Mybatis 源码之Plugin类解析

public class Plugin implements InvocationHandler {

  private Object target; //目标对象
  private Interceptor interceptor;//拦截器对象
  private Map<Class<?>, Set<Method>> signatureMap;//目标对象方法签名

  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) {
  //从拦截器的注解中获取拦截的类名和方法信息
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    Class<?> type = target.getClass();
  //解析被拦截对象的所有接口
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    if (interfaces.length > 0) {
    //生成代理对象，Plugin对象为该代理对象的InvocationHandler
      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)) {
    //调用代理类所属拦截器的intercept方法
        return interceptor.intercept(new Invocation(target, method, args));
      }
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }

  private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
  //获得Interceptor注解，@Signature中的type(要拦截的类),method(拦截类的方法)和args(拦截器用于这些类中)
    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());      
    }
  //获得注解type,method 和args生成一个signature数组
    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 {
    //获得类的方法
        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;
  }
  //获得所有接口
  private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
    Set<Class<?>> interfaces = new HashSet<Class<?>>();
    while (type != null) {
    //获得接口
      for (Class<?> c : type.getInterfaces()) {
        if (signatureMap.containsKey(c)) {
          interfaces.add(c);
        }
      }
    //获得父类
      type = type.getSuperclass();
    }
  //返回一个接口的数组
    return interfaces.toArray(new Class<?>[interfaces.size()]);
  }

}