatements(); } else { throw new BindingException("Unknown execution method for: " + command.getName()); } if (result == null & & method.getReturnType().isPrimitive() & & !method.returnsVoid()) { throw new BindingException("Mapper method ‘" + command.getName() + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ")."); } return result; }

其实这里就回到了第一种模式，该模式是直接指定了statement的Id（这里是command.getName()），而通过mapper的接口方式，则多了这么步骤，最后通过MapperMethod，给sqlSession传入statement的id。
sqlSession其实自己也不执行sql，它只是mybatis对外公布的一个api入口，具体它委托给了executor去执行sql。
什么时候去getMapper

• 手工get，比如
  

  public void createStudent(Student student) { SqlSession sqlSession = MyBatisSqlSessionFactory.getSqlSession(); try { StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class); studentMapper.insertStudent(student); sqlSession.commit(); } finally { sqlSession.close(); } }

  
  
• 【mybatis如何根据mapper接口生成其实现类】集成spring的话
  在spring容器给指定的bean注入mapper的时候get出来(见MapperFactoryBean的getObject方法)
  

二、mybatis-spring帮忙做了什么 通过MapperScannerConfigurer将mapper适配成spring bean

< !-- 配置扫描Mapper接口的包路径 --> < bean class="org.mybatis.spring.mapper.MapperScannerConfigurer"> < property name="sqlSessionFactoryBeanName" value="https://www.songbingjia.com/android/sqlSessionFactory"/> < property name="basePackage" value="https://www.songbingjia.com/android/com.xixicat.dao"/> < /bean>

这里使用 MapperFactoryBean将Mapper接口配置成 Spring bean 实体同时注入sqlSessionFactory。
MapperScannerConfigurer给每个mapper生成对应的MapperFactoryBean

public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException { if (this.processPropertyPlaceHolders) { processPropertyPlaceHolders(); }ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry); scanner.setAddToConfig(this.addToConfig); scanner.setAnnotationClass(this.annotationClass); scanner.setMarkerInterface(this.markerInterface); scanner.setSqlSessionFactory(this.sqlSessionFactory); scanner.setSqlSessionTemplate(this.sqlSessionTemplate); scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName); scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName); scanner.setResourceLoader(this.applicationContext); scanner.setBeanNameGenerator(this.nameGenerator); scanner.registerFilters(); scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS)); }

委托给ClassPathMapperScanner去scan

public Set< BeanDefinitionHolder> doScan(String... basePackages) { Set< BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages); if (beanDefinitions.isEmpty()) { logger.warn("No MyBatis mapper was found in ‘" + Arrays.toString(basePackages) + "‘ package. Please check your configuration."); } else { for (BeanDefinitionHolder holder : beanDefinitions) { GenericBeanDefinition definition = (GenericBeanDefinition) holder.getBeanDefinition(); if (logger.isDebugEnabled()) { logger.debug("Creating MapperFactoryBean with name ‘" + holder.getBeanName() + "‘ and ‘" + definition.getBeanClassName() + "‘ mapperInterface"); }// the mapper interface is the original class of the bean // but, the actual class of the bean is MapperFactoryBean definition.getPropertyValues().add("mapperInterface", definition.getBeanClassName()); definition.setBeanClass(MapperFactoryBean.class); definition.getPropertyValues().add("addToConfig", this.addToConfig); boolean explicitFactoryUsed = false; if (StringUtils.hasText(this.sqlSessionFactoryBeanName)) { definition.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference(this.sqlSessionFactoryBeanName)); explicitFactoryUsed = true; } else if (this.sqlSessionFactory != null) { definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory); explicitFactoryUsed = true; }if (StringUtils.hasText(this.sqlSessionTemplateBeanName)) { if (explicitFactoryUsed) { logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored."); } definition.getPropertyValues().add("sqlSessionTemplate", new RuntimeBeanReference(this.sqlSessionTemplateBeanName)); explicitFactoryUsed = true; } else if (this.sqlSessionTemplate != null) { if (explicitFactoryUsed) { logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored."); } definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate); explicitFactoryUsed = true; }if (!explicitFactoryUsed) { if (logger.isDebugEnabled()) { logger.debug("Enabling autowire by type for MapperFactoryBean with name ‘" + holder.getBeanName() + "‘."); } definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE); } } }return beanDefinitions; }

