[Java知识库]Mybatis启动过程

前置知识，需要了解springBoot自动装配的原理，和bean加载的过程，如不清楚，请看springBoot启动过程二。或者查看其他资料

MybatisAutoConfiguration 注解

@org.springframework.context.annotation.Configuration
@ConditionalOnClass({ SqlSessionFactory.class, SqlSessionFactoryBean.class })
@ConditionalOnSingleCandidate(DataSource.class)
@EnableConfigurationProperties(MybatisProperties.class)
@AutoConfigureAfter({ DataSourceAutoConfiguration.class, MybatisLanguageDriverAutoConfiguration.class })
public class MybatisAutoConfiguration implements InitializingBean {

先分析这几个注解的含义
@Configuration 配置注解，将会被注册成bena，很常见不解释

@Condition*

看与一个方法org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader#loadBeanDefinitionsForBeanMethod

private void loadBeanDefinitionsForBeanMethod(BeanMethod beanMethod) {
		ConfigurationClass configClass = beanMethod.getConfigurationClass();
		MethodMetadata metadata = beanMethod.getMetadata();
		String methodName = metadata.getMethodName();

		// Do we need to mark the bean as skipped by its condition?
		if (this.conditionEvaluator.shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN)) {
			configClass.skippedBeanMethods.add(methodName);
			return;
		}

再进入org.springframework.context.annotation.ConditionEvaluator#shouldSkip(org.springframework.core.type.AnnotatedTypeMetadata, org.springframework.context.annotation.ConfigurationCondition.ConfigurationPhase)

public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
		if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
			return false;
		}

		if (phase == null) {
			if (metadata instanceof AnnotationMetadata &&
					ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
				return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
			}
			return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
		}

		List<Condition> conditions = new ArrayList<>();
		for (String[] conditionClasses : getConditionClasses(metadata)) {
			for (String conditionClass : conditionClasses) {
				Condition condition = getCondition(conditionClass, this.context.getClassLoader());
				conditions.add(condition);
			}
		}

		AnnotationAwareOrderComparator.sort(conditions);

		for (Condition condition : conditions) {
			ConfigurationPhase requiredPhase = null;
			if (condition instanceof ConfigurationCondition) {
				requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
			}
			if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
				return true;
			}
		}

		return false;
	}

分别看条件注解

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnBeanCondition.class)
public @interface ConditionalOnSingleCandidate

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnClassCondition.class)
public @interface ConditionalOnClass {

分别指定了两个处理bean
shouldSkip中遍历每个condition，分别用每个condition去match
最终进入org.springframework.boot.autoconfigure.condition.OnBeanCondition#getMatchOutcome

public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
		ConditionMessage matchMessage = ConditionMessage.empty();
		MergedAnnotations annotations = metadata.getAnnotations();
		if (annotations.isPresent(ConditionalOnBean.class)) {
			Spec<ConditionalOnBean> spec = new Spec<>(context, metadata, annotations, ConditionalOnBean.class);
			MatchResult matchResult = getMatchingBeans(context, spec);
			if (!matchResult.isAllMatched()) {
				String reason = createOnBeanNoMatchReason(matchResult);
				return ConditionOutcome.noMatch(spec.message().because(reason));
			}
			matchMessage = spec.message(matchMessage).found("bean", "beans").items(Style.QUOTE,
					matchResult.getNamesOfAllMatches());
		}
		if (metadata.isAnnotated(ConditionalOnSingleCandidate.class.getName())) {
			Spec<ConditionalOnSingleCandidate> spec = new SingleCandidateSpec(context, metadata, annotations);
			MatchResult matchResult = getMatchingBeans(context, spec);
			if (!matchResult.isAllMatched()) {
				return ConditionOutcome.noMatch(spec.message().didNotFind("any beans").atAll());
			}
			else if (!hasSingleAutowireCandidate(context.getBeanFactory(), matchResult.getNamesOfAllMatches(),
					spec.getStrategy() == SearchStrategy.ALL)) {
				return ConditionOutcome.noMatch(spec.message().didNotFind("a primary bean from beans")
						.items(Style.QUOTE, matchResult.getNamesOfAllMatches()));
			}
			matchMessage = spec.message(matchMessage).found("a primary bean from beans").items(Style.QUOTE,
					matchResult.getNamesOfAllMatches());
		}
		if (metadata.isAnnotated(ConditionalOnMissingBean.class.getName())) {
			Spec<ConditionalOnMissingBean> spec = new Spec<>(context, metadata, annotations,
					ConditionalOnMissingBean.class);
			MatchResult matchResult = getMatchingBeans(context, spec);
			if (matchResult.isAnyMatched()) {
				String reason = createOnMissingBeanNoMatchReason(matchResult);
				return ConditionOutcome.noMatch(spec.message().because(reason));
			}
			matchMessage = spec.message(matchMessage).didNotFind("any beans").atAll();
		}
		return ConditionOutcome.match(matchMessage);
	}

