java barcode reader library Figure 24-3: Administrative Console Persistent Sessions page 418 in Java

Drawer QR in Java Figure 24-3: Administrative Console Persistent Sessions page 418

Figure 24-3: Administrative Console Persistent Sessions page 418
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Finally, the Administrative Console provides a couple of pages where you can tune session management behavior From the Tuning Parameters page (shown in Figure 24-4), you can control the maximum number of sessions stored in memory and the lifetime of an unused session (how long a session should exist after its most recent use) The Allow Overflow setting indicates whether you will allow the number of session objects in memory to temporarily exceed the value you've specified as the maximum number of sessions
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Figure 24-4: Administrative Console Tuning Parameters page You can specify when persistent session information is actually written out to the database The possible values are as follows: END_OF_SERVLET_SERVICE means that persisting takes place when the servlet completes its execution MANUAL_UPDATE leaves the update frequency to the discretion of the servlet programmer To this end, the IBMSession interface must be used (see the section "WebSphere Session-Tracking API," later in this chapter) TIME_BASED_WRITE allows you to save the session at fixed interva ls The interval is selected using the Write Interval parameter You can also control the actual content to be written the whole session object or only attributes that were modified One final option allows you to schedule invalidations Using this option, you can have a garbage collection process remove invalidated sessions from the persistent store in the database You can specify at what time such invalidation takes place on the Invalidation Schedule page, shown in Figure 24-5, which is located under Tuning Parameters
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Figure 24-5: Administrative Console Invalidation Schedule page The session invalidation process takes place twice daily, and on this page you can select the hours at which it takes place; hours are specified as integers between 0 and 23 Sessions in a Clustered Environment In a typical e-business scenario that supports a large number of users, no single processor can ever be powerful enough to support a theoretically unlimited number of users Therefore, to support high-performance sites that scale well, the typical installation will include multiple application server instances, which together provide the illusion of a single-server instance This can be configured on a multiple-processor machine, with each processor running a server, or on multiple machines, each possibly running a number of servers Regardless of the configuration, the outcome is multiple WebSphere servers that behave as one logical server Such a logical scheme is called a cluster WebSphere Application Server Advanced Edition (versus the Single-Site Edition) facilitates the use of clusters It automatically manages the delegation of requests to the cluster instances One of the implications of the one -logical-server scenario is that session management must be conceptually centralized Therefore, all sessions must to be available to all servers in the cluster Clustering using WebSphere Advanced Edition provides a number of benefits First, this type of deployment is a fault-tolerant architecture, in which a backup exists if one of the 420
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application servers goes down as a result of failure or planned maintenance Because all application servers can accommodate any and all sessions, users will not notice this (apart from possible higher load on the live servers) This architecture also support high availability, a term used to describe an architecture that not only provides fault tolerance, but provides it in a way in which all resources (including backup resources) are used constantly One way to provide fault tolerance is to set up a primary server with a backup server that becomes active only if something happens to the primary server In this configuration, the overall system does not benefit in terms of performance High availability is used to describe a system with a backup in place, but this backup is constantly used in normal operation, thereby allowing shorter response times and higher throughput WebSphere supports both scenarios in which session information between cluster instances is shared, as well as scenarios in which it is not In the first scheme, no sharing of session information occurs that is, each WebSphere instance manages its own session information and no sharing is available This is obviously not useful if sessions are heavily used and if users will be serviced by potentially more than one server The second scheme is scalable and fault tolerant in that it supports the distributed nature of session tracking and does not create a bottleneck, all the while supporting complete availability of all session information to all instances In this scheme, persistent sessions are used When an instance needs to access a session, it contacts the database to access the session information It is interesting to note that, ultimately, whenever something needs to have advanced capabilities, a database is always involved Another interesting point is that using persistent sessions merely passes the "hot potato" In such a case, the hard