If you are just starting with RichFaces, it’s likely you will run into this case.
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The page is rendered for the first time and #{bean.show} is evaluated to false. This means the panel grid is now rendered. Keep in mind, the actual component is still in the component tree. No HTML is being rendered back to the browser, the browser doesn’t know anything about this component.
Now, you click the button, #{bean.show} is evaluated to true, and you are trying to re render the panel grid. But, it doesn’t work. Nothing is rendered. Here is what happens. Remember that when the page was shown for the first time, no HTML was rendered. This basically means there is nothing in browser’s DOM about this component (more correct HTML). So, when the button is clicked, content is rendered back to the browser, but the JavaScript client inside the browser doesn’t know what part of the DOM to update with this content as it didn’t exist before. Update is done based on id’s, in this case, it’s looking for id of items, but can’t find it (actually form:id). How do we solve this problem? Well, one way is to create a place holder. The following two are basically identical:
We are putting a place holder around the grid and re rendering it instead:
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You can also use h:panelGrid instead of h:panelGroup.
this is virtually the same thing:
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Why would you use a4j:outputPanel instead of h:panelGroup, because this will also work:
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with ajaxRendered=”true”, we no longer to point to the grid via reRender. Setting this attribute to true tells RichFaces to always re render this component.
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Max Katz 
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