MyBatis Plus插件机制与执行流程原理分析详解

MyBatis Plus插件

MyBatis Plus提供了分页插件PaginationInterceptor、执行分析插件SqlExplainInterceptor、性能分析插件PerformanceInterceptor以及乐观锁插件OptimisticLockerInterceptor。

Mybatis 通过插件 (Interceptor) 可以做到拦截四大对象相关方法的执行 ,根据需求完成相关数据的动态改变。

四大对象是：

• Executor
• StatementHandler
• ParameterHandler
• ResultSetHandler

四大对象的每个对象在创建时，都会执行interceptorChain.pluginAll()，会经过每个插件对象的 plugin()方法，目的是为当前的四大对象创建代理。代理对象就可以拦截到四大对象相关方法的执行，因为要执行四大对象的方法需要经过代理 。

① xml下插件的配置

如下所示：

<bean id="sqlSessionFactoryBean" class="com.baomidou.mybatisplus.spring.MybatisSqlSessionFactoryBean">
<!-- 数据源 -->
<property name="dataSource" ref="dataSource">
</property>
<property name="configLocation" value="classpath:mybatis-config.xml">
</property>
<!-- 别名处理 --><property name="typeAliasesPackage" value="com.jane.mp.beans">
</property>
<!-- 注入全局MP策略配置 --><property name="globalConfig" ref="globalConfiguration">
</property>
<!-- 插件注册 --><property name="plugins"><list><!-- 注册分页插件 -->
<bean class="com.baomidou.mybatisplus.plugins.PaginationInterceptor">
</bean>
<!-- 注册执行分析插件 --><bean class="com.baomidou.mybatisplus.plugins.SqlExplainInterceptor">
<property name="stopProceed" value="true"></property></bean>
<!-- 注册性能分析插件 -->
<bean class="com.baomidou.mybatisplus.plugins.PerformanceInterceptor"><property name="format" value="true">
</property>
<!-- <property name="maxTime" value="5"></property> --></bean>
<!-- 注册乐观锁插件 -->
<bean class="com.baomidou.mybatisplus.plugins.OptimisticLockerInterceptor"></bean>
</list></property></bean>

② springboot下注册插件

这里以分页插件为例：

@Bean public PaginationInterceptor paginationInterceptor()
{
PaginationInterceptor paginationInterceptor = new PaginationInterceptor();
// 设置请求的页面大于最大页后操作， true调回到首页，false 继续请求 默认false
// paginationInterceptor.setOverflow(false);
// 设置最大单页限制数量，默认 500 条，-1 不受限制
// paginationInterceptor.setLimit(500);
// 开启 count 的 join 优化,只针对部分 left join
paginationInterceptor.setCountSqlParser(new JsqlParserCountOptimize(true));
return paginationInterceptor;
}

③ SqlExplainInterceptor

SQL执行分析拦截器，全类名是com.baomidou.mybatisplus.plugins.SqlExplainInterceptor，只支持 MySQL5.6.3以上版本。

该插件的作用是分析 DELETE UPDATE语句 ,防止小白或者恶意进行DELETE UPDATE全表操作，不建议在生产环境中使用会造成性能下降，

在插件的底层通过SQL语句分析命令 Explain 分析当前的 SQL语句，根据结果集中的 Extra列来断定当前是否全表操作。

④ 性能分析插件

性能分析拦截器，全类名是com.baomidou.mybatisplus.plugins.PerformanceInterceptor，用于输出每条 SQL 语句及其执行时间。SQL性能执行分析 ,开发环境使用 超过指定时间，停止运行。

⑤ 乐观锁插件

全类名是com.baomidou.mybatisplus.plugins.OptimisticLockerInterceptor。如果想实现如下需求 : 当要更新一条记录的时候，希望这条记录没有被别人更新，就可以使用该插件进行判断。

乐观锁的实现原理(@Version 用于注解实体字段，必须要有) :

• 取出记录时，获取当前 version
• 更新时，带上这个version
• 执行更新时，set version = yourVersion+1 where version = yourVersion
• 如果 version不对，就更新失败

【2】获取sqlSessionFactoryBean

如下图所示，在系统启动时会初始化定义的bean。DefaultListableBeanFactory.preInstantiateSingletons方法中会从beanDefinitionNames中获取bean name然后依次创建。

这里可以看到RootBeanDefinition是com.baomidou.mybatisplus.spring.MybatisSqlSessionFactoryBean。

