深入理解MyBatis-Spring中间件

Mybatis-Spring

1.应用

mybatis是比较常用的数据库中间件，我们都知道我们来看看怎么在spring中使用mybatis，假设有用户表User，包含四个字段（id,name,sex,mobile），在Spring中使用mybatis操作User表非常简单，这里使用的是mybatis-spring 1.3.0，首先定义接口，

public interface UserMapper {
    int createUser(@Param("user") User user);
}

然后，定义对应的mybatis xml文件，

    
        
        
        
        
    

    
        INSERT INTO
        User
        (name,sex,mobile)
        VALUES
        (#{user.name},#{user.sex},#{user.mobile})
    

紧接着，添加配置文件，命名为spring-dao.mxl，

定义服务UserService接口和实现类，此处没给出UserService接口定义，

@Service
public class UserServiceImpl implements UserService {
    @Resource
    UserMapper userMapper;
    public int createUser(UserDTO userDTO) {
        User user = new User();
        BeanUtils.copyProperties(userDTO,user,new String[]{"id"});
        int row = userMapper.createUser(user);//插入返回值为作用的记录数;生成的主键已经被赋值到user对象上
        if(row >= 1)
            return user.getId();
        return -1;
    }

    public UserDTO getUserById(Integer id) {
        return null;
    }
}

完成接口定义、mybatis xml定义和配置文件，就可以直接使用接口来操作数据库，

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = {"classpath*:spring/spring-*.xml"})
public class TestUser{
    private static UserService userService;

    @BeforeClass
    public static void beforeClass(){
        ApplicationContext context = new ClassPathXmlApplicationContext("classpath:spring/spring-dao.xml");
        userService = context.getBean(UserService.class);
    }
    @Test
    public void testCreatUser(){
        UserDTO userDTO = new UserDTO();
        userDTO.setName("your name");
        userDTO.setSex(true);
        userDTO.setMobile("12134232211");
        Integer userId = userService.createUser(userDTO);
        System.out.println(userId);
        System.out.println(userDTO.getId());
    }

}

2. 原理

从UserServiceImpl实现类中可以看出，服务类直接使用的UserMapper接口来操作数据库，而UserMapper接口没有对应的实现类，这一切都是由spring-mybatis库通过动态代理实现的，接下来分析下它的实现原理，先由SqlSessionFactoryBean生成SQLSessionFactory和并扫描接口，为接口生成动态代理

1. SqlSessionFactoryBean配置

它的主要作用是扫描sql xml文件，查看源码，

public class SqlSessionFactoryBean implements FactoryBean, InitializingBean

SqlSessionFactoryBean实现了FactoryBean和InitializingBean，因此会首先执行afterPropertiesSet()方法，然后根据getObject()方法返回的对象生产Spring Bean，这里生产的是SqlSessionFactory类型的对象，在afterPropertiesSet方法中，执行了buildSqlSessionFactory方法来初始化SqlSessionFactory对象，来看看其中的一段，

if (!isEmpty(this.mapperLocations)) {
      for (Resource mapperLocation : this.mapperLocations) {
        if (mapperLocation == null) {
          continue;
        }

        try {
          XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
              configuration, mapperLocation.toString(), configuration.getSqlFragments());
          xmlMapperBuilder.parse();
        } 

进入XmlMapperBuilder.parse方法，

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);
        }
      }
    }
  }

namespace指定了mybatis dao接口的路径，通过configuration.addMapper(boundType)将接口的动态代理委托给MapperRegistry，MapperRegistry中通过MapperFactory生成动态代理，

public  T getMapper(Class type, SqlSession sqlSession) {
    final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) 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);
    }
  }
  
  public  boolean hasMapper(Class type) {
    return knownMappers.containsKey(type);
  }

  public  void addMapper(Class 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(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);
        }
      }
    }
  }

MapperProxyFactory中使用Proxy.newProxyInstance来生成动态代理

 protected T newInstance(MapperProxy mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

MapperProxy为动态代理的处理类，实际上将SqlSession操作db的过程封装在了MapperMethod类中，

public MapperProxy(SqlSession sqlSession, Class mapperInterface, Map 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);
  }

简单看下结构，

总结，SqlSessionFactoryBean实际上对应的是SqlSessionFactory类，它会扫描sql xml文件，并对接口创建动态代理，将接口类的Class和动态代理关系保存在SqlSessionFactory中，这仅仅是完成了动态代理的生成，而动态代理在哪里被使用到，怎么使用，这些都是由MapperScannerConfigurer完成，接下来看看MapperScannerConfigurer都做了些什么？

2. MapperScannerConfigurer 

从开始的配置文件可以看出，MapperScannerConfigurer依赖于SqlSessionFactoryBean和Mapper接口所在package，之前也说过SqlSessionFactoryBean实际上对应的是SqlSessionFactory，它可以提供Mapper接口的动态代理类，而Mapper所在package提供了扫描的路径，在扫描过程中，会把每个Mapper接口对应到一个MapperFactoryBean，MapperFactoryBean实际上对应的是动态代理类，这一切也就说通了，下面来看看源码，

MapperScannerConfigurer实现了BeanDefinitionRegistryPostProcessor接口，因此会在bean factory初始化的时候，被调用到，调用的方法为postProcessBeanDefinitionRegistry，因此看看postProcessBeanDefinitionRegistry的源码，

public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
    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));
  }

调用了ClassPathMapperScanner的scan()方法，

public int scan(String... basePackages) {
		int beanCountAtScanStart = this.registry.getBeanDefinitionCount();

		doScan(basePackages);

		// Register annotation config processors, if necessary.
		if (this.includeAnnotationConfig) {
			AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
		}

		return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
	}

scan方法又调用了doScan方法，看看ClassPathMapperScanner的doScan方法，Spring会首先把需要实例化的bean加载的BeanDefinitionHolder的集合中，doScan方法，就是添加mybatis mapper接口的bean定义到BeanDefinitionHolder集合，

public Set doScan(String... basePackages) {
    Set beanDefinitions = super.doScan(basePackages);

    if (beanDefinitions.isEmpty()) {
      logger.warn("No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.");
    } else {
      processBeanDefinitions(beanDefinitions);
    }

    return beanDefinitions;
  }

  private void processBeanDefinitions(Set beanDefinitions) {
    GenericBeanDefinition definition;
    for (BeanDefinitionHolder holder : beanDefinitions) {
      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.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); // issue #59
      definition.setBeanClass(this.mapperFactoryBean.getClass());//将其bean Class类型设置为mapperFactoryBean，放入BeanDefinitions

      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);
      }
    }
  }

那MapperFactoryBean究竟又做了什么呢，看源码，

MapperFactoryBean extends SqlSessionDaoSupport implements FactoryBean

通过集成SqlSessionDaoSupport获得SqlSessionFactory，通过实现FactoryBean，生产动态代理对象，

@Override
  public T getObject() throws Exception {
    return getSqlSession().getMapper(this.mapperInterface);
  }

一切到这里就已经很显而易见了，Mapper接口对应的Spring Bean实际上就是getSqlSession().getMapper(this.mapperInterface)返回的动态代理，每次装配Mapper接口时，就相当于装配了此接口对应的动态代理，这样就顺利成章的被代理成功了。