SpringBoot整合MyBatis原理
一. 自動配置
SpringBoot提供了MyBatis的自動配置類MybatisAutoConfiguration，可以自動註冊SqlSessionFactory、SqlSessionTemplate等組件，開發人員只需在配置文件中指定相關屬性即可。
@Configuration
@ConditionalOnClass({ SqlSessionFactory.class, SqlSessionFactoryBean.class })
@ConditionalOnBean(DataSource.class)
@EnableConfigurationProperties(MybatisProperties.class)
@AutoConfigureAfter(DataSourceAutoConfiguration.class)
public class MybatisAutoConfiguration {
private static Log log = LogFactory.getLog(MybatisAutoConfiguration.class);
@Autowired
private MybatisProperties properties;
@Autowired(required = false)
private Interceptor[] interceptors;
@Autowired
private ResourceLoader resourceLoader = new DefaultResourceLoader();
@Autowired(required = false)
private DatabaseIdProvider databaseIdProvider;
//檢查配置文件路徑
@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)");
}
}
//註冊SqlSessionFactory
@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()));
}
factory.setConfiguration(properties.getConfiguration());
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());
}
if (!ObjectUtils.isEmpty(this.properties.resolveMapperLocations())) {
factory.setMapperLocations(this.properties.resolveMapperLocations());
}
return factory.getObject();
}
//註冊SqlSessionTemplate
@Bean
@ConditionalOnMissingBean
public SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
ExecutorType executorType = this.properties.getExecutorType();
if (executorType != null) {
return new SqlSessionTemplate(sqlSessionFactory, executorType);
} else {
return new SqlSessionTemplate(sqlSessionFactory);
}
}
//註冊ClassPathMapperScanner，進行Mapper的自動掃描，也可以使用@MapperScan代替
public static class AutoConfiguredMapperScannerRegistrar
implements BeanFactoryAware, ImportBeanDefinitionRegistrar, ResourceLoaderAware {
private BeanFactory beanFactory;
private ResourceLoader resourceLoader;
@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
log.debug("Searching for mappers annotated with @Mapper'");
ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
try {
if (this.resourceLoader != null) {
scanner.setResourceLoader(this.resourceLoader);
}
List<String> pkgs = AutoConfigurationPackages.get(this.beanFactory);
for (String pkg : pkgs) {
log.debug("Using auto-configuration base package '" + pkg + "'");
}
scanner.setAnnotationClass(Mapper.class);
scanner.registerFilters();
scanner.doScan(StringUtils.toStringArray(pkgs));
} catch (IllegalStateException ex) {
log.debug("Could not determine auto-configuration " + "package, automatic mapper scanning disabled.");
}
}
@Override
public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
this.beanFactory = beanFactory;
}
@Override
public void setResourceLoader(ResourceLoader resourceLoader) {
this.resourceLoader = resourceLoader;
}
}
//如果程序中沒有顯式注入MapperFactoryBean，就打印日誌
@Configuration
@Import({ AutoConfiguredMapperScannerRegistrar.class })
@ConditionalOnMissingBean(MapperFactoryBean.class)
public static class MapperScannerRegistrarNotFoundConfiguration {
@PostConstruct
public void afterPropertiesSet() {
log.debug(String.format("No %s found.", MapperFactoryBean.class.getName()));
}
}
}
SpringBoot爲我們自動註冊了相應的組件：
- SqlSessionFactoryBean：用於構建MyBatis的SqlSessionFactory
- SqlSessionTemplate：MyBatis的代理類，將SqlSession與Spring的事務進行了整合
- ClassPathMapperScanner：提供MyBatis的Mapper的自動掃描
二. SqlSessionFactoryBean
SqlSessionFactoryBean是一個工廠Bean，其作用就是加載用戶自定義的配置，然後使用MyBatis的API創建一個SqlSessionFactory，具體邏輯如下：
protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
Configuration configuration;
XMLConfigBuilder xmlConfigBuilder = null;
if (this.configuration != null) {
configuration = this.configuration;
if (configuration.getVariables() == null) {
configuration.setVariables(this.configurationProperties);
} else if (this.configurationProperties != null) {
configuration.getVariables().putAll(this.configurationProperties);
}
} else if (this.configLocation != null) {
xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
configuration = xmlConfigBuilder.getConfiguration();
} else {
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Property `configuration` or 'configLocation' not specified, using default MyBatis Configuration");
}
configuration = new Configuration();
configuration.setVariables(this.configurationProperties);
}
if (this.objectFactory != null) {
configuration.setObjectFactory(this.objectFactory);
}
if (this.objectWrapperFactory != null) {
