一招解决 Spring Batch 抛出的“Reader must be open before it can be read.”错误

在 Spring Batch 4.+ 中，使用 FlatFileItemReader 从文件中读取数据，抛出“org.springframework.batch.item.ReaderNotOpenException: Reader must be open before it can be read.”错误信息，详细信息如下：

org.springframework.batch.item.ReaderNotOpenException: Reader must be open before it can be read.
	at org.springframework.batch.item.file.FlatFileItemReader.readLine(FlatFileItemReader.java:201) ~[spring-batch-infrastructure-4.3.5.jar:4.3.5]
	at org.springframework.batch.item.file.FlatFileItemReader.doRead(FlatFileItemReader.java:178) ~[spring-batch-infrastructure-4.3.5.jar:4.3.5]
	at org.springframework.batch.item.support.AbstractItemCountingItemStreamItemReader.read(AbstractItemCountingItemStreamItemReader.java:93) ~[spring-batch-infrastructure-4.3.5.jar:4.3.5]
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104) ~[na:na]
	at java.base/java.lang.reflect.Method.invoke(Method.java:578) ~[na:na]
	at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:344) ~[spring-aop-5.3.19.jar:5.3.19]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:198) ~[spring-aop-5.3.19.jar:5.3.19]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163) ~[spring-aop-5.3.19.jar:5.3.19]
	at org.springframework.aop.support.DelegatingIntroductionInterceptor.doProceed(DelegatingIntroductionInterceptor.java:137) ~[spring-aop-5.3.19.jar:5.3.19]
	at org.springframework.aop.support.DelegatingIntroductionInterceptor.invoke(DelegatingIntroductionInterceptor.java:124) ~[spring-aop-5.3.19.jar:5.3.19]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.3.19.jar:5.3.19]
	at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:215) ~[spring-aop-5.3.19.jar:5.3.19]
	at jdk.proxy2/jdk.proxy2.$Proxy48.read(Unknown Source) ~[na:na]
	at org.springframework.batch.core.step.item.SimpleChunkProvider.doRead(SimpleChunkProvider.java:99) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.step.item.SimpleChunkProvider.read(SimpleChunkProvider.java:180) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.step.item.SimpleChunkProvider$1.doInIteration(SimpleChunkProvider.java:126) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.repeat.support.RepeatTemplate.getNextResult(RepeatTemplate.java:375) ~[spring-batch-infrastructure-4.3.5.jar:4.3.5]
	at org.springframework.batch.repeat.support.RepeatTemplate.executeInternal(RepeatTemplate.java:215) ~[spring-batch-infrastructure-4.3.5.jar:4.3.5]
	at org.springframework.batch.repeat.support.RepeatTemplate.iterate(RepeatTemplate.java:145) ~[spring-batch-infrastructure-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.step.item.SimpleChunkProvider.provide(SimpleChunkProvider.java:118) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.step.item.ChunkOrientedTasklet.execute(ChunkOrientedTasklet.java:71) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.step.tasklet.TaskletStep$ChunkTransactionCallback.doInTransaction(TaskletStep.java:407) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.step.tasklet.TaskletStep$ChunkTransactionCallback.doInTransaction(TaskletStep.java:331) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:140) ~[spring-tx-5.3.19.jar:5.3.19]
	at org.springframework.batch.core.step.tasklet.TaskletStep$2.doInChunkContext(TaskletStep.java:273) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.scope.context.StepContextRepeatCallback.doInIteration(StepContextRepeatCallback.java:82) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.repeat.support.RepeatTemplate.getNextResult(RepeatTemplate.java:375) ~[spring-batch-infrastructure-4.3.5.jar:4.3.5]
	at org.springframework.batch.repeat.support.RepeatTemplate.executeInternal(RepeatTemplate.java:215) ~[spring-batch-infrastructure-4.3.5.jar:4.3.5]
	at org.springframework.batch.repeat.support.RepeatTemplate.iterate(RepeatTemplate.java:145) ~[spring-batch-infrastructure-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.step.tasklet.TaskletStep.doExecute(TaskletStep.java:258) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.step.AbstractStep.execute(AbstractStep.java:208) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.job.SimpleStepHandler.handleStep(SimpleStepHandler.java:152) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.job.AbstractJob.handleStep(AbstractJob.java:413) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.job.SimpleJob.doExecute(SimpleJob.java:136) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.job.AbstractJob.execute(AbstractJob.java:320) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.batch.core.launch.support.SimpleJobLauncher$1.run(SimpleJobLauncher.java:149) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.core.task.SyncTaskExecutor.execute(SyncTaskExecutor.java:50) ~[spring-core-5.3.19.jar:5.3.19]
	at org.springframework.batch.core.launch.support.SimpleJobLauncher.run(SimpleJobLauncher.java:140) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104) ~[na:na]
	at java.base/java.lang.reflect.Method.invoke(Method.java:578) ~[na:na]
	at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:344) ~[spring-aop-5.3.19.jar:5.3.19]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:198) ~[spring-aop-5.3.19.jar:5.3.19]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163) ~[spring-aop-5.3.19.jar:5.3.19]
	at org.springframework.batch.core.configuration.annotation.SimpleBatchConfiguration$PassthruAdvice.invoke(SimpleBatchConfiguration.java:128) ~[spring-batch-core-4.3.5.jar:4.3.5]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.3.19.jar:5.3.19]
	at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:215) ~[spring-aop-5.3.19.jar:5.3.19]
	at jdk.proxy2/jdk.proxy2.$Proxy53.run(Unknown Source) ~[na:na]
	at org.springframework.boot.autoconfigure.batch.JobLauncherApplicationRunner.execute(JobLauncherApplicationRunner.java:199) ~[spring-boot-autoconfigure-2.6.7.jar:2.6.7]
	at org.springframework.boot.autoconfigure.batch.JobLauncherApplicationRunner.executeLocalJobs(JobLauncherApplicationRunner.java:173) ~[spring-boot-autoconfigure-2.6.7.jar:2.6.7]
	at org.springframework.boot.autoconfigure.batch.JobLauncherApplicationRunner.launchJobFromProperties(JobLauncherApplicationRunner.java:160) ~[spring-boot-autoconfigure-2.6.7.jar:2.6.7]
	at org.springframework.boot.autoconfigure.batch.JobLauncherApplicationRunner.run(JobLauncherApplicationRunner.java:155) ~[spring-boot-autoconfigure-2.6.7.jar:2.6.7]
	at org.springframework.boot.autoconfigure.batch.JobLauncherApplicationRunner.run(JobLauncherApplicationRunner.java:150) ~[spring-boot-autoconfigure-2.6.7.jar:2.6.7]
	at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:768) ~[spring-boot-2.6.7.jar:2.6.7]
	at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:758) ~[spring-boot-2.6.7.jar:2.6.7]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:310) ~[spring-boot-2.6.7.jar:2.6.7]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1312) ~[spring-boot-2.6.7.jar:2.6.7]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1301) ~[spring-boot-2.6.7.jar:2.6.7]
	at com.hxstrive.spring_batch.flatFileReaderDemo.SpringBatchDemoApplication.main(SpringBatchDemoApplication.java:11) ~[classes/:na]