看这段代码

	if (metadata.isAnnotated(ConditionalOnSingleCandidate.class.getName())) {
			Spec<ConditionalOnSingleCandidate> spec = new SingleCandidateSpec(context, metadata, annotations);
			MatchResult matchResult = getMatchingBeans(context, spec);
			if (!matchResult.isAllMatched()) {
				return ConditionOutcome.noMatch(spec.message().didNotFind("any beans").atAll());
			}
			else if (!hasSingleAutowireCandidate(context.getBeanFactory(), matchResult.getNamesOfAllMatches(),
					spec.getStrategy() == SearchStrategy.ALL)) {
				return ConditionOutcome.noMatch(spec.message().didNotFind("a primary bean from beans")
						.items(Style.QUOTE, matchResult.getNamesOfAllMatches()));
			}
			matchMessage = spec.message(matchMessage).found("a primary bean from beans").items(Style.QUOTE,
					matchResult.getNamesOfAllMatches());
		}

private boolean hasSingleAutowireCandidate(ConfigurableListableBeanFactory beanFactory, Set<String> beanNames,
			boolean considerHierarchy) {
		return (beanNames.size() == 1 || getPrimaryBeans(beanFactory, beanNames, considerHierarchy).size() == 1);
	}

因此表示
Spring提供了很多Condition给我们用，@ConditionalOnSingleCandidate表示当指定Bean在容器中只有一个，或者虽然有多个但是指定首选Bean，这个组合了@Conditional元注解，只是使用了不同的条件（Conditional）

注解@EnableConfigurationProperties

看beanDefination的扫描
org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader#loadBeanDefinitions

public void loadBeanDefinitions(Set<ConfigurationClass> configurationModel) {
		TrackedConditionEvaluator trackedConditionEvaluator = new TrackedConditionEvaluator();
		for (ConfigurationClass configClass : configurationModel) {
			loadBeanDefinitionsForConfigurationClass(configClass, trackedConditionEvaluator);
		}
	}

紧接着注册bena定义org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader#loadBeanDefinitionsForConfigurationClass
会进入

	loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());

其中configClass.getImportBeanDefinitionRegistrars()为指定的注册中心

之后进入EnableConfigurationPropertiesRegistrar
org.springframework.boot.context.properties.EnableConfigurationPropertiesRegistrar#registerBeanDefinitions

	@Override
	public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
		registerInfrastructureBeans(registry);
		ConfigurationPropertiesBeanRegistrar beanRegistrar = new ConfigurationPropertiesBeanRegistrar(registry);
		getTypes(metadata).forEach(beanRegistrar::register);
	}

将MybatisProperties注册到bean定义中

EnableConfigurationProperties实际上就是将配置类做了注入，而不用加@Component

注解AutoConfigureAfter

我们直到SpringBoot自动装配的关键之一是其启动类中的
@EnableAutoConfiguration
指定了org.springframework.boot.autoconfigure.AutoConfigurationImportSelector
用来加载spring.factories配置的org.springframework.boot.autoconfigure.EnableAutoConfiguration

org.springframework.context.annotation.ConfigurationClassParser#parse(java.util.Set<org.springframework.beans.factory.config.BeanDefinitionHolder>)

public void parse(Set<BeanDefinitionHolder> configCandidates) {
		for (BeanDefinitionHolder holder : configCandidates) {
			BeanDefinition bd = holder.getBeanDefinition();
			try {
				if (bd instanceof AnnotatedBeanDefinition) {
					parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
				}
				else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) {
					parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
				}
				else {
					parse(bd.getBeanClassName(), holder.getBeanName());
				}
			}
			catch (BeanDefinitionStoreException ex) {
				throw ex;
			}
			catch (Throwable ex) {
				throw new BeanDefinitionStoreException(
						"Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
			}
		}

		this.deferredImportSelectorHandler.process();
	}

在加载从入口类出发的类之后，进入deferredImportSelectorHandler.process();

进入org.springframework.context.annotation.ConfigurationClassParser.DeferredImportSelectorGroupingHandler#processGroupImports

public void processGroupImports() {
			for (DeferredImportSelectorGrouping grouping : this.groupings.values()) {
				Predicate<String> exclusionFilter = grouping.getCandidateFilter();
				grouping.getImports().forEach(entry -> {
					ConfigurationClass configurationClass = this.configurationClasses.get(entry.getMetadata());
					try {
						processImports(configurationClass, asSourceClass(configurationClass, exclusionFilter),
								Collections.singleton(asSourceClass(entry.getImportClassName(), exclusionFilter)),
								exclusionFilter, false);
					}
					catch (BeanDefinitionStoreException ex) {
						throw ex;
					}
					catch (Throwable ex) {
						throw new BeanDefinitionStoreException(
								"Failed to process import candidates for configuration class [" +
										configurationClass.getMetadata().getClassName() + "]", ex);
					}
				});
			}
		}

