src/main/java/com/cedarsoftware/util/io/JsonWriter.java
private JsonClassWriterBase getCustomWriter(Class c)
    {
        JsonClassWriterBase writer = writerCache.get(c);
        if (writer == null)
        {
            writer = forceGetCustomWriter(c);
            writerCache.put(c, writer);
        }
        return writer == nullWriter ? null : writer;
    }
private JsonClassWriterBase getCustomWriter(Class c)
    {
        JsonClassWriterBase writer = writerCache.get(c);
        if (writer == null)
        {
            writer = forceGetCustomWriter(c);
            writerCache.put(c, writer);
        }
        return writer == nullWriter ? null : writer;
    }
Fetch the customer writer for the passed in Class.  If it is cached (already associated to the
passed in Class), return the same instance, otherwise, make a call to get the custom writer
and store that result.
@param c Class of object for which fetch a custom writer
@return JsonClassWriter/JsonClassWriterEx for the custom class (if one exists), null otherwise.