work to the database is offloaded, and you need to make sure that the database server(s) also have some high availability solution something like Oracle Parallel Server or IBM's HACMP WebSphere Session-Tracking API As mentioned, the interface comibmservletsessionIBMSession presents the programmatic interface to the IBM extension to servlet sessions This interface provides only three methods: getUserName Returns the name of the user authenticated in conjunction with the session The value anonymous is returned if no user is associated with the session isOverflow Indicates whether the memory required for session management has overflowed (that is, whether you've exceeded the limit on the number of sessions in memory) sync Synchronizes to the database (relevant only for persistent sessions) While this programmatic interface doesn't seem very rich, a lot more action goes on behind the scenes with the IBM implementation class for this interface This class implements the required persistence behavior as defined in the Administrative Console settings It also enforces some security, in the sense that a servlet cannot access a session that is associated with an authenticated user unless the servlet is operating under the same credentials (that is, unless it has been authenticated for the same user)
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Personalizing WebSphere Personalization is the process of customizing the content and behavior provided by a Web application to a specific user Customization can be based on explicitly chosen preferences or on previous interactions with the application The former method of 421
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customization is widely available with Web portals such as Yahoo!, whereas the latter is used by e-tailers such as Amazoncom The basis for both methods of personalization is collecting and maintaining information regarding users The collection of all the pieces of information relating to one user is called a user profile WebSphere provides support for maintaining user profiles A user profile is represented by the comibmwebsphereuserprofileUserProfile interface The base implementation for this interface includes the following data items: Address, Name, Phones, City, Nation, Employer, Fax number, Language, E-mail address, State/Province, and Postal Code These data items represent commonly used demographic information that can be read by accessor methods such as getCity or getEmail Obviously, these basic data items are not sufficient for all cases To add more fields, you can have a class exte nd the UserProfile interface and implement the UserProfileExtender interface The latter interface requires that you implement the getNewColumns method, which provides an array of strings holding the names of new columns Another way to implement your own version of user profile information is to extend the UserProfileProperties interface This is essentially a hash table that stores pairs of names and values, similar to the way javautilDictionary does Yet another way of implementing your own user profiling is to leverage the underlying implementation WebSphere implements the user profile as an entity EJB, and you can extend this entity bean We won't go into detail regarding this approach now, since Enterprise JavaBeans (EJB) are explained in detail later in the book It is worth mentioning, though, that an important advantage of extending the user profile bean is that you can import legacy data stored in a relational database into the user profiles managed on the WebSphere Application Server In version 40, you can administer user profiles manually using an XML configuration file (This is a step backward compared to version 35, in which user profiles were administered from the WebSphere Administrative Console, in a fashion similar to the one in which session tracking is administered) To enable user profile support, you need to add the file userprofilexml to the properties directory of your WebSphere installation:
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< xml version="10" > <userprofile> <userprofile-enabled>true</userprofile-enabled> <userprofile-wrapper-class> <classname> comibmservletpersonalizationuserprofileUserProfile </classname> </userprofile-wrapper-class> <userprofile-manager-name> User Profile Manager </userprofile-manager-name> <userprofile-bean> <readonly-interface> comibmservletpersonalizationuserprofileUP_ReadOnly 422
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</readonly-interface> <readwrite-interface> comibmservletpersonalizationuserprofileUP_ReadWrite </readwrite-interface> <readonlyhome-interface> comibmservletpersonalizationuserprofileUP_ReadOnlyHome </readonlyhome-interface> <readwritehome-interface> comibmservletpersonalizationuserprofileUP_ReadWriteHome </readwritehome-interface> <readonly-JNDI-lookupName>UP_ReadOnlyHome</readonly-JNDIlookupName> <readwrite-JNDI-lookupName>UP_ReadWriteHome</readwrite-JNDIlookupName> </userprofile-bean> <userprofile-store> <database-userid>db2admin</database-userid> <database-password>db2passwd</database-password> <database-datasource>jdbc/Profiles</database-datasource> </userprofile-store> </userprofile>