大概逻辑为获取到spring.factories中EnableAutoConfiguration的自动装配类，然后挨个为起始类进行扫描加载。
接着进入org.springframework.boot.autoconfigure.AutoConfigurationImportSelector.AutoConfigurationGroup#selectImports

	public Iterable<Entry> selectImports() {
			if (this.autoConfigurationEntries.isEmpty()) {
				return Collections.emptyList();
			}
			Set<String> allExclusions = this.autoConfigurationEntries.stream()
					.map(AutoConfigurationEntry::getExclusions).flatMap(Collection::stream).collect(Collectors.toSet());
			Set<String> processedConfigurations = this.autoConfigurationEntries.stream()
					.map(AutoConfigurationEntry::getConfigurations).flatMap(Collection::stream)
					.collect(Collectors.toCollection(LinkedHashSet::new));
			processedConfigurations.removeAll(allExclusions);

			return sortAutoConfigurations(processedConfigurations, getAutoConfigurationMetadata()).stream()
					.map((importClassName) -> new Entry(this.entries.get(importClassName), importClassName))
					.collect(Collectors.toList());
		}

这里对这些配置类进行排序。
org.springframework.boot.autoconfigure.AutoConfigurationSorter#getInPriorityOrder

List<String> getInPriorityOrder(Collection<String> classNames) {
		AutoConfigurationClasses classes = new AutoConfigurationClasses(this.metadataReaderFactory,
				this.autoConfigurationMetadata, classNames);
		List<String> orderedClassNames = new ArrayList<>(classNames);
		// Initially sort alphabetically
		Collections.sort(orderedClassNames);
		// Then sort by order
		orderedClassNames.sort((o1, o2) -> {
			int i1 = classes.get(o1).getOrder();
			int i2 = classes.get(o2).getOrder();
			return Integer.compare(i1, i2);
		});
		// Then respect @AutoConfigureBefore @AutoConfigureAfter
		orderedClassNames = sortByAnnotation(classes, orderedClassNames);
		return orderedClassNames;
	}

可以看出来首先根据注解@AutoConfigureOrder进行排序
再根据@AutoConfigureBefore @AutoConfigureAfter分别排序
为什么要指定配置类加载顺序
我们指定@Order注解。可以指定bean实例化的顺序，也能指定aop定义的切面执行顺序，但是对于配置类的执行顺序，其是起不到作用的。

配置类为何需要顺序？
我们已经知道Spring容器它对Bean的初始化是无序的，我们并不能想当然的通过@Order注解来控制其执行顺序。一般来说，对于容器内普通的Bean我们只需要关注依赖关系即可，而并不需要关心其绝对的顺序，而依赖关系的管理Spring的是做得很好的，这不连循环依赖它都可以搞定麽。
@Configuration配置类它也是一个Bean，但对于配置类来说，某些场景下的执行顺序是必须的，是需要得到保证的。比如很典型的一个非A即B的case：若容器内已经存在A了，就不要再把B放进来。这种case即使用中文理解，就能知道对A的“判断”必须要放在B的前面，否则可能导致程序出问题。或者配置B依赖与配置A的配置。那么就需要B在A之后配置

@AutoConfigureAfter({ DataSourceAutoConfiguration.class, MybatisLanguageDriverAutoConfiguration.class })

可以看出来mybatis需要子在数据源自动配置之后再进行自己的配置。
看bean实例化的一个过程
org.springframework.beans.factory.support.DefaultListableBeanFactory#preInstantiateSingletons

public void preInstantiateSingletons() throws BeansException {
		if (logger.isTraceEnabled()) {
			logger.trace("Pre-instantiating singletons in " + this);
		}

		// Iterate over a copy to allow for init methods which in turn register new bean definitions.
		// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
		List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

		// Trigger initialization of all non-lazy singleton beans...
		for (String beanName : beanNames) {
			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
				if (isFactoryBean(beanName)) {
					Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
					if (bean instanceof FactoryBean) {
						FactoryBean<?> factory = (FactoryBean<?>) bean;
						boolean isEagerInit;
						if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
							isEagerInit = AccessController.doPrivileged(
									(PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
									getAccessControlContext());
						}
						else {
							isEagerInit = (factory instanceof SmartFactoryBean &&
									((SmartFactoryBean<?>) factory).isEagerInit());
						}
						if (isEagerInit) {
							getBean(beanName);
						}
					}
				}
				else {
					getBean(beanName);
				}
			}
		}

		// Trigger post-initialization callback for all applicable beans...
		for (String beanName : beanNames) {
			Object singletonInstance = getSingleton(beanName);
			if (singletonInstance instanceof SmartInitializingSingleton) {
				SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
				if (System.getSecurityManager() != null) {
					AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
						smartSingleton.afterSingletonsInstantiated();
						return null;
					}, getAccessControlContext());
				}
				else {
					smartSingleton.afterSingletonsInstantiated();
				}
			}
		}
	}

