MyBatis-ResultHandler

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四大金刚-ResultHandler

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public interface ResultHandler<T> {

  void handleResult(ResultContext<? extends T> resultContext);

}

// 其中一个默认实现
public class DefaultResultHandler implements ResultHandler<Object> {

  private final List<Object> list;

  public DefaultResultHandler() {
    list = new ArrayList<Object>();
  }

  @SuppressWarnings("unchecked")
  public DefaultResultHandler(ObjectFactory objectFactory) {
    list = objectFactory.create(List.class);//核心
  }

  @Override
  public void handleResult(ResultContext<? extends Object> context) {
    list.add(context.getResultObject());
  }

  public List<Object> getResultList() {
    return list;
  }

}

// 核心方法片段
  @Override
  public <T> T create(Class<T> type, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
    Class<?> classToCreate = resolveInterface(type);
    // we know types are assignable
    return (T) instantiateClass(classToCreate, constructorArgTypes, constructorArgs);
  }
  //实质方法，利用反射生成对象
  <T> T instantiateClass(Class<T> type, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
    try {
      Constructor<T> constructor;
      if (constructorArgTypes == null || constructorArgs == null) {
        constructor = type.getDeclaredConstructor();
        if (!constructor.isAccessible()) {
          constructor.setAccessible(true);
        }
        return constructor.newInstance();
      }
      constructor = type.getDeclaredConstructor(constructorArgTypes.toArray(new Class[constructorArgTypes.size()]));
      if (!constructor.isAccessible()) {
        constructor.setAccessible(true);
      }
      return constructor.newInstance(constructorArgs.toArray(new Object[constructorArgs.size()]));
    } catch (Exception e) {
      StringBuilder argTypes = new StringBuilder();
      if (constructorArgTypes != null && !constructorArgTypes.isEmpty()) {
        for (Class<?> argType : constructorArgTypes) {
          argTypes.append(argType.getSimpleName());
          argTypes.append(",");
        }
        argTypes.deleteCharAt(argTypes.length() - 1); // remove trailing ,
      }
      StringBuilder argValues = new StringBuilder();
      if (constructorArgs != null && !constructorArgs.isEmpty()) {
        for (Object argValue : constructorArgs) {
          argValues.append(String.valueOf(argValue));
          argValues.append(",");
        }
        argValues.deleteCharAt(argValues.length() - 1); // remove trailing ,
      }
      throw new ReflectionException("Error instantiating " + type + " with invalid types (" + argTypes + ") or values (" + argValues + "). Cause: " + e, e);
    }
  }