MyBatis-Spring-Boot-Starter源码解析

爪哇优太儿 2020-04-10

290

我们以MyBatis-Spring-Boot-Starter:2.1.2为例，首先打开mybatis-spring-boot-autoconfigure:2.1.2，找到spring.factories：

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.mybatis.spring.boot.autoconfigure.MybatisLanguageDriverAutoConfiguration,\
org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration

这里定义了mybatis的配置类，我们主要看下MybatisAutoConfiguration。

首先看下构造函数：

public MybatisAutoConfiguration(MybatisProperties properties, ObjectProvider<Interceptor[]> interceptorsProvider,
      ObjectProvider<TypeHandler[]> typeHandlersProvider, ObjectProvider<LanguageDriver[]> languageDriversProvider,
      ResourceLoader resourceLoader, ObjectProvider<DatabaseIdProvider> databaseIdProvider,
      ObjectProvider<List<ConfigurationCustomizer>> configurationCustomizersProvider) {
    this.properties = properties;
    this.interceptors = interceptorsProvider.getIfAvailable();
    this.typeHandlers = typeHandlersProvider.getIfAvailable();
    this.languageDrivers = languageDriversProvider.getIfAvailable();
    this.resourceLoader = resourceLoader;
    this.databaseIdProvider = databaseIdProvider.getIfAvailable();
    this.configurationCustomizers = configurationCustomizersProvider.getIfAvailable();
  }

构造函数注入了四个组件：Interceptor、TypeHandler、LanguageDriver、DatabaseIdProvider，同时注入了ConfigurationCustomizer用来自定义配置，还注入了MybatisProperties。可以结合前一篇文章一块来看。

继续往下看，里面定义了两个Bean：

@Bean
@ConditionalOnMissingBean
public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
  SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
  //中间就是向factory中填充各种属性
  ...
  //最终生成出SqlSessionFactory
  return factory.getObject();
}

这是定义SqlSessionFactory。

@Bean
@ConditionalOnMissingBean
public SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
  //利用SqlSessionFactory构建出来SqlSessionTemplate
}

这是定义SqlSessionTemplate，Mybatis-Spring的核心就是SqlSessionTemplate

继续往下面看，下面还定义了一个内嵌的配置类：

@org.springframework.context.annotation.Configuration
@Import(AutoConfiguredMapperScannerRegistrar.class)
@ConditionalOnMissingBean({ MapperFactoryBean.class, MapperScannerConfigurer.class })
public static class MapperScannerRegistrarNotFoundConfiguration implements InitializingBean{
}

这个配置类主要用来做Mapper的扫描的，但是要注意@ConditionalOnMissingBean({ MapperFactoryBean.class, MapperScannerConfigurer.class })，只有在没有定义MapperFactoryBean和MapperScannerConfigurer的情况下，才会启用默认的Mapper扫描。

我们来看下它是如何Mapper扫描的：

public static class AutoConfiguredMapperScannerRegistrar implements BeanFactoryAware, ImportBeanDefinitionRegistrar {
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
  。。。
      BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
      builder.addPropertyValue("processPropertyPlaceHolders", true);
      builder.addPropertyValue("annotationClass", Mapper.class);
      builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(packages));
      BeanWrapper beanWrapper = new BeanWrapperImpl(MapperScannerConfigurer.class);
      Stream.of(beanWrapper.getPropertyDescriptors())
          // Need to mybatis-spring 2.0.2+
          .filter(x -> x.getName().equals("lazyInitialization")).findAny()
          .ifPresent(x -> builder.addPropertyValue("lazyInitialization", "${mybatis.lazy-initialization:false}"));
      registry.registerBeanDefinition(MapperScannerConfigurer.class.getName(), builder.getBeanDefinition());
    }
}

这里是生成了扫描注解的类，这个类的名字是MapperScannerConfigurer，类型是MapperScannerConfigurer，给它设置了几个属性：processPropertyPlaceHolders、annotationClass和basePackage，annotationClass就是要扫描的Mapper的默认的注解类，basePackage就是要扫描的包的跟路径，默认就是SpringBoot的启动类的包路径。

继续往下看MapperScannerConfigurer：

public class MapperScannerConfigurer
    implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {
}

它实现了BeanDefinitionRegistryPostProcessor，因此Spring容器会回调postProcessBeanDefinitionRegistry方法：

@Override
  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.setMapperFactoryBeanClass(this.mapperFactoryBeanClass);
    if (StringUtils.hasText(lazyInitialization)) {
      scanner.setLazyInitialization(Boolean.valueOf(lazyInitialization));
    }
    scanner.registerFilters();
    scanner.scan(
        StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
  }

它里面会创建一个ClassPathMapperScanner的类去做Mapper的扫描，看下具体的doScan()方法：

@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;
  }

继续看下processBeanDefinitions：

private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
    。。。
    for (BeanDefinitionHolder holder : beanDefinitions) {
      definition = (GenericBeanDefinition) holder.getBeanDefinition();
      // 这里就设置了生成的Mappe的类是mapperFactoryBeanClass = MapperFactoryBean.class;
      definition.setBeanClass(this.mapperFactoryBeanClass);
     //给Bean设置了sqlSessionFactory
     definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory);
      // 给Bean设置了sqlSessionFactory
      definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
    }
}

