我们首先介绍一下Mybatis插件相关的几个类，并对源码进行了简单的分析。

Mybatis插件相关的接口或类有：Intercept、InterceptChain、Plugin和Invocation，这几个接口或类实现了整个Mybatis插件流程。

Interceptor：一个接口，是实现自己功能需要实现的接口

源码如下：

/**
 * @author Clinton Begin
 */
public interface Interceptor {

  //在此方法中实现自己需要的功能，最后执行invocation.proceed()方法，实际就是调用method.invoke(target, args)方法，调用代理类
  Object intercept(Invocation invocation) throws Throwable;

  //这个方法是将target生成代理类
  Object plugin(Object target);
  //在xml中注册Intercept是配置一些属性
  void setProperties(Properties properties);

}

InterceptorChain：有一个List<Interceptor> interceptors变量，来保存所有Interceptor的实现类

源码如下：

/**
 * @author Clinton Begin
 */
public class InterceptorChain {

  //插件拦截器链
  private final List<Interceptor> interceptors = new ArrayList<Interceptor>();

  //把target变成代理类，这样在运行target方法之前需要运行Plugin的invoke方法
  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);
  }

}

Invocation：一个比较简单的类，主要的功能就是根据构造函数类执行代理类

源码如下：

/**
 * @author Clinton Begin
 */
public class Invocation {

  private Object target;
  private Method method;
  private 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;
  }
  //其实mybatis的Interceptor最终还是调用的method.invoke方法
  public Object proceed() throws InvocationTargetException, IllegalAccessException {
    return method.invoke(target, args);
  }

}

Plugin：Mybatis插件的核心类，它实现了代理接口InvocationHandler，是一个代理类

源码详解如下：

/**
 * @author Clinton Begin
 */
//这个类是Mybatis拦截器的核心,大家可以看到该类继承了InvocationHandler
//又是JDK动态代理机制
public class Plugin implements InvocationHandler {

  //目标对象
  private Object target;
  //拦截器
  private Interceptor interceptor;
  //记录需要被拦截的类与方法
  private 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;
  }

  //一个静态方法,对一个目标对象进行包装，生成代理类。
  public static Object wrap(Object target, Interceptor interceptor) {
	//首先根据interceptor上面定义的注解 获取需要拦截的信息
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    Class<?> type = target.getClass();
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
	//如果长度为>0 则返回代理类 否则不做处理
    if (interfaces.length > 0) {
	  //创建JDK动态代理对象
      return Proxy.newProxyInstance(
          type.getClassLoader(),
          interfaces,
          new Plugin(target, interceptor, signatureMap));
    }
    return target;
  }

  //在执行Executor、ParameterHandler、ResultSetHandler和StatementHandler的实现类的方法时会调用这个方法
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
	  //通过method参数定义的类 去signatureMap当中查询需要拦截的方法集合
      Set<Method> methods = signatureMap.get(method.getDeclaringClass());
	  //判断是否是需要拦截的方法，如果需要拦截的话就执行实现的Interceptor的intercept方法，执行完之后还是会执行method.invoke方法，不过是放到interceptor实现类中去实现了
      if (methods != null && methods.contains(method)) {
        return interceptor.intercept(new Invocation(target, method, args));
      }
	  //不拦截 直接通过目标对象调用方法
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }
  //根据拦截器接口（Interceptor）实现类上面的注解获取相关信息
  private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
	//获取注解信息
    Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
    // issue #251
	//为空则抛出异常
    if (interceptsAnnotation == null) {
      throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
    }
	//获得Signature注解信息
    Signature[] sigs = interceptsAnnotation.value();
    Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
	//循环注解信息
    for (Signature sig : sigs) {
	  //根据Signature注解定义的type信息去signatureMap当中查询需要拦截方法的集合
      Set<Method> methods = signatureMap.get(sig.type());
	  //第一次肯定为null 就创建一个并放入signatureMap
      if (methods == null) {
        methods = new HashSet<Method>();
        signatureMap.put(sig.type(), methods);
      }
      try {
		//找到sig.type当中定义的方法 并加入到集合
        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;
  }
  //根据对象类型与signatureMap获取接口信息
  private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
    Set<Class<?>> interfaces = new HashSet<Class<?>>();
	//循环type类型的接口信息 如果该类型存在与signatureMap当中则加入到set当中去
    while (type != null) {
      for (Class<?> c : type.getInterfaces()) {
        if (signatureMap.containsKey(c)) {
          interfaces.add(c);
        }
      }
      type = type.getSuperclass();
    }
	//转换为数组返回
    return interfaces.toArray(new Class<?>[interfaces.size()]);
  }

}

对Mybatis插件有一定了解的人应该知道，插件拦截的类是Executor、ParameterHandler、ResultSetHandler和StatementHandler的实现类，为什么会这样呢？在Configuration类中看一下代码就明白了，可以看到在初始化Executor、ParameterHandler、ResultSetHandler和StatementHandler都会调用interceptorChain.pluginAll（）这个函数，其实这样之后各个接口的实现类就被代理类生成为目标类了。

 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) {
    return newExecutor(transaction, defaultExecutorType);
  }

  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;
  }