JBAS-7817: java.lang.IllegalArgumentException

We encounter this issue. The stacktrace is:

java.lang.IllegalArgumentException
at java.nio.Buffer.position(Buffer.java:218)
at org.apache.tomcat.util.buf.B2CConverter.convert(B2CConverter.java:84)
at org.apache.catalina.connector.InputBuffer.realReadChars(InputBuffer.java:403)
at org.apache.tomcat.util.buf.CharChunk.substract(CharChunk.java:383)
at org.apache.catalina.connector.InputBuffer.read(InputBuffer.java:416)
at org.apache.catalina.connector.CoyoteReader.read(CoyoteReader.java:93)
at com.filenet.bp8.api.util.Bp8XMLUtil.getDocumentFromReader(Bp8XMLUtil.java:92)
at com.filenet.bp8.apps.server.servlet.Bp8ActionDispatcherServlet.doPost(Bp8ActionDispatcherServlet.java:67)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:235)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:190)
at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:92)
at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.process(SecurityContextEstablishmentValve.java:126)
at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.invoke(SecurityContextEstablishmentValve.java:70)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:158)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:330)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:829)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:601)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)

What we want to do is to get the content from HTTP Post request:
In the servlet's doPost method, we invoked another utility class's getDocumentFromReader(Reader r) method:

public static Document getDocumentFromReader(Reader r) throws IOException,SAXException {
BufferedReader br = (BufferedReader) r;
int i;
StringBuffer sb = new StringBuffer();
try {
while ((i = br.read()) != -1)
sb.append((char) i);