snaps-guide/src/docs/asciidoc/using-snaps.adoc

[[using-snaps]]
== Using Virgo Snaps in your application

Every snap or host should also be a regular WAB (Web Application
Bundle), all you have to do is add some additional configuration to
benefit from VS.

[[using-snaps-configuring-the-host]]
=== Configuring the Host

In the WAB that is to act as a host to Snaps bundles, all that needs to
be added is a filter in the hosts ' `web.xml`'. This filter can be
mapped to any sub path that you want forwarded to registered snaps. It
is important to note the extra `dispatcher` declarations in the
`filter-mapping`. Without these internal lookups, resources like JSPs
won't get passed to the appropriate snap when needed.

....
<filter>
    <filter-name>host-filter-name</filter-name>
    <filter-class>org.eclipse.virgo.snaps.core.SnapHostFilter</filter-class>
</filter>

<filter-mapping>
    <filter-name>host-filter-name</filter-name>
    <url-pattern>/*</url-pattern>
    <dispatcher>INCLUDE</dispatcher>
    <dispatcher>FORWARD</dispatcher>
    <dispatcher>REQUEST</dispatcher>
</filter-mapping>
        
....

[[using-snaps-configuring-a-snap]]
=== Configuring a Snap

A snap is a normal WAB with the addition of two extra manifest headers.
The first `Snap-Host` is used to resolve the WAB you want to act as a
host for this snap. The second `Snap-ContextPath` gives the path the
snap will answer to. In order to reach the snap a request must be made
for the host's context path, plus any path in the host's Snaps filter
followed by the path given with the `Snap-ContextPath` header.

....
Snap-Host: org.eclipse.virgo.snaps.sample.animal;version="${version:[=.=.=, =.+1)}"
Snap-ContextPath: /dog
        
....

[[using-snaps-taglibs]]
=== Using the Snaps taglibs

There is a tag library available that makes information about the Snaps
environment available to the host from within a JSP page. The prefix and
URI to declare are
`<%@ taglib prefix="snaps" uri="http://www.eclipse.org/virgo/snaps" 
            %>`. You can now access an array of all the Snaps currently
registered with the host. Each snap in the array has two values, the
first is the context path the Snap is registered with, as given in the
snaps manifest header. The second is an array of properties, if any,
given in a property file in the snap. The file must be in the `META-INF
            ` directory and have a name of `snap.properties`. The
following code snippet shows all of these in use to produce links to
each of the installed snaps.

....
<snaps:snaps var="snaps">
    <c:forEach var="snap" items="${snaps}">
        <a href="<c:url value="${snap.contextPath}${snap.properties['link.path']}"/>">
                     ${snap.properties['link.text']}</a>
    </c:forEach>
</snaps:snaps>
        
....

The first line uses the Snaps tag library to place the array of snaps
into the `snaps` variable. Then a `forEach` loop goes through each snap.
The content of the `forEach` loop is the really interesting bit. The
first variable `${snap.contextPath}` returns the context path of the
snap. The two lookups are for properties
`${snap.properties['something']}`. They rely on the snap having the
properties file in place with `link.path` and `link.text` defined in it.
This shows the flexibility you have for defining your own contract
between the host and its snaps. In this case each snap can contribute
extra information about how links in a menu should be constructed.

[[using-snaps-host-prefix]]
=== Referencing Resources

If the snap needs to lookup any resources, this can be done in the
normal way and if the resource cannot be found in the snap, then the
host will be checked. Remember that a host and all its snaps use a
single `ServletContext`. As the snap is always checked first it can hide
resources at the same path in its host. So if you want to look up a
resource in the snaps host that exists in the snap simply prefix the
path with `host:`. This will then cause the Snaps system to bypass the
snap and look only in its host for the requested resource. If it is not
found in its host the snap will NOT be checked, the lookup will return
with nothing.

....
request.getServletContext().getResource("host:/WEB-INF/sample.properties");
            
....

This line of Java code gets the ServletContext from the HttpRequest
object and then tries to get a resource from the host, bypassing the
snap.