关于mybatis:聊聊mybatis的Interceptor机制

序

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