springboot mybatis mapper源码浅析

        MybatisAutoConfiguration是mybatis在springboot下自动配置类，该配置类向spring容器注入了SqlSessionFactory和SqlSessionTemplate，注入的SqlSessionFactory是通过SqlSessionFactoryBean的getObject()获取到的。SqlSessionFactoryBean通过buildSqlSessionFactory()创建SqlSessionFactory时，会通过MapperRegistry添加mapper，源码如下： 

 

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);
      }
    }
  }
}

可以看出在添加mapper时,会将mapper包装成MapperProxyFactory类。MapperRegistry同时提供了getMapper

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);
  }
}

在获取mapper时，先获取到的是MapperProxyFactory包装类，然后通过MapperProxyFactory的newInstance获取最终的mapper对象。

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);
}

从上面源码可以看出，最终返回的是一个由jdk动态代理生成的对象，如图所示（这是mybatisplus生成的代理对象）所以mapper接口中的所有方法最终都是通过MapperProxy的invoke方法执行。我们给其他spring管理的bean注入的mapper就是这个代理对象。

那么getMapper又是如何调用的呢？

在spring创建mapper对象时，通过createbean方法创建出来的bean是一个MapperFactoryBean对象，spring通过调用MapperFactoryBean的getObject()获取最终需要的bean对象，而这时就会调用MapperRegistry的getMapper方法。

那么为什么所有的mapper获取出来的bean都是MapperFactoryBean对象呢？这就得从bean定义说起。下面通过@MapperScan简单说下。

@Import(MapperScannerRegistrar.class)
public @interface MapperScan 

MapperScan定义了扫描mapper接口的路径，MapperScannerRegistrar实现了ImportBeanDefinitionRegistrar接口，该接口是spring对于bean定义的一个扩展接口，spring data也是通过实现该接口处理repository等的定义与生成的。

  @Override
  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
    AnnotationAttributes mapperScanAttrs = AnnotationAttributes
        .fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
    if (mapperScanAttrs != null) {
      registerBeanDefinitions(mapperScanAttrs, registry);
    }
  }

  void registerBeanDefinitions(AnnotationAttributes annoAttrs, BeanDefinitionRegistry registry) {

    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);

 

        mybatis通过ClassPathMapperScanner去扫描MapperScan定义的包路径，ClassPathMapperScanner 继承了ClassPathBeanDefinitionScanner，ClassPathMapperScanner先通过ClassPathBeanDefinitionScanner扫描（在指定的基本包中执行扫描，返回注册的bean定义），然后通过processBeanDefinitions方法将bean定义的beanClass修改成了MapperFactoryBean，spring通过bean定义生成bean对象时就会生成MapperFactoryBean对象。源码如下： 

 

private void processBeanDefinitions(Set beanDefinitions) {
  GenericBeanDefinition definition;
  for (BeanDefinitionHolder holder : beanDefinitions) {
    ...
    definition.setBeanClass(this.mapperFactoryBean.getClass());

@Override
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;
}

 

 有的版本中MapperScannerRegistrar的registerBeanDefinitions方法并未直接通过ClassPathMapperScanner 扫描，而是将MapperScannerConfigurer注册到spring中，MapperScannerConfigurer实现了BeanDefinitionRegistryPostProcessor接口，BeanDefinitionRegistryPostProcessor继承了BeanFactoryPostProcessor，spring在启动时会调用MapperScannerConfigurer的postProcessBeanDefinitionRegistry方法，该方法会去扫描mapper,最终还是调用了MapperScannerRegistrar的processBeanDefinitions方法改变了mapper的bean定义。