计算机科学与技术毕业设计(论文)外文翻译.doc

本科毕业设计(论文)外文翻译（附外文原文） 系 ( 院 )： 信息科学与工程学院 课题名称： 学生信息管理系统 专业(方向)：计算机科学与技术（应用）7.1 Enter ActionMappingsThe Model 2 architecture (see chapter 1) encourages us to use servlets and Java-Server Pages in the same application. Under Model 2, we start by calling a servlet.The servlet handles the business logic and directs control to the appropriate pageto complete the response.The web application deployment descriptor (web.xml) lets us map a URL patternto a servlet. This can be a general pattern, like *.do, or a specific path, likesaveRecord.do.Some applications implement Model 2 by mapping a servlet to each businessoperation. This approach works, but many applications involve dozens or hundredsof business operations. Since servlets are multithreaded, instantiating so manyservlets is not the best use of server resources. Servlets are designed to handle anynumber of parallel requests. There is no performance benefit in simply creatingmore and more servlets.The servlets primary job is to interact with the container and HTTP. Handlinga business operation is something that a servlet could delegate to another component.Struts does this by having the ActionServlet delegate the business operationto an object. Using a servlet to receive a request and route it to a handler is knownas the Front Controller pattern Go3.Of course, simply delegating the business operation to another componentdoes not solve the problem of mapping URIs W3C, URI to business operations.Our only way of communicating with a web browser is through HTTP requests andURIs. Arranging for a URI to trigger a business operation is an essential part ofdeveloping a web application.Meanwhile, in practice many business operations are handled in similar ways.Since Java is multithreaded, we could get better use of our server resources if wecould use the same Action object to handle similar operations. But for this towork, we might need to pass the object a set of configuration parameters to usewith each operation.So whats the bottom line? To implement Model 2 in an efficient and flexibleway, we need to:Enter ActionMappings 195_ Route requests for our business operations to a single servlet_ Determine which business operation is related to the request_ Load a multithreaded helper object to handle the business operation_ Pass the helper object the specifics of each request along with any configurationdetail used by this operationThis is where ActionMappings come in.7.1.1 The ActionMapping beanAn ActionMapping (org.apache.struts.action.ActionMapping) describes howthe framework handles each discrete business operation (or action). In Struts,each ActionMapping is associated with a specific URI through its path property.When a request comes in, the ActionServlet uses the path property to select thecorresponding ActionMapping. The set of ActionMapping objects is kept in anActionMappings collection (org.apache.struts.action.ActionMappings).Originally, the ActionMapping object was used to extend the Action objectrather than the Action class. When used with an Action, a mapping gives a specificAction object additional responsibilities and new functionality. So, it was essentiallyan Action decorator Go4. Along the way, the ActionMapping evolved into anobject in its own right and can be used with or without an Action.DEFINITION The intent of the decorator pattern is to attach additional responsibilities toan object dynamically. Decorators provide a flexible alternative to subclassingfor extending functionality Go4.The ActionMappings are usually created through the Struts configuration file.For more about this file, see chapter The ActionMappings catalogThe ActionMappings catalog the business logic available to a Struts application.When a request comes in, the servlet finds its entry in the ActionMappings catalogand pulls the corresponding bean.The ActionServlet uses the ActionMapping bean to decide what to do next. Itmay need to forward control off to another resource. Or it may need to populateand validate an ActionForm bean. At some point, it may have to pass control to anAction object, and when the Action returns, it may have to look up an Action-Forward associated with this mapping.196 CHAPTER 7Designing with ActionMappingsThe ActionMapping works like a routing slip for the servlet. Depending onhow the mapping is filled out, the request could go just about anywhere.The ActionMappings represent the core design of a Struts application. If youwant to figure out how a Struts application works, start with the ActionMappings. Ifyou want to figure out how to write a new Struts application, start with the Action-Mappings. The mappings are at the absolute center of every Struts application.In this chapter, we take a close look at the ActionMapping properties andexplore how they help you design the flow of a Struts application.1.0 vs 1.1 In Struts 1.1, ActionMapping subclasses ActionConfig (org.apache.struts.config.ActionConfig) and adds API methods required forbackward compatibility. ActionMapping is not deprecated, and how thehierarchy will be handled in future releases has not been determined.For now, we refer to the ActionMapping class, but you should note thatin Struts 1.1 all of the action properties are actually defined by the ActionConfigsuper class. The ActionMapping class otherwise works thesame way in both versions.7.2 ActionMapping propertiesTable 7.1 describes the base ActionMapping properties. As with other configurationcomponents, developers may extend ActionMapping to provide additionalproperties.Table 7.1 The base ActionMapping propertiesProperty Descriptionpath The URI path from the request used to select this mapping. (API command)forward The context-relative path of the resource