Jackson ObjectMapper序列化时保留空字符串的配置问题咨询

我有如下JSON对象 payload：

{"phoneNumbers": [{"id": 1,"number": "01234 567 890",},{"id": 2,"number": "",}]}

对应的Java序列化方法如下：

public String getJson(Object object) {
    String result = null;
    try {
        result = objectMapper.writeValueAsString(object);
    } catch (IOException e) {
        logger.error(e.getMessage(), e);
    }
    return result;
}

通过构造函数注入并配置Jackson的ObjectMapper：

@Autowired
public ServiceImpl(@Qualifier("objectMapper") ObjectMapper objectMapper) {
    this.objectMapper = objectMapper;
    configureObjectMapper(objectMapper);
}

配置方法：

private static void configureObjectMapper(ObjectMapper objectMapper) {
    objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
    objectMapper.setDateFormat(new SimpleDateFormat(DATE_PATTERN));
    objectMapper.setSerializationInclusion(Include.NON_NULL);
    objectMapper.setSerializationInclusion(Include.NON_EMPTY);
}

调用writeValueAsString时，Jackson会忽略第二个对象里的空字符串number字段，返回结果如下：

{"phoneNumbers": [{"id": 1,"number": "01234567890",},{"id": 2,}]}

但改用Codehaus的ObjectMapper实例时：

public String getJson(Object object) {
    org.codehaus.jackson.map.ObjectMapper objectMapper = new org.codehaus.jackson.map.ObjectMapper()
            .configure(SerializationConfig.Feature.FAIL_ON_EMPTY_BEANS, false);

    String result = null;
    try {
        result = objectMapper.writeValueAsString(object);
    } catch (IOException e) {
        logger.error(e.getMessage(), e);
    }
    return result;
}

它不会忽略空字符串，这正是我想要的效果。我使用的是Jackson-core 2.14.0版本，试了多种配置都没解决，求正确的Jackson配置方法，让空字符串""能正常序列化不被忽略。

问题出在你设置的序列化包含规则上：

objectMapper.setSerializationInclusion(Include.NON_NULL);
objectMapper.setSerializationInclusion(Include.NON_EMPTY);

连续调用两次setSerializationInclusion后，最终生效的是后者Include.NON_EMPTY，而该规则会将空字符串视为“空值”进行忽略。

要保留空字符串，可按以下两种方式调整配置：

1. 仅排除null值：如果只需要忽略null字段，保留空字符串、空集合等，直接移除Include.NON_EMPTY的设置，只保留Include.NON_NULL：

private static void configureObjectMapper(ObjectMapper objectMapper) {
    objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
    objectMapper.setDateFormat(new SimpleDateFormat(DATE_PATTERN));
    objectMapper.setSerializationInclusion(Include.NON_NULL);
}

2. 精细组合规则：如果需要排除null值和空集合/数组，但保留空字符串，可使用JsonInclude.Value组合规则：

private static void configureObjectMapper(ObjectMapper objectMapper) {
    objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
    objectMapper.setDateFormat(new SimpleDateFormat(DATE_PATTERN));
    // 排除null值，空字符串保留，空集合/数组排除
    objectMapper.setSerializationInclusion(JsonInclude.Value.construct(
        JsonInclude.Include.NON_NULL,
        JsonInclude.Include.NON_EMPTY
    ));
}

注：Include.NON_EMPTY的默认逻辑是：字符串空值会被忽略，空集合/数组会被忽略，空对象则受FAIL_ON_EMPTY_BEANS配置控制。因此要保留空字符串，最直接的方式就是不要设置Include.NON_EMPTY。

内容的提问来源于stack exchange，提问作者Martin