带列 headers 的 StatefulBeanToCsv

Question

我正在使用 opencsv-4.0 编写一个 csv 文件，我需要在输出文件中添加列 headers。

这是我的代码。

public static void buildProductCsv(final List<Product> product,
        final String filePath) {

    try {

        Writer writer = new FileWriter(filePath);

        // mapping of columns with their positions
        ColumnPositionMappingStrategy<Product> mappingStrategy = new ColumnPositionMappingStrategy<Product>();
        // Set mappingStrategy type to Product Type
        mappingStrategy.setType(Product.class);
        // Fields in Product Bean
        String[] columns = new String[] { "productCode", "MFD", "EXD" };
        // Setting the colums for mappingStrategy
        mappingStrategy.setColumnMapping(columns);

        StatefulBeanToCsvBuilder<Product> builder = new StatefulBeanToCsvBuilder<Product>(writer);

        StatefulBeanToCsv<Product> beanWriter = builder.withMappingStrategy(mappingStrategy).build();
        // Writing data to csv file
        beanWriter.write(product);
        writer.close();

        log.info("Your csv file has been generated!");

    } catch (Exception ex) {
        log.warning("Exception: " + ex.getMessage());
    }

}

以上代码创建了一个包含数据的 csv 文件。但它不包括该文件中的列 headers。

如何添加列 headers 以输出 csv？

Answer 1

ColumnPositionMappingStrategy#generateHeader returns 空数组

/**
 * This method returns an empty array.
 * The column position mapping strategy assumes that there is no header, and
 * thus it also does not write one, accordingly.
 * @return An empty array
 */
@Override
public String[] generateHeader() {
    return new String[0];
}

如果您从 BeanToCsv 构建器中删除 MappingStrategy

// replace 
StatefulBeanToCsv<Product> beanWriter = builder.withMappingStrategy(mappingStrategy).build();
// with
StatefulBeanToCsv<Product> beanWriter = builder.build();

它会将产品的 class 成员写入 CSV header

如果您的产品 class 成员姓名是

"productCode", "MFD", "EXD"

这应该是正确的解决方案

否则，添加@CsvBindByName 注解

import com.opencsv.bean.CsvBindByName;
import com.opencsv.bean.StatefulBeanToCsv;
import com.opencsv.bean.StatefulBeanToCsvBuilder;

import java.io.FileWriter;
import java.io.Writer;
import java.util.ArrayList;
import java.util.List;

public class CsvTest {

    public static void main(String[] args) throws Exception {
        Writer writer = new FileWriter(fileName);

        StatefulBeanToCsvBuilder<Product> builder = new StatefulBeanToCsvBuilder<>(writer);
        StatefulBeanToCsv<Product> beanWriter = builder.build();

        List<Product> products = new ArrayList<>();
        products.add(new Product("1", "11", "111"));
        products.add(new Product("2", "22", "222"));
        products.add(new Product("3", "33", "333"));
        beanWriter.write(products);
        writer.close();
    }

    public static class Product {
        @CsvBindByName(column = "productCode")
        String id;
        @CsvBindByName(column = "MFD")
        String member2;
        @CsvBindByName(column = "EXD")
        String member3;

        Product(String id, String member2, String member3) {
            this.id = id;
            this.member2 = member2;
            this.member3 = member3;
        }

        public String getId() {
            return id;
        }

        public void setId(String id) {
            this.id = id;
        }

        public String getMember2() {
            return member2;
        }

        public void setMember2(String member2) {
            this.member2 = member2;
        }

        public String getMember3() {
            return member3;
        }

        public void setMember3(String member3) {
            this.member3 = member3;
        }
    }

}

输出：

"EXD","MFD","PRODUCTCODE"

"111","11","1"

"222","22","2"

"333","33","3"

注意； class，getters 和 setters 需要 public，因为 OpenCSV 库使用反射

Answer 2

我可能在这里遗漏了一些明显的东西，但你不能将你的 header 字符串附加到作者 object 吗？

Writer writer = new FileWriter(filePath);
writer.append("header1, header2, header3, ...etc \n");

// This will be followed by your code with BeanToCsvBuilder 
// Note: the terminating \n might differ pending env.

Answer 3

您可以通过注释追加

public void export(List<YourObject> list, PrintWriter writer) throws Exception {
        writer.append( buildHeader( YourObject.class ) );
        StatefulBeanToCsvBuilder<YourObject> builder = new StatefulBeanToCsvBuilder<>( writer );
        StatefulBeanToCsv<YourObject> beanWriter = builder.build();
        beanWriter.write( mapper.map( list ) );
        writer.close();
    }

    private String buildHeader(Class<YourObject> clazz) {
        return Arrays.stream( clazz.getDeclaredFields() )
                .filter( f -> f.getAnnotation( CsvBindByPosition.class ) != null
                        && f.getAnnotation( CsvBindByName.class ) != null )
                .sorted( Comparator.comparing( f -> f.getAnnotation( CsvBindByPosition.class ).position() ) )
                .map( f -> f.getAnnotation( CsvBindByName.class ).column() )
                .collect( Collectors.joining( "," ) ) + "\n";
    }

@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
public class YourObject {

    @CsvBindByPosition(position = 0)
    @CsvBindByName(column = "A")
    private Long a;

    @CsvBindByPosition(position = 1)
    @CsvBindByName(column = "B")
    private String b;

    @CsvBindByPosition(position = 2)
    @CsvBindByName(column = "C")
    private String c;

}

Answer 4

您还可以覆盖 generateHeaders 方法和 return 设置的列映射，这将在 csv

中有 header 行

ColumnPositionMappingStrategy<Product> mappingStrategy = new ColumnPositionMappingStrategy<Product>() {
            @Override
            public String[] generateHeader(Product bean) throws CsvRequiredFieldEmptyException {
                return this.getColumnMapping();
            }
        };

Answer 5

使用 HeaderColumnNameMappingStrategy 进行读取，然后使用相同的策略进行写入。在这种情况下，“相同”的意思不仅是相同 class，而且实际上是相同的 object.

来自 StatefulBeanToCsvBuilder.withMappingStrategy 的 javadoc:

It is perfectly legitimate to read a CSV source, take the mapping strategy from the read operation, and pass it in to this method for a write operation. This conserves some processing time, but, more importantly, preserves header ordering.

这样您将获得包含 headers 的 CSV，其中列的顺序与原始 CSV 的顺序相同。

使用 OpenCSV 5.4 为我工作。

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