that should serve this request via a forward.Exactly one of the forward, include, or type properties must be specified.orinclude The context-relative path of the resource that should serve this request via aninclude. Exactly one of the forward, include, or type properties must bespecified.ortype Optionally specifies a subclass of org.apache.struts.action.ActionMappingthat should be used when instantiating this mapping.className The fully qualified name of the Action class used by this mapping.SinceStruts 1.1ActionMapping properties 197In the sections that follow, we take a look at each of these properties.7.2.1 The path propertyThe ActionMapping URI, or path, will look to the user like just another file onthe web server. But it does not represent a file. It is a virtual reference to ourActionMapping.Because it is exposed to other systems, the path is not really a logical name, likethose we use with ActionForward. The path can include slashes and an extensionas if it referred to a file systembut they are all just part of a single name.The ActionMappings themselves are a “flat” namespace with no type of internalhierarchy whatsoever. They just happen to use the same characters that we areused to seeing in hierarchical file The name of the form bean, if any, associated with this action. This is not the classname. It is the logical name used in the form bean configuration.roles The list of security roles that may access this mapping.scope The identifier of the scope (request or session) within which the form bean, if any,associated with this mapping will be created.validate Set to true if the validate method of the form bean (if any) associated with thismapping should be called.input Context-relative path of the input form to which control should be returned if a validationerror is encountered. This can be any URI: HTML, JSP, VM, or another Action-Mapping.parameter General-purpose configuration parameter that can be used to pass extra informationto the Action selected by this ActionMapping.attribute Name of the request-scope or session-scope attribute under which our form bean isaccessed, if it is other than the beans specified name.prefix Prefix used to match request parameter names to form bean property names, if any.suffix Suffix used to match request parameter names when populating the properties ofour ActionForm bean, if any.unknown Can be set to true if this mapping should be configured as the default for this application(to handle all requests not handled by another mapping). Only one mappingcan be defined as the default unknown mapping within an application.forwards(s) Block of ActionForwards for this mapping to use, if any.exception(s) Block of ExceptionHandlers for this mapping to use, if any.Table 7.1 The base ActionMapping properties (continued)Property DescriptionSinceStruts 1.1SinceStruts 1.1198 CHAPTER 7Designing with ActionMappingsOf course, it can still be useful to treat your ActionMappings as if they werepart of a hierarchy and group related commands under the same folder. Theonly restriction is that the names must match whatever pattern is used in theapplications deployment description (web.xml) for the ActionServlet. This is usuallyeither /do/* or *.do, but any similar pattern can be used.If you are working in a team environment, different team members can begiven different ActionMapping namespaces to use. Some people may be workingwith the /customer ActionMappings, others may be working with the /vendorActionMappings. This may also relate to the Java package hierarchy the team isusing. Since the ActionMapping URIs are logical constructs, they can be organizedin any way that suits your project.With Struts 1.1, these types of namespaces can be promoted to applicationmodules. Each team can work independently on its own module, with its own setof configuration files and presentation pages. Configuring your application to usemultiple modules is covered in chapter 4.DEFINITION The web runs on URIs, and most URIs map to physical files. If you want tochange the resource, you change the corresponding file. Some URIs, likeStruts actions, are virtual references. They do not have a correspondingfile but are handled by a programming component. To change the resource,we change how the component is programmed. But since thepath is a URI and interacts with other systems outside our control, thepath is not a true logical referencethe name of an ActionForward, forinstance. We can change the name of an ActionForward without consultingother systems. Its an internal, logical reference. If we change thepath to an ActionMapping, we might need to update other systems thatrefer to the ActionMapping through its public URI.7.2.2 The forward propertyWhen the forward property is specified, the servlet will not pass the request to anAction class but will make a call to RequestDispatcher.forward. Since the operationdoes not use an Action class, it can be used to integrate Struts with otherresources and to prototype systems. The forward, include, and type propertiesare mutually exclusive. (See chapter 6 for more information.)7.2.3 The include propertyWhen the include property is specified, the servlet will not pass the request to anAction class but will make a call to RequestDispatcher.include. The operationActionMapping properties 199does not use an Action class and can be used to integrate Struts with other components.The forward, include, and type properties are mutually exclusive. (Seechapter 6 for more information.)7.2.4 The type propertyMost mappings will specify an Action class type rather than a forward or include.An Action class may be used by more than one mapping. The mappings may specifyform beans, parameters, forwards, or exceptions. The forward, include, andtype