我的代码如下：

// 创建ItemReader对象
@Bean
@StepScope //将 Bean 的生命周期与 Step 执行上下文 绑定
public ItemReader<? extends User> flatFileItemReader() {
    FlatFileItemReader<User> reader = new FlatFileItemReader<>();
    reader.setResource(new ClassPathResource("users.csv"));
    reader.setLinesToSkip(1); // 跳过文件第一行，因为第一行是字段名

    // 解析数据
    DelimitedLineTokenizer tokenizer = new DelimitedLineTokenizer();
    tokenizer.setNames("id", "username", "password");
    // 把解析出的数据映射到 User 对象中
    DefaultLineMapper<User> lineMapper = new DefaultLineMapper<>();
    lineMapper.setLineTokenizer(tokenizer);
    lineMapper.setFieldSetMapper(new FieldSetMapper<User>(){
        @Override
        public User mapFieldSet(FieldSet fieldSet) throws BindException {
            User user = new User();
            user.setId(fieldSet.readInt("id"));
            user.setUsername(fieldSet.readString("username"));
            user.setPassword(fieldSet.readString("password"));
            return user;
        }
    });

    lineMapper.afterPropertiesSet();
    reader.setLineMapper(lineMapper);
    return reader;
}

上述代码中，flatFileItemReader() 返回最基本的 ItemReader<? extends User>，我们只需要将返回值替换成 FlatFileItemReader<? extends User> 就可以了，修改后的代码如下：

// 创建ItemReader对象，使用 JdbcPagingItemReader 实现从数据库读取数据
@Bean
@StepScope //将 Bean 的生命周期与 Step 执行上下文 绑定
public FlatFileItemReader<? extends User> flatFileItemReader() {
    FlatFileItemReader<User> reader = new FlatFileItemReader<>();
    reader.setResource(new ClassPathResource("users.csv"));
    reader.setLinesToSkip(1); // 跳过文件第一行，因为第一行是字段名

    // 解析数据
    DelimitedLineTokenizer tokenizer = new DelimitedLineTokenizer();
    tokenizer.setNames("id", "username", "password");
    // 把解析出的数据映射到 User 对象中
    DefaultLineMapper<User> lineMapper = new DefaultLineMapper<>();
    lineMapper.setLineTokenizer(tokenizer);
    lineMapper.setFieldSetMapper(new FieldSetMapper<User>(){
        @Override
        public User mapFieldSet(FieldSet fieldSet) throws BindException {
            User user = new User();
            user.setId(fieldSet.readInt("id"));
            user.setUsername(fieldSet.readString("username"));
            user.setPassword(fieldSet.readString("password"));
            return user;
        }
    });

    lineMapper.afterPropertiesSet();
    reader.setLineMapper(lineMapper);
    return reader;
}

重新运行代码，即可解决错误。