预处理bean时 按照beanDefinitionNames数组中的顺序实例化，而放入beanDefinitionNames的顺序由AutoConfigureAfter等注解，影响，因此可以保证加载，实例化顺序

@ConditionalOnMissingBean

@ConditionalOnMissingBean，它是修饰bean的一个注解，主要实现的是，当你的bean被注册之后，如果而注册相同类型的bean，就不会成功，它会保证你的bean只有一个，即你的实例只有一个，当你注册多个相同的bean时，会出现异常，以此来告诉开发人员。
分别配置了SqlSessionFactory
SqlSessionTemplate
内部类同时又有一个配置bean
org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration.MapperScannerRegistrarNotFoundConfiguration

SqlSessionFactory

MybatisAutoConfiguration中声明了SqlSessionFactory

先看SqlSessionFactory这个接口

public interface SqlSessionFactory {

  SqlSession openSession();

  SqlSession openSession(boolean autoCommit);

  SqlSession openSession(Connection connection);

  SqlSession openSession(TransactionIsolationLevel level);

  SqlSession openSession(ExecutorType execType);

  SqlSession openSession(ExecutorType execType, boolean autoCommit);

  SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level);

  SqlSession openSession(ExecutorType execType, Connection connection);

  Configuration getConfiguration();

}

可以看出来，SqlSessionFactory就是用来生产sqlSessiond的一个工厂，可以根据不用的sql类型生成不同的sqlSession

具体看时如何创建sqlSessionFactory这个对象吧

 @Bean
  @ConditionalOnMissingBean
  public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
    //创建SqlSessionFactoryBean对象
     SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
      //设置数据源
    factory.setDataSource(dataSource);
    factory.setVfs(SpringBootVFS.class);
    if (StringUtils.hasText(this.properties.getConfigLocation())) {
     //mybatis ，Xml配置文件路径
        、factory.setConfigLocation(this.resourceLoader.getResource(this.properties.getConfigLocation()));
    }
      //应用配置文件
    applyConfiguration(factory);
      
      //Mybatis额外的配置文件路径
    if (this.properties.getConfigurationProperties() != null) {
      factory.setConfigurationProperties(this.properties.getConfigurationProperties());
    }
      //设置mybatis拦截器
    if (!ObjectUtils.isEmpty(this.interceptors)) {
      factory.setPlugins(this.interceptors);
    }
    ···
      //设置*mapper.xml扫描路径
    if (!ObjectUtils.isEmpty(this.properties.resolveMapperLocations())) {
      factory.setMapperLocations(this.properties.resolveMapperLocations());
    }
    ····	

        //获取创建SqlSessionFactoryBean对象
    return factory.getObject();
  }

这里有几个关键类

Interceptor

mybatis拦截器，可以拦截sql查询，更新语句，具体后面可以看具体工作流程
`

TypeHandler

类型处理器 TypeHandler

MyBatis 中的 TypeHandler 类型处理器用于 JavaType 与 JdbcType 之间的转换，用于 PreparedStatement 设置参数值和从 ResultSet 或 CallableStatement 中取出一个值。MyBatis 内置了大部分基本类型的类型处理器，所以对于基本类型可以直接处理，当我们需要处理其他类型的时候就需要自定义类型处理器。

MyBatis 内置的 TypeHandler

在 MyBatis 的 TypeHandlerRegistry 类型中，可以看到内置的类型处理器。内置处理器比较多，这里整理常见的一些。
BooleanTypeHandler：用于 java 类型 boolean，jdbc 类型 bit、boolean
ByteTypeHandler：用于 java 类型 byte，jdbc 类型 TINYINT
ShortTypeHandler：用于 java 类型 short，jdbc 类型 SMALLINT
IntegerTypeHandler：用于 INTEGER 类型
LongTypeHandler：用于 long 类型
FloatTypeHandler：用于 FLOAT 类型
DoubleTypeHandler：用于 double 类型
StringTypeHandler：用于 java 类型 string，jdbc 类型 CHAR、VARCHAR
ArrayTypeHandler：用于 jdbc 类型 ARRAY
BigDecimalTypeHandler：用于 java 类型 BigDecimal，jdbc 类型 REAL、DECIMAL、NUMERIC
DateTypeHandler：用于 java 类型 Date，jdbc 类型 TIMESTAMP
DateOnlyTypeHandler：用于 java 类型 Date，jdbc 类型 DATE
TimeOnlyTypeHandler：用于 java 类型 Date，jdbc 类型 TIME
对于常见的 Enum 类型，内置了 EnumTypeHandler 进行 Enum 名称的转换和 EnumOrdinalTypeHandler 进行 Enum 序数的转换。这两个类型处理器没有在 TypeHandlerRegistry 中注册，如果需要使用必须手动配置。

