public class DataChunk {
    private int pageNumber;
    private byte[] data;
}

public class FooWriter implements ItemWriter<DataChunk> {

    private DataChunkAggregator dataChunkAggregator;

    public void write(List<? extends DataChunk> dataChunks) throws Exception {
        dataChunks.stream().forEach(chunk -> dataChunkAggregator.addChunk(chunk.getPageNumber(), chunk.getData()));
    }
}

public class FooAggregator {
    private Map<int, byte> pagedData; // Key sorted implementation

    public void addChunk(int pageNumber, byte[] data) {
        pagedData.put(pageNumber, data)
    }

    public byte[] aggregate() {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        pagedData.values.stream().forEach(data -> baos.write(data));
        return baos.toByteArray();
    }
}

public class ReportWritingTasklet implements TaskLet {

    private ReportRepository reportRepository;
    private FooAggregator fooAggregator;

    public RepeatStatus execute(StepContribution contribution, ChunkContext context) {
        byte[] data = fooAggregator.aggregate();
        reportRepository.getOne(reportId).setDataBytes(data);
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans>
    <bean id=fooWriter class="FooWriter" scope="step"
        p:fooAggregator-ref="fooAggregator"/>

    <bean id="fooAggregator" class="FooAggregator"/>

    <bean id="reportWritingTasklet" class="ReportWritingTasklet" scope="step"
        p:fooAggregator-ref="fooAggregator"/>

    <batch:job id="fooJob">
        <batch:step id="generateReport" next="assembleReport">
            <batch:chunk reader="fooReader" processor="fooProcessor" writer="fooWriter"/>
        </batch:step>
        <batch:step id="assembleReport">
            <batch:tasklet class="ReportWritingTasklet"/>
        </batch:step>
     </batch:job>
</beans>

Caused by: java.lang.IllegalStateException: Cannot convert value of type    [com.sun.proxy.$Proxy98 implementing org.springframework.aop.scope.ScopedObject,java.io.Serializable,org.springframework.aop.framework.AopInfrastructureBean,org.springframework.aop.SpringProxy,org.springframework.aop.framework.Advised] to required type [FooAggregator] for property 'fooAggregator': no matching editors or conversion strategy found