springboot mybatis mapper源码浅析

addMapper

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 <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);
      }
    }
  }
}

可以看出在添加mapper时,会将mapper包装成MapperProxyFactory类，存入knownMappers中。

getMapper

MapperRegistry同时提供了getMapper

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);
  }
}

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


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

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

getMapper何时调用

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，该接口是spring对于bean定义的一个扩展接口。

  @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扫描生成BeanDefinition，源码如下：

@Override
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 {
    processBeanDefinitions(beanDefinitions);
  }

  return beanDefinitions;
}

然后通过processBeanDefinitions方法将bean定义的beanClass修改成了MapperFactoryBean。

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

所以最终spring通过bean定义生成bean对象时就会生成MapperFactoryBean对象。

不同版本扫描mapper对应的bean定义时的差异

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

public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware {

  @Override
  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
    AnnotationAttributes mapperScanAttrs = AnnotationAttributes
        .fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
    if (mapperScanAttrs != null) {
      registerBeanDefinitions(mapperScanAttrs, registry, generateBaseBeanName(importingClassMetadata, 0));
    }
  }

  void registerBeanDefinitions(AnnotationAttributes annoAttrs, BeanDefinitionRegistry registry, String beanName) {
    //声明MapperScannerConfigurer对应的BeanDefinitionBuilder 
    BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
    ......
    //basePackages会添加到@MapperScan设置的包路径
     List<String> basePackages = new ArrayList<>();
        basePackages.addAll(
     Arrays.stream(annoAttrs.getStringArray("value")).filter(StringUtils::hasText).collect(Collectors.toList()));

    basePackages.addAll(Arrays.stream(annoAttrs.getStringArray("basePackages")).filter(StringUtils::hasText)
        .collect(Collectors.toList()));

    basePackages.addAll(Arrays.stream(annoAttrs.getClassArray("basePackageClasses")).map(ClassUtils::getPackageName)
        .collect(Collectors.toList()));
    //1
    builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(basePackages));
    //MapperScannerConfigurer注册到spring中
    registry.registerBeanDefinition(beanName, builder.getBeanDefinition());

  }

//1设置了mybatis需要扫描的基础包名。spring会在getBean时的填充属性阶段（populateBean）通过反射设置。所以MapperScannerConfigurer的basePackage的值就是我们在@MapperScan上声明的值。

MapperScannerConfigurer实现了BeanDefinitionRegistryPostProcessor接口，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ClassPathMapperScanner的processBeanDefinitions方法改变了mapper的bean定义。

mybatis-spring:2.0.2 MapperScannerConfigurer#postProcessBeanDefinitionRegistry方法

  @Override
  public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
    if (this.processPropertyPlaceHolders) {
      processPropertyPlaceHolders();
    }

    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
    ......
    scanner.registerFilters();
    scanner.scan(
        StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
  }

原文链接：https://blog.csdn.net/sinat_33472737/article/details/106423656