You can use standard tools for it

1. Use the tool xjc from your jdk to generate Java classes from schema

Since Java 9 you must explicitly add JAXB as module with –add-modules java.se.ee See: How to resolve java.lang.NoClassDefFoundError: javax/xml/bind/JAXBException in Java 9

Since Java 11 you have to download xjc in an extra step from https://javaee.github.io/jaxb-v2/

2. Read in as XML write out as JSON using Jackson

Example

With https://schema.datacite.org/meta/kernel-4.1/metadata.xsd

1. Use the tool xjc from your jdk

In my example I will use a fairly complex example based on datacite schemas.

/path/to/jdk/bin/xjc -d /path/to/java/project \
-p stack24174963.datacite \
 https://schema.datacite.org/meta/kernel-4.1/metadata.xsd

This will reply with

parsing a schema...
compiling a schema...
stack24174963/datacite/Box.java
stack24174963/datacite/ContributorType.java
stack24174963/datacite/DateType.java
stack24174963/datacite/DescriptionType.java
stack24174963/datacite/FunderIdentifierType.java
stack24174963/datacite/NameType.java
stack24174963/datacite/ObjectFactory.java
stack24174963/datacite/Point.java
stack24174963/datacite/RelatedIdentifierType.java
stack24174963/datacite/RelationType.java
stack24174963/datacite/Resource.java
stack24174963/datacite/ResourceType.java
stack24174963/datacite/TitleType.java
stack24174963/datacite/package-info.java

If you look into Resource.Creator and Resource.Contributor you will see that the member variables givenName and familyName are not correctly typed. Change their type from Object to String, also apply your changes to the corresponding getter and setter methods!

2. Read in as XML write out as JSON using Jackson

import com.fasterxml.jackson.databind.ObjectMapper;
  import com.fasterxml.jackson.databind.SerializationFeature;

  import stack24174963.datacite.Resource;

  public class HowToXmlToJsonWithSchema {
    @Test
    public void readXmlAndConvertToSchema() throws Exception {
        String example = "schemas/datacite/kernel-4.1/example/datacite-example-complicated-v4.1.xml";
        try (InputStream in = Thread.currentThread().getContextClassLoader().getResourceAsStream(example)) {
            Resource resource = JAXB.unmarshal(in, Resource.class);
            System.out.println(asJson(resource));
        }
    }

    private String asJson(Object obj) throws Exception {
        StringWriter w = new StringWriter();
        new ObjectMapper().configure(SerializationFeature.INDENT_OUTPUT, true).writeValue(w, obj);
        String result = w.toString();
        return result;
    }
  }

Prints:

{
  "identifier" : {
    "value" : "10.5072/testpub",
    "identifierType" : "DOI"
  },
  "creators" : {
    "creator" : [ {
      "creatorName" : {
        "value" : "Smith, John",
        "nameType" : "PERSONAL"
      },
      "givenName" : "John",
      "familyName" : "Smith",
      "nameIdentifier" : [ ],
      "affiliation" : [ ]
    }, {
      "creatorName" : {
        "value" : "つまらないものですが",
        "nameType" : null
      },
      "givenName" : null,
      "familyName" : null,
      "nameIdentifier" : [ {
        "value" : "0000000134596520",
        "nameIdentifierScheme" : "ISNI",
        "schemeURI" : "http://isni.org/isni/"
      } ],
      "affiliation" : [ ]
    } ]
  },
  "titles" : {
    "title" : [ {
      "value" : "Właściwości rzutowań podprzestrzeniowych",
      "titleType" : null,
      "lang" : "pl"
    }, {
      "value" : "Translation of Polish titles",
      "titleType" : "TRANSLATED_TITLE",
      "lang" : "en"
    } ]
  },
  "publisher" : "Springer",
  "publicationYear" : "2010",
  "resourceType" : {
    "value" : "Monograph",
    "resourceTypeGeneral" : "TEXT"
  },
  "subjects" : {
    "subject" : [ {
      "value" : "830 German & related literatures",
      "subjectScheme" : "DDC",
      "schemeURI" : null,
      "valueURI" : null,
      "lang" : "en"
    }, {
      "value" : "Polish Literature",
      "subjectScheme" : null,
      "schemeURI" : null,
      "valueURI" : null,
      "lang" : "en"
    } ]
  },
  "contributors" : {
    "contributor" : [ {
      "contributorName" : {
        "value" : "Doe, John",
        "nameType" : "PERSONAL"
      },
      "givenName" : "John",
      "familyName" : "Doe",
      "nameIdentifier" : [ {
        "value" : "0000-0001-5393-1421",
        "nameIdentifierScheme" : "ORCID",
        "schemeURI" : "http://orcid.org/"
      } ],
      "affiliation" : [ ],
      "contributorType" : "DATA_COLLECTOR"
    } ]
  },
  "dates" : null,
  "language" : "de",
  "alternateIdentifiers" : {
    "alternateIdentifier" : [ {
      "value" : "937-0-4523-12357-6",
      "alternateIdentifierType" : "ISBN"
    } ]
  },
  "relatedIdentifiers" : {
    "relatedIdentifier" : [ {
      "value" : "10.5272/oldertestpub",
      "resourceTypeGeneral" : null,
      "relatedIdentifierType" : "DOI",
      "relationType" : "IS_PART_OF",
      "relatedMetadataScheme" : null,
      "schemeURI" : null,
      "schemeType" : null
    } ]
  },
  "sizes" : {
    "size" : [ "256 pages" ]
  },
  "formats" : {
    "format" : [ "pdf" ]
  },
  "version" : "2",
  "rightsList" : {
    "rights" : [ {
      "value" : "Creative Commons Attribution-NoDerivs 2.0 Generic",
      "rightsURI" : "http://creativecommons.org/licenses/by-nd/2.0/",
      "lang" : null
    } ]
  },
  "descriptions" : {
    "description" : [ {
      "content" : [ "\n      Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea\n      takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores\n      et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.\n    " ],
      "descriptionType" : "ABSTRACT",
      "lang" : "la"
    } ]
  },
  "geoLocations" : null,
  "fundingReferences" : null
}

