为什么会发生以下异常?
2012-08-28 11:41:59,183 ERROR [org.apache.catalina.core.ContainerBase.[jboss.web].[localhost].[/TFO].[tfo]] (http-0.0.0.0-8080-9) Servlet.service() for servlet tfo threw exception: java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793) [:1.6.0_24]
at java.util.HashMap$EntryIterator.next(HashMap.java:834) [:1.6.0_24]
at java.util.HashMap$EntryIterator.next(HashMap.java:832) [:1.6.0_24]
at net.sf.json.JSONObject._fromMap(JSONObject.java:1082) [:]
at net.sf.json.JSONObject.fromObject(JSONObject.java:173) [:]
at net.sf.json.JSONObject._processValue(JSONObject.java:2552) [:]