Pull Service

The Pull service places tool objects in the Velocity context for use by template engineers. It can handle tools with various different "scopes", namely:

  • request-scope tools for which a new instance is needed for each request.
  • global-scope tools for which a single instance can serve the entire application.
  • session-scope tools which are persisted as part of the user object in the servlet engine provided session object.
  • persistent-scope tools which are persisted as part of the user's serialized object data.

The service must be enabled in TurbineResources.properties as shown below, and tools are listed there. The default properties file lists some request-scope tools that are needed for basic Turbine usage ($page, $link and $content), plus the useful UIManager global tool.

Configuration

# -------------------------------------------------------------------
# 
#  S E R V I C E S
#
# -------------------------------------------------------------------
# Classes for Turbine Services should be defined here.
# Format: services.[name].classname=[implementing class]
#
# To specify properties of a service use the following syntax:
# service.[name].[property]=[value]

services.PullService.classname=org.apache.turbine.services.pull.TurbinePullService
.
.
.
# -------------------------------------------------------------------
# 
#  P U L L  S E R V I C E
#
# -------------------------------------------------------------------
# These are the properties for the Pull Service, the service
# that works in conjuction with the Turbine Pull Model API.
# -------------------------------------------------------------------

# This determines whether the non-request tools are refreshed 
# on each request (request tools aren't ever, because they're
# instantiated for the request only anyway).
services.PullService.tools.per.request.refresh=true

# These are tools that are placed in the context by the service
# These tools will be made available to all your
# templates. You list the tools in the following way:
#
# tool.<scope>.<id> = <classname>
#
# <scope>      is the tool scope: global, request, session
#              or persistent (see below for more details)
# <id>         is the name of the tool in the context
#
# For example:
#
# tool.global.ui    = org.apache.turbine.util.pull.UIManager
# tool.global.mm    = org.apache.turbine.util.pull.MessageManager
# tool.request.link = org.apache.turbine.util.template.TemplateLink
# tool.request.page = org.apache.turbine.util.template.TemplatePageAttributes
#
# (the next two examples specify mythical classes) 
#
# tool.session.basket = org.sample.tools.ShoppingBasket
# tool.persistent.ui  = org.sample.tools.PersistentUIManager
#
#
# Tools are accessible in all templates by the <id> given
# to the tool. So for the above listings the UIManager would
# be available as $ui, the MessageManager as $mm, the TemplateLink
# as $link and the TemplatePageAttributes as $page.
#
# Scopes:
#   global:    tool is instantiated once and that instance is available
#              to all templates for all requests. Tool must be threadsafe.
#
#   request:    tool is instantiated once for each request (although the
#               PoolService is used to recycle instances). Tool need not
#               be threadsafe.
#
#   session:    tool is instantiated once for each user session, and is
#               stored in the user's temporary hashtable. Tool should be 
#               threadsafe.
#
#   persistent: tool is instantitated once for each use session, and
#               is stored in the user's permanent hashtable. This means
#               for a logged in user the tool will be persisted in the
#               user's objectdata. Tool should be threadsafe and 
#               Serializable.
#
# Defaults: none

tool.request.link=org.apache.turbine.util.template.TemplateLink
tool.request.page=org.apache.turbine.util.template.TemplatePageAttributes
tool.request.content=org.apache.turbine.util.ContentURI
#tool.request.om=org.apache.turbine.om.OMTool
#tool.request.intake=org.apache.turbine.services.intake.IntakeTool

tool.global.ui=org.apache.turbine.services.pull.util.UIManager

# The UI Manager will allow you to skin your Turbine
# application using simple properties files that are
# located in the WEBAPP/resources/ui/skins/ directory
# hierarchy.

tool.ui.skin=default

Usage

The first step in use of the pull service is deciding on a useful tool API for an object that is available to templates in the Velocity context. This could range from something as simple as the generation of URIs ($link and $content) to a tool for retrieving details of the user's current shopping basket. Define a set of public methods for the tool and they will be available through standard Velocity introspection.

The next step is to decide what scope you need to give the tool. If the tool is retrieving global data in a threadsafe manner, you can make the tool global. If the tool holds data specific to the user look at the session and persistent scopes (choose persistent for a convenient way of having the tools fields persisted across sessions for logged in users). If the tool needs to be instantiated on each request to fulfill its function, or is not threadsafe, then the request scope will be appropriate.

Tools can implement the org.apache.turbine.services.pull.ApplicationTool interface. This will provide a hook for the service to initialise them through the 'init' method and, except for request scope tools, to be refreshed through the 'refresh' method. The type of the init argument 'data' (declared as type 'Object') depends on the scope of the tool. For global tools the argument will be null; for session and persistent scope tools, 'data' will be the current User object; and for request scope tools 'data' will be the current RunData instance.

Important note: request scope tools are recycled by the PoolService. This means they must be written to be reused. This usually means implementing the ApplicationTool interface, and having the 'init' implementation reset all fields.

Current examples of tools include the afore mentioned $link, $page and $content objects, the $ui UI manager, the Intake service tool (org.apache.turbine.services.intake.IntakeTool) and the OM tool (org.apache.turbine.om.OMTool).