这里出现了MapperFactoryBean的身影，然后判断配置文件是指定注入sqlSessionFactory，还是 sqlTemplate(二者不能同时指定，这里是指定了sqlSessionFactory)。这里通过 sqlSessionFactoryBeanName暂时先注入引用，因为此时还在给spring托管的bean进行create，不确定 sqlSessionFactory是否已经被创建。
关于MapperFactoryBean

public class MapperFactoryBean< T> extends SqlSessionDaoSupport implements FactoryBean< T> {private Class< T> mapperInterface; private boolean addToConfig = true; /** * Sets the mapper interface of the MyBatis mapper * * @param mapperInterface class of the interface */ public void setMapperInterface(Class< T> mapperInterface) { this.mapperInterface = mapperInterface; }/** * If addToConfig is false the mapper will not be added to MyBatis. This means * it must have been included in mybatis-config.xml. * < p> * If it is true, the mapper will be added to MyBatis in the case it is not already * registered. * < p> * By default addToCofig is true. * * @param addToConfig */ public void setAddToConfig(boolean addToConfig) { this.addToConfig = addToConfig; }/** * {@inheritDoc} */ @Override protected void checkDaoConfig() { super.checkDaoConfig(); notNull(this.mapperInterface, "Property ‘mapperInterface‘ is required"); Configuration configuration = getSqlSession().getConfiguration(); if (this.addToConfig & & !configuration.hasMapper(this.mapperInterface)) { try { configuration.addMapper(this.mapperInterface); } catch (Throwable t) { logger.error("Error while adding the mapper ‘" + this.mapperInterface + "‘ to configuration.", t); throw new IllegalArgumentException(t); } finally { ErrorContext.instance().reset(); } } }/** * {@inheritDoc} */ public T getObject() throws Exception { return getSqlSession().getMapper(this.mapperInterface); }/** * {@inheritDoc} */ public Class< T> getObjectType() { return this.mapperInterface; }/** * {@inheritDoc} */ public boolean isSingleton() { return true; }}

注意这里继承了SqlSessionDaoSupport，在spring把sqlSessionFactory创建出来后，会去把之前注入的引用改为真的实例，调用SqlSessionDaoSupport的setSqlSessionFactory方法。

public abstract class SqlSessionDaoSupport extends DaoSupport {private SqlSession sqlSession; private boolean externalSqlSession; public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) { if (!this.externalSqlSession) { this.sqlSession = new SqlSessionTemplate(sqlSessionFactory); } }public void setSqlSessionTemplate(SqlSessionTemplate sqlSessionTemplate) { this.sqlSession = sqlSessionTemplate; this.externalSqlSession = true; }/** * Users should use this method to get a SqlSession to call its statement methods * This is SqlSession is managed by spring. Users should not commit/rollback/close it * because it will be automatically done. * * @return Spring managed thread safe SqlSession */ public SqlSession getSqlSession() { return this.sqlSession; }/** * {@inheritDoc} */ protected void checkDaoConfig() { notNull(this.sqlSession, "Property ‘sqlSessionFactory‘ or ‘sqlSessionTemplate‘ are required"); }}

这里值得注意的是setSqlSessionFactory方法new了一个SqlSessionTemplate。
SqlSessionTemplate
它的一个重要的构造器为

public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {notNull(sqlSessionFactory, "Property ‘sqlSessionFactory‘ is required"); notNull(executorType, "Property ‘executorType‘ is required"); this.sqlSessionFactory = sqlSessionFactory; this.executorType = executorType; this.exceptionTranslator = exceptionTranslator; this.sqlSessionProxy = (SqlSession) newProxyInstance( SqlSessionFactory.class.getClassLoader(), new Class[] { SqlSession.class }, new SqlSessionInterceptor()); }

mybatis-srping比传统mybatis方法多做的事情就在于此，生成了一个sqlSessionProxy。这里static import了java.lang.reflect.Proxy.newProxyInstance; 也就是使用使用jdk代理进行了 SqlSessionInterceptor拦截。
SqlSessionInterceptor

private class SqlSessionInterceptor implements InvocationHandler { public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { SqlSession sqlSession = getSqlSession( SqlSessionTemplate.this.sqlSessionFactory, SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator); try { Object result = method.invoke(sqlSession, args); if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) { // force commit even on non-dirty sessions because some databases require // a commit/rollback before calling close() sqlSession.commit(true); } return result; } catch (Throwable t) { Throwable unwrapped = unwrapThrowable(t); if (SqlSessionTemplate.this.exceptionTranslator != null & & unwrapped instanceof PersistenceException) { // release the connection to avoid a deadlock if the translator is no loaded. See issue #22 closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory); sqlSession = null; Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped); if (translated != null) { unwrapped = translated; } } throw unwrapped; } finally { if (sqlSession != null) { closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory); } } } }

到了这里就明白了mybatis-spring帮忙做了session的open和close。
关于一级缓存最后留个问题给大家，如果是使用mybatis-spring，那么mybatis的一级缓存默认开启的话，那么有达到一级缓存的设计初衷的效果么？

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