① 获取bean的过程中bean属性

如下所示，在getBean过程中可以看到bean的属性：

② createBean

第一次获取bean的时候会走到AbstractAutowireCapableBeanFactory.createBean进行bean的创建。

 /** 创建一个bean实例，为bean实例设置属性值，调用post-processors-bean后置处理器 */@Overrideprotected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args)
 throws BeanCreationException
 {//...暂时忽略其他代码
 try {// 这里会首先触发BeanPostProcessors ，如果这里能获取到bean则直接返回，不再走doCreateBean
 Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
 if (bean != null)
 {
 return bean;
 } }//...暂时忽略其他代码

在AbstractAutowireCapableBeanFactory.resolveBeforeInstantiation中就会分别执行bean后置处理器的前置和后置方法。

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd)
{ Object bean = null;
if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved))
{
// Make sure bean class is actually resolved at this point.
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors())
{
Class<?> targetType = determineTargetType(beanName, mbd);
if (targetType != null) {
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null)
{
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = (bean != null);
}
return bean;
}

执行后置处理器的前置方法如下所示：

③ doCreateBean

AbstractAutowireCapableBeanFactory.doCreateBean是创建bean的核心方法，这会为bean属性赋值并会触发bean后置处理器、InitializingBean以及自定init方法等。

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) throws BeanCreationException
{
// Instantiate the bean.--实例化beanBeanWrapper instanceWrapper = null;if (mbd.isSingleton()) { instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}if (instanceWrapper == null)
{//获取bean的包装对象-这里很重要 instanceWrapper = createBeanInstance(beanName, mbd, args);}final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);mbd.resolvedTargetType = beanType;
// Allow post-processors to modify the merged bean definition.synchronized (mbd.postProcessingLock)
{ if (!mbd.postProcessed)
 { try {//调用MergedBeanDefinitionPostProcessors的postProcessMergedBeanDefinition方法  applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
} catch (Throwable ex)
{
 throw new BeanCreationException(mbd.getResourceDescription(), beanName,   "Post-processing of merged bean definition failed", ex);
} mbd.postProcessed = true; }}//...//这里暂时忽略其他代码
// Initialize the bean instance.--初始化bean实例Object exposedObject = bean;try {//如下方法很重要 populateBean(beanName, mbd, instanceWrapper); if (exposedObject != null)
{
 exposedObject = initializeBean(beanName, exposedObject, mbd);
}}//...

④ populateBean

顾名思义，为bean实例属性赋值。

AbstractAutowireCapableBeanFactory.populateBean

protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {//获取属性值列表PropertyValues pvs = mbd.getPropertyValues();
//...该种符号表示暂时忽略其他代码
//在为bean属性赋值前，给InstantiationAwareBeanPostProcessors 机会修改bean的状态//应用场景如支持字段注入boolean continueWithPropertyPopulation = true;
//循环调用InstantiationAwareBeanPostProcessors 的postProcessAfterInstantiation方法if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) { for (BeanPostProcessor bp : getBeanPostProcessors()) { if (bp instanceof InstantiationAwareBeanPostProcessor) {  InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;  if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {  continueWithPropertyPopulation = false;  break;  } } }}
if (!continueWithPropertyPopulation) { return;}//解析autowire注解字段，进行主动注入if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) { MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
 // Add property values based on autowire by name if applicable. if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) { autowireByName(beanName, mbd, bw, newPvs); }
 // Add property values based on autowire by type if applicable. if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) { autowireByType(beanName, mbd, bw, newPvs); }
 pvs = newPvs;}boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);//循环调用InstantiationAwareBeanPostProcessors 的postProcessPropertyValues方法if (hasInstAwareBpps || needsDepCheck) { PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching); if (hasInstAwareBpps) { for (BeanPostProcessor bp : getBeanPostProcessors()) {  if (bp instanceof InstantiationAwareBeanPostProcessor) {  InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;  pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);  if (pvs == null) {   return;  }  } } } if (needsDepCheck) { checkDependencies(beanName, mbd, filteredPds, pvs); }}//在这里为属性赋值，会进行类型转换，这里注意关键词deep copy//如果是引用类型且bean没有存在，则会进行bean的创建过程applyPropertyValues(beanName, mbd, bw, pvs);}

