Spring Boot mybatis-starter原理

一、配置类导入

1、mybatis-spring-boot-starter 引入了如下图5个依赖

 spring-boot-starter是每个starter都要引入的

spring-boot-starter-jdbc 与jdbc相关

后面两个mybatis， mybatis -spring  与mybatis相关

mybatis-spring-boot-autoconfigure 根据之前自定义的starter，它里面spring.factories有一个配置类实现了

2、进入MyBatisAutoConfiguration类

 1）第一个注解是Configuration，标注这个类是配置类

2）接下类是ConditionalOnClass注解，要求容器里有SqlSessionFactory类和SqlSessionfactoryBean类

3）ConditionalOnSingleCandidate注解：要求容器中存在DataSource类

4）接着使用EnableConfigurationProperties注解使MybatisProperties这个类生效。

进入MybatisProperties类

获得mybatis前缀的属性

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

说明当前类要在DataSourceAutoConfiguration和MybatisLanguageDriverAutoConfiguration这两个类之后进行装载

a） DataSourceAutoConfiguration

这个类是对数据源进行配置的

 DataSourceProperties获得spring.datasource 的属性

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

二、关键类注入

MyBatisAutoConfiguration类会注入两个重要的Bean，分别为SqlSessionFactory和sqlSessionTemplate

1、首先进入SqlSessionFactory这个bean方法sqlSessionFactory是单个数据库映射关系经编译后的内存镜像

@Bean
    @ConditionalOnMissingBean
    public 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()));
        }

        this.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 = (Set)Stream.of((new BeanWrapperImpl(SqlSessionFactoryBean.class)).getPropertyDescriptors()).map(FeatureDescriptor::getName).collect(Collectors.toSet());
        Class<? extends LanguageDriver> defaultLanguageDriver = this.properties.getDefaultScriptingLanguageDriver();
        if(factoryPropertyNames.contains("scriptingLanguageDrivers") && !ObjectUtils.isEmpty(this.languageDrivers)) {
            factory.setScriptingLanguageDrivers(this.languageDrivers);
            if(defaultLanguageDriver == null && this.languageDrivers.length == 1) {
                defaultLanguageDriver = this.languageDrivers[0].getClass();
            }
        }

        if(factoryPropertyNames.contains("defaultScriptingLanguageDriver")) {
            factory.setDefaultScriptingLanguageDriver(defaultLanguageDriver);
        }

        return factory.getObject();
    }

2、sqlSessionTemplate类

执行数据库操作的工具类

@Bean
    @ConditionalOnMissingBean
    public SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
        ExecutorType executorType = this.properties.getExecutorType();
        return executorType != null?new SqlSessionTemplate(sqlSessionFactory, executorType):new SqlSessionTemplate(sqlSessionFactory);
    }

进入SqlSessionTemplate类，

里面有SqlSessionFactory和sqlSessionProxy

sqlSessionProxy是代理类，里面的增删改查都是通过它来执行的。处理方法都在SqlSessionInterceptor里面

3、SqlSessionInterceptor 类如下

private class SqlSessionInterceptor implements InvocationHandler {
        private SqlSessionInterceptor() {
        }

        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            SqlSession sqlSession = SqlSessionUtils.getSqlSession(SqlSessionTemplate.this.sqlSessionFactory, SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);

            Object unwrapped;
            try {
                Object result = method.invoke(sqlSession, args);
                if(!SqlSessionUtils.isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
                    sqlSession.commit(true);
                }

                unwrapped = result;
            } catch (Throwable var11) {
                unwrapped = ExceptionUtil.unwrapThrowable(var11);
                if(SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
                    SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
                    sqlSession = null;
                    Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException)unwrapped);
                    if(translated != null) {
                        unwrapped = translated;
                    }
                }

                throw (Throwable)unwrapped;
            } finally {
                if(sqlSession != null) {
                    SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
                }

            }

            return unwrapped;
        }
    }

1) 进入getSqlSession这个方法

public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {
        Assert.notNull(sessionFactory, "No SqlSessionFactory specified");
        Assert.notNull(executorType, "No ExecutorType specified");
        SqlSessionHolder holder = (SqlSessionHolder)TransactionSynchronizationManager.getResource(sessionFactory);
        SqlSession session = sessionHolder(executorType, holder);
        if(session != null) {
            return session;
        } else {
            LOGGER.debug(() -> {
                return "Creating a new SqlSession";
            });
            session = sessionFactory.openSession(executorType);
            registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);
            return session;
        }
    }

a) 通过资源管理器获得资源 (SqlSessionHolder)TransactionSynchronizationManager.getResource(sessionFactory);getResource如下。里面调用了doGetRecource

@Nullable
    public static Object getResource(Object key) {
        Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
        Object value = doGetResource(actualKey);
        if(value != null && logger.isTraceEnabled()) {
            logger.trace("Retrieved value [" + value + "] for key [" + actualKey + "] bound to thread [" + Thread.currentThread().getName() + "]");
        }

        return value;
    }

进入doGetResource

@Nullable
    private static Object doGetResource(Object actualKey) {
        Map<Object, Object> map = (Map)resources.get();
        if(map == null) {
            return null;
        } else {
            Object value = map.get(actualKey);
            if(value instanceof ResourceHolder && ((ResourceHolder)value).isVoid()) {
                map.remove(actualKey);
                if(map.isEmpty()) {
                    resources.remove();
                }

                value = null;
            }

            return value;
        }
    }

resource的数据结构如下

private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal("Transactional resources");

