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本文次要钻研一下mybatis的Interceptor机制
Interceptor
org/apache/ibatis/plugin/Interceptor.java
public interface Interceptor { Object intercept(Invocation invocation) throws Throwable; default Object plugin(Object target) { return Plugin.wrap(target, this); } default void setProperties(Properties properties) { // NOP }}Interceptor定义了intercept办法，其参数为Invocation类型，同时默认提供了plugin办法，通过Plugin.wrap(target, this)进行包装
Invocation
org/apache/ibatis/plugin/Invocation.java
public class Invocation { private final Object target; private final Method method; private final Object[] args; public Invocation(Object target, Method method, Object[] args) { this.target = target; this.method = method; this.args = args; } public Object getTarget() { return target; } public Method getMethod() { return method; } public Object[] getArgs() { return args; } public Object proceed() throws InvocationTargetException, IllegalAccessException { return method.invoke(target, args); }}Invocation定义了target、method、args属性，提供了proceed办法则是反射执行method办法
Plugin
org/apache/ibatis/plugin/Plugin.java
public class Plugin implements InvocationHandler { private final Object target; private final Interceptor interceptor; private final Map<Class<?>, Set<Method>> signatureMap; private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) { this.target = target; this.interceptor = interceptor; this.signatureMap = signatureMap; } public static Object wrap(Object target, Interceptor interceptor) { Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor); Class<?> type = target.getClass(); Class<?>[] interfaces = getAllInterfaces(type, signatureMap); if (interfaces.length > 0) { return Proxy.newProxyInstance(type.getClassLoader(), interfaces, new Plugin(target, interceptor, signatureMap)); } return target; } @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { Set<Method> methods = signatureMap.get(method.getDeclaringClass()); if (methods != null && methods.contains(method)) { return interceptor.intercept(new Invocation(target, method, args)); } return method.invoke(target, args); } catch (Exception e) { throw ExceptionUtil.unwrapThrowable(e); } } //......} Plugin实现了java.lang.reflect.InvocationHandler办法，其invoke办法次要是多了一层判断，判断interceptor的signatureMap有没有蕴含对应的办法，有则执行interceptor.intercept，同时包装了Invocation参数传递过来
而Plugin的wrap办法则是判断interceptor有没有拦挡target对应的接口，如果有则通过Proxy.newProxyInstance返回代理对象不便后续进行拦挡
InterceptorChain
org/apache/ibatis/plugin/InterceptorChain.java
public class InterceptorChain { private final List<Interceptor> interceptors = new ArrayList<>(); public Object pluginAll(Object target) { for (Interceptor interceptor : interceptors) { target = interceptor.plugin(target); } return target; } public void addInterceptor(Interceptor interceptor) { interceptors.add(interceptor); } public List<Interceptor> getInterceptors() { return Collections.unmodifiableList(interceptors); }}InterceptorChain定义了interceptors，它提供了pluginAll办法对target代理所有的interceptor
Configuration
org/apache/ibatis/session/Configuration.java
protected final InterceptorChain interceptorChain = new InterceptorChain(); public void addInterceptor(Interceptor interceptor) { interceptorChain.addInterceptor(interceptor); }Configuration定义了interceptorChain，它通过addInterceptor办法往interceptorChain增加interceptor
XMLConfigBuilder
org/apache/ibatis/builder/xml/XMLConfigBuilder.java
private void pluginElement(XNode parent) throws Exception { if (parent != null) { for (XNode child : parent.getChildren()) { String interceptor = child.getStringAttribute("interceptor"); Properties properties = child.getChildrenAsProperties(); Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).getDeclaredConstructor() .newInstance(); interceptorInstance.setProperties(properties); configuration.addInterceptor(interceptorInstance); } } }XMLConfigBuilder在解析xml的plugin的时候，会获取定义的interceptor，实例化之后通过configuration.addInterceptor增加进去
SqlSessionFactoryBean
org/mybatis/spring/SqlSessionFactoryBean.java
private Interceptor[] plugins; public void addPlugins(Interceptor... plugins) { setPlugins(appendArrays(this.plugins, plugins, Interceptor[]::new)); } public void setPlugins(Interceptor... plugins) { this.plugins = plugins; } protected SqlSessionFactory buildSqlSessionFactory() throws Exception { final Configuration targetConfiguration; XMLConfigBuilder xmlConfigBuilder = null; if (this.configuration != null) { targetConfiguration = this.configuration; if (targetConfiguration.getVariables() == null) { targetConfiguration.setVariables(this.configurationProperties); } else if (this.configurationProperties != null) { targetConfiguration.getVariables().putAll(this.configurationProperties); } } else if (this.configLocation != null) { xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties); targetConfiguration = xmlConfigBuilder.getConfiguration(); } else { LOGGER.debug( () -> "Property 'configuration' or 'configLocation' not specified, using default MyBatis Configuration"); targetConfiguration = new