The guest operations agent could not be contacted

I tried to read an environment variable from the guest using Java API and got :

Caused by: javax.xml.ws.soap.SOAPFaultException: The guest operations agent could not be contacted.

at com.sun.xml.internal.ws.fault.SOAP11Fault.getProtocolException(Unknown Source)

at com.sun.xml.internal.ws.fault.SOAPFaultBuilder.createException(Unknown Source)

at com.sun.xml.internal.ws.client.sei.SyncMethodHandler.invoke(Unknown Source)

at com.sun.xml.internal.ws.client.sei.SyncMethodHandler.invoke(Unknown Source)

at com.sun.xml.internal.ws.client.sei.SEIStub.invoke(Unknown Source)

My question is : How to solve this ?

Some details :

vSphere : 5.1

Guest OS : CentOS 6.4