mybatis如何根据mapper接口生成其实现类

mybatis系列

• SpringBoot集成mybatis

• mybatis的statement的解析与加载

• mybatis如何根据mapper接口生成其实现类

• mybatis的mapper返回map结果集

• mybatis结果的组装

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)

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？