A "List of objects" is not a good encapsulation of an item in the chunk-oriented processing model of Spring Batch.
Option 1: You can change your query to join order and orderDetails and return items of type Order. This way, an item would be an Order.
Option2: Another way to do that is by using the driving query pattern. The idea is to make your reader return Orders, and use a processor to enrich orders with their details. This works for small/medium data sets, but performs poorly for large data sets (because of the additional query per item). This is related to the algorithm itself, not to Spring Batch per se.
You would prefer option 1 in which the database does the join in an efficient way and gives you back order items pre-filled with their details.
EDIT: Add sample for the driving query pattern
The following sample shows the idea of the driving query pattern. The reader returns items of type Person. The processor enriches person items with their addresses. Look how the reader fetches only the id and name of the person, but not the address. You can adapt it to Order and OrderDetails in your case.
/*
* Copyright 2022 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
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package org.springframework.batch.sample;
import javax.sql.DataSource;
import org.springframework.batch.core.Job;
import org.springframework.batch.core.JobParameters;
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.item.ItemProcessor;
import org.springframework.batch.item.ItemWriter;
import org.springframework.batch.item.database.JdbcCursorItemReader;
import org.springframework.batch.item.database.builder.JdbcCursorItemReaderBuilder;
import org.springframework.beans.factory.annotation.Autowired;
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.jdbc.core.BeanPropertyRowMapper;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseBuilder;
import org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType;
/**
* @author Mahmoud Ben Hassine
*/
@Configuration
@EnableBatchProcessing
public class MyJob {
@Bean
public DataSource dataSource() {
return new EmbeddedDatabaseBuilder()
.setType(EmbeddedDatabaseType.H2)
.addScript("/org/springframework/batch/core/schema-drop-h2.sql")
.addScript("/org/springframework/batch/core/schema-h2.sql")
.build();
}
@Bean
public JdbcTemplate jdbcTemplate(DataSource dataSource) {
return new JdbcTemplate(dataSource);
}
@Bean
public JdbcCursorItemReader<Person> itemReader() {
return new JdbcCursorItemReaderBuilder<Person>()
.name("personItemReader")
.dataSource(dataSource())
.sql("select id, name from person")
.beanRowMapper(Person.class)
.build();
}
@Bean
public ItemProcessor<Person, Person> itemProcessor() {
return new ItemProcessor<Person, Person>() {
@Autowired
private JdbcTemplate jdbcTemplate;
@Override
public Person process(Person person) {
Address address = jdbcTemplate.queryForObject("select * from address where personId = ?", new Object[]{person.getId()}, new BeanPropertyRowMapper<>(Address.class));
person.setAddress(address);
return person;
}
};
}
@Bean
public ItemWriter<Person> itemWriter() {
return items -> {
for (Person item : items) {
System.out.println("item = " + item);
}
};
}
@Bean
public Job job(JobBuilderFactory jobs, StepBuilderFactory steps) {
return jobs.get("job")
.start(steps.get("step")
.<Person, Person>chunk(2)
.reader(itemReader())
.processor(itemProcessor())
.writer(itemWriter())
.build())
.build();
}
public static void main(String[] args) throws Exception {
ApplicationContext context = new AnnotationConfigApplicationContext(MyJob.class);
JdbcTemplate jdbcTemplate = context.getBean(JdbcTemplate.class);
jdbcTemplate.update("CREATE TABLE address (id INT IDENTITY NOT NULL PRIMARY KEY, personId INT, street VARCHAR(20));");
jdbcTemplate.update("CREATE TABLE person (id INT IDENTITY NOT NULL PRIMARY KEY, name VARCHAR(20));");
jdbcTemplate.update("INSERT INTO address (id, personId, street) VALUES (1,1, 'oxford street');");
jdbcTemplate.update("INSERT INTO address (id, personId, street) VALUES (2,2, 'howard street');");
jdbcTemplate.update("INSERT INTO person (id, name) VALUES (1, 'foo');");
jdbcTemplate.update("INSERT INTO person (id, name) VALUES (2, 'bar');");
JobLauncher jobLauncher = context.getBean(JobLauncher.class);
Job job = context.getBean(Job.class);
jobLauncher.run(job, new JobParameters());
}
public static class Person {
private long id;
private String name;
private Address address;
public Person() {
}
public Person(long id, String name) {
this.id = id;
this.name = name;
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Address getAddress() {
return address;
}
public void setAddress(Address address) {
this.address = address;
}
@Override
public String toString() {
return "Person{" +
"id=" + id +
", name='" + name + '\'' +
", address=" + address +
'}';
}
}
public static class Address {
private int id;
private int personId;
private String street;
public Address() {
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getStreet() {
return street;
}
public void setStreet(String street) {
this.street = street;
}
public int getPersonId() {
return personId;
}
public void setPersonId(int personId) {
this.personId = personId;
}
@Override
public String toString() {
return "Address{" +
"id=" + id +
", street='" + street + '\'' +
'}';
}
}
}
