插件机制主要组件类：

org.apache.ibatis.plugin.Interceptor

org.apache.ibatis.plugin.InterceptorChain

org.apache.ibatis.executor.parameter.ParameterHandler

org.apache.ibatis.executor.resultset.ResultSetHandler

org.apache.ibatis.executor.statement.StatementHandler

org.apache.ibatis.executor.Executor

org.apache.ibatis.session.Configuration

org.apache.ibatis.plugin.Plugin

org.apache.ibatis.plugin.Intercepts

org.apache.ibatis.plugin.Signature

吐槽 ： mybatis 的插件机制乍一看是很啃爹的 ， 刚看到 Interceptor 这个接口的时候根本不知道该怎么玩 。然后看文档，文档上的列子写的比较详细了，看了之后知道该如何使用了，但是还是有不少疑问。这些疑问就只能通过看源码来找到答案了。

Configuration 中有4个地方用到了 InterceptorChain 的 pluginAll 方法：

newParameterHandler ； newResultSetHandler  ； newStatementHandler ； newExecutor ；

  public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, 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 = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
    resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
    return 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);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }


  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;
  }

public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
}

文档中拦截器的 plugin 方法实现是这样的 ：

public Object plugin(Object target) {
    return Plugin.wrap(target, this);
}

分析 org.apache.ibatis.plugin.Plugin 这个类 ：

wrap 方法 ： 如果传入的 interceptor 上有 @Intercepts ， @Signature 注解 ，target 有接口 ， 就为这个 target  生成一个代理对象 （使用的依旧是 jdk 的Proxy ）, 否则返回原来的对象 。

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;
}

getSignatureMap 方法 ： 作用就是获取到传入的拦截器类上注解 @Intercepts ， @Signature 中指定拦截的对象类型和需要进行拦截的方法。@Signature 的三个属性 type , method , args  就可以定位一个方法了。

  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;
}

getAllInterfaces 方法 ： 获取指定 type 的所有接口类型 ， 因为 jdk 的 Proxy 是基于接口的。

private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
    Set<Class<?>> interfaces = new HashSet<Class<?>>();
    while (type != null) {
      for (Class<?> c : type.getInterfaces()) {
        if (signatureMap.containsKey(c)) {
          interfaces.add(c);
        }
      }
      type = type.getSuperclass();
    }
    return interfaces.toArray(new Class<?>[interfaces.size()]);
}

org.apache.ibatis.plugin.Plugin 类是 java.lang.reflect.InvocationHandler 的实现类，这次 Plugin  在内部保存了被代理对象 target  。

invoke 方法 ： 每次调用的方法如果是 @Signature 注解指定的方法 ，就去在调用目标方法前先去调用拦截器的 intercept 方法 。

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);
    }
}