三、mapper类扫描

1、进入MybatisAutoConfiguration类。因为没有mapperFactoryBean和MapperScannerConfigurer类，所有会把MapperScannerRegisterarNoFoundConfiguration 注入到容器中，并且导入AutoConfiguredMapperScannerRegistrar类

2、进入AutoConfiguredMapperScannerRegistrar类。实现了ImportBanDefinitionRegistrar接口

 最后注册MapperScannerConfigurer这个Bean。

3、MapperScannerConfigurer

作用：扫描mapper接口注册到容器中

1）而MapperScannerConfigurer实现了BeanDefinitionRegitryPostProcessor接口，实现了postProcessBeanDefinitionRegistry这个方法

 2） 我们进入这个方法scanner.registerFilters();

进入registerFilters

public void registerFilters() {
        boolean acceptAllInterfaces = true;
        if(this.annotationClass != null) {
            this.addIncludeFilter(new AnnotationTypeFilter(this.annotationClass));
            acceptAllInterfaces = false;
        }

        if(this.markerInterface != null) {
            this.addIncludeFilter(new AssignableTypeFilter(this.markerInterface) {
                protected boolean matchClassName(String className) {
                    return false;
                }
            });
            acceptAllInterfaces = false;
        }

        if(acceptAllInterfaces) {
            this.addIncludeFilter((metadataReader, metadataReaderFactory) -> {
                return true;
            });
        }

        this.addExcludeFilter((metadataReader, metadataReaderFactory) -> {
            String className = metadataReader.getClassMetadata().getClassName();
            return className.endsWith("package-info");
        });
    }

3)对包路径进行扫描 scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ",; \t\n"));

public int scan(String... basePackages) {
		int beanCountAtScanStart = this.registry.getBeanDefinitionCount();

		doScan(basePackages);

		// Register annotation config processors, if necessary.
		if (this.includeAnnotationConfig) {
			AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
		}

		return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
	}

核心是doScan方法。

首先会调用父类的doScan方法： 

public Set<BeanDefinitionHolder> doScan(String... basePackages) {
        Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);
        if(beanDefinitions.isEmpty()) {
            LOGGER.warn(() -> {
                return "No MyBatis mapper was found in ‘" + Arrays.toString(basePackages) + "‘ package. Please check your configuration.";
            });
        } else {
            this.processBeanDefinitions(beanDefinitions);
        }

        return beanDefinitions;
    }

然后调用this.processBeanDefinitions(beanDefinitions);方法

四、mapper类生成

1、进入processBeanDefinitions方法

private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
        GenericBeanDefinition definition;
        for(Iterator var3 = beanDefinitions.iterator(); var3.hasNext(); definition.setLazyInit(this.lazyInitialization)) {
            BeanDefinitionHolder holder = (BeanDefinitionHolder)var3.next();
            definition = (GenericBeanDefinition)holder.getBeanDefinition();
            String beanClassName = definition.getBeanClassName();
            LOGGER.debug(() -> {
                return "Creating MapperFactoryBean with name ‘" + holder.getBeanName() + "‘ and ‘" + beanClassName + "‘ mapperInterface";
            });
            definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName);
            definition.setBeanClass(this.mapperFactoryBeanClass);
            definition.getPropertyValues().add("addToConfig", Boolean.valueOf(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(() -> {
                        return "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(() -> {
                        return "Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.";
                    });
                }

                definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
                explicitFactoryUsed = true;
            }

            if(!explicitFactoryUsed) {
                LOGGER.debug(() -> {
                    return "Enabling autowire by type for MapperFactoryBean with name ‘" + holder.getBeanName() + "‘.";
                });
                definition.setAutowireMode(2);
            }
        }

    }

里面会将beanClass替换为mapperFactoryBeanClass 。definition.setBeanClass(this.mapperFactoryBeanClass);

2、进入MapperFactoryBean方法

里面有一个getObject方法

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

进入getMapper方法，类型为接口TestMapper

进入getMapper方法

 进入getMapper

 进入newInstance方法。最终返回一个代理对象。MapperProxy是Mybatis源码里的内容，这里不做过多的介绍。

五、mapper类执行

1、进入MapperProxy中的invoke方法

 进入execute方法

进入增删改查其中一种类型，然后通过sqlSession进行执行，执行完毕后将结果返回

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
      case INSERT: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      case SELECT:
        if (method.returnsVoid() && method.hasResultHandler()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
          Object param = method.convertArgsToSqlCommandParam(args);
          result = sqlSession.selectOne(command.getName(), param);
          if (method.returnsOptional()
              && (result == null || !method.getReturnType().equals(result.getClass()))) {
            result = Optional.ofNullable(result);
          }
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        throw new BindingException("Unknown execution method for: " + command.getName());
    }
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
      throw new BindingException("Mapper method ‘" + command.getName()
          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
    }
    return result;
  }