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Developing Web Applications for WebLogic Server
Configuring Web Application Components
The following sections describe how to configure Web Application components:
You define servlets as a part of a Web Application in several entries in the Web Application deployment descriptor. The first entry, under the <servlet> element, defines a name for the servlet and specifies the compiled class that executes the servlet. (Or, instead of specifying a servlet class, you can specify a JSPs.) This element also contains definitions for initialization parameters and security roles for the servlet. The second entry, under the <servlet-mapping> element, defines the URL pattern that calls this servlet.
Servlet mapping controls how you access a servlet. The following examples demonstrate how you can use servlet mapping in your Web Application. In the examples, a set of servlet configurations and mappings (from the web.xml deployment descriptor) is followed by a table (see url-patterns and Servlet Invocation showing the URLs used to invoke these servlets.
For more information on servlet mappings, such as general servlet mapping rules and conventions, refer to Section 11 of the Servlet 2.3 specification at: http://www.jcp.org/aboutJava/communityprocess/final/jsr053/
Listing 3-1 Servlet Mapping Example
Servlet Initialization Parameters
You define initialization parameters for servlets in the Web Application deployment descriptor, web.xml, in the <init-param> element of the <servlet> element, using <param-name> and <param-value> tags. For example:
Listing 3-2 Example of Configuring Servlet Initialization Parameters in web.xml
For more information on editing the Web Application deployment descriptor, see Deploying Web Applications
Configuring Java Server Pages (JSPs)
In order to deploy Java Server Pages (JSP) files, you must place them in the root (or in a subdirectory below the root) of a Web Application. You define JSP configuration parameters in the <jsp-descriptor> element of the WebLogic-specific deployment descriptor, weblogic.xml. These parameters define the following functionality:
For a complete description of these parameters, see JSP Parameter Names and Values.
Registering JSPs as a Servlet
You can register a JSP as a servlet using the <servlet> element. A servlet container maintains a map of the servlets known to it. This map is used to resolve requests that are made to the container. Adding entries into this map is known as "registering" a servlet. You add entries to this map by referencing a <servlet> element in web.xml through the <servlet-mapping> entry.
A JSP is a type of servlet; registering a JSP is a special case of registering a servlet. Normally, JSPs are implicitly registered the first time you invoke on them, based on the name of the JSP file. Therefore, the myJSPfile.jsp file would be registered as myJSPfile.jsp in the mapping table. You can implicity register JSPs, as illustrated in the provided example. In this example, you request the JSP with the name /main instead of the implicit name myJSPfile.jsp.
In this example, a URL containing /main will invoke myJSPfile.jsp:
Registering a JSP as a servlet allows you to specify the load order, initialization parameters, and security roles for a JSP, just as you would for a servlet.
Configuring JSP Tag Libraries
Weblogic Server lets you create and use custom JSP tags. Custom JSP tags are Java classes you can call from within a JSP page. To create custom JSP tags, you place them in a tag library and define their behavior in a tag library descriptor (TLD) file. You make this TLD available to the Web Application containing the JSP by defining it in the Web Application deployment descriptor. It is a good idea to place the TLD file in the WEB-INF directory of your Web Application, because that directory is never available publicly.
In the Web Application deployment descriptor, you define a URI pattern for the tag library. This URI pattern must match the value in the taglib directive in your JSP pages. You also define the location of the TLD. For example, if the taglib directive in the JSP page is:
<%@ taglib uri="myTaglib" prefix="taglib" %>
and the TLD is located in the WEB-INF directory of your Web Application, you would create the following entry in the Web Application deployment descriptor:
You can also deploy a tag library as a .jar file.
For more information on creating custom JSP tag libraries, see Programming JSP Tag Extensions.
WebLogic Server also includes several custom JSP tags that you can use in your applications. These tags perform caching, facilitate query parameter-based flow control, and facilitate iterations over sets of objects. For more information, see:
Configuring Welcome Pages
WebLogic Server allows you to set a page that is served by default if the requested URL is a directory. This feature can make your site easier to use, because the user can type a URL without giving a specific filename.
Welcome pages are defined at the Web Application level. If your server is hosting multiple Web Applications, you need to define welcome pages separately for each Web Application.
To define Welcome pages, you edit the Web Application deployment descriptor, web.xml. If you do not define Welcome Pages, WebLogic Server looks for the following files in the following order and serves the first one it finds:
Setting Up a Default Servlet
Each Web Application has a default servlet. This default servlet can be a servlet that you specify, or, if you do not specify a default servlet, WebLogic Server uses an internal servlet called the FileServlet as the default servlet.
You can register any servlet as the default servlet. Writing your own default servlet allows you to use your own logic to decide how to handle a request that falls back to the default servlet.
Setting up a default servlet replaces the FileServlet and should be done carefully because the FileServlet is used to serve most files, such as text files, HTML file, image files, and more. If you expect your default servlet to serve such files, you will need to write that functionality into your default servlet.
