submit form to convert to a Java Bean model.
实体类中无需构造函数.
Since we haven’t specified a constructor, Java will provide a default constructor that will set all instance fields to their null values.
未找到对应set方法的会被设置成null.因此尽量使用包装类.比如boolean的包装类Boolean会被set成null.
非包装类会自动进行类型转换.