500 java.io.IOException: Error writing to server

[HeroLi]

Dear all

        when i use PutQuery(Pivot-web-2.0.jar) to execute DB update.got a 
exception as below:

        org.apache.pivot.web.QueryException: 500 java.io.IOException: 
Error writing to server

        Code:
                        PutQuery putQuery
                                        = new PutQuery(PivotConstants.
hostName,
                                                        PivotConstants.
hostPort,
                                                        PivotConstants.
hostPath + "XX.jsp",false);
                        putQuery.getParameters().put("SQL", sql string);
                        try {
                                // update
                                putQuery.execute();
                        } catch (Exception ex) {
                                ...
                        }

when sql is simple like "update a set ...".It's OK.
when sql'length over some limit(actually still very simple just like 100 
update sql),the exception above comes out.

My test server is tomcat.and IDE is eclipse.

I really don't understand where this exception comes .Can anyone give me a 
hint or sth else?

Thanks and regards

Hero