Custom ItemReader with spring-data-jpa

Question

I'm creating a Spring batch project using existing entities and repositories. I need to use custom ItemReader for the job which reads data using the existing jpa repositories.

Custom reader

public class InMemoryReader implements ItemReader<Product> {

    @Autowired
    private ProductService productService;


    private int nextStudentIndex;
    private List<Product> studentData;

    public InMemoryReader() {
        initialize();
    }

    private void initialize() {
        studentData = new ArrayList<Product>();
        studentData.add(new Product("hi"));

        for (Product p : productService.get())
            studentData.add(p);
        nextStudentIndex = 0;
    }

    @Override
    public Product read() throws Exception {
        Product nextStudent = null;

        if (nextStudentIndex < studentData.size()) {
            nextStudent = studentData.get(nextStudentIndex);
            nextStudentIndex++;
        }

        return nextStudent;
    }
}

But I can't Autowire the ProductService in the itemreader . It throwing error as follows:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'reader' defined in class path resource [wariyum/sb/emailNotifier/BatchConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.batch.item.ItemReader]: Factory method 'reader' threw exception; nested exception is java.lang.NullPointerException
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:599)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1123)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1018)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:510)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:305)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:301)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:196)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:772)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:834)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:537)
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:689)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:321)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:969)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:958)
    at wariyum.sb.emailNotifier.Application.main(Application.java:15)
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.batch.item.ItemReader]: Factory method 'reader' threw exception; nested exception is java.lang.NullPointerException
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:189)
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:588)
    ... 16 common frames omitted
Caused by: java.lang.NullPointerException: null
    at wariyum.sb.emailNotifier.service.InMemoryReader.initialize(InMemoryReader.java:31)
    at wariyum.sb.emailNotifier.service.InMemoryReader.<init>(InMemoryReader.java:24)
    at wariyum.sb.emailNotifier.BatchConfiguration.reader(BatchConfiguration.java:41)
    at wariyum.sb.emailNotifier.BatchConfiguration$$EnhancerBySpringCGLIB$$df186b4e.CGLIB$reader$0(<generated>)
    at wariyum.sb.emailNotifier.BatchConfiguration$$EnhancerBySpringCGLIB$$df186b4e$$FastClassBySpringCGLIB$$724d6a16.invoke(<generated>)
    at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:228)
    at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:318)
    at wariyum.sb.emailNotifier.BatchConfiguration$$EnhancerBySpringCGLIB$$df186b4e.reader(<generated>)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:162)
    ... 17 common frames omitted

Batch configuration file

@Configuration
@EnableBatchProcessing
public class BatchConfiguration {


    @Autowired
    public JobBuilderFactory jobBuilderFactory;

    @Autowired
    private DataSource dataSource;

    @Autowired
    public StepBuilderFactory stepBuilderFactory;

    @Bean
    public ItemReader<Product> reader() throws Exception {
        return new InMemoryReader();
    }


    @Bean
    public ProductItemProcessor processor() {
        return new ProductItemProcessor();
    }

    @Bean
    public ItemWriter<Product> writer() {
        return new ItemWriter<Product>() {
            @Override
            public void write(List<? extends Product> items) throws Exception {

            }
        };
    }
    // end::readerwriterprocessor[]

    // tag::listener[]

    @Bean
    public JobExecutionListener listener() {
        return null;
    }

    // end::listener[]

    // tag::jobstep[]

    @Bean
    public Job importPerson(JobBuilderFactory jobs, Step s1) {

        return jobs.get("import")
                .incrementer(new RunIdIncrementer()) // because a spring config bug, this incrementer is not really useful
                .flow(s1)
                .end()
                .build();
    }

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

Product service implementation

@Service("productService")
public class ProductServiceImpl implements ProductService {

    private ProductRepository repository;

    @Autowired
    public ProductServiceImpl(ProductRepository repository) {
        this.repository = repository;
    }

    @Override
    public List<Product> get() {
        List<Product> list = new ArrayList<Product>();
        for (Product p : repository.findAll())
            list.add(p);
        return list;
    }
}

Answer 1

You are calling initialize() from the constructor ie before the ProductService dependency has been wired.

Remove the call to initialize() from the constructor and instead have Spring invoke it after the dependencies have been wired by annotating it with @PostConstruct or using one of the other methods outlined here:

How to call a method after bean initialization is complete?