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mybatis如何根据mapper接口生成其实现类

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mybatis系列

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?

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mybatis如何根据mapper接口生成其实现类

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mybatis如何根据mapper接口生成其实现类

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