Write out headers even if no data rows written

[ansell]

If a schema is setup for a CSV writer and headers are setup to be used, the headers should be written when the writer is closed even if no data rows have been written.

Currently (unless I am using the library incorrectly), the header line is not written out unless there is at least one data row written out. I am not sure if this is relevant here or in another issue tracker such as databind.

My test code that I have been using to verify is:

List<String> headers = Arrays.asList("TestHeader1", "TestHeader2");
List<List<String>> dataSource = Arrays.asList();
// Or alternatively,
// List<List<String>> dataSource = Arrays.asList(Arrays.asList("TestValue1", "TestValue2"));
java.io.Writer writer = new StringWriter();
CsvSchema.Builder builder = CsvSchema.builder();
for (String nextHeader : headers) {
    builder = builder.addColumn(nextHeader);
}
CsvSchema schema = builder.setUseHeader(true).build();
try(SequenceWriter csvWriter = new CsvMapper().writerWithDefaultPrettyPrinter().with(schema).forType(List.class).writeValues(writer);) {
    for(List<String> nextRow : dataSource) {
        csvWriter.write(nextRow);
    }
    // Check to see whether dataSource is empty 
    // and if so write a single empty list to trigger header output
    if(dataSource.isEmpty()) {
        csvWriter.write(Arrays.asList());
    }
}
System.out.println(writer.toString());

If dataSource contains a single row, there are two lines sent to the Writer, but if it contains zero rows (as happens sometimes), I would like one line to be sent to the writer. One way as shown above is to send an empty List if we know that nothing was sent, but it would be simpler to use if that check were done internally by Jackson if it fits within the architecture.

[cowtowncoder]

This is the right place to report this. I agree in that it'd be better to force output; current behavior stems from writing of the headers being triggered by code that starts a new line (via JsonToken.START_OBJECT, I think), and not something that was designed.

The challenge is mostly that of figuring out what to use as a trigger instead: construction of a SequenceWriter (or underlying JsonGenerator) is not enough due to some problems with setup (that is, figuring when everything is completely ready). One possibility would be ensure header has been written when CsvGenerator.close() gets called; that might be a reasonable way.

[lukas0krupa]

I am using 2.8.1 and it seems to be an issue (unless I am doing something wrong). If stream is empty, I got empty file - no headers.

   private void saveData(
            final Supplier<OutputStream> responseOutputSupplier,
            final Stream<List<String>> lines) {
        try (final OutputStreamWriter osw = new OutputStreamWriter(responseOutputSupplier.get(), StandardCharsets.ISO_8859_1);
             final SequenceWriter sequenceWriter = MAPPER.writer(csvSchema()).writeValues(osw)) {
            lines.forEach(line -> {
                try {
                    sequenceWriter.write(line);
                } catch (IOException e) {
                    log.error("Unable to write to the file", e);
                }
            });
            sequenceWriter.writeAll(emptyList());
        } catch (IOException e) {
            log.error("Unable to write to the file", e);
        }
   }

   private CsvSchema csvSchema() {
        final CsvSchema.Builder builder = new CsvSchema.Builder();
        builder.setUseHeader(true);
        builder.addColumn("column1");
        builder.addColumn("column2");
        return builder.build();
   }

[cowtowncoder]

@lukas0krupa Make sure to use a later version, 2.8.3. We keep multiple branches open, so 2.7.7 was actually released after 2.8.1, so fix has gone in either 2.8.2 or 2.8.3.