Скачать или смотреть How to Successfully Pass a List as a Parameter to p:commandButton in PrimeFaces

Discover how to resolve issues with passing a `List` to a `p:commandButton` in PrimeFaces while using JSF. Learn about scope management and proper view handling.
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How to Successfully Pass a List as a Parameter to p:commandButton in PrimeFaces

When working with JSF and PrimeFaces, developers often find themselves faced with various challenges regarding data handling within their UI components. One common issue is trying to pass a List as a parameter to a p:commandButton within a p:dataTable. If you’ve run into this problem and noticed that the list is always returning null, you’re not alone! Let’s walk through the solution and understand how to handle this situation effectively.

The Problem

In JSF, when trying to pass a list from a data table to a command button, you might find that the list appears empty or returns as null upon submission. This can be particularly frustrating since the data is displayed correctly in the view before the form is submitted. The scenario we’re addressing involves using PrimeFaces to create a data table that allows interaction with user selections and submitting those selections through a command button.

Code Overview

Let's look at a simplified version of the code that illustrates the issue:

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In this code, chamaService(listaRestricoes) appears to be set up to receive a List. However, during submission, listaRestricoes ends up as null.

The Solution

Understanding Scope

The root cause of the issue lies in the scope of the backing bean used in the controller. In this case, the bean was initially defined with @ RequestScoped, which meant that after the request was processed, all data inside the bean was discarded. Thus, the command button could not access the list values upon form submission.

Step-by-Step Fix

Change bean scope: Switch the bean from @ RequestScoped to @ ViewScoped. This change ensures that the state of the bean is preserved across multiple requests while the user interacts with the same view.

Here's how you can update the bean:

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Clean View After Submission: After successfully processing the list, you may want to clear the view to prevent any unwanted retention of data. You can do this by adding an onsuccess attribute to your command button, which will refresh the page upon completion:

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Final Outcome

After implementing the changes, the command button can now successfully access the list passed from the data table, and you should see the correct values handled by your service.

Conclusion

Understanding the differences between scope types in JSF is crucial for maintaining data integrity across web forms. By switching to @ ViewScoped, you allow your backing bean to hold on to its state between requests, solving the issue of receiving a null list parameter on submission. Now you can confidently utilize p:dataTable and p:commandButton together without worry!

By addressing these key areas, you'll improve your PrimeFaces applications significantly and navigate similar challenges more confidently in the future.