MyBatis Interceptor执行顺序详解（plugin机制、责任链模式）

目录一、引言二、Interceptor的注册顺序2.1 配置文件注册mybatis-config.xml2.2 代码注册2.3 SpringBoot Order2.4 扩展 - PageHelper链最后即最先执行三、plugin机制与InterceptorChain3.1 InterceptorChain.pluginAll3.2 Interceptor.plugin3.3 Plugin.wrap(target, this)四、Configuration调用pluginAll生成Executor五、责任链模式的应用六、执行顺序重点先注册后执行七、示例代码八、结论一、引言在MyBatis中Interceptor拦截器是实现自定义功能的核心扩展点。通过插件机制开发者可以在SQL执行、参数处理、结果映射等关键流程中插入自定义逻辑。本文将深入介绍MyBatis中Interceptor的执行顺序重点关注Interceptor的注册顺序、plugin机制、Configuration如何调用pluginAll方法生成Executor以及责任链模式的应用并详细说明了“先注册后执行”的机制和Plugin.wrap底层实现。二、Interceptor的注册顺序MyBatis的Interceptor通常通过配置文件或代码注册到Configuration对象中。注册顺序非常重要因为它直接影响拦截器的执行顺序。2.1 配置文件注册mybatis-config.xmlpluginsplugininterceptorcom.example.MyInterceptor1/plugininterceptorcom.example.MyInterceptor2//plugins拦截器会按照配置文件中出现的顺序依次注册。2.2 代码注册configuration.addInterceptor(newMyInterceptor1());configuration.addInterceptor(newMyInterceptor2());同样拦截器会按照addInterceptor的调用顺序注册。2.3 SpringBoot Order如果你使用的是 mybatis-spring-boot-starter推荐方式可以通过Order注解明确指定顺序BeanOrder(1)publicInterceptorinterceptorA(){...}BeanOrder(2)publicInterceptorinterceptorB(){...}注册顺序说明Spring 会将所有类型为Interceptor的 Bean 注入到 MyBatis 的SqlSessionFactory中。默认情况下Spring 注入集合类型如 List时顺序为 Bean上Order注解声明的顺序。2.4 扩展 - PageHelper链最后即最先执行PageHelper通过实现InitializingBean.afterPropertiesSet方法强制将PageInterceptor放到拦截器列表的最后面即保证了PageInterceptor会最先被执行具体代码如下packagecom.github.pagehelper.autoconfigure;importcom.github.pagehelper.PageInterceptor;importorg.apache.ibatis.plugin.Interceptor;importorg.apache.ibatis.session.SqlSessionFactory;importorg.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration;importorg.springframework.beans.factory.InitializingBean;importorg.springframework.boot.autoconfigure.AutoConfigureAfter;importorg.springframework.boot.autoconfigure.condition.ConditionalOnBean;importorg.springframework.boot.context.properties.EnableConfigurationProperties;importorg.springframework.context.annotation.Configuration;importorg.springframework.context.annotation.Lazy;importjava.util.List;/** * 自定注入分页插件 * * author liuzh */ConfigurationConditionalOnBean(SqlSessionFactory.class)EnableConfigurationProperties({PageHelperProperties.class,PageHelperStandardProperties.class})AutoConfigureAfter(MybatisAutoConfiguration.class)//Import(PageHelperProperties.class)Lazy(false)publicclassPageHelperAutoConfigurationimplementsInitializingBean{privatefinalListSqlSessionFactorysqlSessionFactoryList;privatefinalPageHelperPropertiesproperties;publicPageHelperAutoConfiguration(ListSqlSessionFactorysqlSessionFactoryList,PageHelperStandardPropertiesstandardProperties){this.sqlSessionFactoryListsqlSessionFactoryList;this.propertiesstandardProperties.getProperties();}OverridepublicvoidafterPropertiesSet()throwsException{PageInterceptorinterceptornewPageInterceptor();interceptor.setProperties(this.properties);for(SqlSessionFactorysqlSessionFactory:sqlSessionFactoryList){org.apache.ibatis.session.ConfigurationconfigurationsqlSessionFactory.getConfiguration();if(!containsInterceptor(configuration,interceptor)){// 重点关注 configuration.addInterceptor(interceptor);}}}/** * 是否已经存在相同的拦截器 * * param configuration * param interceptor * return */privatebooleancontainsInterceptor(org.apache.ibatis.session.Configurationconfiguration,Interceptorinterceptor){try{// getInterceptors since 3.2.2returnconfiguration.getInterceptors().stream().anyMatch(config-interceptor.getClass().isAssignableFrom(config.getClass()));}catch(Exceptione){returnfalse;}}}三、plugin机制与InterceptorChain3.1 InterceptorChain.pluginAllMyBatis通过plugin机制实现拦截器的动态包装。