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This guide demonstrates how to read data from a SQL database and write it to a .dat file using Spring Batch in a Spring Boot application. The .dat file will contain comma-separated values (CSV format), and Spring Batch handles the job configuration, reading, and writing processes.

📦 Step 1: Add Dependencies (Maven)

Add the following to your pom.xml:

xml

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-batch</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-jpa</artifactId>
    </dependency>
    <dependency>
        <groupId>com.h2database</groupId>
        <artifactId>h2</artifactId>
        <scope>runtime</scope>
    </dependency>
</dependencies>

⚙️ Step 2: Configure application.properties

properties

spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.driverClassName=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=password
spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
spring.batch.job.enabled=true

🧱 Step 3: Define Entity and Repository

MyEntity.java

java

import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
public class MyEntity {
    @Id
    private Long id;
    private String name;

    // Getters and setters
}

MyEntityRepository.java

java

import org.springframework.data.jpa.repository.JpaRepository;

public interface MyEntityRepository extends JpaRepository<MyEntity, Long> {
}

🔁 Step 4: Batch Configuration

BatchConfig.java

java

import org.springframework.batch.core.Job;
import org.springframework.batch.core.Step;
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.support.RunIdIncrementer;
import org.springframework.batch.item.ItemProcessor;
import org.springframework.batch.item.ItemReader;
import org.springframework.batch.item.ItemWriter;
import org.springframework.batch.item.database.JpaPagingItemReader;
import org.springframework.batch.item.file.FlatFileItemWriter;
import org.springframework.batch.item.file.transform.DelimitedLineAggregator;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.io.FileSystemResource;

import javax.persistence.EntityManagerFactory;

@Configuration
@EnableBatchProcessing
public class BatchConfig {

    @Autowired
    private JobBuilderFactory jobBuilderFactory;

    @Autowired
    private StepBuilderFactory stepBuilderFactory;

    @Autowired
    private EntityManagerFactory entityManagerFactory;

    @Bean
    public Job job() {
        return jobBuilderFactory.get("sqlToDatFileJob")
                .incrementer(new RunIdIncrementer())
                .start(step1())
                .build();
    }

    @Bean
    public Step step1() {
        return stepBuilderFactory.get("step1")
                .<MyEntity, MyEntity>chunk(10)
                .reader(reader())
                .processor(processor())
                .writer(writer())
                .build();
    }

    @Bean
    public ItemReader<MyEntity> reader() {
        JpaPagingItemReader<MyEntity> reader = new JpaPagingItemReader<>();
        reader.setQueryString("SELECT e FROM MyEntity e");
        reader.setEntityManagerFactory(entityManagerFactory);
        reader.setPageSize(10);
        return reader;
    }

    @Bean
    public ItemProcessor<MyEntity, MyEntity> processor() {
        return item -> item; // No transformation
    }

    @Bean
    public ItemWriter<MyEntity> writer() {
        FlatFileItemWriter<MyEntity> writer = new FlatFileItemWriter<>();
        writer.setResource(new FileSystemResource("output.dat"));
        writer.setLineAggregator(new DelimitedLineAggregator<MyEntity>() {{
            setDelimiter(",");
            setFieldExtractor(item -> new Object[]{item.getId(), item.getName()});
        }});
        return writer;
    }
}

📝 Explanation

• Job: The batch job is composed of a single step.
• Step: Configured to read entities from the database, process them (optional), and write them to a .dat file.
• JpaPagingItemReader: Reads from the database using JPA.
• ItemProcessor: Currently a pass-through; you can modify it to transform data.
• FlatFileItemWriter: Writes each record as a line to output.dat, using comma-separated fields.

▶️ Running the Job

Run the Spring Boot application (main method), and Spring Batch will:

1. Read records from the SQL database (MyEntity table).
2. Write the records to a .dat file named output.dat.

Each line in output.dat will look like:

python-repl

1,John Doe
2,Jane Smith
...