Spring batch DB to JSON files

Using a chunk-oriented tasklet won't work, because there will be a single item writer on which the resource is set upfront and will be fixed during the entire step. Using a composite item writer might work but you need to know how many distinct writers to create and configure upfront.

The most straightforward option I see is to use a tasklet, something like:

import java.util.Collections;
import java.util.HashMap;

import javax.sql.DataSource;

import org.springframework.batch.core.Job;
import org.springframework.batch.core.JobParameters;
import org.springframework.batch.core.StepContribution;
import org.springframework.batch.core.configuration.annotation.EnableBatchProcessing;
import org.springframework.batch.core.configuration.annotation.JobBuilderFactory;
import org.springframework.batch.core.configuration.annotation.StepBuilderFactory;
import org.springframework.batch.core.launch.JobLauncher;
import org.springframework.batch.core.scope.context.ChunkContext;
import org.springframework.batch.core.step.tasklet.Tasklet;
import org.springframework.batch.item.ExecutionContext;
import org.springframework.batch.item.database.JdbcPagingItemReader;
import org.springframework.batch.item.database.Order;
import org.springframework.batch.item.database.builder.JdbcPagingItemReaderBuilder;
import org.springframework.batch.item.file.FlatFileItemWriter;
import org.springframework.batch.item.file.builder.FlatFileItemWriterBuilder;
import org.springframework.batch.repeat.RepeatStatus;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.io.FileSystemResource;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.jdbc.datasource.embedded.EmbeddedDatabase;
import org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseBuilder;
import org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType;

@Configuration
@EnableBatchProcessing
public class MyJob {

    @Bean
    public JdbcPagingItemReader<Person> itemReader() {
        return new JdbcPagingItemReaderBuilder<Person>()
                .name("personItemReader")
                .dataSource(dataSource())
                .beanRowMapper(Person.class)
                .selectClause("select *")
                .fromClause("from person")
                .sortKeys(new HashMap<String, Order>() {{ put("id", Order.DESCENDING);}})
                .build();
    }

    @Bean
    public Job job(JobBuilderFactory jobs, StepBuilderFactory steps) {
        return jobs.get("job")
                .start(steps.get("step")
                        .tasklet(new MyTasklet(itemReader()))
                        .build())
                .build();
    }
    
    private static class MyTasklet implements Tasklet {

        private boolean readerInitialized;
        private JdbcPagingItemReader<Person> itemReader;

        public MyTasklet(JdbcPagingItemReader<Person> itemReader) {
            this.itemReader = itemReader;
        }

        @Override
        public RepeatStatus execute(StepContribution contribution, ChunkContext chunkContext) throws Exception {
            ExecutionContext executionContext = chunkContext.getStepContext().getStepExecution().getExecutionContext();
            if (!readerInitialized) {
                itemReader.open(executionContext);
                readerInitialized = true;
            }
            Person person = itemReader.read();
            if (person == null) {
                itemReader.close();
                return RepeatStatus.FINISHED;
            }
            // process the item
            process(person);
            // write the item in its own file (dynamically generated at runtime)
            write(person, executionContext);
            // save current state in execution context: in case of restart after failure, the job would resume where it left off.
            itemReader.update(executionContext);
            return RepeatStatus.CONTINUABLE;
        }

        private void process(Person person) {
            // do something with the item
        }
        
        private void write(Person person, ExecutionContext executionContext) throws Exception {
            FlatFileItemWriter<Person> itemWriter = new FlatFileItemWriterBuilder<Person>()
                    .resource(new FileSystemResource("person" + person.getId() + ".csv"))
                    .name("personItemWriter")
                    .delimited()
                    .names("id", "name")
                    .build();
            itemWriter.open(executionContext);
            itemWriter.write(Collections.singletonList(person));
            itemWriter.close();
        }
        
    }

    public static void main(String[] args) throws Exception {
        ApplicationContext context = new AnnotationConfigApplicationContext(MyJob.class);
        JobLauncher jobLauncher = context.getBean(JobLauncher.class);
        Job job = context.getBean(Job.class);
        jobLauncher.run(job, new JobParameters());
    }

    @Bean
    public DataSource dataSource() {
        EmbeddedDatabase embeddedDatabase = new EmbeddedDatabaseBuilder()
                .setType(EmbeddedDatabaseType.H2)
                .addScript("/org/springframework/batch/core/schema-drop-h2.sql")
                .addScript("/org/springframework/batch/core/schema-h2.sql")
                .build();
        JdbcTemplate jdbcTemplate = new JdbcTemplate(embeddedDatabase);
        jdbcTemplate.execute("create table person (id int primary key, name varchar(20));");
        for (int i = 1; i <= 10; i++) {
            jdbcTemplate.execute(String.format("insert into person values (%s, 'foo%s');", i, i));
        }
        return embeddedDatabase;
    }

    static class Person {
        private int id;
        private String name;

        public Person() {
        }

        public int getId() {
            return id;
        }

        public void setId(int id) {
            this.id = id;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String toString() {
            return "Person{id=" + id + ", name='" + name + '\'' + '}';
        }
    }

}

This example reads 10 persons from a db table and generates 10 csv files (person1.csv, person2.csv, etc)