To set up a user-defined default servlet:
If you also want your default servlet to respond to files ending in *.htm or *.html, then you must map those extensions to your default servlet, in addition to mapping "/". For instructions on mapping servlets, see Configuring Servlets.
Customizing HTTP Error Responses
You can configure WebLogic Server to respond with your own custom Web pages or other HTTP resources when particular HTTP errors or Java exceptions occur, instead of responding with the standard WebLogic Server error response pages.
You define custom error pages in the <error-page> element of the Web Application deployment descriptor (web.xml). For more information on error pages, see error-page.
Using CGI with WebLogic Server
WebLogic Server provides functionality to support your legacy Common Gateway Interface (CGI) scripts. For new projects, we suggest you use HTTP servlets or JavaServer Pages.
WebLogic Server supports all CGI scripts through an internal WebLogic servlet called the CGIServlet. To use CGI, register the CGIServlet in the Web Application deployment descriptor (see Sample Web Application Deployment Descriptor Entries for Registering the CGIServlet). For more information, see Configuring Servlets.
Configuring WebLogic Server to Use CGI
To configure CGI in WebLogic Server:
Listing 3-3 Sample Web Application Deployment Descriptor Entries for Registering the CGIServlet
Requesting a CGI Script
The URL used to request a perl script must follow the pattern:
Serving Resources from the CLASSPATH with the ClasspathServlet
If you need to serve classes or other resources from the system CLASSPATH, or from the WEB-INF/classes directory of a Web Application, you can use a special servlet called the ClasspathServlet. The ClasspathServlet is useful for applications that use applets or RMI clients and require access to server-side classes. The ClasspathServlet is implicitly registered and available from any application.
There are two ways that you can use the ClasspathServlet:
In this case, the resource is located in the following directory, relative to the root of the Web Application:
Warning: Since the ClasspathServlet serves any resource located in the system CLASSPATH, do not place resources that should not be publicly available in the system CLASSPATH.
Configuring Resources in a Web Application
The resources that you use in a Web Application are generally deployed externally to the application. JDBC Datasources can optionally be deployed within the scope of the Web Application as part of an EAR file.
Prior to WebLogic Server 7.0, JDBC DataSources were always deployed externally to the Web Application. To use external resources in the Web Application, you resolve the JNDI resource name that the application uses with the global JNDI resource name using the web.xml and weblogic.xml deployment descriptors. See Configuring External Resources for more information.
WebLogic Server versions 7.x and later enable you to deploy JDBC DataSources as part of the Web Application EAR file by configuring those resources in the weblogic-application.xml deployment descriptor. Resources deployed as part of the EAR file are referred to as application-scoped resources. These resources remain private to the Web Application, and application components can access the resource names directly from the local JNDI tree at java:comp/env. See Configuring Application-Scoped Resources for more information.
Configuring External Resources
When accessing external resources (resources not deployed with the application EAR file) such as a DataSource from a Web Application via Java Naming and Directory Interface (JNDI), you can map the JNDI name you look up in your code to the actual JNDI name as bound in the global JNDI tree. This mapping is made using both the web.xml and weblogic.xml deployment descriptors and allows you to change these resources without changing your application code. You provide a name that is used in your Java code, the name of the resource as bound in the JNDI tree, and the Java type of the resource, and you indicate whether security for the resource is handled programmatically by the servlet or from the credentials associated with the HTTP request.
To configure external resources:
This example assumes that you have defined a data source called accountDataSource. For more information, see JDBC Data Sources Online Help.
Listing 3-4 Using an External DataSource
javax.sql.DataSource ds = (javax.sql.DataSource) ctx.lookup
. . .
. . .
Configuring Application-Scoped Resources
WebLogic Server binds application-scoped resource names to the application's local JNDI tree. The Web Application code accesses these resources by looking up the actual JNDI resource name relative to java:comp/env.
If your Web Application uses only application-scoped resources, you do not need to enter global JNDI resources names in the weblogic.xml deployment descriptor, as described in Configuring External Resources. (In fact, you can omit weblogic.xml entirely if you do not require any other features of that deployment descriptor.)
To configure application-scoped resources:
Note: If Web Application code uses a different JNDI name to reference the resource, you must treat the resource as an external DataSource and configure the weblogic.xml deployment descriptor as illustrated in Listing 3-5.
Listing 3-5 Web Application Code Using a Different JNDI Name to Reference the Resource: Treated as External DataSource
javax.sql.DataSource ds = (javax.sql.DataSource) ctx.lookup
Referencing EJBs in a Web Application
Web Application can access EJBs that are deployed as part of a different application (a different EAR file) by using an external reference, or they can be deployed within the scope of the Web Application as part of an EAR file. The procedures for referencing an EJB differ depending on whether the EJB is external or application-scoped.