继续看下MapperFactoryBean：

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {
}
public abstract class SqlSessionDaoSupport extends DaoSupport {
}
public abstract class DaoSupport implements InitializingBean{
}

MapperFactoryBean是一个FactoryBean，所以容器会调用getObject()来生成Mapper的具体的类：

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

同时它也实现了InitializingBean，因此容器会回调afterPropertiesSet()，最终会回调到checkDaoConfig():

@Override
  protected void checkDaoConfig() {
    super.checkDaoConfig();
    Configuration configuration = getSqlSession().getConfiguration();
    if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
      try {
        //这里就把Mapper接口添加到了configuratiuon中
        configuration.addMapper(this.mapperInterface);
      } catch (Exception e) {
        logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", e);
        throw new IllegalArgumentException(e);
      } finally {
        ErrorContext.instance().reset();
      }
    }
  }

继续跟一下configuration.addMapper，一直到MapperRegistry.addMapper():

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<>(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);
        }
      }
    }
  }

看一下parser.parse()：

public void parse() {
    String resource = type.toString();
    if (!configuration.isResourceLoaded(resource)) {
      //这里面就会去解析与Mapper接口相同包名下面的xml文件
      loadXmlResource();
      configuration.addLoadedResource(resource);
      assistant.setCurrentNamespace(type.getName());
      parseCache();
      parseCacheRef();
      Method[] methods = type.getMethods();
      for (Method method : methods) {
        try {
          // issue #237
          if (!method.isBridge()) {
            //这里回去解析接口方法上的SQL注解
            parseStatement(method);
          }
        } catch (IncompleteElementException e) {
          configuration.addIncompleteMethod(new MethodResolver(this, method));
        }
      }
    }
    parsePendingMethods();
  }

至此，Mapper解析加载完成。

此外，如果不使用默认的Mapper扫描机制，也可以使用@MapperScan注解，看下它的源码：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(MapperScannerRegistrar.class)
@Repeatable(MapperScans.class)
public @interface MapperScan {
}

看下MapperScannerRegistrar：

public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware {
}

容器会回调registerBeanDefinitions()：

void registerBeanDefinitions(AnnotationMetadata annoMeta, AnnotationAttributes annoAttrs,
      BeanDefinitionRegistry registry, String beanName) {
    //这里其实也是注册了一个MapperScannerConfigurer的Bean，后面的处理则与默认相同
    BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
    builder.addPropertyValue("processPropertyPlaceHolders", true);
    Class<? extends Annotation> annotationClass = annoAttrs.getClass("annotationClass");
    if (!Annotation.class.equals(annotationClass)) {
      builder.addPropertyValue("annotationClass", annotationClass);
    }
    Class<?> markerInterface = annoAttrs.getClass("markerInterface");
    if (!Class.class.equals(markerInterface)) {
      builder.addPropertyValue("markerInterface", markerInterface);
    }
    Class<? extends BeanNameGenerator> generatorClass = annoAttrs.getClass("nameGenerator");
    if (!BeanNameGenerator.class.equals(generatorClass)) {
      builder.addPropertyValue("nameGenerator", BeanUtils.instantiateClass(generatorClass));
    }
    Class<? extends MapperFactoryBean> mapperFactoryBeanClass = annoAttrs.getClass("factoryBean");
    if (!MapperFactoryBean.class.equals(mapperFactoryBeanClass)) {
      builder.addPropertyValue("mapperFactoryBeanClass", mapperFactoryBeanClass);
    }
    String sqlSessionTemplateRef = annoAttrs.getString("sqlSessionTemplateRef");
    if (StringUtils.hasText(sqlSessionTemplateRef)) {
      builder.addPropertyValue("sqlSessionTemplateBeanName", annoAttrs.getString("sqlSessionTemplateRef"));
    }
    String sqlSessionFactoryRef = annoAttrs.getString("sqlSessionFactoryRef");
    if (StringUtils.hasText(sqlSessionFactoryRef)) {
      builder.addPropertyValue("sqlSessionFactoryBeanName", annoAttrs.getString("sqlSessionFactoryRef"));
    }
    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()));
    if (basePackages.isEmpty()) {
      basePackages.add(getDefaultBasePackage(annoMeta));
    }
    String lazyInitialization = annoAttrs.getString("lazyInitialization");
    if (StringUtils.hasText(lazyInitialization)) {
      builder.addPropertyValue("lazyInitialization", lazyInitialization);
    }
    builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(basePackages));
    registry.registerBeanDefinition(beanName, builder.getBeanDefinition());
}

跟上面就一样了。

整个流程还是很清晰的，需要注意下starter首先是注册了BeanDefinitionRegistryPostProcessor，然后在它里面又去做的Mapper的扫描，下一篇我们将讲解下如何来自定义组件扫描，欢迎关注。

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MyBatis-Spring-Boot-Starter源码解析

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