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15.3 Deep Dive: ItemReader, ItemProcessor, ItemWriter

1. FlatFileItemReader — CSV File Processing​

@Bean
public FlatFileItemReader<MemberCsvDto> csvMemberReader() {
    return new FlatFileItemReaderBuilder<MemberCsvDto>()
            .name("csvMemberReader")
            .resource(new ClassPathResource("data/members.csv"))
            .delimited()
            .names("name", "email", "age")            // Map CSV columns to field names
            .targetType(MemberCsvDto.class)
            .linesToSkip(1)                           // Skip header row
            .build();
}

2. ItemProcessor — Transformation and Filtering​

ItemProcessor transforms each item from input to output type. Returning null filters out that item — it won't be passed to the Writer.

@Bean
public ItemProcessor<MemberCsvDto, Member> memberCsvToEntityProcessor() {
    return csvDto -> {
        // Filter out minors
        if (csvDto.getAge() < 18) {
            return null; // null = filtered out, not passed to writer
        }
        return Member.builder()
                .name(csvDto.getName())
                .email(csvDto.getEmail())
                .age(csvDto.getAge())
                .build();
    };
}

3. CompositeItemProcessor — Chaining Processors​

Use this when you want to chain multiple transformation/validation steps in order.

@Bean
public CompositeItemProcessor<MemberCsvDto, Member> compositeProcessor() {
    CompositeItemProcessor<MemberCsvDto, Member> processor = new CompositeItemProcessor<>();
    processor.setDelegates(List.of(
        new AgeFilterProcessor(),      // Step 1: Age filter
        new EmailValidateProcessor(),  // Step 2: Email format validation
        new CsvToEntityProcessor()     // Step 3: Final entity transformation
    ));
    return processor;
}

4. ItemWriteListener — Post-Write Success/Failure Handling​

@Component
public class BatchWriteListener implements ItemWriteListener<Member> {

    @Override
    public void afterWrite(Chunk<? extends Member> items) {
        log.info("Successfully saved {} items", items.size());
    }

    @Override
    public void onWriteError(Exception exception, Chunk<? extends Member> items) {
        log.error("Write failed! Failed items: {}", items);
        // Send Slack alert or push to a Dead Letter Queue
    }
}
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