Guidelines, Patterns, and code for end-to-end Java applications.
Questions and Answers - Client Tier
- How should my applets talk to my enterprise beans ?
- Can enterprise bean data be imported directly into a StarOffice spreadsheet?
- Is there a way to import data from EJBs directly into an Excel spreadsheet?
1. How should my applets talk to my enterprise beans ?
We recommend making services from EJB technology-based components
("enterprise bean services") available to applets through a web-tier
servlet. The servlet acts as a facade that unifies the applet's access
to back-end data, and isolates the applet code from changes in the
enterprise bean classes.
 It's especially useful for
the servlet to serve XML to the applet, if the applet has need of data
that is structured. If you choose to serve XML, be sure that your
applet uses a small-footprint XML parsing mechanism, such as a
lightweight SAX implementation.
Placing a servlet between the applet and the enterprise beans
the applet uses is good since:
-
You may don't have control over the Java Runtime Environment
which is running the applet, so you don't know if you can access
enterprise beans at all. Any version of the JRE can handle a
simple XML parser, which is all that is needed to provide
services to the applet.
-
Clients behind firewalls may not be able to make RMI-IIOP connections,
so they can't access the enterprise beans directly. The servlet can serve
any format deliverable over HTTP through the firewall.
-
Exposing the EJB tier to the outside world is a security risk. The
servlet acts as a single point of control for what enterprise bean
functions are externally available.
-
The servlet can decouple applets (and other clients) from the EJB
tier, allowing their implementations to vary independently. This
ability may be less important for applets, which are typically loaded
at runtime, but custom clients that require installation can benefit
from this decoupling. The web tier implementation can be changed as
needed changed to correspond to changes in the entity beans' APIs, while
maintaining the original contract with the installed base of clients.
(This is a general benefit of the Facade design pattern.)
Using a web-tier component as a facade is a powerful design pattern
for making enterprise bean functionality available to medium- and
heavyweight clients without exposing the EJB tier. For example, the
servlet described here could also make enterprise bean data accessible
to a spreadsheet, as described
below. This pattern is similar to the
Session Facade design pattern, where a web-tier stateless session
bean is used as a facade for access to entity beans.
2. Can enterprise bean data be imported directly into a StarOfficeTM spreadsheet?
StarOfficeTM spreadsheets cannot directly communicate with enterprise
beans, but a servlet or page created with JavaServer PagesTM technology (JSP page) in the web tier can access
services in the EJB tier and present the results as XML, which the
StarOffice spreadsheet can then import.
It's best that communication between J2EE technology-enabled servers
and spreadsheets be loosely-coupled. While many spreadsheets
incorporate full-fledged application development languages which
could, in theory, access enterprise beans directly, such a solution
would probably be complicated and brittle.
XML makes it easy to present EJB services on the web to non-J2EE
clients. Both StarOffice and Excel provide an easy way to transport
data over HTTP, and XML manipulation facilities are available from
within both applications. When interacting with XML-aware, non-J2EE
technology-enabled clients, we recommend using a servlet to expose
selected enterprise bean services as XML data sources, and using the
client's HTTP transport and XML processing facilities.
The Java Pet Store sample application provides an example of this
technique. All pending orders in the Java Pet Store are available in
XML format from the web page estore/pendingorders.jsp.
This page produces an XML description of the current pending orders.
StarOffice accesses the abovementioned web page, receives the data in
XML format, and displays the data in the spreadsheet. See the
document docs/starofficeDemo.html in the Java Pet Store
install directory for a description of how to access the Java Pet
Store from StarOffice.
This technique is not limited to spreadsheets. Any client capable of
making HTTP requests and using the XML returned from the servlet can
access the Java Pet Store in this way. In fact, this is an identical
solution to the one described in the discussion on
applets and enterprise beans above, except that a spreadsheet is
accessing the XML data source, instead of an applet doing so.
3. Is there a way to import data from EJBs directly into an Excel spreadsheet?
Excel can access the Java Pet Store just as can StarOffice. See the
document docs/excelDemo.html in the Java Pet Store
install directory for a description of how to access the Java Pet
Store from Excel.
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