Spring Batch — ItemReader

ItemReader — Strategy interface for providing the data. Implementations are expected to be stateful and will be called multiple times for each batch, with each call to read() returning a different value and finally returning null when all input data is exhausted. Implementations need not be thread-safe and clients of a ItemReader need to be aware that this is the case. A richer interface (e.g. with a look ahead or peek) is not feasible because we need to support transactions in an asynchronous batch.

SimpleItemReader.java

public class SimpleItemReader implements ItemReader<String> {
   
   private List<String> dataSet = new ArrayList<>();
   
   private Iterator<String> iterator;
   
   public SimpleItemReader() {
      this.dataSet.add("1");
      this.dataSet.add("2");
      this.dataSet.add("3");
      this.dataSet.add("4");
      this.dataSet.add("5");
      this.iterator = this.dataSet.iterator();
   }

   @Override
   public String read() throws Exception, UnexpectedInputException, ParseException, NonTransientResourceException {
      return iterator.hasNext() ? iterator.next():null; 
   }
}

MainApp.java

@SpringBootApplication
@EnableBatchProcessing
public class MainBatchApplication {
   
   @Autowired
   public JobBuilderFactory jobBuilderFactory;
   
   @Autowired
   public StepBuilderFactory stepBuilderFactory;

   @Bean
   public ItemReader<String> itemReader() {
      return new SimpleItemReader(); 
   }
   
   @Bean
   public Step chunkBasedStep() {
      return this.stepBuilderFactory.get("chunkBasedStep")
            .<String,String>chunk(3)
            .reader(itemReader())
            .writer(getWriter())
            .build();
   }

   private ItemWriter<String> getWriter() {
      return items -> {
         System.out.println(String.format("Received list of size: %s", items.size()));
         items.forEach(System.out::println);
      };
   }


   @Bean
   public Job job() {
      return this.jobBuilderFactory.get("job")
            .start(chunkBasedStep())
            .build();
   }

   public static void main(String[] args) {
      SpringApplication.run(MainBatchApplication.class, args);
   }
}

Console

.   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
            ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
            \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
            '  |____| .__|_| |_|_| |_\__, | / / / /
            =========|_|==============|___/=/_/_/_/
            :: Spring Boot ::                (v2.5.2)

            2021-07-18 00:04:17.611  INFO 16664 --- [           main] com.example.demo.DemoApplication         : Starting DemoApplication using Java 15.0.1 on DESKTOP-NQ639DU with PID 16664 (C:\Users\pc\Downloads\demo\demo\target\classes started by pc in C:\Users\pc\Downloads\demo\demo)
            2021-07-18 00:04:17.615  INFO 16664 --- [           main] com.example.demo.DemoApplication         : No active profile set, falling back to default profiles: default
2021-07-18 00:04:20.210  INFO 16664 --- [           main] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Starting...
            2021-07-18 00:04:20.920  INFO 16664 --- [           main] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Start completed.
            2021-07-18 00:04:21.265  INFO 16664 --- [           main] o.s.b.c.r.s.JobRepositoryFactoryBean     : No database type set, using meta data indicating: H2
2021-07-18 00:04:21.621  INFO 16664 --- [           main] o.s.b.c.l.support.SimpleJobLauncher      : No TaskExecutor has been set, defaulting to synchronous executor.
            2021-07-18 00:04:21.868  INFO 16664 --- [           main] com.example.demo.DemoApplication         : Started DemoApplication in 5.317 seconds (JVM running for 6.122)
2021-07-18 00:04:21.872  INFO 16664 --- [           main] o.s.b.a.b.JobLauncherApplicationRunner   : Running default command line with: []
            2021-07-18 00:04:22.008  INFO 16664 --- [           main] o.s.b.c.l.support.SimpleJobLauncher      : Job: [SimpleJob: [name=job]] launched with the following parameters: [{}]
            2021-07-18 00:04:22.102  INFO 16664 --- [           main] o.s.batch.core.job.SimpleStepHandler     : Executing step: [chunkBasedStep]
    Received list of size: 3
            1
            2
            3
    Received list of size: 2
            4
            5
            2021-07-18 00:04:22.175  INFO 16664 --- [           main] o.s.batch.core.step.AbstractStep         : Step: [chunkBasedStep] executed in 45ms
2021-07-18 00:04:22.185  INFO 16664 --- [           main] o.s.b.c.l.support.SimpleJobLauncher      : Job: [SimpleJob: [name=job]] completed with the following parameters: [{}] and the following status: [COMPLETED] in 122ms
2021-07-18 00:04:22.218  INFO 16664 --- [ionShutdownHook] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Shutdown initiated...
            2021-07-18 00:04:22.223  INFO 16664 --- [ionShutdownHook] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Shutdown completed.