要使用Sping Boot 和Jackson在XML prolog中包含standalone=“yes”属性，您可以通过提供自定义Jackson2ObjectMapperBuilder示例来自定义XmlMapper配置。下面是包含standalone属性的代码的更新版本：

import com.fasterxml.jackson.dataformat.xml.XmlMapper;
import com.fasterxml.jackson.dataformat.xml.ser.ToXmlGenerator;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.xml.MappingJackson2XmlHttpMessageConverter;
import org.springframework.http.converter.xml.Jackson2ObjectMapperBuilder;

@Configuration
public class JacksonXmlConfig {

    @Bean
    public MappingJackson2XmlHttpMessageConverter mappingJackson2XmlHttpMessageConverter(Jackson2ObjectMapperBuilder builder) {
        XmlMapper xmlMapper = builder.createXmlMapper(true).build();
        xmlMapper.configure(ToXmlGenerator.Feature.WRITE_XML_DECLARATION, true);
        xmlMapper.configure(ToXmlGenerator.Feature.WRITE_XML_1_0_BOM, false);
        xmlMapper.configure(ToXmlGenerator.Feature.STANDALONE, "yes"); // Set the standalone attribute
        
        return new MappingJackson2XmlHttpMessageConverter(xmlMapper);
    }

}

在这个更新的代码中，xmlMapper.configure（ToXmlGenerator.Feature.STANDALONE，“yes”）行将XML序言中的standalone属性设置为“yes”。这将在生成的XML响应中包含standalone=“yes”属性。
请注意，ToXmlGenerator.Feature.STANDALONE特性从Jackson版本2.13.0开始可用。既然你提到你的Jackson版本是2.14.2，它应该可以正常工作。
请确保在Sping Boot 应用程序中或适当的组件扫描位置包含此配置类，以使其生效。

要在XML序言中包含standalone=“yes”属性，可以使用Jackson的XmlMapper实现自定义XML编写器，并覆盖负责编写XML序言的方法。下面是一个例子：

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.dataformat.xml.ser.ToXmlGenerator;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;

import java.io.IOException;
import java.io.Writer;

public class CustomXmlMapper extends XmlMapper {

    @Override
    protected ToXmlGenerator _createGenerator(Writer out, JsonGenerator originalGenerator) throws IOException {
        ToXmlGenerator xmlGenerator = super._createGenerator(out, originalGenerator);
        xmlGenerator.enable(ToXmlGenerator.Feature.WRITE_XML_DECLARATION);
        xmlGenerator.configure(ToXmlGenerator.Feature.WRITE_XML_1_1, true);
        xmlGenerator.configure(ToXmlGenerator.Feature.WRITE_XML_1_0_BOM, false);

        // Write the XML prolog with the standalone attribute
        xmlGenerator.writeRaw("<?xml version=\"1.0\" encoding=\"" + xmlGenerator.getOutputContext().getEncoding().getJavaName() + "\" standalone=\"yes\"?>\n");

        return xmlGenerator;
    }

    // Additional customization methods can be added here if needed

}

在此示例中，通过扩展XmlMapper创建了一个名为CustomXmlMapper的自定义XmlMapper类。_createGenerator（）方法被重写以自定义XML生成过程。在该方法中，XML序言是用standalone=“yes”属性手动编写的。
要在Sping Boot 应用程序中使用此自定义XmlMapper，请按如下方式修改配置代码：

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.xml.MappingJackson2XmlHttpMessageConverter;
import org.springframework.http.converter.xml.Jackson2ObjectMapperBuilder;

@Configuration
public class JacksonXmlConfig {

    @Bean
    public MappingJackson2XmlHttpMessageConverter mappingJackson2XmlHttpMessageConverter(Jackson2ObjectMapperBuilder builder) {
        XmlMapper xmlMapper = new CustomXmlMapper(); // Use the custom XmlMapper
        xmlMapper.configure(SerializationFeature.WRAP_ROOT_VALUE, true);

        return new MappingJackson2XmlHttpMessageConverter(xmlMapper);
    }

}

在此更新的代码中，我们使用CustomXmlMapper而不是默认的XmlMapper，以确保使用自定义XML编写器。
请注意，此解决方案要求您使用XmlMapper的自定义实现并覆盖XML生成过程。