I wanna query and collect work items information of a specific project
by a java plain client. The Java plain client for Jazz has been
downloaded and installed, but I CANNOT find the API interface for
retrieving existing work item of a project. The Snippet sample just
explains how to create work item. Please give a hand. Any hint is
appreciated.

Also take a look at the Developer's Guide to Querying Work Items:
http://jazz.net/wiki/bin/view/Main/QueryDevGuide

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Regards,
Patrick
Jazz Work Item Team

you should really ask in this forum: http://jazz.net/forums/viewforum.php?f=2 since they are talking about exactly your questions there. You could also search the forum, since I am positive I found the answer to your question there.

In the meantime here some code that I happen to have and I believe should also work with the Plain Java Api. This is only one possible way to do it. You can create and run queries etc.

IWorkItemClient workItemService = (IWorkItemClient) targetRepository
.getClientLibrary(IWorkItemClient.class);
IQueryClient queryClient = workItemService.getQueryClient();
IAuditableClient auditableClient = (IAuditableClient)targetRepository.getClientLibrary(IAuditableClient.class);
IQueryableAttribute attribute=QueryableAttributes.getFactory(IWorkItem.ITEM_TYPE).findAttribute(targetProjectArea, IWorkItem.PROJECT_AREA_PROPERTY, auditableClient, monitor);
Expression inProjectArea= new AttributeExpression(attribute,AttributeOperation.EQUALS, targetProjectArea);
IQueryResult<IResolvedResult<IWorkItem>> results=queryClient.getResolvedExpressionResults(targetProjectArea, inProjectArea,IWorkItem.SMALL_PROFILE);

while (results.hasNext(monitor)) {
IResolvedResult<IWorkItem> resresult = (IResolvedResult<IWorkItem>) results.next(monitor);
IWorkItem item = resresult.getItem();
System.out.println(item.getId()+" " + item.getHTMLSummary().toString());
...............