下面的語句中，如果throw t1 換成 throw t 就會有編譯錯誤，說是must be caught or declared to be thrown，為啥？

    try{
        FileInputStream fis = new FileInputStream(file);
        Throwable t = null;
        try{
            fis.read();
        }catch (Throwable t1){
            t = t1;
            throw t1;
        } finally{
            ...
        }
    } catch(Exception e){
        ...
    }