为什么 Jackson 在重建用于为 CSV 构建记录行的配置模式时包装包含逗号的字符串？

Question

我正在从存储库获取数据并将其放入 CSV 文件中。为了建立一条记录线，我使用杰克逊。如果该字段值包含逗号，我的目标是用双引号将字段（字符串类型）括起来。所以输出应该是这样的：

some-uuid-value，一些不带逗号的字符串，SOMETHING，123456，www.some.url，等等
some-uuid-value，"some string, but with comma"，一些东西，123456，www.some.url，等等
some-uuid-value，一些不带逗号的字符串，SOMETHING，123456，www.some.url，等等

我想出了这个代码：

private String toCsvString(EntityCsvRecord entity) {

        CsvMapper mapper = new CsvMapper();
        CsvSchema schema = mapper.schemaFor(EntityCsvRecord.class).withoutQuoteChar();

        if (entity.getName() == null) {
            entity.setName("");
        }

        if (entity.getName().contains(",")) {
            String columnName = "name";
            int nameColumnIndex = schema.column(columnName).getIndex();
            schema = mapper
                .configure(CsvGenerator.Feature.STRICT_CHECK_FOR_QUOTING, true)
                .schemaFor(EntityCsvRecord.class)
                .rebuild()
                .replaceColumn(nameColumnIndex, new CsvSchema.Column(nameColumnIndex, columnName))
                .build();
        }

        try {
            return mapper.writer(schema).writeValueAsString(entity);
        } catch (Exception e) {
            ...
        }
    }

但是，我不明白为什么会这样，我在文档中找不到任何下降线索。

有人能解开这个谜团吗？

Answer 1

整个技巧就是启用 CsvGenerator.Feature.STRICT_CHECK_FOR_QUOTING 功能。来自文档：

Feature that determines how much work is done before determining that a column value requires quoting: when set as true, full check is made to only use quoting when it is strictly necessary; but when false, a faster but more conservative check is made, and possibly quoting is used for values that might not need it. Trade-offs is basically between optimal/minimal quoting (true), and faster handling (false). Faster check involves only checking first N characters of value, as well as possible looser checks.

Note, however, that regardless setting, all values that need to be quoted will be: it is just that when set to false, other values may also be quoted (to avoid having to do more expensive checks).

Default value is false for "loose" (approximate, conservative) checking.

您可以删除模式和映射器的所有其他配置，它们将以相同的方式工作。您可以将其简化为以下代码：

class CsvEntityGenerator {

    private final CsvMapper mapper;
    private final CsvSchema schema;

    public CsvEntityGenerator(Class clazz) {
        mapper = new CsvMapper();
        mapper.enable(CsvGenerator.Feature.STRICT_CHECK_FOR_QUOTING);

        schema = mapper.schemaFor(clazz).withNullValue("");
    }

    public String toCsvString(Object entity) throws IOException {
        return mapper.writer(schema).writeValueAsString(entity);
    }
}

简单用法：

CsvEntityGenerator gen = new CsvEntityGenerator(EntityCsvRecord.class);
System.out.print(gen.toCsvString(new EntityCsvRecord("Na,me")));
System.out.print(gen.toCsvString(new EntityCsvRecord(null)));
System.out.print(gen.toCsvString(new EntityCsvRecord("Name")));

打印：

8b572b1b-17c1-429d-887b-ec9af1c30d05,"Na,me",SOMETHING,123456,www.some.url
e86eacb1-d45e-4614-91bb-45f0d8840ea9,,SOMETHING,123456,www.some.url
e9627c32-6736-44a5-8eb2-7d153f86af20,Name,SOMETHING,123456,www.some.url

如您所见，我们只创建了一次 CsvMapper 和 CsvSchema 并在我们想要序列化实体时重用它。这是更快的方法。

为什么 Jackson 在重建用于为 CSV 构建记录行的配置模式时包装包含逗号的字符串？

Why Jackson wrap the string containing comma when rebuilding the configuration schema for building a record line for a CSV?

java

csv

jackson

export-to-csv