configuration.setObjectWrapperFactory(this.objectWrapperFactory);
}
if (this.vfs != null) {
configuration.setVfsImpl(this.vfs);
}
if (hasLength(this.typeAliasesPackage)) {
String[] typeAliasPackageArray = tokenizeToStringArray(this.typeAliasesPackage,
ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
for (String packageToScan : typeAliasPackageArray) {
configuration.getTypeAliasRegistry().registerAliases(packageToScan,
typeAliasesSuperType == null ? Object.class : typeAliasesSuperType);
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Scanned package: '" + packageToScan + "' for aliases");
}
}
}
if (!isEmpty(this.typeAliases)) {
for (Class<?> typeAlias : this.typeAliases) {
configuration.getTypeAliasRegistry().registerAlias(typeAlias);
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Registered type alias: '" + typeAlias + "'");
}
}
}
if (!isEmpty(this.plugins)) {
for (Interceptor plugin : this.plugins) {
configuration.addInterceptor(plugin);
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Registered plugin: '" + plugin + "'");
}
}
}
if (hasLength(this.typeHandlersPackage)) {
String[] typeHandlersPackageArray = tokenizeToStringArray(this.typeHandlersPackage,
ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
for (String packageToScan : typeHandlersPackageArray) {
configuration.getTypeHandlerRegistry().register(packageToScan);
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Scanned package: '" + packageToScan + "' for type handlers");
}
}
}
if (!isEmpty(this.typeHandlers)) {
for (TypeHandler<?> typeHandler : this.typeHandlers) {
configuration.getTypeHandlerRegistry().register(typeHandler);
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Registered type handler: '" + typeHandler + "'");
}
}
}
if (this.databaseIdProvider != null) {//fix #64 set databaseId before parse mapper xmls
try {
configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
} catch (SQLException e) {
throw new NestedIOException("Failed getting a databaseId", e);
}
}
if (this.cache != null) {
configuration.addCache(this.cache);
}
if (xmlConfigBuilder != null) {
try {
xmlConfigBuilder.parse();
if (LOGGER.isDebugEnabled()) {
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();
}
}
if (this.transactionFactory == null) {
this.transactionFactory = new SpringManagedTransactionFactory();
}
configuration.setEnvironment(new Environment(this.environment, this.transactionFactory, this.dataSource));
if (!isEmpty(this.mapperLocations)) {
for (Resource mapperLocation : this.mapperLocations) {
if (mapperLocation == null) {
continue;
}
try {
XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
configuration, mapperLocation.toString(), configuration.getSqlFragments());
xmlMapperBuilder.parse();
} catch (Exception e) {
throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
} finally {
ErrorContext.instance().reset();
}
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Parsed mapper file: '" + mapperLocation + "'");
}
}
} else {
if (LOGGER.isDebugEnabled()) {
LOGGER.debug("Property 'mapperLocations' was not specified or no matching resources found");
}
}
return this.sqlSessionFactoryBuilder.build(configuration);
}
三. SqlSessionTemplate
SqlSessionTemplate是Spring提供的一個對MyBatis的SqlSession的一個增強類，它的作用就是將SqlSession與當前的事務所綁定，而且是線程安全的，一個SqlSessionTemplate可以被多個dao所共享。
SqlSessionTemplate基於動態代理模式，內部委託了一個SqlSession對象，並且在其基礎上進行了增強。代碼如下：
//傳入的SqlSessionFactory
private final SqlSessionFactory sqlSessionFactory;
//Executor類型
private final ExecutorType executorType;
//SqlSession的動態代理
private final SqlSession sqlSessionProxy;
private final PersistenceExceptionTranslator exceptionTranslator;
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;
//創建SqlSession的動態代理
this.sqlSessionProxy = (SqlSession) newProxyInstance(
SqlSessionFactory.class.getClassLoader(),
new Class[] { SqlSession.class },
new SqlSessionInterceptor());
}
在創建SqlSessionTemplate傳入SqlSessionFactory，而通過SqlSessionFactory創建的SqlSession其實是一個動態代理類，其增強的邏輯在SqlSessionInterceptor中定義，如下：
private class SqlSessionInterceptor implements InvocationHandler {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//獲取當前事務下綁定的SqlSession，由於Spring通過ThreadLocal將線程與事務綁定，所以也可以認爲獲取的是當前線程綁定的SqlSession
SqlSession sqlSession = getSqlSession(
SqlSessionTemplate.this.sqlSessionFactory,
SqlSessionTemplate.this.executorType,
SqlSessionTemplate.this.exceptionTranslator);
try {
//執行SqlSession的方法
Object result = method.invoke(sqlSession, args);
