What can be the cause of "Etat HTTP 500" error? how can resolve it?

I have a problem in dynamic web application runned via eclipse, every time I try to test it, this error message is outputed (knowing that some links work but after short time are crashed):

Etat HTTP 500 - 

--------------------------------------------------------------------------------

type Rapport d'exception

message 

desc开发者_高级运维ription Le serveur a rencontré une erreur interne () qui l'a empêché de satisfaire la requête.

exception 

javax.servlet.ServletException
    org.n52.oxf.rest.sos.RestSosFrontend.doGet(RestSosFrontend.java:462)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:802)


cause mère 

javax.servlet.ServletException
    org.n52.oxf.rest.sos.RestSosFrontend.showResultsInPreferredMIMEType(RestSosFrontend.java:696)
    org.n52.oxf.rest.sos.RestSosFrontend.doGet(RestSosFrontend.java:456)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:802)


cause mère 

org.n52.oxf.OXFException
    org.n52.oxf.feature.sos.SOSObservationStore.unmarshalFeatures100(SOSObservationStore.java:114)
    org.n52.oxf.feature.sos.SOSObservationStore.unmarshalFeatures(SOSObservationStore.java:63)
    org.n52.oxf.rest.sos.RestSosFrontend.showResultsInPreferredMIMEType(RestSosFrontend.java:669)
    org.n52.oxf.rest.sos.RestSosFrontend.doGet(RestSosFrontend.java:456)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:802)


note La trace complète de la cause mère de cette erreur est disponible dans les fichiers journaux de Apache Tomcat/5.5.12.


--------------------------------------------------------------------------------

Apache Tomcat/5.5.12

HTTP 500 generally means that your program on the server side crashed or otherwise errored out.

In your case, a servlet had an error at

org.n52.oxf.rest.sos.RestSosFrontend.showResultsInPreferredMIMEType(RestSosFrontend.java:696)

and the exception was

org.n52.oxf.OXFException