Spring Batch Writer to write Map<Key,Values> to file - spring

Question

I am using the Spring batch to develop CSV feed files. I had used a writer similar to the one given below to generate my output file.
#Bean
public FlatFileItemWriter<CustomObj> writer()
{
BeanWrapperFieldExtractor<CustomObj> extractor = new BeanWrapperFieldExtractor<>();
extractor.setNames(new String[] {"name", "emailAddress", "dob"});
DelimitedLineAggregator<CustomObj> lineAggregator = new DelimitedLineAggregator<>();
lineAggregator.setDelimiter(";");
FieldExtractor<CustomObj> fieldExtractor = createStudentFieldExtractor();
lineAggregator.setFieldExtractor(fieldExtractor);
FlatFileItemWriter<CustomObj> writer = new FlatFileItemWriter<>();
writer.setResource(outputResource);
writer.setAppendAllowed(true);
//writer.setHeaderCallback(headerCallback);
writer.setLineAggregator(lineAggregator);
return writer;
}
output
name;emailAddress;dob
abc;abc#xyz.com;10-10-20
But now we have a requirement to make this writer generic such that we no longer pass the object, instead pass a Map<String, String> and the object values are now stored in the Map of key Value pairs
Eg: name-> abc , emailAddress->abc#xyz.com, dob -> 10-10-20
We tried to use the writer similar to the below one,
But the problem here is that as there is no FieldExtractor set and thus the header and the values may become out of sync.
The PassThroughFieldExtractor just passes all the values in the collection(Map) in any order. even if the Map contains more fields, it prints all the fields.
Header and the values are not bound together in this case.
Is there any way to implement a custom field extractor which will make sure even if we change the ordering of the header, the ordering of the values remain consistent with the header.
#Bean
public FlatFileItemWriter<Map<String,String>> writer()
{
DelimitedLineAggregator<Map<String,String>> lineAggregator = new DelimitedLineAggregator<>();
lineAggregator.setDelimiter(";");
FieldExtractor<Map<String,String>> fieldExtractor = createStudentFieldExtractor();
lineAggregator.setFieldExtractor(new PassThroughFieldExtractor<>());
FlatFileItemWriter<Map<String,String>> writer = new FlatFileItemWriter<>();
writer.setResource(outputResource);
writer.setAppendAllowed(true);
writer.setLineAggregator(lineAggregator);
return writer;
}
output
name;emailAddress;dob
abc#xyz.com;abc;10-10-20
expected Output
case 1:
name;emailAddress;dob
abc;abc#xyz.com;10-10-20
case 2:
emailAddress;dob
abc#xyz.com;10-10-20

Answer 1

you need a custom fieldset extractor that extracts values from the map in the same order as the headers. Spring Batch does not provides such an extractor so you need to implement it yourself. For example, you can pass the headers at construction time to the extractor, and extract values from the map according to the headers order.