mybatis配置的時候需要注意一下幾點：

1. 當yml中配置了config-location的時候方椎，java代碼配置的配置將不再生效
2. 當mybatis.config-location=classpath:config/config.xml
   mybatis.configuration.map-underscore-to-camel-case=true
   config-location和configuration.**的配置不能共存

換而言之， yml蛋济、xml甜无、java都能進行配置
yml撒璧、xml的優(yōu)先級高于java代碼 ==== 符合了spring的設計，配置大于編碼
yml和xml不能共存

ConfigurationCustomizer.class
public interface ConfigurationCustomizer {
    void customize(Configuration var1);
}

MybatisAutoConfiguration.class
@Configuration//配置類
//SqlSessionFactory及刻、SqlSessionFactoryBean存在的時候配置生效
@ConditionalOnClass({SqlSessionFactory.class, SqlSessionFactoryBean.class})
//DataSource類存在時生效
@ConditionalOnBean({DataSource.class})
//啟用配置屬性MybatisProperties類
@EnableConfigurationProperties({MybatisProperties.class})
//在DataSourceAutoConfiguration配置之后配置
@AutoConfigureAfter({DataSourceAutoConfiguration.class})
public class MybatisAutoConfiguration {
    //日志
    private static final Logger logger = LoggerFactory.getLogger(MybatisAutoConfiguration.class);
//配置類
    private final MybatisProperties properties;
    private final Interceptor[] interceptors;
//資源加載器
    private final ResourceLoader resourceLoader;
    private final DatabaseIdProvider databaseIdProvider;
//重點：放置接口進行配置的list
    private final List<ConfigurationCustomizer> configurationCustomizers;

    public MybatisAutoConfiguration(MybatisProperties properties, ObjectProvider<Interceptor[]> interceptorsProvider, ResourceLoader resourceLoader, ObjectProvider<DatabaseIdProvider> databaseIdProvider, ObjectProvider<List<ConfigurationCustomizer>> configurationCustomizersProvider) {
        this.properties = properties;
        this.interceptors = (Interceptor[])interceptorsProvider.getIfAvailable();
        this.resourceLoader = resourceLoader;
        this.databaseIdProvider = (DatabaseIdProvider)databaseIdProvider.getIfAvailable();
//拿到當前工程下邊所有申明@bean的ConfigurationCustomizer的實例
        this.configurationCustomizers = (List)configurationCustomizersProvider.getIfAvailable();
    }
//構(gòu)造行數(shù)執(zhí)行之后進行檢查
    @PostConstruct
    public 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)");
        }

    }

    @Bean
    @ConditionalOnMissingBean
    public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
        SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
        factory.setDataSource(dataSource);
        factory.setVfs(SpringBootVFS.class);
//獲取mybatis.config-location=classpath:config/config.xml配置
        if (StringUtils.hasText(this.properties.getConfigLocation())) {
            factory.setConfigLocation(this.resourceLoader.getResource(this.properties.getConfigLocation()));
        }
//此處永遠為空
        org.apache.ibatis.session.Configuration configuration = this.properties.getConfiguration();
//在yml不存在config-location屬性是創(chuàng)建configuration 
        if (configuration == null && !StringUtils.hasText(this.properties.getConfigLocation())) {
            configuration = new org.apache.ibatis.session.Configuration();
        }
//生效java代碼的配置
        if (configuration != null && !CollectionUtils.isEmpty(this.configurationCustomizers)) {
            Iterator var4 = this.configurationCustomizers.iterator();

            while(var4.hasNext()) {
//迭代拿到申明@bean接口的實例
                ConfigurationCustomizer customizer = (ConfigurationCustomizer)var4.next();
                customizer.customize(configuration);
            }
        }
        factory.setConfiguration(configuration);
        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 (StringUtils.hasLength(this.properties.getTypeHandlersPackage())) {
            factory.setTypeHandlersPackage(this.properties.getTypeHandlersPackage());
        }
//設置mybatis的mapper xml
        if (!ObjectUtils.isEmpty(this.properties.resolveMapperLocations())) {
            factory.setMapperLocations(this.properties.resolveMapperLocations());
        }
        return factory.getObject();
    }
}


MybatisProperties .class
@ConfigurationProperties(
    prefix = "mybatis"
)
//一堆get set方法
public class MybatisProperties {
    public static final String MYBATIS_PREFIX = "mybatis";
    private String configLocation;
    private String[] mapperLocations;
    private String typeAliasesPackage;
    private String typeHandlersPackage;
    private boolean checkConfigLocation = false;
    private ExecutorType executorType;
    private Properties configurationProperties;
    @NestedConfigurationProperty//這個注解相當于把Configuration類里的屬性 嵌套進來了
    private Configuration configuration;
}