看一个IntegerTypeHandler了解一下共组流程，具体工作流程看后面mybatis的sql执行过程

public class IntegerTypeHandler extends BaseTypeHandler<Integer> {

  @Override
  public void setNonNullParameter(PreparedStatement ps, int i, Integer parameter, JdbcType jdbcType)
      throws SQLException {
    ps.setInt(i, parameter);
  }

  @Override
  public Integer getNullableResult(ResultSet rs, String columnName)
      throws SQLException {
    int result = rs.getInt(columnName);
    return result == 0 && rs.wasNull() ? null : result;
  }

  @Override
  public Integer getNullableResult(ResultSet rs, int columnIndex)
      throws SQLException {
    int result = rs.getInt(columnIndex);
    return result == 0 && rs.wasNull() ? null : result;
  }

  @Override
  public Integer getNullableResult(CallableStatement cs, int columnIndex)
      throws SQLException {
    int result = cs.getInt(columnIndex);
    return result == 0 && cs.wasNull() ? null : result;
  }
}

buildSqlSessionFactory

可以看住了SqlSessionFactoryBean的大致功能就是设置一系列属性，最终生成SqlSessionFactory对象

 @Override
  public SqlSessionFactory getObject() throws Exception {
    if (this.sqlSessionFactory == null) {
      afterPropertiesSet();
    }

    return this.sqlSessionFactory;
  }

 protected SqlSessionFactory buildSqlSessionFactory() throws Exception {

    final Configuration targetConfiguration;

    XMLConfigBuilder xmlConfigBuilder = null;
    if (this.configuration != null) {
      targetConfiguration = this.configuration;
      if (targetConfiguration.getVariables() == null) {
        targetConfiguration.setVariables(this.configurationProperties);
      } else if (this.configurationProperties != null) {
        targetConfiguration.getVariables().putAll(this.configurationProperties);
      }
    } else if (this.configLocation != null) {
      xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
      targetConfiguration = xmlConfigBuilder.getConfiguration();
    } else {
      LOGGER.debug(
          () -> "Property 'configuration' or 'configLocation' not specified, using default MyBatis Configuration");
      targetConfiguration = new Configuration();
      Optional.ofNullable(this.configurationProperties).ifPresent(targetConfiguration::setVariables);
    }

    Optional.ofNullable(this.objectFactory).ifPresent(targetConfiguration::setObjectFactory);
    Optional.ofNullable(this.objectWrapperFactory).ifPresent(targetConfiguration::setObjectWrapperFactory);
    Optional.ofNullable(this.vfs).ifPresent(targetConfiguration::setVfsImpl);

    if (hasLength(this.typeAliasesPackage)) {
      scanClasses(this.typeAliasesPackage, this.typeAliasesSuperType).stream()
          .filter(clazz -> !clazz.isAnonymousClass()).filter(clazz -> !clazz.isInterface())
          .filter(clazz -> !clazz.isMemberClass()).forEach(targetConfiguration.getTypeAliasRegistry()::registerAlias);
    }

        if (!isEmpty(this.typeAliases)) {
          Stream.of(this.typeAliases).forEach(typeAlias -> {
            targetConfiguration.getTypeAliasRegistry().registerAlias(typeAlias);
            LOGGER.debug(() -> "Registered type alias: '" + typeAlias + "'");
          });
        }

    if (!isEmpty(this.plugins)) {
      Stream.of(this.plugins).forEach(plugin -> {
        targetConfiguration.addInterceptor(plugin);
        LOGGER.debug(() -> "Registered plugin: '" + plugin + "'");
      });
    }

    if (hasLength(this.typeHandlersPackage)) {
      scanClasses(this.typeHandlersPackage, TypeHandler.class).stream().filter(clazz -> !clazz.isAnonymousClass())
          .filter(clazz -> !clazz.isInterface()).filter(clazz -> !Modifier.isAbstract(clazz.getModifiers()))
          .forEach(targetConfiguration.getTypeHandlerRegistry()::register);
    }

    if (!isEmpty(this.typeHandlers)) {
      Stream.of(this.typeHandlers).forEach(typeHandler -> {
        targetConfiguration.getTypeHandlerRegistry().register(typeHandler);
        LOGGER.debug(() -> "Registered type handler: '" + typeHandler + "'");
      });
    }

    targetConfiguration.setDefaultEnumTypeHandler(defaultEnumTypeHandler);

    if (!isEmpty(this.scriptingLanguageDrivers)) {
      Stream.of(this.scriptingLanguageDrivers).forEach(languageDriver -> {
        targetConfiguration.getLanguageRegistry().register(languageDriver);
        LOGGER.debug(() -> "Registered scripting language driver: '" + languageDriver + "'");
      });
    }
    Optional.ofNullable(this.defaultScriptingLanguageDriver)
        .ifPresent(targetConfiguration::setDefaultScriptingLanguage);

    if (this.databaseIdProvider != null) {// fix #64 set databaseId before parse mapper xmls
      try {
        targetConfiguration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
      } catch (SQLException e) {
        throw new NestedIOException("Failed getting a databaseId", e);
      }
    }

