PyServlet
A great advantage of HttpServlet programming for Jython programmers
is that the Jython distribution comes with the Servlet org.python.util.
PyServlet. This Servlet loads, executes and caches Jython files so that you
can write and view a Jython Servlet without any compiling step in the middle.
This works with a Servlet mapping. A Servlet mapping is an association
between a specific Servlet, PyServlet in this case, and a certain URL
pattern, such as *.py. With a proper web.xml file,
Jython's PyServlet class is mapped to all *.py files, so
as requests for *.py files arrive, the PyServlet class loads,
caches, and invokes methods in the *.py files as needed to respond.
PyServlet's design creates some restrictions on the Jython
files it serves. A Jython file must contain a class that subclasses
javax.servlet.http. HttpServlet. The name of that class must match the
name of the file without the .py extension. In other words, the file
Test.py must contain the class Test, which must be a subclass
of javax.servlet.http.HttpServlet. Additionally, it is unsafe to use
module-global identifiers other than the one class which subclasses
HttpServlet.
A note on naming: With Servlets, PyServlet, and Jython modules that
implement servlets, names get a bit confusing. To clarify any confusion, the
term servlet appears in a generic sense regardless of which language implements
it. PyServlet refers to the specific Servlet
org.python.util.PyServlet. The confusion arises with those Jython
modules that PyServlet actually serves. Files matching the
*.py pattern within the context that are loaded, executed, and cached
by PyServlet, need a name of their own to distinguish them. There is no
precedence for any such term, but dubbed jylets they are for the sake of
this chapter.
Installing PyServlet
All that is required to install the PyServlet is to define a Servlet
mapping and make sure the jython.jar file is in the context's
lib directory. A Servlet mapping is defined in the context's
deployment descriptor file: web.xml. The web.xml file for the
Jython context belongs in the location $TOMCAT_HOME/
webapps/jython/WEB-INF/web.xml. In Tomcat, the default web.xml
file from $TOMCAT_HOME/conf/web.xml is used for all the settings that
are not explicitly included in the context's web.xml. Therefore,
there may not be a web.xml file in your context yet, and you need not
define all properties in that context because default values already exist.
Listing 12.5 is an example deployment descriptor for the Jython
context, which establishes PyServlet as the handler for context
requests matching the *.py pattern. The essential additions to the
web.xml file are a Servlet definition for PyServlet and a
Servlet mapping that associates URLs with the PyServlet. Because Tomcat
has default values for items not defined, Listing 12.5 is a sufficiently
complete web.xml file for the Tomcat server.
Listing 12.5 A Sample Deployment Descriptor for PyServlet
<web-app>
<servlet>
<servlet-name>PyServlet</servlet-name>
<servlet-class>
org.python.util.PyServlet
</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>PyServlet</servlet-name>
<url-pattern>*.py</url-pattern>
</servlet-mapping>
</web-app>
The Servlet definition defines the Servlet name and the Servlet's class.
Listing 12.5 defines the name as PyServlet and the class as
org.python.util.PyServlet. The class is the full package-class
hierarchy that uniquely identifies the Servlet class: PyServlet. The
actual class resides in the jython.jar file, which should be in the
context's lib directory.
The Servlet definition for PyServlet may optionally include
initialization parameters (init-params). PyServlet uses such parameters
in the initialization of Jython; therefore, this is where you would place any of
the Jython property settings such as python.home, python.path,
python.respectJavaAccessibility or any of the other properties normally
set in Jython's registry file. Listing 12.6 is a web.xml file
supplies a python.home and python.path value as
init-params.
Listing 12.6 PyServlet init-params in web.xml
<web-app>
<servlet>
<servlet-name>PyServlet</servlet-name>
<servlet-class>
org.python.util.PyServlet
</servlet-class>
<load-on-startup>1</load-on-startup>
<init-param>
<param-name>python.home</param-name>
<param-value>c:\jython-2.1</param-value>
</init-param>
<init-param>
<param-name>python.path</param-name>
<param-value>
c:\jython-2.1\lib\site-packages
</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>PyServlet</servlet-name>
<url-pattern>*.py</url-pattern>
</servlet-mapping>
</web-app>
Properties that define locations of Jython resources, such as
python.home and python.path are of special concern. These
affect whether a context is self contained and affect cross-platform
portability. If the python.home property points outside the context, or
if the python.path property includes a directory outside the context,
the context is no longer self contained. Additionally, the property values must
be platform-specific paths. Notice the python.home and
python.path properties in Listing 12.6. They would not work on other
platforms, and there is no mechanism to create platform-neutral paths for these
property values. Fortunately, PyServlet has a default value for the
python.home property that creates both a self-contained and a
platform-neutral context.
The default python.home value for PyServlet is the
context's lib directory. This makes the default
python.home value for the Jython context one of the following
(depending on platform).
%TOMCAT_HOME\webapps\jython\WEB-INF\lib
$TOMCAT_HOME/webapps/jython/WEB-INF/lib
Additionally, Jython's lib directory automatically becomes one
of the following:
%TOMCAT_HOME%\webapps\jython\WEB-INF\lib\Lib
$TOMCAT_HOME/webapps/jython/WEB-INF/lib/Lib
This keeps all of Jython's resources within the context, keeping it self
contained. The additional advantage is that PyServlet adds the default
python.home path in a platform-independent manner, making those
contexts that use the default value platform independent.
Testing PyServlet
You con confirm that the Servlet mapping is working by starting Tomcat and
viewing a simple jylet (Jython-Servlet) in a browser. Below is a simple test
jylet:
# File ServletMappingTest.py
from javax.servlet.http import HttpServlet
class ServletMappingTest(HttpServlet):
def doGet(self, req, res):
out = res.writer
res.contentType = "text/html"
print >> out, "Greetings from a jylet."
Save this test file as
%TOMCAT_HOME%\webapps\jython\ServletMappingTest.py, then point your
browser to
http://localhost:8080/jython/ServletMappingTest.py.
If you see the greeting message, the PyServlet mapping is correct. To
complete the installation, create Jython's lib directory
%TOMCAT_HOME%\webapps\jython\WEB-INF\lib\Lib. This is the directory
where you will place any Jython modules used in your jylets. After that,
installation is complete.
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