properties are mutually exclusive.7.2.5 The className propertyWhen specified, className is the fully qualified Java classname of the ActionMappingsubclass that should be used for this object. This allows you to use your ownActionMapping subclass with specialized methods and properties. See alsosection .6 The name propertyThis property specifies the logical name for the form bean, as given in the formbeansegment of the Struts configuration file. By default, this is also the name tobe used when placing the form bean in the request or session context. Use theattribute property of this class to specify a different attribute key.7.2.7 The roles propertyThis property is a comma-delimited list of the security role names that are allowedaccess to this ActionMapping object. By default, the same system that is used withstandard container-based security is applied to the list of roles given here. Thismeans you can use action-based security in lieu of specifying URL patterns in thedeployment descriptor, or you can use both together.The security check is handled by the processRoles method of the Request-Processor (org.apache.struts.action.RequestProcessor). By subclassingRequestProcessor, you can also use the roles property with application-basedsecurity. See chapter 9 for more about subclassing RequestProcessor.7.2.8 The scope propertyThe ActionForm bean can be stored in the current request or in the session scope(where it will be available to additional requests). While most developers userequest scope for the ActionForm, the framework default is session scope. Tomake request the default, see section 7.4.SinceStruts 1.1SinceStruts 1.1200 CHAPTER 7Designing with ActionMappings7.2.9 The validate propertyAn important step in the lifecycle of an ActionForm is to validate its data beforeoffering it to the business layer. When the validate property for a mapping is true,the ActionServlet will call the ActionForms validate method. If validate returnsfalse, the request is forwarded to the resource given by the input property.Often, developers will create a pair of mappings for each data entry form. Onemapping will have validate set to false, so you can create an empty form. Theother has validate set to true and is used to submit the completed form.NOTE Whether or not the ActionForm validate method is called does not relateto the ActionServlets validating property. That switch controlshow the Struts configuration file is processed.7.2.10 The input propertyWhen validate is set to true, it is important that a valid path for input be provided.This is where control will pass should the ActionForm validate methodreturn false. Often, this is the address for a presentation page. Sometimes it willbe another Action path (with validate set to false) that is required to generatedata objects needed by the page.NOTE The input path often leads back to the page that submitted the request.While it seems natural for the framework to return the request to whereit originated, this is not a simple task in a web application. A request is oftenpassed from component to component before a response is sent backto the browser. The browser only knows the path it used to retrieve theinput page, which may or may not also be the correct path to use for theinput property. While it may be possible to try and generate a default inputpage based on the HTTP referrer attribute, the Struts designersdeemed that approach unreliable.inputForwardIn Struts 1.0, the ActionMapping input property is always a literal URI. InStruts 1.1, it may optionally be the name of an ActionForward instead. TheActionForward is retrieved and its path property is used as the input property.This can be a global or local ActionForward.To use ActionForwards here instead of literal paths, set the inputForwardattribute on the element for this module to true:SinceStruts 1.1ActionMapping properties 201For more about configuring Struts, see chapter 4. For more about ActionForwards,see chapter 1 The parameter propertyThe generic parameter property allows Actions to be configured at runtime. Severalof the standard Struts Actions make use of this property, and the standardScaffold Actions often use it, too. The parameter property may contain a URI, thename of a method, the name of a class, or any other bit of information an Actionmay need at runtime. This flexibility allows some Actions to do double and tripleduty, slashing the number of distinct Action classes an application needs on hand.Within an Action class, the parameter property is retrieved from the mappingpassed to perform:parameter = mapping.getParameter();Multiple parametersWhile multiple parameters are not supported by the standard ActionMappingsclass, there are some easy ways to implement this, including using HttpUtils, aStringTokenizer, or a Properties file (java.util.Properties).HttpUtils. Although deprecated as of the Servlet API 2.3 specification, theHttpUtils package (javax.servlet.http.HttpUtils) provides a static methodthat parses any string as if it were a query string and returns a Hashtable(java.util.Hashtable):Hashtable parameters = parseQueryString(parameter);The parameter property for your mapping then becomes just another querystring, because you might use it elsewhere in the Struts configuration.stringTokenizer. Another simple approach is to delimit the parameters using thetoken of your choicesuch as a comma, colon, or semicolonand use theStringTokenizer to read them back:StringTokenizer incoming =new StringTokenizer(mapping.getParameter(),;);int i = 0;String parameters = new Stringincoming.countTokens();while (incoming.hasMoreTokens() parametersi+ = incoming.nextToken().trim();202 CHAPTER 7Designing with ActionMappingsProperties file. While slightly