    Optional.ofNullable(this.cache).ifPresent(targetConfiguration::addCache);

    if (xmlConfigBuilder != null) {
      try {
        xmlConfigBuilder.parse();
        LOGGER.debug(() -> "Parsed configuration file: '" + this.configLocation + "'");
      } catch (Exception ex) {
        throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
      } finally {
        ErrorContext.instance().reset();
      }
    }

    targetConfiguration.setEnvironment(new Environment(this.environment,
        this.transactionFactory == null ? new SpringManagedTransactionFactory() : this.transactionFactory,
        this.dataSource));

    if (this.mapperLocations != null) {
      if (this.mapperLocations.length == 0) {
        LOGGER.warn(() -> "Property 'mapperLocations' was specified but matching resources are not found.");
      } else {
        for (Resource mapperLocation : this.mapperLocations) {
          if (mapperLocation == null) {
            continue;
          }
          try {
            XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
                targetConfiguration, mapperLocation.toString(), targetConfiguration.getSqlFragments());
            xmlMapperBuilder.parse();
          } catch (Exception e) {
            throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
          } finally {
            ErrorContext.instance().reset();
          }
          LOGGER.debug(() -> "Parsed mapper file: '" + mapperLocation + "'");
        }
      }
    } else {
      LOGGER.debug(() -> "Property 'mapperLocations' was not specified.");
    }

    return this.sqlSessionFactoryBuilder.build(targetConfiguration);
  }

看这一段代码 myabtis设置别名

 if (hasLength(this.typeAliasesPackage)) {
      scanClasses(this.typeAliasesPackage, this.typeAliasesSuperType).stream()
          .filter(clazz -> !clazz.isAnonymousClass()).filter(clazz -> !clazz.isInterface())
          .filter(clazz -> !clazz.isMemberClass()).forEach(targetConfiguration.getTypeAliasRegistry()::registerAlias);
    }

    if (!isEmpty(this.typeAliases)) {
      Stream.of(this.typeAliases).forEach(typeAlias -> {
        targetConfiguration.getTypeAliasRegistry().registerAlias(typeAlias);
        LOGGER.debug(() -> "Registered type alias: '" + typeAlias + "'");
      });
    }

这里介绍mybatis别用的用处
问题描述
Mybatis有个代码生成工具，生成的代码里面有mapper.xml文件，mapper.xml中的sql语句会用parameterType这个属性，而这个值可能是我们自定义的对象，此时，如果没有typealiases，我们就需要为parameterType指定全路径：

<select id="getStudent" parameterType="com.csct.domain.StudentEntity" resultType="com.csct.domain.StudentEntity" resultMap="studentResultMap"></span>  

代码自动生成工具生成的mapper.xml文件中不带全路径，那么，如果有三、四十个实体对象，我们岂不是要修改的地方太多了。
解决办法
这个时候，我们是不是很期望有一个方式，能把com.csct.domain下的实体全部给加载进来，mapper.xml遇到不识别的类就在这里查找呢？你都想到了，Mybatis当然也能想到吧，typealiases就是为此而生的。使用特简单，如下：

<span style="font-size:14px;"><configuration>  
    <typeAliases>  
      <!--  
      通过package, 可以直接指定package的名字， mybatis会自动扫描你指定包下面的javabean,  
      并且默认设置一个别名，默认的名字为： javabean 的首字母小写的非限定类名来作为它的别名。  
      也可在javabean 加上注解@Alias 来自定义别名， 例如： @Alias(user)   
      <package name="com.dy.entity"/>  
       -->  
      <typeAlias alias="UserEntity" type="com.dy.entity.User"/>  
  </typeAliases>  
    
  ......  
    
</configuration></span>  

这样，mapper.xml的sql语句就如下了：

<span style="font-size:14px;"><select id="getStudent" parameterType="StudentEntity" resultType="StudentEntity" resultMap="studentResultMap"></span>  

创建真正的SqlSessionFactory

  public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
  }

public DefaultSqlSessionFactory(Configuration configuration) {
    this.configuration = configuration;
  }

SqlSessionTemplate

顾名思义。SqlSessionTemplate就是一个selssiong的执行模板，继承了SqlSession接口，并且做了简单的实现

看一下构造

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());
  }

设置executorType，异常转化类。exceptionTranslator
重要的时代理的这这一句
为sqlSessionProxy创建了一个代理对象具体逻辑由SqlSessionInterceptor处理