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This file is easy to follow and includes the definition of a data wrapper class, which is UserProfile by default, but could be any other extension alternative that we've mentioned In addition, you can choose the underlying EJB implementation In fact, two separate implementations are provided: one for read-only access and another for read/write access For each of these, you can specify the home interface, remote interface, and Java Naming Directory Interface (JNDI) name (These terms are all related to EJB deployment, and are explained in 11 and in Part VI of the book) Finally, you are required to provide the database user, ID, passwords, and JDBC datasource used for user-profile persistence After you have installed the userprofilexml file, you can access user profiles fro within your enterprise applications Note that such applications should use the default (or your own) beans for representing the user profiles The default EJBs for profiles are located in lib/userprofilejar under you WebSphere installation root
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Other IBM Servlet Extensions When building servlets, you should be aware of some additional support provided by the WebSphere Application Server This additional functionality is not part of the servlet specification, but it saves you from doing a lot of repetitive work Error Handling Servlet errors are reported when a servlet throws an uncaught exception or when it explicitly invokes ServletResponsesendError By default, WebSphere shows an error that indicates the thread stack where the error occurred While this information is 423
valuable for developers, it is no help to users (In fact, it might be a little confusing or even intimidating for a typical user to confront these error pages) To solve this problem, WebSphere provides the comibmwebsphereservleterrorServletErrorReporter class, which takes care of error handling As a developer, you do not need to interact directly with this class Instead, you specify a New Error Page from the Application Assembly Tool, as shown here, when assembling an application
Filtering and Chaining Servlets Multipurpose Internet Mail Extensions (MIME)-based filtering involves configuring the Web container to forward HTTP responses with a specific MIME type to a designated servlet for further processing The servlet is designated with the Application Assembly Tool's New MIME Filter dialog box:
Servlet chaining is a process in which servlets forward their output and responses to other servlets for processing This obviously promotes modularity The mechanics of servlet chains are such that the output of one servlet is the input for the next servlet, and the last servlet in the chain returns its response to the client To use servlet chaining, your application needs to use WebSphere's built-in servlet comibm websphereservletfilterChainerServlet In the initialization parameters for this servlet, you need to specify a list of URLs to be invoked under the parameter chainerpathlist Servlet Events The package comibmwebsphereservletevent allows you to track servletrelated events mainly those stemming from the servlet lifecycle To use this package from within a servlet, you use the ServletContextEventSource interface, as follows:
ServletContextEventSource source = (ServletContextEventSource) getServletContext()getAttribute( ServletContextEventSourceATTRIBUTE_NAME); sourceaddServletErrorListener(listener);
The object listener should implement the ServletListener interface, which provides methods that correspond to servlet lifecycle events: the beginning and end of the init method, the beginning and end of the destroy method, loading, and unloading Note that you can also define listeners for application-level events by implementing the ApplicationListener interface This interface offers methods that allow listeners to track the application's availability Proxy Requests and Responses The package comibmwebsphereservletrequest allows a servlet to generate a request that will be forwarded to another servlet This is useful when a request is to be forwarded to a different servlet but you need to change something in the request For example, suppose you want to change the request from GET to POST Because HttpServletRequest does not provide an interface for modifying the request, WebSphere allows you to use HttpSerlvetRequestProxy You can extend this class in any way to define your own requests To make the request a POST request, you implement the method getMethod() so that it always returns POST The same package also provides the class ServletInputStreamAdapter, which allows you to wrap any Java stream so that it will be used as the input stream for the servlet that takes your request Similarly, the package comibmwebsphereservletresponse supports proxies using invocations to the underlying HttpServletResponse This is not as useful as the request proxy, but it can cache servlet responses that are not expected to change frequently To use this package, you need to implement a response object extending the HttpServletResponseProxy This proxy can provide an output stream using the ServletOutputStreamAdapter class to capture the output Subsequent requests can then use the cached output Invoking Servlets by Name and Serving Files WebSphere includes two built-in servlets that provide optional functionality for Web applications The first servlet lets you access a servlet using its classname (for example, SnoopServlet) To enable this feature, you need to enable the corresponding attribute in the Application Assembly Tool entry for Web module IBM extensions, as shown in Figure 24-6 This is not recommended, however, because it exposes what we consider to be the internals of the application and provides information that may be useful to potential hackers
