I know how to use a custom serializer in Jackson (by extending `JsonSerializer`), but I want the default serializer to work for all fields, except for just 1 field, which I want to override using the custom serializer. 

Annotations are not an option, because I am serializing a generated class (from Thrift). 

How do I specify only certain fields to be overridden when writing a custom jackson serializer?

*Update:*

Here's the class I want to serialize:

```
class Student {
    int age;
    String firstName;
    String lastName;
    double average;
    int numSubjects

    // .. more such properties ...
}

```

The above class has many properies, most of which use native types. I want to just override a few properties in the custom serializer and let Jackson deal with the rest as usual. For e.g. I just want to convert the "age" field to a custom output.