SqlSessionInterceptor

 private class SqlSessionInterceptor implements InvocationHandler {
    public SqlSessionInterceptor() {
        // Prevent Synthetic Access
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      final SqlSession sqlSession = SqlSessionManager.this.localSqlSession.get();
      if (sqlSession != null) {
        try {
          return method.invoke(sqlSession, args);
        } catch (Throwable t) {
          throw ExceptionUtil.unwrapThrowable(t);
        }
      } else {
        try (SqlSession autoSqlSession = openSession()) {
          try {
            final Object result = method.invoke(autoSqlSession, args);
            autoSqlSession.commit();
            return result;
          } catch (Throwable t) {
            autoSqlSession.rollback();
            throw ExceptionUtil.unwrapThrowable(t);
          }
        }
      }
    }
  }

SqlSessionTemplate 如何实现线程安全
在进行反射的时候，invoke通过getSqlSession方法拿到DefaultSqlSession实例，getSqlSession方法里面处理了sqlSession的线程安全问题（通过ThreadLocal实现）。

从threadLocal中取出对应sqlSession进行操作

MapperScannerRegistrar

mapper文件扫描类
在MybatisAutoConfiguration中并没有指定此类，而是在注解@MapperScan中导入

导入了这个类
看下去类结构

ImportBeanDefinitionRegistrar

上面说的注解@EnableConfigurationProperties，也是导入了这个类型的类
在加载BeanDefination时也会执行ImportBeanDefinitionRegistrar

这里会执行org.mybatis.spring.annotation.MapperScannerRegistrar#registerBeanDefinitions(org.springframework.core.type.AnnotationMetadata, org.springframework.beans.factory.support.BeanDefinitionRegistry)

会将注解和mybatis中的其他信息组装放入beanDefination中的属性去

 void registerBeanDefinitions(AnnotationMetadata annoMeta, AnnotationAttributes annoAttrs,
      BeanDefinitionRegistry registry, String beanName) {

    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());

  }

然后注册了MapperScannerConfigurerbean定义
再看MapperScannerConfigurerbean

MapperScannerConfigurer

看其结构可以看出来其继承iBeanFactoryPostProcessor就可以看出来是用来加载bean定义的一个类

在刷新springBoot容器会进入
org.mybatis.spring.mapper.MapperScannerConfigurer#postProcessBeanDefinitionRegistry

 @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));
  }

这里构造了ClassPathMapperScanner

/**
 * Configures parent scanner to search for the right interfaces. It can search
 * for all interfaces or just for those that extends a markerInterface or/and
 * those annotated with the annotationClass
 */
public void registerFilters() {
    boolean acceptAllInterfaces = true;

    // if specified, use the given annotation and / or marker interface
    // 对于annotationClass属性的处理
    if (this.annotationClass != null) {
        addIncludeFilter(new AnnotationTypeFilter(this.annotationClass));
        acceptAllInterfaces = false;
    }

    // override AssignableTypeFilter to ignore matches on the actual marker interface
    // 对于markerInterface属性的处理
    if (this.markerInterface != null) {
        addIncludeFilter(new AssignableTypeFilter(this.markerInterface) {
            @Override
            protected boolean matchClassName(String className) {
                return false;
            }
        });
        acceptAllInterfaces = false;
    }

    if (acceptAllInterfaces) {
        // default include filter that accepts all classes
        addIncludeFilter((metadataReader, metadataReaderFactory) -> true);
    }

    // exclude package-info.java
    // 不扫描package-info.java文件
    addExcludeFilter((metadataReader, metadataReaderFactory) -> {
        String className = metadataReader.getClassMetadata().getClassName();
        return className.endsWith("package-info");
    });
}

代码中得知，根据之前属性的配置生成了对应的过滤器。
（1）annotationClass 属性处理。
如果annotationClass不为空，表示用户设置了此属性，那么就要根据此属性生成过滤器以保证达到用户想要的效果，而封装此属性的过滤器就是AnnotationTypeFilter。AnnotationTypeFilter保证在扫描对应Java文件时只接受标记有注解为annotationClass的接口。
（2）markerlnterface 属性处理。
如果markerlnterface不为空，表示用户设置了此属性，那么就要根据此属性生成过滤器以保证达到用户想要的效果，而封装此属性的过滤器就是实现AssignableTypeFilter接口的局部类。表示扫描过程中只有实现markerlnterface接口的接口才会被接受。
（3）全局默认处理。
在上面两个属性中如果存在其中任何属性，acceptAllInterfaces的值将会改变，但是如果用户没有设定以上两个属性，那么，Spring会为我们增加一个默认的过滤器实现TypeFilter接口的局部类，旨在接受所有接口文件。
（4）package-info.java 处理。
对于命名为package-info的Java文件，默认不作为逻辑实现接口，将其排除掉，使用TypeFilter接口的局部类实现match方法。
从上面的函数我们看出，控制扫描文件Spring通过不同的过滤器完成，这些定义的过滤器记录在了includeFilters和excludeFilters属性中。
设置好basepackage,交给父类的doScan
org.springframework.context.annotation.ClassPathBeanDefinitionScanner#doScan

	protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
		for (String basePackage : basePackages) {
			Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
			for (BeanDefinition candidate : candidates) {
				ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
				candidate.setScope(scopeMetadata.getScopeName());
				String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
				if (candidate instanceof AbstractBeanDefinition) {
					postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
				}
				if (candidate instanceof AnnotatedBeanDefinition) {
					AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
				}
				if (checkCandidate(beanName, candidate)) {
					BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
					definitionHolder =
							AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
					beanDefinitions.add(definitionHolder);
					registerBeanDefinition(definitionHolder, this.registry);
				}
			}
		}
		return beanDefinitions;
	}

public Set<BeanDefinition> findCandidateComponents(String basePackage) {
    if (this.componentsIndex != null && indexSupportsIncludeFilters()) {
        return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);
    }
    else {
        return scanCandidateComponents(basePackage);
    }
}

private Set<BeanDefinition> scanCandidateComponents(String basePackage) {
    Set<BeanDefinition> candidates = new LinkedHashSet<>();
    try {
        String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
                resolveBasePackage(basePackage) + '/' + this.resourcePattern;
        Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);
        boolean traceEnabled = logger.isTraceEnabled();
        boolean debugEnabled = logger.isDebugEnabled();
        for (Resource resource : resources) {
            if (traceEnabled) {
                logger.trace("Scanning " + resource);
            }
            if (resource.isReadable()) {
                try {
                    MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);
                    if (isCandidateComponent(metadataReader)) {
                        ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
                        sbd.setResource(resource);
                        sbd.setSource(resource);
                        if (isCandidateComponent(sbd)) {
                            if (debugEnabled) {
                                logger.debug("Identified candidate component class: " + resource);
                            }
                            candidates.add(sbd);
                        }
                        else {
                            if (debugEnabled) {
                                logger.debug("Ignored because not a concrete top-level class: " + resource);
                            }
                        }
                    }
                    else {
                        if (traceEnabled) {
                            logger.trace("Ignored because not matching any filter: " + resource);
                        }
                    }
                }
                catch (Throwable ex) {
                    throw new BeanDefinitionStoreException(
                            "Failed to read candidate component class: " + resource, ex);
                }
            }
            else {
                if (traceEnabled) {
                    logger.trace("Ignored because not readable: " + resource);
                }
            }
        }
    }
    catch (IOException ex) {
        throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
    }
    return candidates;
}
复制代码

org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider#findCandidateComponents
findCandidateComponents方法根据传人的包路径信息并结合类文件路径拼接成文件的绝对路径，同时完成了文件的扫描过程并且根据对应的文件生成了对应的bean，使用 ScannedGenericBeanDefinition 类型的 bean 承载信息，bean 中只记录了resource 和 source 信息。 这里，我们更感兴趣的是isCandidateComponent(metadataReader)，此句代码用于判断当前扫描的文件是否符合要求，而我们之前注册的一些过滤器信息也正是在此时派上用场的。

我们看到了之前加入过滤器的两个属性excludeFilters、includeFilters，并且知道对应的文件是否符合要求是根据过滤器中的match方法所返回的信息来判断的，当然用户可以实现并注册满足自己业务逻辑的过滤器来控制扫描的结果，metadataReader中有你过滤所需要的全部文件信息。

默认扫描所有接口并且过滤掉packageInfo.java
这样即使不在mapper文件上加@component也会被注册成为bean

读取到对应basePackage包中的bean定义后做处理

 private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
    GenericBeanDefinition definition;
    for (BeanDefinitionHolder holder : beanDefinitions) {
      definition = (GenericBeanDefinition) holder.getBeanDefinition();
      String beanClassName = definition.getBeanClassName();
      LOGGER.debug(() -> "Creating MapperFactoryBean with name '" + holder.getBeanName() + "' and '" + beanClassName
          + "' mapperInterface");

      // the mapper interface is the original class of the bean
      // but, the actual class of the bean is MapperFactoryBean
      definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName); // issue #59
      definition.setBeanClass(this.mapperFactoryBeanClass);

      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) {
        LOGGER.debug(() -> "Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
        definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
      }
      definition.setLazyInit(lazyInitialization);
    }
  }

把beanClass设置成org.mybatis.spring.mapper.MapperFactoryBean，继承自FactoryBean
有点偷天换日的赶紧，把扫描的BenaDefination设置成本MapperFactoryBean，后面实例化bean的时候通过MapperFactoryBean进行生成，MapperFactoryBean继承自FactoryBean,可自定义生产mapper,bean的工厂.

至此，我们完成了mybatis启动和文件的扫描过程的分析。