Web applications are configured via elements contained in the web application deployment descriptor. The following sections give a brief introduction to the web application features you will usually want to configure. A number of security parameters can be specified; these are covered in Securing Web Applications (page 987). In the following sections, examples demonstrate procedures for configuring the Hello, World application. If Hello, World does not use a specific configuration feature, the section gives references to other examples that illustrate how to specify the deployment descriptor element. Mapping URLs to Web ComponentsWhen a request is received by the web container it must determine which web component should handle the request. It does so by mapping the URL path contained in the request to a web application and a web component. A URL path contains the context root and an alias: http://host:port/context_root/alias Setting the Component AliasThe alias identifies the web component that should handle a request. The alias path must start with a forward slash (/) and end with a string or a wildcard expression with an extension (for example, *.jsp). Since web containers automatically map an alias that ends with *.jsp, you do not have to specify an alias for a JSP page unless you wish to refer to the page by a name other than its file name. The hello2 application has two servlets that need to be mapped in the web.xml file. You can edit a web application's web.xml file in NetBeans 5.5 by doing the following:
The following steps detail how we made the necessary edits to the web.xml file, including how to set the display name and how to map the servlet components. Because we've already made the edits, you can just use the steps to view the settings we've made. To set the display name:
To perform the servlet mappings:
If you are not using NetBeans 5.5, you can add these settings using a text editor. To package the example with NetBeans 5.5, do the following:
To package the example with the ant utility, do the following:
To deploy the example using NetBeans 5.5, right-click on the project in the Projects pane and select Deploy Project. To deploy the example using ant, run ant deploy. The deploy target in this case gives you an incorrect URL to run the application. To run the application, please use the URL shown at the end of this section. To run the application, first deploy the web module, and then open the URL http://localhost:8080/hello2/greeting in a browser. Declaring Welcome FilesThe welcome files mechanism allows you to specify a list of files that the web container will use for appending to a request for a URL (called a valid partial request) that is not mapped to a web component. For example, suppose you define a welcome file welcome.html. When a client requests a URL such as host:port/webapp/directory, where directory is not mapped to a servlet or JSP page, the file host:port/webapp/directory/welcome.html is returned to the client. If a web container receives a valid partial request, the web container examines the welcome file list and appends to the partial request each welcome file in the order specified and checks whether a static resource or servlet in the WAR is mapped to that request URL. The web container then sends the request to the first resource in the WAR that matches. If no welcome file is specified, the Application Server will use a file named index. XXX, where XXX can be html or jsp, as the default welcome file. If there is no welcome file and no file named index. XXX, the Application Server returns a directory listing. To specify a welcome file in the web application deployment descriptor using NetBeans 5.5, do the following:
The example discussed in Encapsulating Reusable Content Using Tag Files (page 212) has a welcome file. Setting Initialization ParametersThe web components in a web module share an object that represents their application context (see Accessing the Web Context, page 88). You can pass initialization parameters to the context or to a web component. To add a context parameter using NetBeans 5.5, do the following:
Alternatively, you can edit the XML of the web.xml directly by clicking XML at the top of the editor pane and using the following elements to add a context parameter:
For a sample context parameter, see the example discussed in The Example JSP Pages (page 101). To add a web component initialization parameter using NetBeans 5.5, do the following:
Alternatively, you can edit the XML of the web.xml file directly by clicking XML at the top of the editor pane and using the following elements to add a context parameter:
Mapping Errors to Error ScreensWhen an error occurs during execution of a web application, you can have the application display a specific error screen according to the type of error. In particular, you can specify a mapping between the status code returned in an HTTP response or a Java programming language exception returned by any web component (see Handling Errors, page 66) and any type of error screen. To set up error mappings using NetBeans 5.5, do the following:
Alternatively, you can click XML at the top of the editor pane and enter the error page mapping by hand using the following elements:
You can have multiple error-page elements in your deployment descriptor. Each one of the elements identifies a different error that causes an error page to open. This error page can be the same for any number of error-page elements. Note You can also define error screens for a JSP page contained in a WAR. If error screens are defined for both the WAR and a JSP page, the JSP page's error page takes precedence. See Handling Errors (page 109). For a sample error page mapping, see the example discussed in The Example Servlets (page 60). Declaring Resource ReferencesIf your web component uses objects such as enterprise beans, data sources, or web services, you use Java EE annotations to inject these resources into your application. Annotations eliminate a lot of the boilerplate lookup code and configuration elements that previous versions of Java EE required. Although resource injection using annotations can be more convenient for the developer, there are some restrictions from using it in web applications. First, you can only inject resources into container-managed objects. This is because a container must have control over the creation of a component so that it can perform the injection into a component. As a result, you cannot inject resources into objects such as simple JavaBeans components. However, JavaServer Faces managed beans are managed by the container; therefore, they can accept resource injections. Additionally, JSP pages cannot accept resource injections. This is because the information represented by annotations must be available at deployment time, but the JSP page is compiled after that; therefore, the annotation will not be seen when it is needed. Those components that can accept resource injections are listed in Table 21.
This section describes how to use a couple of the annotations supported by a servlet container to inject resources. Chapter 25 describes how web applications use annotations supported by the Java Persistence API. Chapter 30 describes how to use annotations to specify information about securing web applications. Declaring a Reference to a ResourceThe @Resource annotation is used to declare a reference to a resource such as a data source, an enterprise bean, or an environment entry. This annotation is equivalent to declaring a resource-ref element in the deployment descriptor. The @Resource annotation is specified on a class, method or field. The container is responsible for injecting references to resources declared by the @Resource annotation and mapping it to the proper JNDI resources. In the following example, the @Resource annotation is used to inject a data source into a component that needs to make a connection to the data source, as is done when using JDBC technology to access a relational database: @Resource javax.sql.DataSource catalogDS; public getProductsByCategory() { // get a connection and execute the query Connection conn = catalogDS.getConnection(); .. } The container injects this data source prior to the component being made available to the application. The data source JNDI mapping is inferred from the field name catalogDS and the type, javax.sql.DataSource. If you have multiple resources that you need to inject into one component, you need to use the @Resources annotation to contain them, as shown by the following example: @Resources ({ @Resource (name="myDB" type=java.sql.DataSource), @Resource(name="myMQ" type=javax.jms.ConnectionFactory) }) The web application examples in this tutorial use the Java Persistence API to access relational databases. This API does not require you to explicitly create a connection to a data source. Therefore, the examples do not use the @Resource annotation to inject a data source. However, this API supports the @PersistenceUnit and @PersistenceContext annotations for injecting EntityManagerFactory and EntityManager instances, respectively. Chapter 25 describes these annotations and the use of the Java Persistence API in web applications. Declaring a Reference to a Web ServiceThe @WebServiceRef annotation provides a reference to a web service. The following example shows uses the @WebServiceRef annotation to declare a reference to a web service. WebServiceRef uses the wsdlLocation element to specify the URI of the deployed service's WSDL file: ... import javax.xml.ws.WebServiceRef; ... public class ResponseServlet extends HTTPServlet { @WebServiceRef(wsdlLocation= "http://localhost:8080/helloservice/hello?wsdl") static HelloService service; |