Configuration(); Optional.ofNullable(this.configurationProperties).ifPresent(targetConfiguration::setVariables); } Optional.ofNullable(this.objectFactory).ifPresent(targetConfiguration::setObjectFactory); Optional.ofNullable(this.objectWrapperFactory).ifPresent(targetConfiguration::setObjectWrapperFactory); Optional.ofNullable(this.vfs).ifPresent(targetConfiguration::setVfsImpl); //...... if (!isEmpty(this.plugins)) { Stream.of(this.plugins).forEach(plugin -> { targetConfiguration.addInterceptor(plugin); LOGGER.debug(() -> "Registered plugin: '" + plugin + "'"); }); } if (hasLength(this.typeHandlersPackage)) { scanClasses(this.typeHandlersPackage, TypeHandler.class).stream().filter(clazz -> !clazz.isAnonymousClass()) .filter(clazz -> !clazz.isInterface()).filter(clazz -> !Modifier.isAbstract(clazz.getModifiers())) .forEach(targetConfiguration.getTypeHandlerRegistry()::register); } if (!isEmpty(this.typeHandlers)) { Stream.of(this.typeHandlers).forEach(typeHandler -> { targetConfiguration.getTypeHandlerRegistry().register(typeHandler); LOGGER.debug(() -> "Registered type handler: '" + typeHandler + "'"); }); } targetConfiguration.setDefaultEnumTypeHandler(defaultEnumTypeHandler); //...... return this.sqlSessionFactoryBuilder.build(targetConfiguration); }SqlSessionFactoryBean的buildSqlSessionFactory办法在判断plugins不为空时，通过targetConfiguration.addInterceptor(plugin)将interceptor注册进去
MybatisAutoConfiguration
org/mybatis/spring/boot/autoconfigure/MybatisAutoConfiguration.java
@org.springframework.context.annotation.Configuration@ConditionalOnClass({ SqlSessionFactory.class, SqlSessionFactoryBean.class })@ConditionalOnSingleCandidate(DataSource.class)@EnableConfigurationProperties(MybatisProperties.class)@AutoConfigureAfter({ DataSourceAutoConfiguration.class, MybatisLanguageDriverAutoConfiguration.class })public class MybatisAutoConfiguration implements InitializingBean { private static final Logger logger = LoggerFactory.getLogger(MybatisAutoConfiguration.class); private final MybatisProperties properties; private final Interceptor[] interceptors; //...... public MybatisAutoConfiguration(MybatisProperties properties, ObjectProvider<Interceptor[]> interceptorsProvider, ObjectProvider<TypeHandler[]> typeHandlersProvider, ObjectProvider<LanguageDriver[]> languageDriversProvider, ResourceLoader resourceLoader, ObjectProvider<DatabaseIdProvider> databaseIdProvider, ObjectProvider<List<ConfigurationCustomizer>> configurationCustomizersProvider, ObjectProvider<List<SqlSessionFactoryBeanCustomizer>> sqlSessionFactoryBeanCustomizers) { this.properties = properties; this.interceptors = interceptorsProvider.getIfAvailable(); this.typeHandlers = typeHandlersProvider.getIfAvailable(); this.languageDrivers = languageDriversProvider.getIfAvailable(); this.resourceLoader = resourceLoader; this.databaseIdProvider = databaseIdProvider.getIfAvailable(); this.configurationCustomizers = configurationCustomizersProvider.getIfAvailable(); this.sqlSessionFactoryBeanCustomizers = sqlSessionFactoryBeanCustomizers.getIfAvailable(); } @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())); } applyConfiguration(factory); if (this.properties.getConfigurationProperties() != null) { factory.setConfigurationProperties(this.properties.getConfigurationProperties()); } if (!ObjectUtils.isEmpty(this.interceptors)) { factory.setPlugins(this.interceptors); } //...... } MybatisAutoConfiguration的sqlSessionFactory办法，在判断interceptors不为空时，通过SqlSessionFactory的setPlugins办法把interceptors增加进去；MybatisAutoConfiguration标注了@Configuration注解，该注解标注了@Component，因此这些interceptors则是通过结构器从spring中注入的
Configuration.pluginAll
org/apache/ibatis/session/Configuration.java
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) { ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql); return (ParameterHandler) interceptorChain.pluginAll(parameterHandler); } public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler, ResultHandler resultHandler, BoundSql boundSql) { ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds); return (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler); } public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql); return (StatementHandler) interceptorChain.pluginAll(statementHandler); } public Executor newExecutor(Transaction transaction, ExecutorType executorType) { executorType = executorType == null ? defaultExecutorType : executorType; Executor executor; if (ExecutorType.BATCH == executorType) { executor = new BatchExecutor(this, transaction); } else if (ExecutorType.REUSE == executorType) { executor = new ReuseExecutor(this, transaction); } else { executor = new SimpleExecutor(this, transaction); } if (cacheEnabled) { executor = new CachingExecutor(executor); } return (Executor) interceptorChain.pluginAll(executor); }Configuration提供了newParameterHandler、newResultSetHandler、newStatementHandler、newExecutor办法，这些办法会对ParameterHandler、ResultSetHandler、StatementHandler、Executor执行interceptorChain.pluginAll办法，则创立作用了所有interceptor的代理对象，从而实现对这些对象的拦挡成果
小结
- mybatis的Interceptor机制应用的是jdk的Proxy.newProxyInstance的形式
- 在扫描xml的时候把interceptor注册到configuration中，针对spring的场景，在MybatisAutoConfiguration中注入所有托管的interceptor，之后在结构SqlSessionFactory的时候把interceptor注册到configuration中
- 最初Configuration提供了newParameterHandler、newResultSetHandler、newStatementHandler、newExecutor办法，创立作用了所有interceptor的代理对象，从而实现对这些对象的拦挡成果