MyBatis自动配置原理

在springboot中，mybatis引用的jar包

 <dependency><groupId>org.mybatis.spring.bootgroupId><artifactId>mybatis-spring-boot-starterartifactId><version>2.1.3version>
dependency>

mybatis的配置一般放在application.yml文件中

mybatis:mapper-locations: classpath:mybatis/mapper/*Mapper.xmltype-aliases-package: com.xxx.xlt.model.po

mybatis自动配置主要跟mybatis-spring-boot-autoconfigure有关，该jar包中的MybatisProperties可以读取application.properties文件中以mybatis开头的配置信息

@ConfigurationProperties(prefix = MybatisProperties.MYBATIS_PREFIX)
public class MybatisProperties {public static final String MYBATIS_PREFIX = "mybatis";private static final ResourcePatternResolver resourceResolver = new PathMatchingResourcePatternResolver();/*** Location of MyBatis xml config file.*/private String configLocation;/*** Locations of MyBatis mapper files.*/private String[] mapperLocations;/*** Packages to search type aliases. (Package delimiters are ",; \t\n")*/private String typeAliasesPackage;/*** The super class for filtering type alias. If this not specifies, the MyBatis deal as type alias all classes that* searched from typeAliasesPackage.*/private Class<?> typeAliasesSuperType;/*** Packages to search for type handlers. (Package delimiters are ",; \t\n")*/private String typeHandlersPackage;/*** Indicates whether perform presence check of the MyBatis xml config file.*/private boolean checkConfigLocation = false;/*** Execution mode for {@link org.mybatis.spring.SqlSessionTemplate}.*/private ExecutorType executorType;/*** The default scripting language driver class. (Available when use together with mybatis-spring 2.0.2+)*/private Class<? extends LanguageDriver> defaultScriptingLanguageDriver;/*** Externalized properties for MyBatis configuration.*/private Properties configurationProperties;/*** A Configuration object for customize default settings. If {@link #configLocation} is specified, this property is* not used.*/@NestedConfigurationPropertyprivate Configuration configuration;
}

在应用启动过程中就会读取配置文件中的mybatis配置信息

MybatisAutoConfiguration可以实现mybatis的自动配置，他的说明原文是这样的

Auto-Configuration for Mybatis. Contributes a SqlSessionFactory and a SqlSessionTemplate. If org.mybatis.spring.annotation.MapperScan is used, or a configuration file is specified as a property, those will be considered, otherwise this auto-configuration will attempt to register mappers based on the interface definitions in or under the root auto-configuration package.

意思是说：自动配置mybatis, 生成一个SqlSessionFactory和SqlSessionTemplate的bean。如果使用了@MapperScan注解或者是一个配置文件被指定为一个属性，这些将会被考虑。否则，自动配置将会尝试基于在自动配置根package下的接口定义去注册mappers.

@org.springframework.context.annotation.Configuration
// SqlSessionFactory和SqlSessionFactoryBean位于类路径上时实例化bean   
@ConditionalOnClass({ SqlSessionFactory.class, SqlSessionFactoryBean.class })
// 自动装配一般使用，beanFactory包含了指定的bean的时候，条件匹配
@ConditionalOnSingleCandidate(DataSource.class)
// 开启MybatisProperties.class的自动参数映射
@EnableConfigurationProperties(MybatisProperties.class)
// 在DataSourceAutoConfiguration,MybatisLanguageDriverAutoConfiguration初始化之后，进行自动配置
@AutoConfigureAfter({ DataSourceAutoConfiguration.class, MybatisLanguageDriverAutoConfiguration.class })
public class MybatisAutoConfiguration implements InitializingBean {private final MybatisProperties properties;private final Interceptor[] interceptors;private final TypeHandler[] typeHandlers;private final LanguageDriver[] languageDrivers;private final ResourceLoader resourceLoader;private final DatabaseIdProvider databaseIdProvider;private final List<ConfigurationCustomizer> configurationCustomizers;// bean初始化时执行@Overridepublic void afterPropertiesSet() {checkConfigFileExists();}private void checkConfigFileExists() {if (this.properties.isCheckConfigLocation() && StringUtils.hasText(this.properties.getConfigLocation())) {Resource resource = this.resourceLoader.getResource(this.properties.getConfigLocation());Assert.state(resource.exists(),"Cannot find config location: " + resource + " (please add config file or check your Mybatis configuration)");}}// beanFactory中缺少SqlSessionFactory时会初始化一个@Bean@ConditionalOnMissingBeanpublic SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {SqlSessionFactoryBean factory = new SqlSessionFactoryBean();factory.setDataSource(dataSource);factory.setVfs(SpringBootVFS.class);if (StringUtils.hasText(this.properties.getConfigLocation())) {factory.setConfigLocation(this.resourceLoader.getResource(this.properties.getConfigLocation()));}applyConfiguration(factory);if (this.properties.getConfigurationProperties() != null) {factory.setConfigurationProperties(this.properties.getConfigurationProperties());}if (!ObjectUtils.isEmpty(this.interceptors)) {factory.setPlugins(this.interceptors);}if (this.databaseIdProvider != null) {factory.setDatabaseIdProvider(this.databaseIdProvider);}if (StringUtils.hasLength(this.properties.getTypeAliasesPackage())) {factory.setTypeAliasesPackage(this.properties.getTypeAliasesPackage());}if (this.properties.getTypeAliasesSuperType() != null) {factory.setTypeAliasesSuperType(this.properties.getTypeAliasesSuperType());}if (StringUtils.hasLength(this.properties.getTypeHandlersPackage())) {factory.setTypeHandlersPackage(this.properties.getTypeHandlersPackage());}if (!ObjectUtils.isEmpty(this.typeHandlers)) {factory.setTypeHandlers(this.typeHandlers);}if (!ObjectUtils.isEmpty(this.properties.resolveMapperLocations())) {factory.setMapperLocations(this.properties.resolveMapperLocations());}Set<String> factoryPropertyNames = Stream.of(new BeanWrapperImpl(SqlSessionFactoryBean.class).getPropertyDescriptors()).map(PropertyDescriptor::getName).collect(Collectors.toSet());Class<? extends LanguageDriver> defaultLanguageDriver = this.properties.getDefaultScriptingLanguageDriver();if (factoryPropertyNames.contains("scriptingLanguageDrivers") && !ObjectUtils.isEmpty(this.languageDrivers)) {// Need to mybatis-spring 2.0.2+factory.setScriptingLanguageDrivers(this.languageDrivers);if (defaultLanguageDriver == null && this.languageDrivers.length == 1) {defaultLanguageDriver = this.languageDrivers[0].getClass();}}if (factoryPropertyNames.contains("defaultScriptingLanguageDriver")) {// Need to mybatis-spring 2.0.2+factory.setDefaultScriptingLanguageDriver(defaultLanguageDriver);}return factory.getObject();}// beanFactory中缺少SqlSessionTemplate时会初始化一个 @Bean@ConditionalOnMissingBeanpublic SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {ExecutorType executorType = this.properties.getExecutorType();if (executorType != null) {return new SqlSessionTemplate(sqlSessionFactory, executorType);} else {return new SqlSessionTemplate(sqlSessionFactory);}}
}

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