核心类是InterceptorChain它维护一个Interceptor列表并负责将目标对象如Executor、StatementHandler等按顺序包装。InterceptorChain源码片段publicclassInterceptorChain{privatefinalListInterceptorinterceptorsnewArrayList();publicObjectpluginAll(Objecttarget){for(Interceptorinterceptor:interceptors){targetinterceptor.plugin(target);}returntarget;}}可以看到pluginAll方法会按照Interceptor注册顺序依次调用每个Interceptor的plugin方法将目标对象层层包装。3.2 Interceptor.pluginInterceptor接口publicinterfaceInterceptor{Objectintercept(Invocationinvocation)throwsThrowable;defaultObjectplugin(Objecttarget){returnPlugin.wrap(target,this);}defaultvoidsetProperties(Propertiesproperties){}}3.3 Plugin.wrap(target, this)MyBatis的plugin机制核心在于Plugin.wrap(target, this)方法。其底层实现主要依赖Java的动态代理JDK Proxy实现了对目标对象的拦截。主要流程判断目标对象是否需要拦截Plugin.wrap会根据Interceptor的Signature注解判断目标对象是否实现了需要拦截的接口如Executor、StatementHandler等。创建代理对象如果需要拦截Plugin.wrap会使用JDK动态代理生成一个代理对象。代理对象会在目标方法被调用时自动调用Interceptor的intercept方法。责任链传递多个Interceptor包装后代理对象会层层嵌套形成责任链。每个intercept方法内部通常会调用invocation.proceed()继续传递到下一个Interceptor或原始对象。源码简要publicclassPluginimplementsInvocationHandler{privateObjecttarget;privateInterceptorinterceptor;privateMapClass?,SetMethodsignatureMap;publicstaticObjectwrap(Objecttarget,Interceptorinterceptor){MapClass?,SetMethodsignatureMapgetSignatureMap(interceptor);Class?typetarget.getClass();// 判断是否需要拦截if(shouldIntercept(type,signatureMap)){returnProxy.newProxyInstance(type.getClassLoader(),getInterfaces(type,signatureMap),newPlugin(target,interceptor,signatureMap));}returntarget;}OverridepublicObjectinvoke(Objectproxy,Methodmethod,Object[]args)throwsThrowable{// 判断是否拦截该方法if(shouldIntercept(method)){returninterceptor.intercept(newInvocation(target,method,args));}returnmethod.invoke(target,args);}}四、Configuration调用pluginAll生成Executor在MyBatis初始化时Configuration会创建Executor等核心组件并通过InterceptorChain的pluginAll方法进行包装典型流程以Executor为例ExecutorexecutornewSimpleExecutor();executorinterceptorChain.pluginAll(executor);这意味着Executor会被所有Interceptor按注册顺序依次包装形成一条“责任链”。五、责任链模式的应用MyBatis的plugin机制本质上是责任链模式Chain of Responsibility Pattern的应用。每个Interceptor都可以选择拦截或放行操作最终形成如下调用链Executor被第一个Interceptor包装返回一个代理对象。代理对象再被第二个Interceptor包装依次类推。执行SQL时调用链会从最外层Interceptor开始逐层向内传递直到原始Executor。这种模式带来的好处是拦截器可以灵活地插入、组合且执行顺序完全由注册顺序决定。六、执行顺序重点先注册后执行需要特别强调的是Interceptor的执行顺序遵循“先注册后执行”原则。注册时Configuration会将Interceptor按顺序添加到InterceptorChain的interceptors列表。包装时pluginAll方法会依次调用每个Interceptor的plugin方法将目标对象层层包裹。执行时调用链会从最外层最后注册的Interceptor开始逐层向内传递直到最先注册的Interceptor最后到原始对象。举例说明假设注册顺序为InterceptorA → InterceptorB → InterceptorC执行包装的顺序InterceptorChain.pluginAllInterceptorA.plugin → InterceptorB.plugin → InterceptorC.plugin最终目标对象的包装顺序为C包裹BB包裹AA包裹原始对象执行顺序为C → B → A → 原始对象七、示例代码假设有两个拦截器publicclassMyInterceptor1implementsInterceptor{publicObjectintercept(Invocationinvocation)throwsThrowable{System.out.println(MyInterceptor1 before);Objectresultinvocation.proceed();System.out.println(MyInterceptor1 after);returnresult;}}publicclassMyInterceptor2implementsInterceptor{publicObjectintercept(Invocationinvocation)throwsThrowable{System.out.println(MyInterceptor2 before);Objectresultinvocation.proceed();System.out.println(MyInterceptor2 after);returnresult;}}注册顺序为1→2执行顺序如下MyInterceptor2 before MyInterceptor1 before [原始Executor执行] MyInterceptor1 after MyInterceptor2 after八、结论Interceptor注册顺序决定执行顺序先注册的拦截器会被后执行责任链模式保证了拦截器的顺序性和可扩展性。Plugin.wrap(target, this)通过JDK动态代理实现拦截器包装只有目标对象实现了需要拦截的接口才会被代理拦截器通过intercept方法实现自定义逻辑。实际开发中务必关注拦截器的注册顺序合理设计拦截器的职责和顺序确保业务逻辑的正确性和可维护性。如需进一步了解可以参考MyBatis源码中的InterceptorChain和Plugin类实现以及项目中的拦截器注册代码。