Figure 24-6: Application Assembly Tool, IBM extensions for Web module Another convenience is provided by the file-serving servlet To enable this feature, click the File Serving Enabled checkbox in the Application Assembly Tool, shown in Figure 24-6 The effect of file serving is that files under the Web application's document root are served automatically; no additional configuration steps are required This reduces the amount of work you must do when setting up the application in the Application Assembly Tool, since you need not explicitly configure servlets and HTML pages in the application (as long as they reside under the application document root) Caution Not only does the File Serving Enabled option provide great ease of use for serving out static pages, but it also may be a security risk, because any files dropped into the document root will be served out Security measures (appropriate operating system file/directory permissions, and so on) should be taken to ensure that only the appropriate files are stored in the document root
Summary Servlets are among the most important constructs supported by WebSphere Application Server and the J2EE framework They provide the entry point to practically all Web applications It is no wonder then that so much in WebSphere helps developers management application content and session information within the context of servlets This chapter covered these features some of which are IBM-specific, and some that are part of the J2EE specification Servlets in WebSphere 4 are usually employed as central access points and interfaces to back-end functionality Not too long ago, these servlets also formed the presentation layer in a Web application the layer that would dynamically create the HTML to be served up to the browser This is no longer the norm, however; in today's WebSphere applications this task is done using JSPs, which are the focus of s 26 and 27
25: JSP Elements Overview JSP is the Java technology best suited for building dynamic user interfaces for Webbased applications Such applications use presentation technologies such as Hypertext Markup Language (HTML), Dynamic HTML (DHTML), Extensible Markup Language (XML), and Java applets, while using Java server-side code for business processing Java Server Pages (JSP) is specifically designed to support a convenient and smooth integration between these technologies by allowing dynamic content to be generated using Java server code We have already covered the functional model and processing foundations of JSP in previous chapters; in this chapter and the next, we will delve into the details about how JSP are written and how they are deployed in WebSphere We will start with a detailed discussion and examples illustrating the JSP tag set and functional capabilities, all the while discussing how the JSP engine performs its feats of magic The next chapter continues with some examples of JSP
JSP Syntax JSP files are text files that include HTML (or XML) tags, scripts, and any other tag sets, in addition to a set of elements that conform to the JSP syntax The syntax described here conforms to the JSP 11 specification The JSP specification used in WebSphere 40 is JSP 11 The JSP specification defines two syntaxes the standard syntax, which is based on an Active Server Pages (ASP) like tag directive, and one that defines the tag set as an application of XML and uses XML namespaces In XML syntax, tags imply what they are, as opposed to the standard syntax that just switches between HTML code and Java code For example, a tag for the include directive following the JSP standard syntax looks like this: <%@ include file=" " %> The JSP XML syntax, in this case, will look like this: <jsp:directiveinclude file=" " /> While it is generally better to express the JSP files as XML documents and use tags, there are also a number of disadvantages For example, an XML document must have a single root element, while JSP pages do not have such a natural root; such a node can be defined, but it is somewhat artificial In addition, the XML syntax is more cumbersome to use, and because one of the fundamental reasons for JSP is the fast and easy coding of Web presentations, this formalism can be a serious drawback The advantages of the XML syntax of JSP pages include the fact that the documents are well formatted and that they are similar to the notion of tags and attributes This becomes crucial when you start using tag libraries, which is the direction in which the industry is going We will talk more about tag libraries in 27 While the XML syntax may be more useful for tools and programs, the standard syntax, which we will call the JSP syntax throughout the rest of this chapter, is easier to use for simple programs and has become the norm for many of us who have been writing JSP pages for a long time The specification, therefore, does not abandon the JSP syntax and neither will we The JSP syntax defines a number of possible elements Anything that does not fall into one of these categories is not processed by the JSP engine and passes unchanged to the resulting page These elements are normally called template elements because they usually form the presentation code into which dynamic content is injected Each JSP element falls into one of the following categories: 429
Comments Standard implicit Actions Directives Declarations Scriptlets Expressions Bean references
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