Spring Batch简单入门（三）- ItemReader 与 ItemWriter

﹏ヽ暗。殇╰゛Y 2022-05-13 07:00 407阅读 0赞

接[上一章][Link 1]，Spring Batch本身提供了许多开箱即用的ItemReader与ItemWriter实现。Spring Batch 4针对这些组件又提供了Builder实现，用户可以很方便的通过Builder模式来使用这些组件。

![这里写图片描述][70]

本章，我们重点讨论以下几种ItemReader和ItemWriter的使用。

### **ItemReader** ###

*  **FlatFileItemReader**

对于FlatFileItemReader，[官方示例][Link 2]已给出代码实现。我们这里不多赘述。

> Reads from a flat file. Includes ItemStream and Skippable functionality

@Bean
        public FlatFileItemReader<User> flatFileItemReader() {
            FlatFileItemReader<User> reader = new FlatFileItemReader<>();
            DefaultLineMapper<User> lineMapper = new DefaultLineMapper<User>() {
                {
                    setLineTokenizer(new DelimitedLineTokenizer() {
                        {
                            setNames(new String[]{
       "firstName", "lastName"});
                        }
                    });
                    setFieldSetMapper(new BeanWrapperFieldSetMapper<User>() {
                        {
                            setTargetType(User.class);
                        }
                    });
                }
            };
            reader.setResource(new ClassPathResource("sample-data.csv"));
            reader.setLineMapper(lineMapper);
            return reader;
        }

*  **JpaPagingItemReader**

> Given a JPQL statement, pages through the rows, such that large datasets can be read without running out of memory.

以**JPQL**方式读取海量数据库记录，为避免内存泄漏，以分页方式读取。

@Bean(destroyMethod = "")
        @Qualifier("jpaPagingItemReader")
        public ItemReader<User> jpaPagingItemReader() {
            JpaPagingItemReader<User> reader = new JpaPagingItemReader<User>();
            String sqlQuery = "select * from user where id like :limit ";
            JpaNativeQueryProvider<User> queryProvider = new JpaNativeQueryProvider<User>();
            queryProvider.setSqlQuery(sqlQuery);
            queryProvider.setEntityClass(User.class);
            reader.setEntityManagerFactory(emf);
            reader.setPageSize(3);
            reader.setQueryProvider(queryProvider);
            reader.setParameterValues(Collections.<String, Object>singletonMap("limit", "%"));
            reader.setSaveState(true);
            return reader;
        }

*  **RepositoryItemReader**

> Given a Spring Data PagingAndSortingRepository object, a Sort, and the name of method to execute, returns items provided by the Spring Data repository implementation.

以Spring Data JPA(Spring Data Repository)方式读取数据，传入参数包含：PagingAndSortingRepository对象，结果集排序方式，调用目标方法等。返回Spring Data JPA方法实现执行结果。

比如，对于**UserReaderRepository**：

public interface UserReaderRepository extends PagingAndSortingRepository<User, Long> { 
        @Override
        Page<User> findAll(Pageable pageable);
    
        Page<User> findAllByFirstNameLike(String firstName, Pageable pageable);
    }

接口中定义**`findAllByFirstNameLike`**方法，查询FirstName以特定字符开始的数据库记录，因此，我们的**`RepositoryItemReader`**定义如下：

@Bean
        @Qualifier("repositoryItemReaderWithParams")
        public RepositoryItemReader<User> repositoryItemReaderWithParams() {
            Map<String, Sort.Direction> map = new HashMap<>();
            map.put("id", Sort.Direction.DESC);
            List<String> params = new ArrayList();
            params.add("i%");
            RepositoryItemReader<User> repositoryItemReader = new RepositoryItemReader<>();
            repositoryItemReader.setRepository(userReaderRepository);
            repositoryItemReader.setPageSize(5);
            repositoryItemReader.setMethodName("findAllByFirstNameLike");
            repositoryItemReader.setArguments(params);
            repositoryItemReader.setSort(map);
            return repositoryItemReader;
        }

### **ItemWriter** ###

*  **JdbcBatchItemWriter**

> Uses batching features from a PreparedStatement, if available, and can take rudimentary steps to locate a failure during a flush.

利用PreparedStatement批处理特性保存数据。

@Bean
        @Qualifier("jdbcBatchItemWriter")
        public JdbcBatchItemWriter<People> jdbcBatchItemWriter() {
            JdbcBatchItemWriter<People> writer = new JdbcBatchItemWriter<>();
            writer.setItemSqlParameterSourceProvider(new BeanPropertyItemSqlParameterSourceProvider<>());
            writer.setSql("insert into people (person_id, first_name, last_name) VALUES (uuid(), :firstName, :lastName)");
            writer.setDataSource(w_dataSource);
            return writer;
        }

*  **RepositoryItemWriter**

> Given a Spring Data CrudRepository implementation, items are saved through the method specified in the configuration.

调用Spring Data JPA (Spring Data CrudRepository)指定方法实现，将数据保存到目标数据库中。比如，我们定义**PeopleCrudRepository**接口：

public interface PeopleCrudRepository extends CrudRepository<People, Long> { 
    }

我们调用**CrudRepository**中的默认**save**方法实现。

@Bean
        @Qualifier("repositoryItemWriter")
        public RepositoryItemWriter<People> repositoryItemWriter() {
            RepositoryItemWriter<People> peopleRepositoryItemWriter = new RepositoryItemWriter<>();
            peopleRepositoryItemWriter.setRepository(peopleCrudRepository);
            peopleRepositoryItemWriter.setMethodName("save");
            return peopleRepositoryItemWriter;
        }

*更多源码请参考:*

[https://github.com/ypmc/spring-cloud/tree/master/spring-batch][https_github.com_ypmc_spring-cloud_tree_master_spring-batch]

[Link 1]: https://blog.csdn.net/kangkanglou/article/details/82596326
[70]: /images/20220513/15cc7dac1289413d993002742a90b69e.png
[Link 2]: https://github.com/spring-guides/gs-batch-processing
[https_github.com_ypmc_spring-cloud_tree_master_spring-batch]: https://github.com/ypmc/spring-cloud/tree/master/spring-batch