Multi line record reader using PeekableItemReader

MultiLineTradeItemReader

package com.trmsmy.spring.boot.batch.springbootbatch;


import java.util.ArrayList;
import java.util.List;

import org.springframework.batch.item.ExecutionContext;
import org.springframework.batch.item.ItemReader;
import org.springframework.batch.item.ItemStream;
import org.springframework.batch.item.ItemStreamException;
import org.springframework.batch.item.file.FlatFileItemReader;
import org.springframework.batch.item.support.SingleItemPeekableItemReader;
import org.springframework.util.Assert;
import org.springframework.util.StringUtils;
 
public class MultiLineTradeItemReader implements ItemReader<String>, ItemStream {
	private SingleItemPeekableItemReader<String> delegate;
 
	@Override
	public String read() throws Exception {
		List<String> record = null;

		for (String line; (line = this.delegate.read()) != null;) {
			
			if (isRecordStart(line)) {
				record = new ArrayList<>();
				record.add(line);
			}
			else {
				record.add(line);
				String nextLine = this.delegate.peek();
				
				if (nextLine == null || isRecordStart(nextLine)) {
					return StringUtils.collectionToDelimitedString(record, "\n");				
				}
				
			}
		}
		
		Assert.isNull(record, "No 'END' was found.");
		return null;
	}

	private boolean isRecordStart(String line) {
		return line != null && line.equals("START");
	}

	public void setDelegate(FlatFileItemReader<String> delegate) {
		this.delegate = new SingleItemPeekableItemReader<>();
		this.delegate.setDelegate(delegate);
	}

	@Override
	public void close() throws ItemStreamException {
		this.delegate.close();
	}

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

	@Override
	public void update(ExecutionContext executionContext) throws ItemStreamException {
		this.delegate.update(executionContext);
	}
}

This is so useful, thanks a lot!

great

Can you suggest how to use this to read a file (instead of or in conjunction with FlatFileTemReader)?

Can the read method return the collection and processor receive the same? Can you please share how the step configuration would look like. I am having hard time the generics.

My usecase needs the delegate to read a book(item) and group by authors and pass the collection of books of an author to processor and then build a different aggregation object from processor to be send to writer

Can this be used in MultiResourceItemReader?
I found it was thrown with this exception message Input resource must be set.