CompositeItemStreamReader wraps multiple ItemStreamReaders and reads them simultaneously.

CompositeItemStreamReader.java

import java.util.ArrayList;
import java.util.List;
import org.springframework.batch.item.ExecutionContext;
import org.springframework.batch.item.ItemStream;
import org.springframework.batch.item.ItemStreamException;
import org.springframework.batch.item.ItemStreamReader;
import org.springframework.batch.item.NonTransientResourceException;
import org.springframework.batch.item.ParseException;
import org.springframework.batch.item.UnexpectedInputException;

/**
 * Copies the CompositeItemStream implementation, wraps its registered readers 
 * under one hood, so all read operations are simultaneously. 
 * Is not threadsafe, at least not on purpose.
 * 
 * @author Michael R. Lange <[email protected]>
 */
public class CompositeItemStreamReader<T> implements ItemStreamReader<T> {

    /** Registered ItemStreamReaders. */
    private List<ItemStreamReader<?>> itemReaderStreams;
    /** Mandatory Unifying Mapper Implementation. */
    private UnifyingItemsMapper<T> unifyingMapper;

    @Override
    public T read() throws Exception, UnexpectedInputException, ParseException, NonTransientResourceException {
        // read from all registered readers
        List items = new ArrayList();
        for (ItemStreamReader<?> itemReaderStream : itemReaderStreams) {
            items.add(itemReaderStream.read());
        }
        // delegate to mapper
        return unifyingMapper.mapItems(items);
    }

    /**
     * Broadcast the call to update to all registered readers.
     * 
     * @see org.springframework.batch.item.ItemStream#update(ExecutionContext)
     */
    @Override
    public void update(ExecutionContext executionContext) {
        for (ItemStream itemStream : itemReaderStreams) {
            itemStream.update(executionContext);
        }
    }

    /**
     * Broadcast the call to close to all registered readers.
     * 
     * @throws ItemStreamException
     */
    @Override
    public void close() throws ItemStreamException {
        for (ItemStream itemStream : itemReaderStreams) {
            itemStream.close();
        }
    }

    /**
     * Broadcast the call to open to all registered readers.
     * 
     * @throws ItemStreamException
     */
    @Override
    public void open(ExecutionContext executionContext) throws ItemStreamException {
        for (ItemStream itemStream : itemReaderStreams) {
            itemStream.open(executionContext);
        }
    }

    public void setUnifyingMapper(UnifyingItemsMapper<T> mapper) {
        this.unifyingMapper = mapper;
    }

    /**
     * Register ItemStreamReaders.
     *
     * @param itemReaderStreams 
     */
    public void setItemReaderStreams(List<ItemStreamReader<?>> itemReaderStreams) {
        this.itemReaderStreams = itemReaderStreams;
    }
}