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本文主要研究一下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方法，其参数为Invocation类型，同时默认提供了plugin方法，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方法则是反射执行method方法

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方法，其invoke方法主要是多了一层判断，判断interceptor的signatureMap有没有包含对应的方法，有则执行interceptor.intercept，同时包装了Invocation参数传递过去
而Plugin的wrap方法则是判断interceptor有没有拦截target对应的接口，如果有则通过Proxy.newProxyInstance返回代理对象方便后续进行拦截

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方法对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方法往interceptorChain添加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添加进去

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方法在判断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方法，在判断interceptors不为空时，通过SqlSessionFactory的setPlugins方法把interceptors添加进去；MybatisAutoConfiguration标注了@Configuration注解，该注解标注了@Component，因而这些interceptors则是通过构造器从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方法，这些方法会对ParameterHandler、ResultSetHandler、StatementHandler、Executor执行interceptorChain.pluginAll方法，则创建作用了所有interceptor的代理对象，从而实现对这些对象的拦截效果

小结

• mybatis的Interceptor机制使用的是jdk的Proxy.newProxyInstance的方式
• 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，之后在构造SqlSessionFactory的时候把interceptor注册到configuration中
• 最后Configuration提供了newParameterHandler、newResultSetHandler、newStatementHandler、newExecutor方法，创建作用了所有interceptor的代理对象，从而实现对这些对象的拦截效果