qr code scanner java app Binding pages to stateful Seam components in Java

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17.2.2 Binding pages to stateful Seam components
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The search.xhtml page doesn t change at all; review the code in listing 17.2. This page has a value binding to itemEditor.itemId and an action binding to itemEditor.doSearch. When the page is rendered by the JSF servlet, these expressions are evaluated, and the widgets are bound to the respective methods in the itemEditor bean.
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Introducing JBoss Seam
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The EJB component interface The itemEditor bean is now an EJB. The interface of this EJB is EditItem.java (listing 17.8).
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Listing 17.8 The interface of a stateful component
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package auction.beans; import ... public interface EditItem { // Value binding methods public Long getItemId(); public void setItemId(Long itemId); public Item getItem(); // Action binding methods public String doSearch(); public String doSave(); // Cleanup routine public void destroy(); }
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The first two methods are the getter and setter for the value binding of the search input text field of the page. The getItem() method (you don t need a setter here) will be used later by the edit page. The doSearch() method is bound to the Search button, doSave() will be bound to a button on the edit page. This is an interface for a stateful component. A stateful component is instantiated when it s first requested for example, because a page is rendered for the first time. Every stateful component needs a method that the runtime environment can call when the component is destroyed. You could use the doSave() method and say that the component s lifecycle ends when this method completes, but you ll see in a minute why a separate method is cleaner. Next, let s look at the implementation of this interface. The EJB component implementation The standard stateful component in EJB 3.0 is a stateful session bean. The implementation in EditItemBean.java is a POJO class with a few extra annotations. In listing 17.9, all Seam annotations are shown in bold.
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Listing 17.9 The implementation of a stateful component
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package auction.beans; import ...
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B C D
@Name("itemEditor") @Scope(ScopeType.CONVERSATION)
@Stateful public class EditItemBean implements EditItem { @PersistenceContext EntityManager em;
private Long itemId; public Long getItemId() { return itemId; } public void setItemId(Long itemId) { this.itemId = itemId; } private Item item; public Item getItem() { return item; } @Begin public String doSearch() { item = em.find(Item.class, itemId); if (item == null) FacesMessages.instance().add( "itemSearchField", new FacesMessage("No item found.") ); return item != null "found" : null; }
@End public String doSave() { item = em.merge(item); return "success"; }
@Destroy @Remove public void destroy() {} }
The Seam @Name annotation declares the name for this Seam component. It also turns this EJB into a Seam component. You can now reference this component anywhere under this name. When an instance of this component is required, Seam instantiates it for you. Seam puts the instance into a context under its name. Here s a formal description:
Introducing JBoss Seam
An instance of EditItem is managed by Seam in the conversation context, as a value of the contextual variable itemEditor.
A POJO needs the EJB 3.0 @Stateful annotation to become a stateful session bean. The EJB 3.0 container injects an EntityManager with a fresh persistence context into this bean, before a method is called by any client of the bean. The persistence context is closed when the method returns (assuming this method call is also the scope of a system transaction, which is the default here). This stateful component holds state internally, in the fields itemId and item. The state is exposed with property accessor methods. A Seam @Begin annotation marks a method that begins a long-running conversation. If a JSF action triggers a call to this method, Seam maintains the state of this component across HTTP requests. The doSearch() method returns a string outcome (or null) and generates a JSF message that can be rendered on a page. The Seam FacesMessages helper makes this message-passing easy. A Seam @End annotation marks a method that ends a long-running conversation. If a JSF action triggers a call to this method, and the method returns, Seam will destroy the component s state and no longer maintain it across HTTP requests. The Seam @Destroy annotation marks the method that is called by Seam when the component state has to be destroyed (when the end of the conversation has been reached). This is useful for internal cleanup (there is nothing to do in this case). The EJB 3.0 @Remove annotation marks the method that a client (Seam, in this case) has to call to remove the stateful session bean instance. These two annotations usually appear on the same method. Why don t you mark the doSave() method with @End, @Destroy, and @Remove The doSave() method might throw an exception, and this exception has to roll back any system transaction. Seam, however, logs and swallows any exception thrown by its @Destroy method, so you frequently see empty destroy methods in stateful Seam components. Furthermore, the component instance is needed for a little while after the saving of an item, to render a response. This EJB implementation encapsulates all application logic; there is no more Java code anywhere else (well, there is the Item entity class). If you ignore trivial code, the application logic is only four lines in the two action methods. Two more changes are necessary to make the application work. Some value bindings in edit.xhtml need to be modified, and the block of XML that defined the old JSF backing bean can be removed from faces-config.xml.
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