private SystemStreamPartitionMapper getSystemStreamPartitionMapper(Config config) {
    JobConfig jobConfig = new JobConfig(config);
    String systemStreamPartitionMapperClass = jobConfig.getSystemStreamPartitionMapperFactoryName();
    SystemStreamPartitionMapperFactory systemStreamPartitionMapperFactory =
        ReflectionUtil.getObj(systemStreamPartitionMapperClass, SystemStreamPartitionMapperFactory.class);
    return systemStreamPartitionMapperFactory.getStreamPartitionMapper(config, new MetricsRegistryMap());
  }
Creates a instance of {@link SystemStreamPartitionMapper} using the stream partition expansion factory class
defined in the {@param config}.
@param config the configuration of the samza job.
@return the instantiated {@link SystemStreamPartitionMapper} object.
samza-core/src/main/java/org/apache/samza/container/grouper/stream/SSPGrouperProxy.java