Steps to reproduce

1. run the project https://github.com/hawkchen/zssperformance on Tomcat 7
2. use Jmeter 2.10 to open test plan IntensiveFormula.jmx (the file is under [PROJECT-BASE]\TestPlan)(original test plan https://docs.google.com/a/potix.com/document/d/1pK-mi4hO14mKkmlf1qZLwi-wp3uU04wcMVzHk6MuS5I/edit#heading=h.x9e2wlg0mqa7)
3. set number of thread to 50 and run
   

Expected

The tomcat 7 console shows no error.

What I actually see

The tomcat 7 console shows many ConcurrentModificationException.

SEVERE:
java.util.ConcurrentModificationException
at java.util.LinkedHashMap$LinkedHashIterator.nextEntry(LinkedHashMap.ja
va:373)
at java.util.LinkedHashMap$EntryIterator.next(LinkedHashMap.java:392)
at java.util.LinkedHashMap$EntryIterator.next(LinkedHashMap.java:391)
at org.zkoss.util.CacheMap.expunge(CacheMap.java:228)
at org.zkoss.util.CacheMap.doExpunge(CacheMap.java:208)
at org.zkoss.util.CacheMap.tryExpunge(CacheMap.java:202)
at org.zkoss.util.CacheMap.get(CacheMap.java:391)
at org.zkoss.poi.ss.format.CellNumberFormatter.getFormatter(CellNumberFo
rmatter.java:112)
at org.zkoss.poi.ss.format.CellFormat$1.apply(CellFormat.java:119)
at org.zkoss.zss.model.impl.sys.FormatEngineImpl.format0(FormatEngineImp
l.java:94)
at org.zkoss.zss.model.impl.sys.FormatEngineImpl.format(FormatEngineImpl
.java:62)
at org.zkoss.zss.ui.impl.CellFormatHelper.getInnerHtmlStyle(CellFormatHe
lper.java:323)
at org.zkoss.zss.ui.Spreadsheet$ExtraCtrl.getCellAttr(Spreadsheet.java:3
290)
at org.zkoss.zss.ui.Spreadsheet$ExtraCtrl.getRangeAttrs(Spreadsheet.java
:3144)
at org.zkoss.zss.ui.Spreadsheet.responseUpdateCell(Spreadsheet.java:2778
)
at org.zkoss.zss.ui.Spreadsheet.updateCell(Spreadsheet.java:2747)
at org.zkoss.zss.ui.Spreadsheet.access$4000(Spreadsheet.java:213)
at org.zkoss.zss.ui.Spreadsheet$InnerModelEventDispatcher.onCellContentC
hange(Spreadsheet.java:2346)
at org.zkoss.zss.ui.Spreadsheet$InnerModelEventDispatcher.access$1200(Sp
readsheet.java:2058)
at org.zkoss.zss.ui.Spreadsheet$InnerModelEventDispatcher$7.onEvent(Spre
adsheet.java:2102)
at org.zkoss.zss.ui.ModelEventDispatcher.onEvent(ModelEventDispatcher.ja
va:49)
at org.zkoss.zss.model.impl.EventQueueListenerAdaptor$WrappedListener.on
Event(EventQueueListenerAdaptor.java:105)
at org.zkoss.zk.ui.event.impl.DesktopEventQueue$QueueListener.onEvent(De
sktopEventQueue.java:152)
at org.zkoss.zk.ui.AbstractComponent.onEvent(AbstractComponent.java:2746
)
at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2717
)
at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2658
)
at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:136)
at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1765
)
at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1550)
at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1260)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.ja
va:603)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java
:485)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.jav
a:494)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Appl
icationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationF
ilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperV
alve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextV
alve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(Authentica
torBase.java:501)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.j
ava:171)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.j
ava:102)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:
950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineVal
ve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.jav
a:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp
11Processor.java:1040)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(
AbstractProtocol.java:607)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpo
int.java:2442)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoin
t.java:2431)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExec
utor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor
.java:908)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskTh
read.java:61)
at java.lang.Thread.run(Thread.java:662)