Springbatch dynamic multiple xml File writer

I have to do a batch that :
read some data from DB (each row is an item, this is fine)
then do some process to add some more data (more data is always better ;) )
then here is my problem, I have to write each item in an xml file who's name depends of the item's data.

for exemple I have
ItemA (attr1=toto, attr2=foo, attr3=myNonKeyData...)=>goes in toto_foo.xml
ItemB (attr1=toto, attr2=foo, attr3=myNonKeyData...)=>goes in toto_foo.xml

ItemC (attr1=tata, attr2=foo...)=>goes in tata_foo.xml
...

I can't see how to do this with only one batch run once.
I have too much keys and possible output files to do a classifier.
Maybe using the partitioner could be a good idea even it seems not designed for that.

This is what I have understood.

• Arbitrary number of files.
• Process data from DB in a single pass - read once from DB and write to multiple files.
• One or more attributes of the item being written determine the name of the target file to write to.

Create an composite ItemWriter(inspired by org.springframework.batch.item.support.CompositeItemWriter). Here is some pseudocode

public class MyCompositeItemWriter<T> implements ItemStreamWriter<T> {

// The String key is the file-name(toto_foo.xml, tata_foo.xml etc)
//The map will be empty to start with as we do not know how many files will be created. 
private Map<String, ItemWriter<? super T>> delegates;

private boolean ignoreItemStream = false;

public void setIgnoreItemStream(boolean ignoreItemStream) {
    this.ignoreItemStream = ignoreItemStream;
}

@Override
public void write(List<? extends T> items) throws Exception {

    for(T item : items) {
        ItemWriter<? super T> writer = getItemWriterForItem(item);
        // Writing one item ata time might be inefficent. You can optimize this by grouping items by fileName. 
        writer.write(item);     
    }       
}


private getItemWriterForItem(T item) {
    String fileName = getFileNameForItem(item); 
    ItemWriter<? super T> writer = delegates.get(fileName);
    if(writer == null) {
        // There is no writer for the fileName. 
        //create one
        writer = createMyItemWriter(fileName);
        delegates.put(fileName, writer);
    }
    return writer;
}

ItemWriter<? super T> createMyItemWriter(String fileName) {
    // create the writer. Maybe a org.springframework.batch.item.xml.StaxEventItemWriter 
    // set the resource(fielName)
    //open the writer
}




// Identify the name of the target file - toto_foo.xml, tata_foo.xml etc
private String getFileNameForItem(Item item) {
    .....
}

@Override
public void close() throws ItemStreamException {
    for (ItemWriter<? super T> writer : delegates) {
        if (!ignoreItemStream && (writer instanceof ItemStream)) {
            ((ItemStream) writer).close();
        }
    }
}

@Override
public void open(ExecutionContext executionContext) throws ItemStreamException {
    // Do nothing as we do not have any writers to begin with. 
    // Writers will be created as needed. And will be opened after creation. 
}

@Override
public void update(ExecutionContext executionContext) throws ItemStreamException {
    for (ItemWriter<? super T> writer : delegates) {
        if (!ignoreItemStream && (writer instanceof ItemStream)) {
            ((ItemStream) writer).update(executionContext);
        }
    }
}