For XML input:

<?xml version="1.0" encoding="UTF-8"?>
  <resource xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://datacite.org/schema/kernel-4" xsi:schemaLocation="http://datacite.org/schema/kernel-4 http://schema.datacite.org/meta/kernel-4.1/metadata.xsd">
    <identifier identifierType="DOI">10.5072/testpub</identifier>
    <creators>
      <creator>
        <creatorName nameType="Personal">Smith, John</creatorName>
        <givenName>John</givenName>
        <familyName>Smith</familyName>
      </creator>
      <creator>
        <creatorName>つまらないものですが</creatorName>
        <nameIdentifier nameIdentifierScheme="ISNI" schemeURI="http://isni.org/isni/">0000000134596520</nameIdentifier>
      </creator>
    </creators>
    <titles>
      <title xml:lang="pl">Właściwości rzutowań podprzestrzeniowych</title>
      <title xml:lang="en" titleType="TranslatedTitle">Translation of Polish titles</title>
    </titles>
    <publisher>Springer</publisher>
    <publicationYear>2010</publicationYear>
    <subjects>
      <subject xml:lang="en" subjectScheme="DDC">830 German &amp; related literatures</subject>
      <subject xml:lang="en">Polish Literature</subject>
    </subjects>
    <contributors>
      <contributor contributorType="DataCollector">
        <contributorName nameType="Personal">Doe, John</contributorName>
        <givenName>John</givenName>
        <familyName>Doe</familyName>
        <nameIdentifier nameIdentifierScheme="ORCID" schemeURI="http://orcid.org/">0000-0001-5393-1421</nameIdentifier>
      </contributor>
    </contributors>
    <language>de</language>
    <resourceType resourceTypeGeneral="Text">Monograph</resourceType>
    <alternateIdentifiers>
      <alternateIdentifier alternateIdentifierType="ISBN">937-0-4523-12357-6</alternateIdentifier>
    </alternateIdentifiers>
    <relatedIdentifiers>
      <relatedIdentifier relatedIdentifierType="DOI" relationType="IsPartOf">10.5272/oldertestpub</relatedIdentifier>
    </relatedIdentifiers>
    <sizes>
      <size>256 pages</size>
    </sizes>
    <formats>
      <format>pdf</format>
    </formats>
    <version>2</version>
    <rightsList>
      <rights rightsURI="http://creativecommons.org/licenses/by-nd/2.0/">Creative Commons Attribution-NoDerivs 2.0 Generic</rights>
    </rightsList>
    <descriptions>
      <description xml:lang="la" descriptionType="Abstract">
        Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea
        takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores
        et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.
      </description>
    </descriptions>
  </resource>

See also:

https://stackoverflow.com/a/49977184/1485527

and

https://github.com/jschnasse/overflow/tree/master/src/test/java/stack24174963