Mybatis的核心实现Plugin、Interceptor

今天药忘吃喽~ 2021-12-23 03:51 142阅读 0赞

本章讲Interceptor其实在分享怎样的设计才是具备良好的拓展性的程序，注重内功修为的童靴可能更感兴趣。Mybatis中的插件允许你针对核心组件接口Executor 、StatementHandler、ParameterHandler、ResultSetHandler中任何一个方法进行拦截调用。而每个Interceptor（拦截的接口）其实是通过[JDK的动态代理技术][JDK]生成的代理类，每当执行这4种接口中的方法时，就会进入拦截方法（具体就是InvocationHandler的invoke()方法）。

关于Mybatis的系统架构及Executor、ParameterHandler、ResultSetHandler、StatementHandler可参阅[之前的相关的分享][Link 1]。

> **责任链模式**
> 
> 责任链模式是一种对象行为模式，插件用的是它，很多对象由每一个对象对其下家的引用而连接起来形成一条链，请求在这个链上传递，直到链上的某一个对象决定处理此请求。

### 用法 ###

用法一点也不复杂：只需实现 Interceptor 接口，并指定想要拦截的方法签名即可，更多用法可参阅[官方教程][Link 2]

除了用插件来修改 MyBatis 核心行为之外，还可以通过完全覆盖配置类来达到目的。只需继承后覆盖其中的每个方法，再把它传递到 SqlSessionFactoryBuilder.build(myConfig) 方法即可。再次重申，这可能会严重影响 MyBatis 的行为，务请慎之又慎。

<plugins>
      <plugin interceptor="org.mybatis.example.ExamplePlugin">
        <property name="someProperty" value="100"/>
      </plugin>
    </plugins>

//执行Executor.update时拦截
    @Intercepts({@Signature(type= Executor.class, method = "update", args = {MappedStatement.class,Object.class})})
    public class ExamplePlugin implements Interceptor {
      public Object intercept(Invocation invocation) throws Throwable {//干你想干的事情
        return invocation.proceed();
      }
      public Object plugin(Object target) {
        return Plugin.wrap(target, this);
      }
      public void setProperties(Properties properties) {
      }
    }

### 源码 ###

在上面的源码中Plugin.wrap()，它就是当前拦截器（ExamplePlugin）的代理类。而InterceptorChain.pluginAll()则定义了一个组件接口（比如：ParameterHandler）执行的责任链。

public interface Interceptor {
      //拦截器具体实现
      Object intercept(Invocation invocation) throws Throwable;
      //拦截器的代理类
      Object plugin(Object target);
      //添加属性
      void setProperties(Properties properties);
    }
    //源码Plugin.wrap其实是一个代理
    public class Plugin implements InvocationHandler {
      private Object target;
      private Interceptor interceptor;//拦截器
      private Map<Class<?>, Set<Method>> signatureMap;
      //为拦截器生成代理类 
      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) {//目标列的接口与拦截器匹配，则生成代理类
          return Proxy.newProxyInstance(
              type.getClassLoader(),
              interfaces,
              new Plugin(target, interceptor, signatureMap));
        }
        return target;//原生类返回
      }
      //代理类执行入口
      @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)) {//执行插件类的拦截规则
            return interceptor.intercept(new Invocation(target, method, args));
          }
          return method.invoke(target, args);
        } catch (Exception e) {
          throw ExceptionUtil.unwrapThrowable(e);
        }
      }
      //处理拦截器类上的各种注解
      private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
        Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
        // issue #251
        if (interceptsAnnotation == null) {
          throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());      
        }
        Signature[] sigs = interceptsAnnotation.value();
        Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
        for (Signature sig : sigs) {
          Set<Method> methods = signatureMap.get(sig.type());
          if (methods == null) {
            methods = new HashSet<Method>();
            signatureMap.put(sig.type(), methods);
          }
          try {
            Method method = sig.type().getMethod(sig.method(), sig.args());
            methods.add(method);
          } catch (NoSuchMethodException e) {
            throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
          }
        }
        return signatureMap;
      }
    }
    //拦截器的请求
    public class Invocation {
      private Object target;
      private Method method;
      private Object[] args;
      //调用原先真实方法(非代理)
      public Object proceed() throws InvocationTargetException, IllegalAccessException {
        return method.invoke(target, args);
      }
    }

> **拦截器链执行过程**
> 
> 拦截器代理类对象->拦截器->目标方法，即：InvocationHandler.invoke()->Interceptor.intercept()->Invocation.proceed()。

也就是说其实在执行每个Executor、ParameterHandler、ResultSetHandler、StatementHandler都会构建一个拦截器链，如果针对每个配置了plugin，一个sql执行下来会陆续经过四个拦截器链。

//拦截器管理类
    public class InterceptorChain {
      private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
      //构建责任链
      public Object pluginAll(Object target) {
        for (Interceptor interceptor : interceptors) {//存在拦截器则构建
          //有多个同类型的拦截器则构成链，将代理类作为target再成代理类
          target = interceptor.plugin(target);
        }
        return target;//可以没有拦截器
      }
      public void addInterceptor(Interceptor interceptor) {
        interceptors.add(interceptor);
      }
      public List<Interceptor> getInterceptors() {
        return Collections.unmodifiableList(interceptors);
      }
    }
    //重要的配置类
    public class Configuration {
      protected final InterceptorChain interceptorChain = new InterceptorChain();
      //有匹配的拦截器则构建责任链，没有则不生成
      public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
        ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
        parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
        return parameterHandler;
      }
      //同newParameterHandler
      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);
        resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
        return resultSetHandler;
      }
      //同newParameterHandler
      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);
        statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
        return statementHandler;
      }
    
      public Executor newExecutor(Transaction transaction) {
        return newExecutor(transaction, defaultExecutorType);
      }
      //同newParameterHandler
      public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
        executorType = executorType == null ? defaultExecutorType : executorType;
        executorType = executorType == null ? ExecutorType.SIMPLE : 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);
        }
        executor = (Executor) interceptorChain.pluginAll(executor);
        return executor;
      }
    }
    //负责解析Mybatis 全局配置文件
    public class XMLConfigBuilder extends BaseBuilder {
      private void pluginElement(XNode parent) throws Exception {//解析interceptor
        if (parent != null) {
          for (XNode child : parent.getChildren()) {
            String interceptor = child.getStringAttribute("interceptor");
            Properties properties = child.getChildrenAsProperties();
            Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
            interceptorInstance.setProperties(properties);
            configuration.addInterceptor(interceptorInstance);
          }
        }
      }
    }
    public class SimpleExecutor extends BaseExecutor {
      @Override
      public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
        Statement stmt = null;
        try {
          Configuration configuration = ms.getConfiguration();
          //生成责任链
          StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
          stmt = prepareStatement(handler, ms.getStatementLog());
          return handler.<E>query(stmt, resultHandler);//可拦截query()
        } finally {
          closeStatement(stmt);
        }
      }
      private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
        Statement stmt;
        Connection connection = getConnection(statementLog);
        stmt = handler.prepare(connection, transaction.getTimeout());//可拦截prepare()
        handler.parameterize(stmt);//可拦截parameterize()
        return stmt;
      }
    }

mybatis用拦截器的设计非常巧妙的利用了jdk的动态代理，其实复杂度并不高。![Link 1][]

[JDK]: https://blog.csdn.net/alex_xfboy/article/details/88966201
[Link 1]: /images/20211223/94805cf46351432cb3d5e87bd32222e9.png
[Link 2]: http://www.mybatis.org/mybatis-3/zh/configuration.html#plugins