//如果當前在非事務環境下允許，則強制commit一下，因爲有些數據庫要求在close()方法前要先調用commit()或rollback()
if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
sqlSession.commit(true);
}
return result;
} catch (Throwable t) {
Throwable unwrapped = unwrapThrowable(t);
throw unwrapped;
} finally {
//關閉sqlSession
if (sqlSession != null) {
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
}
}
}
}
這樣一來，Spring就將事務與SqlSession整合到了一起。
四. Mapper自動掃描
通過@MapperScan註解，可以指定Mapper自動掃描的包路徑。自動掃描的處理是通過ClassPathMapperScanner實現的，其doScan()方法如下：
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;
}
ClassPathMapperScanner會將所有的Mapper掃描進來，並且將每個Mapper包裝成一個類型爲MapperFactoryBean的BeanDefinition，註冊到IoC容器中。
MapperScannerConfigurer實現了BeanDefinitionRegistryPostProcessor，是一個BeanFactoryPostProcessor，它的功能就是在容器啓動階段動態向容器中註冊BeanDefinition。經過MapperScannerConfigurer處理後，所有Mapper接口的BeanDefinition就以MapperFactoryBean的形式註冊到Spring IoC容器中了。 代碼可見ClassPathMapperScanner#processBeanDefinitions()：
private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
GenericBeanDefinition definition;
for (BeanDefinitionHolder holder : beanDefinitions) {
definition = (GenericBeanDefinition) holder.getBeanDefinition();
if (logger.isDebugEnabled()) {
logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName()
+ "' and '" + definition.getBeanClassName() + "' mapperInterface");
}
// the mapper interface is the original class of the bean
// but, the actual class of the bean is MapperFactoryBean
//這裏實際註冊的BeanDefinition是Mapper接口所對應的MapperFactoryBean
definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); // issue #59
definition.setBeanClass(this.mapperFactoryBean.getClass());
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) {
if (logger.isDebugEnabled()) {
logger.debug("Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
}
definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
}
}
}
五. MapperFactoryBean
MapperFactoryBean也是一個工廠Bean，可以通過它創建Mapper實例：
@Override
public T getObject() throws Exception {
return getSqlSession().getMapper(this.mapperInterface);
}
這樣一來，所有程序中定義的Mapper就都以Bean的形式加載到Spring的IoC容器中了。
六. Spring對MyBatis生命週期的修改
-
SqlSession
在MyBatis原生API中，SqlSession的生命週期是方法級別的，在每個方法中創建一個新的SqlSession實例，用完了之後就銷燬。而Spring對MyBatis原生的SqlSession進行了事務的增強，通過一個單例的SqlSessionTemplate，保證每次獲取的SqlSession其實都是與當前線程綁定的。
-
Mapper
MyBatis原生API中，通過SqlSession#getMapper()獲取到的Mapper，是一個MapperProxy動態代理類，它的生命週期與SqlSession一致，都是方法級別的。但是Spring通過MapperFactoryBean創建的Mapper是單例的，也就是將其聲明週期提升到了全局級別。
七. MyBatis一級緩存失效問題
我們知道MyBatis有兩級緩存，其中一級緩存是默認開啓的。但是在Spring整合了MyBatis後，卻經常出現一級緩存失效的問題，其原因是在SqlSessionTemplate對SqlSession進行了代理後，在非事務環境下，每次執行完SqlSession的操作後都會進行一次commit()，且最後後關閉SqlSession，因此會清空一級緩存(強調下僅在非事務環境下，在事務中調用不會出現這個問題)，源碼如下：
private class SqlSessionInterceptor implements InvocationHandler {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
SqlSession sqlSession = getSqlSession(
SqlSessionTemplate.this.sqlSessionFactory,
SqlSessionTemplate.this.executorType,
SqlSessionTemplate.this.exceptionTranslator);
try {
Object result = method.invoke(sqlSession, args);
if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
//在非事務環境下執行，會強制commit()
sqlSession.commit(true);
}
return result;
} catch (Throwable t) {
Throwable unwrapped = unwrapThrowable(t);
if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
// release the connection to avoid a deadlock if the translator is no loaded. See issue #22
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
sqlSession = null;
Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
if (translated != null) {
unwrapped = translated;
}
}
throw unwrapped;
} finally {
//關閉SqlSession
//在非事務環境下，就是直接調用SqlSession.close()方法
//在事務環境下，並不會關閉SqlSession,而僅是對當前事務的引用計數-1
if (sqlSession != null) {
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
}
}
}
}
八. SpringBoot整合MyBatis簡單演示
-
引入MyBatis的Starter依賴
<dependency> <groupId>org.mybatis.spring.boot</groupId> <artifactId>mybatis-spring-boot-starter</artifactId> <version>1.1.1</version> </dependency> -
在application.properties中增加MyBatis相關配置
#數據源配置 spring.datasource.type=com.alibaba.druid.pool.DruidDataSource spring.datasource.url=jdbc:mysql://localhost:3306/test spring.datasource.driver-class-name=com.mysql.jdbc.Driver spring.datasource.username=xxx spring.datasource.password=xxx #MyBatis配置 mybatis.type-aliases-package=xxx mybatis.mapper-locations=classpath:mappers/*.xml -
編寫Mapper和對應的映射文件(或註解)
-
開啓Mapper自動掃描，並指定包路徑
@MapperScan("xxx.mapper")