Chapter 6. Conversations and workspace management - JBoss Seam 1.2.1 正式版英文参考手册

The examples we have seen so far make use of a very simple conversation model that follows these rules:

Seam transparently propagates the conversation context across JSF postbacks and redirects. If you don't do anything special, a non-faces request (a GET request for example) will not propagate the conversation context and will be processed in a new temporary conversation. This is usually - but not always - the desired behavior.

If you want to propagate a Seam conversation across a non-faces request, you need to explicitly code the Seam conversation id as a request parameter:

<a href="main.jsf?conversationId=#{conversation.id}">Continue</a>

Or, the more JSF-ish:

<h:outputLink value="main.jsf">
    <f:param name="conversationId" value="#{conversation.id}"/>
    <h:outputText value="Continue"/>
</h:outputLink>

If you use the Seam tag library, this is equivalent:

<h:outputLink value="main.jsf">
    <s:conversationId/>
    <h:outputText value="Continue"/>
</h:outputLink>

If you wish to disable propagation of the conversation context for a postback, a similar trick is used:

<h:commandLink action="main" value="Exit">
    <f:param name="conversationPropagation" value="none"/>
</h:commandLink>

If you use the Seam tag library, this is equivalent:

<h:commandLink action="main" value="Exit">
    <s:conversationPropagation type="none"/>
</h:commandLink>

Note that disabling conversation context propagation is absolutely not the same thing as ending the conversation.

The conversationPropagation request parameter, or the <s:conversationPropagation> tag may even be used to begin and end conversation, or begin a nested conversation.

<h:commandLink action="main" value="Exit">
    <s:conversationPropagation type="end"/>
</h:commandLink>

<h:commandLink action="main" value="Select Child">
    <s:conversationPropagation type="nested"/>
</h:commandLink>

<h:commandLink action="main" value="Select Hotel">
    <s:conversationPropagation type="begin"/>
</h:commandLink>

<h:commandLink action="main" value="Select Hotel">
    <s:conversationPropagation type="join"/>
</h:commandLink>

This conversation model makes it easy to build applications which behave correctly with respect to multi-window operation. For many applications, this is all that is needed. Some complex applications have either or both of the following additional requirements:

A nested conversation is created by invoking a method marked @Begin(nested=true) inside the scope of an existing conversation. A nested conversation has its own conversation context, and also has read-only access to the context of the outer conversation. (It can read the outer conversation's context variables, but not write to them.) When an @End is subsequently encountered, the nested conversation will be destroyed, and the outer conversation will resume, by "popping" the conversation stack. Conversations may be nested to any arbitrary depth.

Certain user activity (workspace management, or the back button) can cause the outer conversation to be resumed before the inner conversation is ended. In this case it is possible to have multiple concurrent nested conversations belonging to the same outer conversation. If the outer conversation ends before a nested conversation ends, Seam destroys all nested conversation contexts along with the outer context.

A conversation may be thought of as a continuable state. Nested conversations allow the application to capture a consistent continuable state at various points in a user interaction, thus insuring truly correct behavior in the face of backbuttoning and workspace management.

TODO: an example to show how a nested conversation prevents bad stuff happening when you backbutton.

Usually, if a component exists in a parent conversation of the current nested conversation, the nested conversation will use the same instance. Occasionally, it is useful to have a different instance in each nested conversation, so that the component instance that exists in the parent conversation is invisible to its child conversations. You can achieve this behavior by annotating the component @PerNestedConversation.

JSF does not define any kind of action listener that is triggered when a page is accessed via a non-faces request (for example, a HTTP GET request). This can occur if the user bookmarks the page, or if we navigate to the page via an <h:outputLink>.

Sometimes we want to begin a conversation immediately the page is accessed. Since there is no JSF action method, we can't solve the problem in the usual way, by annotating the action with @Begin.

A further problem arises if the page needs some state to be fetched into a context variable. We've already seen two ways to solve this problem. If that state is held in a Seam component, we can fetch the state in a @Create method. If not, we can define a @Factory method for the context variable.

If none of these options works for you, Seam lets you define a page action in the pages.xml file.

<pages>
    <page view-id="/messageList.jsp" action="#{messageManager.list}"/>
    ...
</pages>

This action method is called at the beginning of the render response phase, any time the page is about to be rendered. If a page action returns a non-null outcome, Seam will process any appropriate JSF and Seam navigation rules, possibly resulting in a completely different page being rendered.

If all you want to do before rendering the page is begin a conversation, you could use a built-in action method that does just that:

<pages>
    <page view-id="/messageList.jsp" action="#{conversation.begin}"/>
    ...
</pages>

Note that you can also call this built-in action from a JSF control, and, similarly, you can use #{conversation.end} to end conversations.

If you want more control, to join existing conversations or begin a nested conversion, to begin a pageflow or an atomic conversation, you should use the <begin-conversation> element.

<pages>
    <page view-id="/messageList.jsp">
       <begin-conversation nested="true" pageflow="AddItem"/>
    <page>
    ...
</pages>

There is also an <end-conversation> element.

<pages>
    <page view-id="/home.jsp">
       <end-conversation/>
    <page>
    ...
</pages>

To solve the first problem, we now have five options:

JSF command links always perform a form submission via JavaScript, which breaks the web browser's "open in new window" or "open in new tab" feature. In plain JSF, you need to use an <h:outputLink> if you need this functionality. But there are two major limitations to <h:outputLink>.

Seam provides the notion of a page action to help solve the first problem, but this does nothing to help us with the second problem. We could work around this by using the RESTful approach of passing a request parameter and requerying for the selected object on the server side. In some cases—such as the Seam blog example application—this is indeed the best approach. The RESTful style supports bookmarking, since it does not require server-side state. In other cases, where we don't care about bookmarks, the use of @DataModel and @DataModelSelection is just so convenient and transparent!

To fill in this missing functionality, and to make conversation propagation even simpler to manage, Seam provides the <s:link> JSF tag.

The link may specify just the JSF view id:

<s:link view=“/login.xhtml” value=“Login”/>

Or, it may specify an action method (in which case the action outcome determines the page that results):

<s:link action=“#{login.logout}” value=“Logout”/>

If you specify both a JSF view id and an action method, the 'view' will be used unless the action method returns a non-null outcome:

<s:link view="/loggedOut.xhtml"  action=“#{login.logout}” value=“Logout”/>

The link automatically propagates the selected row of a DataModel using inside <h:dataTable>:

<s:link view=“/hotel.xhtml” action=“#{hotelSearch.selectHotel}” value=“#{hotel.name}”/>

You can leave the scope of an existing conversation:

<s:link view=“/main.xhtml” propagation=“none”/>

You can begin, end, or nest conversations:

<s:link action=“#{issueEditor.viewComment}” propagation=“nest”/>

If the link begins a conversation, you can even specify a pageflow to be used:

<s:link action=“#{documentEditor.getDocument}” propagation=“begin” 
        pageflow=“EditDocument”/>

The taskInstance attribute if for use in jBPM task lists:

<s:link action=“#{documentApproval.approveOrReject}” taskInstance=“#{task}”/>

(See the DVD Store demo application for examples of this.)

Finally, if you need the "link" to be rendered as a button, use <s:button>:

<s:button action=“#{login.logout}” value=“Logout”/>

It is quite common to display a message to the user indicating success or failure of an action. It is convenient to use a JSF FacesMessage for this. Unfortunately, a successful action often requires a browser redirect, and JSF does not propagate faces messages across redirects. This makes it quite difficult to display success messages in plain JSF.

The built in conversation-scoped Seam component named facesMessages solves this problem. (You must have the Seam redirect filter installed.)

@Name("editDocumentAction")
@Stateless
public class EditDocumentBean implements EditDocument {
    @In EntityManager em;
    @In Document document;
    @In FacesMessages facesMessages;
    
    public String update() {
        em.merge(document);
        facesMessages.add("Document updated");
    }
}

Any message added to facesMessages is used in the very next render response phase for the current conversation. This even works when there is no long-running conversation since Seam preserves even temporary conversation contexts across redirects.

You can even include JSF EL expressions in a faces message summary:

facesMessages.add("Document #{document.title} was updated");

You may display the messages in the usual way, for example:

<h:messages globalOnly="true"/>

Ordinarily, Seam generates a meaningless unique id for each conversation in each session. You can customize the id value when you begin the conversation.

This feature can be used to customize the conversation id generation algorithm like so:

@Begin(id="#{myConversationIdGenerator.nextId}") 
public void editHotel() { ... }

Or it can be used to assign a meaningful conversation id:

@Begin(id="hotel#{hotel.id}") 
public String editHotel() { ... }

@Begin(id="hotel#{hotelsDataModel.rowData.id}") 
public String selectHotel() { ... }

@Begin(id="entry#{params['blogId']}")
public String viewBlogEntry() { ... }

@BeginTask(id="task#{taskInstance.id}") 
public String approveDocument() { ... }

Clearly, these example result in the same conversation id every time a particular hotel, blog or task is selected. So what happens if a conversation with the same conversation id already exists when the new conversation begins? Well, Seam detects the existing conversation and redirects to that conversation without running the @Begin method again. This feature helps control the number of workspaces that are created when using workspace management.

Workspace management is the ability to "switch" conversations in a single window. Seam makes workspace management completely transparent at the level of the Java code. To enable workspace management, all you need to do is:

<pages>
    <page view-id="/main.xhtml">Search hotels: #{hotelBooking.searchString}</page>
    <page view-id="/hotel.xhtml">View hotel: #{hotel.name}</page>
    <page view-id="/book.xhtml">Book hotel: #{hotel.name}</page>
    <page view-id="/confirm.xhtml">Confirm: #{booking.description}</page>
</pages>

<pageflow-definition name="shopping">

   <start-state name="start">
      <transition to="browse"/>
   </start-state>
   
   <page name="browse" view-id="/browse.xhtml">
      <description>DVD Search: #{search.searchPattern}</description>
      <transition to="browse"/>
      <transition name="checkout" to="checkout"/>
   </page>
   
   <page name="checkout" view-id="/checkout.xhtml">
      <description>Purchase: $#{cart.total}</description>
      <transition to="checkout"/>
      <transition name="complete" to="complete"/>
   </page>
   
   <page name="complete" view-id="/complete.xhtml">
      <end-conversation />
   </page>
   
</pageflow-definition>

6.7.3. The conversation switcher

Include the following fragment in your JSP or facelets page to get a drop-down menu that lets you switch to any current conversation, or to any other page of the application:

<h:selectOneMenu value="#{switcher.conversationIdOrOutcome}">
    <f:selectItem itemLabel="Find Issues" itemValue="findIssue"/>
    <f:selectItem itemLabel="Create Issue" itemValue="editIssue"/>
    <f:selectItems value="#{switcher.selectItems}"/>
</h:selectOneMenu>
<h:commandButton action="#{switcher.select}" value="Switch"/>

In this example, we have a menu that includes an item for each conversation, together with two additional items that let the user begin a new conversation.

6.7.4. The conversation list

The conversation list is very similar to the conversation switcher, except that it is displayed as a table:

<h:dataTable value="#{conversationList}" var="entry"
        rendered="#{not empty conversationList}">
    <h:column>
        <f:facet name="header">Workspace</f:facet>
        <h:commandLink action="#{entry.select}" value="#{entry.description}"/>
        <h:outputText value="[current]" rendered="#{entry.current}"/>
    </h:column>
    <h:column>
        <f:facet name="header">Activity</f:facet>
        <h:outputText value="#{entry.startDatetime}">
            <f:convertDateTime type="time" pattern="hh:mm a"/>
        </h:outputText>
        <h:outputText value=" - "/>
        <h:outputText value="#{entry.lastDatetime}">
            <f:convertDateTime type="time" pattern="hh:mm a"/>
        </h:outputText>
    </h:column>
    <h:column>
        <f:facet name="header">Action</f:facet>
        <h:commandButton action="#{entry.select}" value="#{msg.Switch}"/>
        <h:commandButton action="#{entry.destroy}" value="#{msg.Destroy}"/>
    </h:column>
</h:dataTable>

We imagine that you will want to customize this for your own application.

The conversation list is nice, but it takes up a lot of space on the page, so you probably don't want to put it on every page.

Notice that the conversation list lets the user destroy workspaces.

6.7.5. Breadcrumbs

Breadcrumbs are useful in applications which use a nested conversation model. The breadcrumbs are a list of links to conversations in the current conversation stack:

<t:dataList value="#{conversationStack}" var="entry">
    <h:outputText value=" | "/> 
    <h:commandLink value="#{entry.description}" action="#{entry.select}"/>
</t:dataList>

Notice that here we are using the MyFaces <t:dataList> component, since JSF amazingly does not provide any standard component for looping.

Please refer to the Seam Issue Tracker demo to see all this functionality in action!

Conversational components have one minor limitation: they cannot be used to hold bindings to JSF components. (We generally prefer not to use this feature of JSF unless absolutely necessary, since it creates a hard dependency from application logic to the view.) On a postback request, component bindings are updated during the Restore View phase, before the Seam conversation context has been restored.

To work around this use an event scoped component to store the component bindings and inject it into the conversation scoped component that requires it.

@Name("grid")
@Scope(ScopeType.EVENT)
public class Grid
{
    private HtmlPanelGrid htmlPanelGrid;

    // getters and setters
    ...
}

@Name("gridEditor")
@Scope(ScopeType.CONVERSATION)
public class GridEditor
{
    @In(required=false)
    private Grid grid;
    
    ...
}