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="true"/>          <setting name="lazyLoadingEnabled" value="true"/>          <setting name="multipleResultSetsEnabled" value="true"/>          <setting name="useColumnLabel" value="true"/>          <setting name="useGeneratedKeys" value="false"/>          <setting name="defaultExecutorType" value="SIMPLE"/>          <setting name="defaultStatementTimeout" value="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="com.mysql.jdbc.Driver"/>                  <property name="url" value="jdbc:mysql://localhost:3306/demo"/>                  <property name="username" value="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="classpath:mybatis-config.xml"/>          <property name="typeAliasesPackage" value="com.xixicat.domain"/>          <!-- 配置扫描Mapper XML的位置 -->          <property name="mapperLocations" value="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();          }      }

• 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 )

  </li> </ul>

  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="sqlSessionFactory"/>          <property name="basePackage" value="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？

  </div>

  来自： http://segmentfault.com/a/1190000004238437