PCollection to Array -如何在WriteToText PTransform中动态输入头？

import org.apache.beam.sdk.Pipeline;
import org.apache.beam.sdk.io.TextIO;
import org.apache.beam.sdk.transforms.DoFn;
import org.apache.beam.sdk.transforms.ParDo;
import org.apache.beam.sdk.transforms.PTransform;
import org.apache.beam.sdk.values.PCollection;

public class WriteToTextWithHeader extends PTransform<PCollection<String>, PCollection<String>> {
    private final String header;

    public WriteToTextWithHeader(String header) {
        this.header = header;
    }

    @Override
    public PCollection<String> expand(PCollection<String> input) {
        PCollection<String> output = input.apply(TextIO.write().to("output.txt").withoutSharding());

        output = output.apply(ParDo.of(new AddHeaderFn(header)));

        return output;
    }

    private static class AddHeaderFn extends DoFn<String, String> {
        private final String header;

        public AddHeaderFn(String header) {
            this.header = header;
        }

        @ProcessElement
        public void processElement(ProcessContext c) {
            if (c.element().equals(c.element().trim())) {
                c.output(header);
            }
            c.output(c.element());
        }
    }

    public static void main(String[] args) {
        Pipeline pipeline = Pipeline.create();

        PCollection<String> data = pipeline.apply(TextIO.read().from("input.txt"));

        String header = "Column1,Column2,Column3"; // 头的内容

        data.apply(new WriteToTextWithHeader(header));

        pipeline.run().waitUntilFinish();
    }
}