Spring Batch: Writing data to multiple files with dynamic File Name

I'm not sure but I don't think there is an easy way of obtaining this. You could try to build your own ItemWriter like this:

public class DynamicItemWriter  implements ItemStream, ItemWriter<YourEntry> {

    private Map<String, FlatFileItemWriter<YourEntry>> writers = new HashMap<>();

    private LineAggregator<YourEntry> lineAggregator;

    private ExecutionContext executionContext;

    @Override
    public void open(ExecutionContext executionContext) throws ItemStreamException {
        this.executionContext = executionContext;
    }

    @Override
    public void update(ExecutionContext executionContext) throws ItemStreamException {
    }

    @Override
    public void close() throws ItemStreamException {
        for(FlatFileItemWriter f:writers.values()){
            f.close();
        }
    }

    @Override
    public void write(List<? extends YourEntry> items) throws Exception {
        for (YourEntry item : items) {
            FlatFileItemWriter<YourEntry> ffiw = getFlatFileItemWriter(item);
            ffiw.write(Arrays.asList(item));
        }
    }

    public LineAggregator<YourEntry> getLineAggregator() {
        return lineAggregator;
    }

    public void setLineAggregator(LineAggregator<YourEntry> lineAggregator) {
        this.lineAggregator = lineAggregator;
    }


    public FlatFileItemWriter<YourEntry> getFlatFileItemWriter(YourEntry item) {
        String key = item.FileName();
        FlatFileItemWriter<YourEntry> rr = writers.get(key);
        if(rr == null){
            rr = new FlatFileItemWriter<>();
            rr.setLineAggregator(lineAggregator);
            try {
                UrlResource resource = new UrlResource("file:"+key);
                rr.setResource(resource);
                rr.open(executionContext);
            } catch (MalformedURLException e) {
                e.printStackTrace();
            }
            writers.put(key, rr);
            //rr.afterPropertiesSet();
        }
        return rr;
    }
}

and configure it as a writer:

<bean id="csvWriter" class="com....DynamicItemWriter">
        <property name="lineAggregator">
        <bean
         class="org.springframework.batch.item.file.transform.DelimitedLineAggregator">
            <property name="delimiter" value=","/>
            <property name="fieldExtractor" ref="csvFieldExtractor"/>
        </bean>
    </property>

In spring-batch, you can do this using ClassifierCompositeItemWriter.

Since ClassifierCompositeItemWriter gives you access to your object during write, you can write custom logic to instruct spring to write to different files.

Take a look at below sample. The ClassifierCompositeItemWriter needs an implementation of Classifier interface. Below you can see that I have created a lambda where I am implementing the classify() method of the Classifier interface. The classify() method is where you will create your ItemWriter. In our example below, we have created a FlatFileItemWriter which gets the name of the file from the item itself and then creates a resource for that.

@Bean
public ClassifierCompositeItemWriter<YourDataObject> yourDataObjectItemWriter(
    Classifier<YourDataObject, ItemWriter<? super YourDataObject>> itemWriterClassifier
) {
  ClassifierCompositeItemWriter<YourDataObject> compositeItemWriter = new ClassifierCompositeItemWriter<>();
  compositeItemWriter.setClassifier(itemWriterClassifier);
  return compositeItemWriter;
}

@Bean
public Classifier<YourDataObject, ItemWriter<? super YourDataObject>> itemWriterClassifier() {
  return yourDataObject -> {
    String fileName = yourDataObject.getFileName();

    BeanWrapperFieldExtractor<YourDataObject> fieldExtractor = new BeanWrapperFieldExtractor<>();
    fieldExtractor.setNames(new String[]{"recId", "name"});
    DelimitedLineAggregator<YourDataObject> lineAggregator = new DelimitedLineAggregator<>();
    lineAggregator.setFieldExtractor(fieldExtractor);

    FlatFileItemWriter<YourDataObject> itemWriter = new FlatFileItemWriter<>();
    itemWriter.setResource(new FileSystemResource(fileName));
    itemWriter.setAppendAllowed(true);
    itemWriter.setLineAggregator(lineAggregator);
    itemWriter.setHeaderCallback(writer -> writer.write("REC_ID,NAME"));

    itemWriter.open(new ExecutionContext());
    return itemWriter;
  };
}

Finally, you can attach your ClassifierCompositeItemWriter in your batch step like you normally attach your ItemWriter.

@Bean
public Step myCustomStep(
    StepBuilderFactory stepBuilderFactory
) {
  return stepBuilderFactory.get("myCustomStep")
      .<?, ?>chunk(1000)
      .reader(myCustomReader())
      .writer(yourDataObjectItemWriter(itemWriterClassifier(null)))
      .build();
}

NOTE: As pointed out in comments by @Ping, a new writer will be created for each chunk, which is usually a bad practice and not an optimal solution. A better solution would be to maintain a hashmap of filename and writer so that you can reuse the writer.