Migrating JSF 1.1 with Ajax4jsf 1.x to JSF 2

We are migrating JSF 1.1 (MyFaces) project to JSF 2. The idea is to migrate periodically by keeping both JSP and XHTML together for some time. We use many ajax4jsf-1.1.1 tags in JSP pages. We don't use RichFaces. After configuring the system to JSF 2 (with all config changes mentioned in tutorial by Balusc) When tried to access the JSP page with ajax4jsf.jar in classpath, we get an exception:

Caused by: java.lang.IllegalStateException: setViewHandler may not be executed after a lifecycle request has been completed
    at org.apache.myfaces.application.ApplicationImpl.setViewHandler(ApplicationImpl.java:853)
    at org.ajax4jsf.framework.ajax.InitPhaseListener.beforePhase(InitPhaseListener.java:92)
    at org.apache.myfaces.lifecycle.PhaseListenerManager.informPhaseListenersBefore(PhaseListenerManager.java:76)
    at org.apache.myfaces.lifecycle.LifecycleImpl.executePhase(LifecycleImpl.java:131)

It looks ajax4jsf.jar is not compatible with JSF 2. Looks some issue with LifeCycle configuration.

Is there any way we can make a4j work with JSF 2 JSPs? I know when we use XHTML we don't need all this.

Get rid of Ajax4jsf 1.x altogether. It's indeed not compatible with JSF2. Instead, JSF2 offers a new main ajax tag <f:ajax> which covers all the core functionality as previously offered by Ajax4jsf 1.x.

If upgrading to RichFaces 4 is not an option (because, as you said yourself, you aren't using RichFaces components anywhere), then just remove Ajax4jsf 1.x and replace all <a4j:xxx> tags by standard JSF2 equivalents.

<a4j:ajaxListener>: use <f:ajax listener>.
<a4j:keepAlive>: just put managed bean in the view scope by @ViewScoped.
<a4j:log>: use jsf.ajax.addOnEvent() or jsf.ajax.addOnError() in JS context.
<a4j:commandLink>: just nest <f:ajax> inside <h:commandLink>.
<a4j:outputPanel>: use <h:panelGroup> and remember to include its ID in <f:ajax render> or PrimeFaces <p:outputPanel>.
<a4j:repeat>: just use standard <ui:repeat>.
<a4j:form>: just use <h:form>, it will autorecognize <f:ajax>.
<a4j:htmlCommandLink>: just nest <f:ajax> inside <h:commandLink>.
<a4j:jsFunction>: no replacement. Consider OmniFaces <o:commandScript> or PrimeFaces <p:remoteCommand>.
<a4j:region>: just use <f:ajax execute>, you can even wrap <f:ajax> around a group of components.
<a4j:loadBundle>: just use standard <f:loadBundle>.
<a4j:status>: use jsf.ajax.addOnEvent() or jsf.ajax.addOnError() in JS context.
<a4j:actionparam>: just use standard <f:param>.
<a4j:loadScript>: just use standard <h:outputScript>.
<a4j:mediaOutput>: no replacement. Consider PrimeFaces <p:media>.
<a4j:poll>: no replacement. Consider OmniFaces <o:commandScript> or PrimeFaces <p:poll>.
<a4j:commandButton>: just nest <f:ajax> inside <h:commandButton>.
<a4j:include>: just use standard <ui:include>.
<a4j:loadStyle>: just use standard <h:outputStylesheet>.
<a4j:support>: just use standard <f:ajax>.

You also need to rename/rewrite JSP files to Facelets files. In simple cases, this is usually just a matter of changing root declarations and file extensions. Facelets makes it easier to replace all duplicated code by a single template. The following answer applies: