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## 6.20. < a4j:queue >

### 6.20.1. Description

The <a4j:queue> component creates queues that other components can use and reference.

User can create a default queue for a form ( overriding the global default queue ).

Table 6.39. a4j : queue attributes

Attribute Name | Description
--- | ---
binding | The attribute takes a value-binding expression for a component property of a backing bean
disabled | If "true", disables this component on page.
id | Every component may have a unique id that is automatically created if omitted
ignoreDupResponses | Attribute allows you to ignore an Ajax response produced by a request if the newest 'similar' request is in the queue already. ignoreDupResponses="true" does not cancel the request while it is processed on the server, but just allows avoiding unnecessary updates on the client side if the response isn't actual now
name | The name of the queue
onbeforedomupdate | JavaScript code for call before DOM has been updated on client side
oncomplete | JavaScript code for call after request completed on client side
onerror | HTML: a script expression; event fires whenever an JavaScript server side error occurs
onrequestdequeue | onrequestdequeue
onrequestqueue | onrequestqueue
onsizeexceeded | A script expression; a size is exceed
onsubmit | JavaScript code for call before submission of an ajax request
requestDelay | Attribute defines the time (in ms) the request will be waiting in the queue before it is ready to be sent.
size | Defines the number of requests allowed in the queue at one time.
sizeExceededBehavior | Defines the strategies of the queue's behavior if the number of the requests waiting in the queue is exceeded. There are four strategies: dropNext (by default), dropNew, fireNext , fireNew.
status | status
timeout | Waiting time for response on a particular request. If no response is received during this time, the request is aborted

Table 6.40. Component identification parameters

Name | Value
--- | ---
component-family | org.ajax4jsf.Queue
component-class | org.ajax4jsf.component.html.HtmlQueue
renderer-type | org.ajax4jsf.QueueRenderer
tag-class | org.ajax4jsf.taglib.html.jsp.QueueTag

### 6.20.2. Creating on a page

Here is the simplest way for a component creation on a page.

Example:

    <h:form>

           <a4j:queue name="fooQueue" ... />

    </h:form>

### 6.20.3. Creating the Component Dynamically Using Java

Example:

    import org.ajax4jsf.component.html.HtmlQueue;

    ...

    HtmlQueue myQueue = new HtmlQueue();

### 6.20.4. Key attributes and ways of usage

By means of the <a4j:queue> component a user can create several types of queues associated with it.

- A user can set a queue globally in "web.xml". You don't need set the queue on the page itself. The global queue can be used by all components. You can see the example below:

      <context-param>

         <param-name>org.richfaces.queue.global.enabled</param-name>

         <param-value>true</param-value>

         </context-param>

  In order to enable/disable a global queue see the following examples:

      <a4j:queue name="org.richfaces.global_queue" disabled="false"... />

  or:

      <a4j:queue name="org.richfaces.global_queue" disabled="true"... />

- A user can create a separate queue for specific form with the help of the <a4j:form> or <h:form> tags. It will be the global queue only for this form.

  Example:

      <h:form ... >

          ...

          <a4j:queue ... />

          ...

          </h:form>

  or:

  Example:

      <a4j:form ... >

          ...

          <a4j:queue <a4j:queue name="fooQueue" ... />

          ...

      </a4j:form>

  If you use the <a4j:form> tag, you can reference a named queue as the form's default.

  Example:

      <a4j:form eventsQueue="fooQueue" ...>

         ...

         </a4j:form>

- With the help of the "eventsQueue" attribute a user can reference a named queue from any a4j or rich component that supports this attribute.

  Example:

      <a4j:queue name="fooQueue" ... />

      ...

      <h:inputText value="#{foo2.bar1}">

          <a:support id="onblur" event="onblur" reRender="bar21" eventsQueue="fooQueue"/>

      </h:inputText>

      <a4j:commandButton eventsQueue="fooQueue" ... >

  As a result all requests generated through the onblur of the <h:inputText> component, and clicking of the <h:commandButton> will be funneled through the `"fooQueue"`.

  If you try to refer to the nonexistent queue the new named queue will be created with all default settings.

The "size" attribute helps you to set the number of requests allowed in the queue at one time.

The "sizeExceededBehavior" attribute defines four strategies of the queue's behavior if the number of the requests waiting in the queue is exceeded:

- dropNext - this strategy is default. It drops next request that should be fired

- dropNew - it drops the incoming request

- fireNext - with the help of this strategy you can immediately fire the next request in line to be fired

- fireNew - This type of the strategy immediately fires the incoming request.

The "requestDelay" attribute defines time delay for all the requests which fired using this queue.

Example:

    <a4j:queue requestDelay="2000" ... />

As a result all the requests which were fired using this queue will have 2 sec delay.

The requests collect in the queue, combining similar ones, during request delay. User should remember that such requests can combine, if they are raised sequentially, in order to not block the queue and not to change the requests order.

### 6.20.5. Relevant resources links

Here you can see the example of <a4j:queue> usage and sources for the given example.

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