mybatis-插件详解

基本原理

使用了JDK动态代理，基于 interceptor 实现

public class Plugin implements InvocationHandler {private final Object target;private final Interceptor interceptor;private final Map<Class<?>, Set<Method>> signatureMap;@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {Set<Method> methods = signatureMap.get(method.getDeclaringClass());if (methods != null && methods.contains(method)) {//走插件return interceptor.intercept(new Invocation(target, method, args));}//正常查询return method.invoke(target, args);} catch (Exception e) {throw ExceptionUtil.unwrapThrowable(e);}}//为目标对象target创建代理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;}
}

看下Interceptor接口

public interface Interceptor {Object intercept(Invocation invocation) throws Throwable;//创建代理对象并返回default Object plugin(Object target) {return Plugin.wrap(target, this);}
}

mybatis是如何使用的

创建 MybatisAutoConfiguration 的时候进行注入

@Configuration
@ConditionalOnClass({ SqlSessionFactory.class, SqlSessionFactoryBean.class })
@ConditionalOnSingleCandidate(DataSource.class)
@EnableConfigurationProperties(MybatisProperties.class)
@AutoConfigureAfter({ DataSourceAutoConfiguration.class, MybatisLanguageDriverAutoConfiguration.class })
public class MybatisAutoConfiguration implements InitializingBean {private final MybatisProperties properties;private final Interceptor[] interceptors;private final TypeHandler[] typeHandlers;private final List<ConfigurationCustomizer> configurationCustomizers;private final List<SqlSessionFactoryBeanCustomizer> sqlSessionFactoryBeanCustomizers;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;//通过构造器进行注入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@ConditionalOnMissingBeanpublic SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {SqlSessionFactoryBean factory = new SqlSessionFactoryBean();factory.setDataSource(dataSource);...............//赋值到sqlSessionFactory中，当开启会话时，便可以进行拦截if (!ObjectUtils.isEmpty(this.interceptors)) {factory.setPlugins(this.interceptors);}//获取SqlSessionFactoryreturn factory.getObject();}
}

获取SqlSessionFactory时，Configuration初始化 interceptorChain

public class Configuration {//实例化protected final InterceptorChain interceptorChain = new InterceptorChain();public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);return parameterHandler;}public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,ResultHandler resultHandler, BoundSql boundSql) {resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);return resultSetHandler;}public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);return statementHandler;}public Executor newExecutor(Transaction transaction, ExecutorType executorType) {executor = (Executor) interceptorChain.pluginAll(executor);return executor;}
}

可见 ，插件作用于 ParameterHandler、ResultSetHandler、StatementHandler、Executor 这四种类型

拦截器责任链 InterceptorChain

public class InterceptorChain {private final List<Interceptor> interceptors = new ArrayList<>();//给指定对象应用所有的拦截器public Object pluginAll(Object target) {for (Interceptor interceptor : interceptors) {//有多少个拦截器，就创建多少个代理对象，拦截器不要太多，影响性能target = interceptor.plugin(target);}return target;}
}

最后，开启会话session的时候，就会进行增强

自定义插件

实现Interceptor接口，根据方法签名@Signature声明需要拦截的方法

@Intercepts({@Signature(type= Executor.class, //上面说的四种类型之一method = "query", //方法名称args = {MappedStatement.class ,Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class}), //方法参数@Signature(type= Executor.class,method = "query",args = {MappedStatement.class ,Object.class, RowBounds.class, ResultHandler.class})})
public class MyIntercepter  implements Interceptor {@Overridepublic Object intercept(Invocation invocation) throws Throwable {// 增强..................return invocation.proceed();}}

注册到容器中

@Configuration
public class MybatisConfig {@Beanpublic Interceptor myInterceptor() {return new MyIntercepter();}
}

使用场景：分页，字段加密、自动赋值，监控等等。