FasterXML jackson-dataformat-xml serialization version and encoding not added to xml

You can configure your XmlMapper to write the XML header.

XmlMapper xmlMapper = new XmlMapper();
xmlMapper.configure( ToXmlGenerator.Feature.WRITE_XML_DECLARATION, true );

As an example:

import com.fasterxml.jackson.dataformat.xml.XmlMapper;
import com.fasterxml.jackson.dataformat.xml.ser.ToXmlGenerator;
import java.io.IOException;

public class Xml {
    public static void main(String[] args) throws IOException {
     // Important: create XmlMapper; it will use proper factories, workarounds
        XmlMapper xmlMapper = new XmlMapper();
        xmlMapper.configure(ToXmlGenerator.Feature.WRITE_XML_DECLARATION, true);
        xmlMapper.writeValue(System.out, new SampleRequest());
    }
}

class SampleRequest{
    public int x = 1;
    public int y = 2;
}

This generates the output:

<?xml version="1.0" encoding="UTF-8"?>
<SampleRequest>
   ...
</SampleRequest>

In case you want to set the version to 1.1 instead of 1.0, use ToXmlGenerator.Feature.WRITE_XML_1_1.

Notice that Faster-XML team recommends to use Woodstox library. In case you use it, some other configurations can be set. Among all of them there is one related to setting double quotes:

public static final String P_USE_DOUBLE_QUOTES_IN_XML_DECL="com.ctc.wstx.useDoubleQuotesInXmlDecl";

at WstxOutputProperties.java

For more details check out configuring Woodstox parser.

For those wondering how to change single quotes to double quotes:

String propName = com.ctc.wstx.api.WstxOutputProperties.P_USE_DOUBLE_QUOTES_IN_XML_DECL;
xmlMapper.getFactory()
         .getXMLOutputFactory()
         .setProperty(propName, true);

import com.ctc.wstx.api.WstxOutputProperties;
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.json.Jackson2ObjectMapperBuilder;
import org.springframework.http.converter.xml.MappingJackson2XmlHttpMessageConverter;

@Configuration
public class XmlConfig {
    @Bean
    public MappingJackson2XmlHttpMessageConverter mappingJackson2XmlHttpMessageConverter(Jackson2ObjectMapperBuilder builder) {
        XmlMapper xmlMapper = builder.createXmlMapper(true).build();
        xmlMapper.enable(ToXmlGenerator.Feature.WRITE_XML_DECLARATION);
        xmlMapper.getFactory().getXMLOutputFactory().setProperty(WstxOutputProperties.P_USE_DOUBLE_QUOTES_IN_XML_DECL, true);
        return new MappingJackson2XmlHttpMessageConverter(xmlMapper);
    }
}