PrimeFaces LazyDataModel resolves rows incorrectly after live scroll? - jsf

Question

I'm trying to use PrimeFaces 5.3 (with JSF 2.2) to implement a data table that contains items, and each item has a list of command buttons to perform server side actions. I use primefaces LazyDataModel because I have huge datasets and cannot afford to load all the items when the view loads. But after livescrolling happens, rows do not behave as expected.
I have setup a small project to illustrate my issue. (full sources at the end of the post)
The issue
When I press A0, "Executed A0" must be printed on the console (and so on).
Everything works fine for the first data set. Here's what happens when I press the first three buttons:
Fetching starting at 0 //expected result
Executed A0 //A0
Executed A1 //A1
Executed A2 //A2
But when I scroll, and 3 other items are loaded, the first three rows do not work correctly.
Here's what happens when I press all the buttons in order:
Fetching starting at 3 //expected result
Executed A3 //A0
Executed A4 //A1
Executed A5 //A2
Executed A3 //A3
Executed A4 //A4
Executed A5 //A5
The first three buttons execute the actions of the last three ones :(
A server-side issue
It is not a client-side issue, I have monitored the requests sent.
When I press A0, the request contains the following POST parameters:
j_idt8:resultsTable:0:j_idt12:0:j_idt13=j_idt8:resultsTable:0:j_idt12:0:j_idt13
javax.faces.source=j_idt8:resultsTable:0:j_idt12:0:j_idt13
And when I press A3:
j_idt8:resultsTable:3:j_idt12:0:j_idt13=j_idt8:resultsTable:3:j_idt12:0:j_idt13
javax.faces.source=j_idt8:resultsTable:3:j_idt12:0:j_idt13
The correct row is referenced after resultsTable:
Debug mode
I also messed with the debug mode. When I press A1, the UIData#brodacast(FacesEvent) is called with an event that contains a field rowIndex=1. So it works fine at this point.
Then, I don't really understand what happens. But at the end of the process, the target method is executed on the wrong instance...
Source code
Item.java
public class Item {
private int id;
private String name;
private List<ItemAction> actions;
public Item(int id, String name, List<ItemAction> actions) {
this.id = id;
this.name = name;
this.actions = actions;
}
public int getId() {
return id;
}
public String getName() {
return name;
}
public List<ItemAction> getActions() {
return actions;
}
}
ItemAction.java
public class ItemAction {
private String label;
public ItemAction(String label) {
this.label = label;
}
public void execute(){
System.out.println("Executed " + label);
}
public String getLabel() {
return label;
}
}
TheService.java
public class TheService {
private static final int ITEMS_COUNT = 8;
public int getItemsCount() {
return ITEMS_COUNT;
}
public List<Item> retrieveItems(int startIndex, int pageSize){
List<Item> result = new ArrayList<>(pageSize);
int endIndex = startIndex + pageSize;
if(endIndex > ITEMS_COUNT){
endIndex = ITEMS_COUNT;
}
for(int index=startIndex; index<endIndex; index++){
result.add(generateItem(index));
}
return result;
}
private Item generateItem(int id){
List<ItemAction> actions = new ArrayList<>(2);
actions.add(new ItemAction("A" + id));
actions.add(new ItemAction("B" + id));
return new Item(id, "Item " + id, actions);
}
}
DatatableLazyModel.java
#ManagedBean
#ViewScoped
public class DatatableLazyModel extends LazyDataModel<Item>{
private TheService service;
private List<Item> allResults;
public DatatableLazyModel() {
this.service = new TheService();
this.allResults = new ArrayList<>();
}
#Override
public int getRowCount() {
return service.getItemsCount();
}
#Override
public List<Item> load(int first, int pageSize, String sortField, SortOrder sortOrder,
Map<String, Object> filters) {
System.out.println("Fetching starting at " + first);
List<Item> pageResults = service.retrieveItems(first, pageSize);
allResults.addAll(pageResults);
return pageResults;
}
}
index.xhtml
<?xml version='1.0' encoding='UTF-8' ?>
<!DOCTYPE composition PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<ui:composition xmlns="http://www.w3.org/1999/xhtml"
xmlns:ui="http://java.sun.com/jsf/facelets"
xmlns:h="http://java.sun.com/jsf/html"
xmlns:f="http://java.sun.com/jsf/core"
xmlns:p="http://primefaces.org/ui" template="template/ui.xhtml">
<ui:define name="body">
<h3>Datatablelazy load</h3>
<h:form>
<p:dataTable id="resultsTable" var="item"
value="#{datatableLazyModel}" liveScroll="true" scrollRows="3"
scrollHeight="100" scrollable="true" lazy="true">
<p:column headerText="Name">
<h:outputText value="#{item.name}" />
</p:column>
<p:column headerText="Actions">
<ui:repeat var="action" value="#{item.actions}">
<p:commandButton action="#{action.execute()}" value="#{action.label}">
</p:commandButton>
</ui:repeat>
</p:column>
</p:dataTable>
</h:form>
</ui:define>
</ui:composition>
pom.xml dependencies
<dependency>
<groupId>com.sun.faces</groupId>
<artifactId>jsf-api</artifactId>
<version>2.2.4</version>
</dependency>
<dependency>
<groupId>com.sun.faces</groupId>
<artifactId>jsf-impl</artifactId>
<version>2.2.4</version>
</dependency>
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>3.0.1</version>
</dependency>
<dependency>
<groupId>org.primefaces</groupId>
<artifactId>primefaces</artifactId>
<version>5.3</version>
</dependency>
Questions
I have tried this with more items loaded. Each time, only methods on the last loaded set are executed. Even from old components.
So basically, the question is : "How can I make all the buttons work as expected?"
Why do methods do not get executed on the correct instance? How is the target instance resolved?
Thank you for your help :)

Answer 1

The issue is with the LazyDataModel implementation that is meant to be used with pages and not live scroll, especially this snippet :
if(rowIndex == -1 || pageSize == 0)
this.rowIndex = -1;
else
this.rowIndex = (rowIndex % pageSize);
I subclassed LazyDataModel with my own implementation to solve the problem.
LiveScrollLazyDataModel.java
public abstract class LiveScrollLazyModel<T> extends LazyDataModel<T> {
private int rowIndex;
#Override
public void setRowIndex(int rowIndex) {
int oldIndex = getRowIndex();
if (rowIndex == -1 || getPageSize() == 0){
super.setRowIndex(-1);
this.rowIndex = -1;
}else{
super.setRowIndex(rowIndex);
this.rowIndex = rowIndex;
}
if (getResults() == null) {
return;
}
DataModelListener[] listeners = getDataModelListeners();
if (listeners != null && oldIndex != this.rowIndex) {
Object rowData = null;
if (isRowAvailable()) {
rowData = getRowData();
}
DataModelEvent dataModelEvent = new DataModelEvent(this, rowIndex, rowData);
for (int i = 0; i < listeners.length; i++) {
listeners[i].rowSelected(dataModelEvent);
}
}
}
#Override
public T getRowData() {
return getResults().get(this.rowIndex);
}
#Override
public boolean isRowAvailable() {
if(getResults() == null) {
return false;
}
return rowIndex >= 0 && rowIndex < getResults().size();
}
/**
* The complete list of results
* #return
*/
public abstract List<T> getResults();
}
The implementor needs to implement the getResults() method that provides the complete list of results.