All the code is in http://code.google.com/p/pagingappfuse go to http://code.google.com/p/pagingappfuse/source/checkout for instructions on checking out the whole project, or you can browse the source from http://code.google.com/p/pagingappfuse/source/browse

Overview

I saw the blog post by Ram Gorti at http://i-work-i-blog.blogspot.com/2007/06/display-tag-and-external-pagination.html and have re-used code and adapted the implementation to fit into a web project built on appfuse. Thanks Ram!

Getting setup

The following will get a new project into a state needed to build the paging and sorting: Start with a SpringMVC basic project and generate in the same was as the appfuse quickstart guide shows here: http://appfuse.org/display/APF/AppFuse+QuickStart e.g.

mvn archetype:create -DarchetypeGroupId=org.appfuse.archetypes -DarchetypeArtifactId=appfuse-basic-spring -DremoteRepositories=http://static.appfuse.org/releases -DarchetypeVersion=2.0.2 -DgroupId=com.pobox.paging -DartifactId=pagingappfuse

Run the app to download dependencies and so on - I'm using h2 as an RDBMS here: mvn -Ph2 then generate eclipse project files: mvn eclipse:eclipse

Now add an entity to persist then try to page and sort with. create a Hibernate annotated POJO called Authority in the model package like this: Authority.java

Generate the CRUD logic and screens - mvn appfuse:gen -Dentity=Authority Because the pluralisation as generated is slightly wrong in this example, fix the bean definition for urlMapping in /paging/src/main/webapp/WEB-INF/dispatcher-servlet.xml by adding /authorities.html=authorityController and also you should rename the file /paging/src/main/webapp/WEB-INF/pages/authoritys.jsp to /paging/src/main/webapp/WEB-INF/pages/authorities.jsp

Add some sample data into /paging/src/test/resources/sample-data.xml (you could also add it via the web app itself but that would be tedious) You'll need more than 25 records to trigger paging on the authorties screen Display Tag table. The version here has over 100 records which you could re-use.

Run the app: mvn jetty:run-war -Ph2 browse to http://localhost:8080 login and navigate via the menu to Authority List page - see the table with records in it.

The Problem

With the project as is you should be able to see the problem: Display Tag allows paging and sorting, if you click on a column header to sort ascending or descending. Note that it only sorts the records that are on the page - all the other records are excluded from this 'internal' Display Tag sort. Similarly when the page was generated every single record was retrieved, but only 25 were shown. With big datasets this is going to slow your web app down, (a lot), and the inconsistent sorting will confuse your users. This is what we are going to fix.

Proposed Solution

Display Tag allows you to implement a PaginatedList interface, if it gets passed one of these on your page to use as a model it assumes you are doing all paging and sorting externally and modifies the request URL's it send back from a page to your controller to something slightly less cryptic than the usual. Our controller can take advantage of this to only find the number of records required to display this page full of info, and the total number of records that the query would return if we didn't limit it. Similarly we can amend our query to get the RDBMS (via Hibernate) to sort on the users request column and then return the X number records required - fixing the sorting as well. To make this work we will need to keep track of what page we are on, number of records per page, the total number of records etc. and modify queries accordingly; which is easiest by adding some helper objects and a new PagingDAO similar to the existing LookupDAO in Appfuse.

THE STEPS

extend Paginated List

In the blog entry by Ram Gorti he has a nice implementation of an extended PaginatedList, (original is here http://javanotes.wikispaces.com/PaginatedListImpl ) keeping track of everything we need and doing the sums about pagesizes and numbers. So I reused this wholesale - the code for this is in PaginatedListImpl.java

Convenience factory method for PaginatedList from HttpRequest

Add a convenience creation method that unwraps the HttpRequest getting all the Display Tag info out of it returning a new populated object that extends PaginatedList, already correctly setup - all that's needed is the subset (page) of List records and the total to be set. I put this into a helper object at PaginateListFactory.java This has a null check for HttpRequest so out appfuse unit tests work outside of a web container, it has sensible defaults if it can't unwrap the http request.

Add a Manager in the service layer

Similar to the exisitng LookupDAO in appfuse, add a manager that accepts a class , and one of our extended PaginatedList objects. It will call down onto a new PagingLookupDAO to get the page of records, correctly sorted as requested, and the Total count of records. It sets the results of these DAO calls into the extended PaginateList object itself and returns. This is implemented in PagingLookupManagerImpl.java, there is an interface for this too.

Add the DAO

This object has two methods, find the page of records required (correctly sorted) and find the total number of records. This is implemented using Hibernate queries, I'd be interested to hear if there are ways of improving these two methods.Source is in PagingLookupDaoHibernate.java there is an interface for this too.

Amend the controller

AuthorityController now uses the new manager instead of GenericManager and has the helper PaginatedListFactory injected too by Spring. It uses this to created our new PaginatedList object, correctly populated (using the Factory method in the helper), calls the manager to get the the page required with sort options etc, then puts the PaginatedList into the ModelAndView. When you add it to the model, give it the same name as used in your display Tag tag id on the jsp page - it didn't need to have an explicit name before because of Springs magic name discovery stuff, but now we want to explicitly name it "authorityList". Code for this is in AuthorityController.java

Despatcher-servlet amend controller bean

add a property for the helper to be injected into authorityController in despatcher-servlet.xml, also change the manager property to inject pagingLookupManager instead of GenericManager. source is here dispatcher-servlet.xml

Add new manager, dao and helper beans to app context

In your applicationContext.xml add in bean definitions for the pagingLookupDao, pagingLookupManager and paginatedListFactory. example source is here: application-context.xml

Change AuthorityControllerTest test

The last line of the unit test does an cast for a List - change this to assert an instance of PaginatedList then getList before the size assertion. see AuthorityControllerTest.java

Run it and try new paging

execute mvn jetty:run-war -Ph2 then login and browse to the Authority List page - try the new paging and see that sorting of columns now works. This places much less load on the database as it only retrieves required records. Note also the URL of the requests from Display tag for paging and sorting are now less cryptic - it has recognized that a PaginatedList was supplied and is handing off all calls for sorting and paging correctly.