Referencing External EJBs
Web Applications can access EJBs that are deployed as part of a different application (a different EAR file) by using an external reference. The EJB being referenced exports a name to the global JNDI tree in its weblogic-ejb-jar.xml deployment descriptor. An EJB reference in the Web Application module can be linked to this global JNDI name by adding an <ejb-reference-description> element to its weblogic.xml deployment descriptor.
This procedure provides a level of indirection between the Web Application and an EJB and is useful if you are using third-party EJBs or Web Applications and cannot modify the code to directly call an EJB. In most situations, you can call the EJB directly without using this indirection. For more information, see "Invoking Deployed EJBs" inProgramming WebLogic Enterprise JavaBeans.
To reference an external EJB for use in a Web Application:
If the Web Application is part of an Enterprise Application Archive (EAR file), you can reference an EJB by the name used in the EAR with the <ejb-link> element.
More about the ejb-ref* Elements
The ejb-ref element in the web.xml deployment descriptor declares that either a servlet, JSP, or HTML page is going to be using a particular EJB. The ejb-reference-description element in the weblogic.xml deployment descriptor binds that reference to an EJB, which is advertised in the global JNDI tree.
The ejb-reference-descriptor element indicates which ejb-ref elment it is resolving with the ejb-ref-name element. That is, the ejb-reference-descriptor and ejb-ref elements with the same ejb-ref-name element go together.
With the addition of the ejb-link syntax, the ejb-reference-descriptor element is no longer required if the EJB being used is in the same application as the servlet, JSP, or HTML page that is using the EJB.
The ejb-ref-name element serves two purposes in the web.xml deployment descriptor:
Assuming the ejb-ref-name element is ejb1, the code in your servlet, JSP, or HTML page should look like:
Context ctx = new InitialContext();
ctx = (Context)ctx.lookup("java:comp/env");
Object o = ctx.lookup("ejb1");
Ejb1Home home = (Ejb1Home) PortableRemoteObject.narrow(o, Ejb1Home.class);
Referencing Application-Scoped EJBs
Within an application, WebLogic Server binds any EJBs referenced by other application components to the environments associated with those referencing components. These resources are accessed at runtime through a JNDI name lookup relative to java:comp/env.
The following is an example of an application deployment descriptor (application.xml) for an application containing an EJB and a Web Application, also called an Enterprise Application. (For the sake of brevity, the XML header is not included in this example.)
Listing 3-6 Example Deployment Descriptor
To allow the code in the Web application to use an EJB in ejb1.jar, the Web application deployment descriptor (web.xml) must include an <ejb-ref> stanza that contains an <ejb-link> referencing the JAR file and the name of the EJB that is being called.
The format of the <ejb-link> entry must be as follows:
where filename is the name of the JAR file, relative to the Web application, and ejbname is the EJB within that JAR file. The <ejb-link> element should look like the following:
Note that since the JAR path is relative to the WAR file, it begins with "../". Also, if the ejbname is unique across the application, the JAR path may be dropped. As a result, your entry may look like the following:
The <ejb-link> element is a sub-element of an <ejb-ref> element contained in the Web application's web.xml descriptor. The <ejb-ref> element should look like the following:
Listing 3-7 <ejb-ref> Element
Referring to the syntax for the <ejb-link> element in the above example,
the portion of the syntax to the left of the # is a relative path to the EJB module being referenced. The syntax to the right of # is the particular EJB being referenced in that module. In the above example, the EJB JAR and WAR files are at the same level.
The name referenced in the <ejb-link> (in this example, myejb) corresponds to the <ejb-name> element of the referenced EJB's descriptor. As a result, the deployment descriptor (ejb-jar.xml) of the EJB module that this <ejb-ref> is referencing should have an entry an entry similar to the following:
Notice the <ejb-name> element is set to myejb.
At runtime, the Web Application code looks up the EJB's JNDI name relative to java:/comp/env. The following is an example of the servlet code:
MyHome home = (MyHome)ctx.lookup("java:/comp/env/ejb1");
The name used in this example (ejb1) is the <ejb-ref-name> defined in the <ejb-ref> element of the web.xml segment above.
Determining the Encoding of an HTTP Request
WebLogic Server needs to convert character data contained in an HTTP request from its native encoding to the Unicode encoding used by the Java servlet API. In order to perform this conversion, WebLogic Server needs to know which codeset was used to encode the request.
There are two ways you can define the codeset:
Mapping IANA Character Sets to Java Character Sets
The names assigned by the Internet Assigned Numbers Authority (IANA) to describe character sets are sometimes different from the names used by Java. Because all HTTP communication uses the IANA character set names and these names are not always the same, WebLogic Server internally maps IANA character set names to Java character set names and can usually determine the correct mapping. However, you can resolve any ambiguities by explicitly mapping an IANA character set to the name of a Java character set.
To map a IANA character set to a Java character set the character set names in the <charset-mapping> element of the WebLogic-specific deployment descriptor, weblogic.xml. Define the IANA character set name in the <iana-charset-name> element and the Java character set name in the <java-charset-name> element. For example: