The XML output contains the wrapper "credentials" altough I excluded it with defaultUseWrapper(false).

UserDAO

public class UserDAO {
    @JacksonXmlProperty(isAttribute = true)
    private int id;

    private Credentials credentials;
    
    @JacksonXmlProperty(localName = "todos")
    private TasksDAO tasks;

    // getter, setter
}

Credentials

public class Credentials {
    @JacksonXmlProperty(localName = "name")
    @JsonProperty("name")
    private String username;
    private String password;
   
    // getter, setter
}

XmlWrapper config

XmlMapper
                .builder()
                .defaultUseWrapper(false)
                .addModule(new JavaTimeModule())
                .disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS)
                .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
                .build()

XML output

<users>
    <user id="1">
        <credentials>
            <password>123</password>
            <name>Steve</name>
        </credentials>
        <todos/>
    </user>
</users>

My goal is to have it flat with user and password. How should I do to remove the wrapper credentials?

Desired Output

<users>
    <user id="1">
        <password>123</password>
        <name>Steve</name>
        <todos/>
    </user>
</users>

Dirty Solution

UserDAOSerializer.java

package org.example;

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.ser.std.StdSerializer;
import com.fasterxml.jackson.dataformat.xml.ser.ToXmlGenerator;
import java.io.IOException;

public class UserDAOSerializer extends StdSerializer<UserDAO> {
    public UserDAOSerializer() {
        this(null);
    }

    public UserDAOSerializer(Class<UserDAO> t) {
        super(t);
    }

    @Override
    public void serialize(UserDAO value, JsonGenerator gen, SerializerProvider provider) throws IOException {
        Object jsonGenerator;
        if (gen instanceof ToXmlGenerator) {
            final ToXmlGenerator xmlGenerator = (ToXmlGenerator) gen;
            gen.writeStartObject();
            xmlGenerator.setNextIsAttribute(true);
            gen.writeStringField("id",String.valueOf(value.getId()));

            xmlGenerator.setNextIsAttribute(false);
            gen.writeStringField("password",value.getCredentials().getPassword());
            gen.writeStringField("name",value.getCredentials().getUsername());
            gen.writeEndObject();
        } else {
            //TODO JSON MAPPING
        }
    }

}

UserDAO.java

package org.example;

import com.fasterxml.jackson.databind.annotation.JsonSerialize;
import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlProperty;

@JsonSerialize(using = UserDAOSerializer.class)
public class UserDAO {
    @JacksonXmlProperty(isAttribute = true)
    private int id;
    private Credentials credentials;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }

    public Credentials getCredentials() {
        return credentials;
    }

    public void setCredentials(Credentials credentials) {
        this.credentials = credentials;
    }

}

Main2.java

package org.example;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;

public class Main2 {
    public static void main(String[] args) throws JsonProcessingException {
        Credentials c1=new Credentials();
        c1.setUsername("Steve");
        c1.setPassword("123");
        UserDAO u1=new UserDAO();
        u1.setId(1);
        u1.setCredentials(c1);
        XmlMapper xmlMapper = new XmlMapper();
        String s2=xmlMapper.writeValueAsString(u1);
        System.out.println(s2);
    }
}

Credentials.java

package org.example;

import com.fasterxml.jackson.annotation.JsonProperty;

public class Credentials {
   // @JacksonXmlProperty(localName = "name")
    @JsonProperty("name")
    private String username;
    private String password;

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }
}

Run Result

<UserDAO id="1">
    <password>123</password>
    <name>Steve</name>
</UserDAO>

REF

Jackson: Registering a custom XML serializer for Map data structure

@Bender answer

Conclusion

I know this is not exactly what you asked for, this is just a POC.

I think you can handle others.

I hope this answer can solve your problem.