Can a Java ManagedBean method return null?

Question

I have always returned null when I wanted the user to return to the same page. For example, if the user requests to Edit Groups, but he doesn't have access to this, I usually return null and print some access denied message. If the user is allowed, I just return the name of the next page, like "Edit".

For example,

@ManagedBean(name = "groupController")
@SessionScoped
public class GroupController {

    public String prepareEdit() {
        if (userIsAllowed()) {
            return "Edit";
        } else {
            JsfUtil.addAccessDeniedMessage("Access denied.");
            return null; // return to the same page
        }
    }

    private boolean userIsAllowed() {
        return false; // for testing purpose
    }
}

When I return null, the same page is returned to the user. So far, so good. But the problem is that after 2 requests I always get a NullPointerException like this:

WARNING: StandardWrapperValve[Faces Servlet]: PWC1406: Servlet.service() for servlet Faces Servlet threw exception java.lang.NullPointerException at com.sun.faces.application.view.StateManagementStrategyImpl$4.invokeContextCallback(StateManagementStrategyImpl.java:289) at javax.faces.component.UIComponent.invokeOnComponent(UIComponent.java:1253) at javax.faces.component.UIComponentBase.invokeOnComponent(UIComponentBase.java:672) at javax.faces.component.UIComponent.invokeOnComponent(UIComponent.java:1262) at javax.faces.component.UIComponentBase.invokeOnComponent(UIComponentBase.java:672) at com.sun.faces.application.view.StateManagementStrategyImpl.restoreView(StateManagementStrategyImpl.java:284) at com.sun.faces.application.StateManagerImpl.restoreView(StateManagerImpl.java:177) at com.sun.faces.application.view.ViewHandlingStrategy.restoreView(ViewHandlingStrategy.java:131) at com.sun.faces.application.view.FaceletViewHandlingStrategy.restoreView(FaceletViewHandlingStrategy.java:430) at com.sun.faces.application.view.MultiViewHandler.restoreView(MultiViewHandler.java:143) at com.sun.faces.lifecycle.RestoreViewPhase.execute(RestoreViewPhase.java:199) at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101) at com.sun.faces.lifecycle.RestoreViewPhase.doPhase(RestoreViewPhase.java:110) at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118) at javax.faces.webapp.FacesServlet.service(FacesServlet.java:312) at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1523) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:279) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:188) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:641) at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:97) at com.sun.enterprise.web.PESessionLockingStandardPipeline.invoke(PESessionLockingStandardPipeline.java:85) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:185) at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:332) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:233) at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:165) at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:791) at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:693) at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:954) at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:170) at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:135) at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:102) at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:88) at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:76) at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:53) at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:57) at com.sun.grizzly.ContextTask.run(ContextTask.java:69) at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:330) at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:309) at java.lang.Thread.run(Thread.java:619)

Does anybody know if is correct to return null to get the same page? I didn't find any specification.

BalusC BalusC · Accepted Answer · 2011-10-18T14:07:24

Returning null is perfectly fine.

Your problem is caused by something else. I bet that you're using a very early Mojarra 2.0 version which has some serious bugs in partial state saving. I suggest to upgrade to the latest 2.0 or even 2.1. You can find them all at Mojarra homepage.

Can a Java ManagedBean method return null?

1 Answers