Keywords: XML, Web-based Editor

Transcription

1 A WEB-BASED XML EDITOR Rahul Shrivastava, Sherif Elfayoumy, and Sanjay Ahuja Department of Computer and Information Sciences University of North Florida 4567 St. Johns Bluff Rd., South Jacksonville, FL ABSTRACT Many businesses and developers use XML to deliver formatted contents from a wide variety of applications to the desktop and over the web. The need for extensive knowledge and technical skills to creating such XML based applications and websites, raised a need for XML supporting software tools, which facilitate faster development of these applications. Many standalone commercially available tools are used for designing, building and deploying such applications. Consider a company mostly relying on XML for its information display through its website, going through all the process of XML DTD and Schema development, validation and publication. The developers developing these documents, on standalone XML-editors, need to take extra precaution in validating such document using parsers with their DTD or Schema and then loading them to website for display. What if all the steps involved in XML, DTD and schema document design, validation and display could be automated? The web-based XML editor presented in this paper is an attempt to automate and expedite these steps and providing development ease even to XML-unaware users. Keywords: XML, Web-based Editor 1. INTRODUCTION The software, which is web-based and facilitates faster and easier development of XML documents can give the organization the required vital business edge. The main advantage of such software, for corporations mostly relying on XML for its information display through its website, would be that content creation could be accomplished from anywhere in the world with a web browser and an Internet connection besides automating the workflow completely and providing access to the web services. This paper will describe a prototype that is built on this concept of webbased development. Over the past few years, XML is fast emerging as the dominant standard for representing data in the World Wide Web. One expectation is that XML will simplify web site management and the publishing of documents; for this purpose XML was initially designed. A second vision is that XML will be a lingua franca, which makes it possible to exchange data and thus develop truly interoperable applications. As XML separates the underlying data from how it is displayed, the data itself can easily be organized, programmed, edited and exchanged between any web site and application device. It standardizes the way information is searched, exchanged, adaptively presented, and personalized. Business data such as customer information, credit card transactions, purchase orders, and fulfillment requests can be converted to XML and shared across applications without changing legacy systems. XML applications can now exchange data between Web server and browser without needing any prior description of the data's structure. To communicate with customers in today's rich-content world, one needs to provide them with information. Until very recently, such information was inevitably encapsulated in proprietary, document-based formats that are not shared easily. The first step to overcome this was Standard Generalized Markup Language (SGML) [1]. Although it was designed in the late 1960s, it became the international standard for defining markup languages in 1986 after the creation of the ISO standard. In the late 1980s, companies and government agencies started to adopt this tag-based language. It allowed them to create and manage paper documentation in a way that was easy to share with others. But, due to its complexity it was difficult to work with and unsuited in Web application design. Hence, instead of SGML, the developers of the Internet adopted the Hypertext Markup Language (HTML), a simple markup language used to create hypertext documents that are portable from one platform to another. HTML was a simplified subset of SGML [2]. It was defined to organize, view, and transfer scientific documents across different platforms and used HTTP (the Hypertext Transfer Protocol) to transfer information over the Internet. Unfortunately, HTML had some disadvantages too. HTML was designed to describe only how information should appear-that is, its format. It was not designed to define the syntax (logical structure) or semantics (meaning) of a document. Another problem with HTML is that it is not extensible. It is not possible to create new tags. Thus, in 1996 to address some of the problems of HTML and SGML, a group working under the auspices of the World Wide Web Consortium (W3C) created a new standard tagged language called XML (Extensible Markup Language). XML is a standardized document formatting language that enables a publisher to create a single document source that can be viewed, displayed, or printed 1