如下图所示在创建sqlSessionFactoryBean过程中会创建其属性globalConfiguration对象：

如下图所示在创建sqlSessionFactoryBean过程中(从左侧的方法顺序就可以看出来)会创建其属性PaginationInterceptor对象：

如下图所示在创建sqlSessionFactoryBean过程中(从左侧的方法顺序就可以看出来)会创建其属性SqlExplainInterceptor对象：

如下图所示在创建sqlSessionFactoryBean过程中(从左侧的方法顺序就可以看出来)会创建其属性PerformanceInterceptor对象：

如下图所示在创建sqlSessionFactoryBean过程中(从左侧的方法顺序就可以看出来)会创建其属性OptimisticLockerInterceptor对象：

AbstractAutowireCapableBeanFactory.initializeBean源码如下：

protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd)
{
if (System.getSecurityManager() != null)
{
AccessController.doPrivileged(new PrivilegedAction<Object>()
{
@Override  public Object run()
{
invokeAwareMethods(beanName, bean);
return null;
}
}, getAccessControlContext()); } else { //调用意识/通知方法 invokeAwareMethods(beanName, bean);
}
 Object wrappedBean = bean;
 if (mbd == null || !mbd.isSynthetic())
 { //调用bean后置处理器的前置方法
 wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
 } //调用初始化方法
 try {
 invokeInitMethods(beanName, wrappedBean, mbd);
 } catch (Throwable ex)
 {
 throw new BeanCreationException(
 (mbd != null ? mbd.getResourceDescription() : null),
 beanName, "Invocation of init method failed", ex);
 }

AbstractAutowireCapableBeanFactory.invokeInitMethods方法源码如下：

protected void invokeInitMethods(String beanName, final Object bean, RootBeanDefinition mbd)
throws Throwable {
 boolean isInitializingBean = (bean instanceof InitializingBean);
 if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet")))
 {
 if (logger.isDebugEnabled())
 {
 logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
 }
 //调用InitializingBean.afterPropertiesSet
 if (System.getSecurityManager() != null)
 {
 try {
 AccessController.doPrivileged(new PrivilegedExceptionAction<Object>()
 {
 @Override   public Object run() throws Exception
 {
 ((InitializingBean) bean).afterPropertiesSet();
 return null;
 }  }, getAccessControlContext());
 }
 catch (PrivilegedActionException pae)
 {
 throw pae.getException();
 } } else {  ((InitializingBean) bean).afterPropertiesSet();
 } }//调用自定义初始化方法 if (mbd != null)
 {
 String initMethodName = mbd.getInitMethodName();
 if (initMethodName != null && !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
 !mbd.isExternallyManagedInitMethod(initMethodName))
 {
 invokeCustomInitMethod(beanName, bean, mbd);
 }
 }}

如下图所示，MybatisSqlSessionFactoryBean同样实现了InitializingBean接口。那么我们就需要注意其afterPropertiesSet方法了。

⑥ MybatisSqlSessionFactoryBean.afterPropertiesSet

如下所示，代码很简短只是创建了sqlSessionFactory。

@Overridepublic void afterPropertiesSet() throws Exception
{
notNull(dataSource, "Property 'dataSource' is required"); //sqlSessionFactoryBuilder在populateBean的applyPropertyValues过程中已经存在！
notNull(sqlSessionFactoryBuilder, "Property 'sqlSessionFactoryBuilder' is required");
state((configuration == null && configLocation == null) || !(configuration != null && configLocation != null),
"Property 'configuration' and 'configLocation' can not specified with together");
 this.sqlSessionFactory = buildSqlSessionFactory();
 }

