Programming Jakarta Struts, 2nd Edition [Electronic resources] نسخه متنی

4.4 The Web Application Deployment Descriptor

The web
application deployment descriptor conveys configuration information
between application developers, deployers, and assemblers. Web
containers also use the descriptor to configure and load web
applications when the container is started.

All servlet containers that are
compliant with the 2.3 Servlet specification are required to support
the following types of deployment information:

• Initialization
  parameters

• Session configuration

• Servlet
  declarations

• Servlet
  mappings

• Application lifecycle
  listener classes

• Filter definitions and mappings

• MIME type
  mappings

• Welcome
  file list

• Error
  pages

Security configuration information
is not required unless the servlet container is part of a J2EE
implementation. The following elements are not required unless the
servlet container is using JSP pages or
is part of a J2EE application server:

• Tag
  library mappings

• JNDI
  References

Much of the functionality described in this list was added in the 2.3 version of the Servlet specification. If you are using a 2.2-compliant container, it will not be available to you. Struts 1.1 supports the 2.2 and 2.3 Servlet specifications.

4.4.1 Web Application Deployment Descriptor DTD

The format for both the web application
deployment descriptor and the Struts configuration file is based on a
Document
Type Definition (DTD), which defines the legal building
blocks that may be used in the XML files. From the DTD point of view,
all XML documents, including the web application deployment
descriptor and the Struts configuration file, are made up of the
following elements:

• Elements

• Tags

• Attributes

• Entities

• PCDATA

• CDATA

Using these components, DTDs help to specify valid and well-formed
XML documents.^[2] The DTD
for the 2.3 web application deployment descriptor can be downloaded
from http://java.sun.com/dtd/indexl.

^[2] A well-formed XML document is one
that is properly formatted with all beginning tags closed with end
tags, all attributes quoted properly, all entities declared, and so
on. When an XML document is well-formed, it is easier for a computer
program to parse it and deliver it over a network. A
valid XML document is one that
declares a DTD and adheres to the rules set forth in that DTD. For
more information, see Java & XML by Brett
McLaughlin (O'Reilly).

The following DTD declaration shows the top-level elements that make
up the deployment descriptor for a web application:

<!ELEMENT web-app (icon?, display-name?, description?,
distributable?, context-param*, filter*, filter-mapping*,
listener*, servlet*, servlet-mapping*, session-config?, mime-
mapping*, welcome-file-list?, error-page*, taglib*, resource-
env-ref*, resource-ref*, security-constraint*, login-config?,
security-role*, env-entry*, ejb-ref*, ejb-local-ref*)
>

The web-app element is
the root of the deployment descriptor for a web application. The
other elements inside the parentheses are child elements, which must
be placed inside the root web-app element within
the XML file. The symbols next to the child elements indicate the
allowed multiplicity of the child elements within the XML file. Table 4-1 provides a brief explanation of the symbols.

The order of the child elements is implied by their order inside of the parent element. For instance, in the web-app syntax, the servlet element must come before the servlet-mapping element, the servlet-mapping element must come before the taglib element, and so on.