Atomic update in solr using solrj - solr

Question

I've web page which using for update user's password. When I run my servlet, I want to update user's password in my solr document. I wrote this code for it.
String username = request.getParameter("changeUser");
String newPass = request.getParameter("changePassword");
String userPassMsg = "Changing user's password is successful!";
String url = "http://localhost:8983/solr/userCore";
SolrClient solr = new HttpSolrClient.Builder(url).build();
((HttpSolrClient) solr).setParser(new XMLResponseParser());
SolrQuery query = new SolrQuery();
query.setQuery("userName:\""+username+"\"");
query.setFields("password");
query.setStart(0);
query.set("defType", "edismax");
try {
QueryResponse resp = solr.query(query);
SolrDocumentList list = resp.getResults();
String SolrID = (String) list.get(0).getFieldValue("id");
SolrInputDocument solrDocToIndex = new SolrInputDocument();
solrDocToIndex.setField("id", SolrID);
Map<String,String> partialUpdate = new HashMap<>();
partialUpdate.put("set", newPass);
solrDocToIndex.setField("password", partialUpdate);
solr.add(solrDocToIndex);
solr.close();
request.getSession().setAttribute("userPassMsg", userPassMsg);
request.getRequestDispatcher("upload.jsp").forward(request, response);
} catch (SolrServerException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
When I'm running this code it gives me this error
org.apache.solr.client.solrj.impl.HttpSolrClient$RemoteSolrException: Error from server at http://localhost:8983/solr/userCore: Document is missing mandatory uniqueKey field: id
at org.apache.solr.client.solrj.impl.HttpSolrClient.executeMethod(HttpSolrClient.java:610)
at org.apache.solr.client.solrj.impl.HttpSolrClient.request(HttpSolrClient.java:279)
at org.apache.solr.client.solrj.impl.HttpSolrClient.request(HttpSolrClient.java:268)
at org.apache.solr.client.solrj.SolrRequest.process(SolrRequest.java:149)
at org.apache.solr.client.solrj.SolrClient.add(SolrClient.java:173)
at org.apache.solr.client.solrj.SolrClient.add(SolrClient.java:138)
at org.apache.solr.client.solrj.SolrClient.add(SolrClient.java:152)
at org.solr.changeUserPassServlet.doPost(changeUserPassServlet.java:71)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:661)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:493)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:650)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:800)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:806)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1498)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Unknown Source)
I set id as unique key in my schema.xml. Why it gives me this error??