进入afterPropertiesSet()方法前MybatisSqlSessionFactoryBean如下所示：

我们看一下sqlSessionFactory，这是一段很长的过程：

protected SqlSessionFactory buildSqlSessionFactory() throws Exception
{
Configuration configuration; // TODO 加载自定义 MybatisXmlConfigBuilder
MybatisXMLConfigBuilder 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) { //通常如果配置了configLocation会从这里创建MybatisXMLConfigBuilder， //其构造方法又创建了MybatisConfiguration
 xmlConfigBuilder = new MybatisXMLConfigBuilder(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");
 }
 // TODO 使用自定义配置
 configuration = new MybatisConfiguration();
 if (this.configurationProperties != null)
 {
 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))
 {
 // TODO 支持自定义通配符
 String[] typeAliasPackageArray;
 if (typeAliasesPackage.contains("*") && !typeAliasesPackage.contains(",")
 && !typeAliasesPackage.contains(";"))
 {
 typeAliasPackageArray = PackageHelper.convertTypeAliasesPackage(typeAliasesPackage);
 } else {   typeAliasPackageArray = tokenizeToStringArray(this.typeAliasesPackage,
  ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
  }
  if (typeAliasPackageArray == null) {
  throw new MybatisPlusException("not find typeAliasesPackage:" + typeAliasesPackage);
  }  for (String packageToScan : typeAliasPackageArray)
  {
  configuration.getTypeAliasRegistry().registerAliases(packageToScan,
  typeAliasesSuperType == null ? Object.class : typeAliasesSuperType);
  if (LOGGER.isDebugEnabled())
  {
  LOGGER.debug("Scanned package: '" + packageToScan + "' for aliases");
  }
  }
  }

 // TODO 自定义枚举类扫描处理
 if (hasLength(this.typeEnumsPackage))
 {
 Set<Class> classes = null;
 if (typeEnumsPackage.contains("*") && !typeEnumsPackage.contains(",")
 && !typeEnumsPackage.contains(";"))
 {
 classes = PackageHelper.scanTypePackage(typeEnumsPackage);
 } else {   String[] typeEnumsPackageArray = tokenizeToStringArray(this.typeEnumsPackage,
 ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
 if (typeEnumsPackageArray == null)
 {
 throw new MybatisPlusException("not find typeEnumsPackage:" + typeEnumsPackage);
 }
 classes = new HashSet<Class>();
 for (String typePackage : typeEnumsPackageArray)
 {
 classes.addAll(PackageHelper.scanTypePackage(typePackage));
 }
 }  // 取得类型转换注册器
 TypeHandlerRegistry typeHandlerRegistry = configuration.getTypeHandlerRegistry();
 for (Class cls : classes) {   if (cls.isEnum()) {    if (IEnum.class.isAssignableFrom(cls)) {
 typeHandlerRegistry.register(cls.getName(), com.baomidou.mybatisplus.handlers.EnumTypeHandler.class.getCanonicalName());    } else {
 // 使用原生 EnumOrdinalTypeHandler     typeHandlerRegistry.register(cls.getName(), org.apache.ibatis.type.EnumOrdinalTypeHandler.class.getCanonicalName());
 }
 }  } }
 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));
 // 设置元数据相关
 GlobalConfigUtils.setMetaData(dataSource, globalConfig); SqlSessionFactory sqlSessionFactory = this.sqlSessionFactoryBuilder.build(configuration);
 // TODO SqlRunner
 SqlRunner.FACTORY = sqlSessionFactory; // TODO 缓存 sqlSessionFactory globalConfig.setSqlSessionFactory(sqlSessionFactory); // TODO 设置全局参数属性
 globalConfig.signGlobalConfig(sqlSessionFactory);
 if (!isEmpty(this.mapperLocations)) {
 if (globalConfig.isRefresh()) {
 //TODO 设置自动刷新配置 减少配置
 new MybatisMapperRefresh(this.mapperLocations, sqlSessionFactory, 2,
 2, true);  }
 for (Resource mapperLocation : this.mapperLocations)
 {
 if (mapperLocation == null) {
 continue;
 }
   try {
	 // TODO 这里也换了噢噢噢噢
	 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 sqlSessionFactory;
	 }

上面代码主要做了如下事情：

• 获取MybatisXMLConfigBuilder对象
• 获取Configuration对象-MybatisConfiguration
• 配置对象Configuration添加插件configuration.addInterceptor(plugin);
• xmlConfigBuilder.parse()对configuration做进一步处理
• 获取SpringManagedTransactionFactory用来创建SpringManagedTransaction
• 获取一个DefaultSqlSessionFactory实例对象
• globalConfig.setSqlSessionFactory(sqlSessionFactory)中会创建MybatisSqlSessionTemplate
• 解析mapperLocation对应的一个个mapper配置文件，使用助手builderAssistant的addMappedStatement方法将一个个结点添加配置对象中
• 其他属性设置等等

也就是说，在MybatisSqlSessionFactoryBean.afterPropertiesSet方法执行结束后，SqlSessionFactory、SqlSessionTemplate、Configuration等都已存在！

【3】查询执行流程分析

示例代码如下：

List<Employee > emps = employeeMapper.selectPage(page, null);

如下图所示，此时我们获取到的employeeMapper其实是个代理对象：

总结

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