2 in a variety of ways [3]. XML addresses issues relating to data and object structure and provides a standard mechanism for any document builder to define new XML tags within any XML document. Its features lower the barriers for creation of integrated, multi-platform, application-to-application protocols. Below is a list of a few reasons why XML is becoming so popular in the computing world. Object oriented nature Easy data exchange format Customizable languages Self describing data Structured and integrated data 2. XML AND DTD CONCEPTS In XML, data and markup are stored as text that can be configured by the user. This text data is not encoded in any way making it more accessible. XML provides a very efficient way of storing most Data. Customized markup languages can be created using XML and that represents in extraordinary power. Customizable browsers or applications are possible to handle customized XML. Applications such as Internet Financial Exchange (IFX), Bank Internet Payment System (BIPS), Telecommunication Interchange Markup (TIM), and The Text Encoding Initiative (TEI) and several others are example of such customized XML. The user can define XML s tags; XML is self-describing and lets users specify not only data, but also the structure of that data and how various elements are integrated into other elements. This is important when dealing with complex and important data. Hence, XML emphasizes the need for the correctness of the XML Document. XML Documents whose syntax and wellformedness has been checked successfully are called valid documents; in particular, an XML document is considered valid if it adheres to all the rules laid down by the DTD (document type definition) associated with it. DTD or grammar for XML document is all about specifying the structure and syntax of XML document not their content. Well-formed documents are the least stringent: they simply require that all elements are cleanly nested. Valid documents, on the other hand, must include a DTD and adhere to it. The rules and specification regarding wellformed and valid XML documents are defined in [3]. Thus, there are two levels of document quality in XML: i) well-formed documents, and ii) valid documents. An XML document is said to be a well-formed document when there is one and only one root element, all elements that are not empty are marked with start and end tags, the order of the elements is hierarchical, that is, element A that starts within element B also ends within element B, attributes do not occur twice in one element, and all entities used have been declared. An XML document is said to be a valid document when the XML document is well formed, and the document complies with a specified DTD document. The concept of a valid document has been imported to XML from SGML. In SGML all documents must be valid. XML is not so strict. It is possible to use an XML document even without a DTD document. If the user agent knows how to use the XML document without the DTD, then the DTD need not even be sent over the Internet. Below is an example of an XML file with a DTD defined within the document. <?xml version = "1.0" encoding = "UTF-8"?> <DOCUMENT> <GREETING> Hello from XML </GREETING> <MESSAGE> Welcome to XML s World. </MESSAGE> </DOCUMENT> <!DOCTYPE DOCUMENT [ <!ELEMENT DOCUMENT (GREETING, MESSAGE)> <!ELEMENT GREETING (#PCDATA)> <!ELEMENT MESSAGE (#PCDATA)> ]> Figure 1: XML file with DTD XML document DTD An XML document is a hierarchical representation of data. Hence there would be a root node, which should be unique in the document. Here ELEMENT DOCUMENT is the root node of the XML file. It contains two more tags, defined as GREETING and MESSAGE. Looking at first line of the above DTD, one could see how the DTD defines the elements and tags. Lines 2 and 3 of DTD define the tags which state that tags contain text data defined as #PCDATA. The elements and tags are case sensitive. Each element/tag opened must be closed. Thus the example above is well formed and valid. The job of an XML editor is to provide an interface to help create XML documents. These XML editors are designed explicitly for the job of handling XML. Editors ease the job of creating such XML documents. Several standalone commercially available editors are used for designing, building and deploying these XML documents and XML applications. XML Pro [4] and XML Spy [5] are few examples. But in case of such standalone editors the validation of XML against its DTD is not an easy task. The DTD has to reside in the system where the XML is being developed. It becomes difficult for the companies or organizations, which maintain a pre-defined set of DTD to develop XML adhering to them. The only way out is to distribute these pre-defined DTD to each and every of its developers to carry out XML validation. Once the validation is achieved such validated XML document need to be uploaded to the web-server where they could be published. 3. SOFTWARE ARCHITECTURE AND DESIGN The Web-based XML editor can be accessed through any web browser on the client machine. The editing interface, shown in figure 2 above, could be used for creating new and editing existing XML documents, DTD or XML Schemas. These files could be uploaded from local client machine, or a central database on the server side. The editor provides a means to check the well-formedness and 2

3 Interface to create new / edit existing XML / DTD Document Area to Display Syntax/Validatio n errors of any XML Opens file from client Opens file from Server Saves file in local Machine Saves file in the webserve Checks wellformednes Checks validity with DTD Figure 2. User Interface of XML Editor validity of an XML document against its document type definition. The browser-based software displays error messages about wrong syntax, missing elements or tags upon validation, as per the XML specifications. The editor recognizes the following three types of errors that can occur in an XML document as defined by the XML specification. Fatal Error: This is kind of well-formedness error. As soon as the editor detects it, it points out the line number in which the error occurred and the reason why the error occurred. Error: This is an error but not a well-formedness error. The most common such error is a validity error, though there are a few other kinds as well. These sorts of errors are a source of some interoperability problems in XML. It also points out the line number in which the error occurred and the reason why the error occurred. Warning: This is not itself an error. However, it may nonetheless indicate a mistake of some kind in the document. For example, the editor issues a warning if it encountered an element named XMLDocument because all names beginning with XML (in any arrangement of case) are reserved by the W3C for future standards. All these and other features provided are discussed in later sections. The architecture adopted in the development of the webbased XML editor is client-server based where many clients can access the server concurrently. The editor uses Tomcat web server, Apache's Xerces parser, Microsoft s DOM parser. A bird's eye view of the current implementation of the editor follows, after which the components will be discussed in greater detail. The server machine hosts Apache web server, Apache s Xerces Parser and servlet container that is used to present 3 an html interface to the user [6]. The user, through a web browser such as Microsoft Internet Explorer, interacts with the server via JSP [7] and Java servlets [8]. JSP provides a simplified, fast and easy way to create web pages that display dynamically generated content. JSP pages uses XML/HTML tags and scripts written in Java to encapsulate the logic that generates the content for the page. It passes any formatting (HTML or XML) tags directly back to the response page. In this way, JSP pages separate the page logic from its design and display. The logic part of the scrip is executed on the server while the html helps displaying content. JSP pages are compiled into servlets. JSP pages may call JavaBeans components to perform processing on the server. A typical use scenario for servlets would be: a user views an html/jsp web page, enters information into a form on the page, clicks the 'submit' button, the user's information is sent to a servlet which processes the information, and a new html web page reflecting the results of processing the user's information is sent back to the user for viewing. The description of functionalities provided by the editor and their design implications are discussed below. Opening local client s XML/DTD file: This functionality was made possible in the editor by using ActiveXObject Object. The ActiveXOject allows referring to an application in the code. An ActiveXObject object has no intrinsic properties or methods; it allows accessing the properties and methods of the automation object. XML/DTD type files were passed as parameter to Scripting.FileSystemOject, which allows reading the content of the file. It is very important to keep fileaccessing permission from the client machine in mind when using the FileSystemObject and ActiveXObject. This permission can be selected in Security Controls

4 under Internet Options under Tools in Internal Explorer browser. Opening Server file: The JSP page (getfilename.jsp) to get the user to select the file name in the drop down menu through the Open Server File button is invoked using Javascript (editorcontrol.jsp). This Java server page calls another JSP (editor.jsp) and passes the name of file selected as a parameter. The called JSP file has embedded Java code in a script to read the content of file whose name was passed as a parameter. Once the file is read the content is again displayed to the user by reading it to the screen through embedded HTML code in JSP. Saving File on Server: The content of editing interface in the editor is passed to a JSP file (filename.jsp) using the Save on Server button through Javascript (editorcontrol.jsp). This file requests the user to enter the File name under which it will be saved and passes the filename parameter to another JSP file (savefile.jsp), which has Java code embedded in it to write the file content. The name and content of the file are passed as Objects to file read object in the Servlet using the POST method, which writes the file to the server. Saving file locally: The content of editing interface in the Editor is passed to a JSP file (writelocal.jsp) using the Save locally button through Javascript (editorcontrol.jsp). The file write the content to the server with the default file name EnterFileName.xml and then reads it back as an attachment, using file read object. Client Check: Client Check, checks the well-formedness of the XML file. To provide this functionality Microsoft s XMLDOM, a non-validating parser was employed. This functionality was again made possible in the editor by using ActiveXObject Object. The ActiveXOject allows referring to an application in the code. Here, Microsoft s XMLDOM was passed as a parameter to an instance of ActiveXObject. The content of the editing interface was passed to this XMLDOM parser using Client Check button through the Javascript code (editorcontrol.jsp). The parser checks for wellformedness error and passes the result to the editor s error frame using a jsp (showerror.jsp). Server Check: Server Check, checks the validity of the XML file with its DTD. To facilitate this, Apache s SAX parser was installed on the web-server. The content of the editing interface on the browser is passed, on the click of Server Check button, using JavaScript (editorcontrol.jsp) to a JSP page (showerror2.jsp). The JSP page writes the content to a temporary file called validate.xml in the web-server. To validate this file against its DTD, a new instance of parser is invoked. The parser has certain factory-defined features, which are supposed to be turned true from their default value. The features provided are described in the following chunks of code. "validation/dynamic", true); //turns DTD validation on "validation/schema-full-checking", true); // turns Schema validation on "continue-after-fatal-error", true); // continue after fatal error exception feature is turned on All the errors occurred during the XML file parsing are caught by the seterrorhandler method under specific categories. Error Handler defines three methods as shown below: public void fatal (SAXParseException e){} //catches fatal error public void error (SAXParseException e){} // catches error public void warning (SAXParseException e){} //catches warnings The parser looks for the XML and its DTD file in its working directory. Once all the errors are caught, they are sent back to the editor s interface through a JSP page. 4. FUTURE EXTENSIONS The features of the developed editor can be extended in several ways as described below: The software can be made secure by providing the users a login/authentication facility. Functionality to add new users/delete old users, password recovery, and user account change notification through can be added. Certain privileges can be assigned to a group of users through password maintenance. Provision to convert text and other common file formats to XML can be added. Other files could as well be loaded to the database where they can be converted to XML. Graphical user interface could be provided for viewing the XML document as per its Style Sheet (XSLT). Functionality to view XML as drop down tree structure could be added. Provide database at the back end to store the XML, DTD, and Schema files. 6. CONCLUSION The web-based XML editor presented in this paper provides various functionalities such as a browser-based editing interface for XML, DTD, and Schema documents along with validating and publishing features. The user can create new XML document, and modify the existing ones residing in the server or in the client, which can be then validated as per their DTD or Schema through the XML-parser software developed on the server side. This browser-based editor makes it easy for business users to quickly generate XML and web content. REFERENCES [1] Overview of SGML Resources, [2] HuperText Markup Language (HTML), [3] Extensible Markup Language (XML) 1.0, 2 nd Ed., [4] XMLPro, [5] XML Spy, 4

5 [6] Xerces2 Java Parser Readme, [7] JavaServer Pages Technology, [8] Java Servlet Technology, 5