Friday, January 29, 2010

URL rewriting

Both of these methods revolve around the concept of URL rewriting. Basically it involves suffixing the URL with a HTTP_QUERY_STRING, which mostly is a session id used to keep track of a particular users context.

URL re-writing is used get over the usage of cookies which are dependent on browser configuration, thus not reliable at all.

The difference between the methods is in their usage and not their functionality. Both perform the same task of re-writing a URL.

When you send a redirect header to the browser using response.sendRedirect(string URL)

Example, you want to redirect to different page.

if (name == null){

On the other hand, you provide a link to shopping cart page to a user.

printWriter.println("A HREF=" + response.encodeURL("shoppingCart.jasp") + ">")

Essentially they do same thing of re-writing the URl for non-cookie compliant browser.

encodeURL() is used for all URLs in a servlet's output. It helps session ids to be encoded with the URL.

encodeRedirectURL() is used with res.sendRedirect only.It is also used for encoding session ids with URL but only while redirecting.

Monday, January 25, 2010


Xerces is a collection of software libraries for parsing, validating, serializing and manipulating XML. The library implements a number of standard APIs for XML parsing, including DOM, SAX and SAX2.

XML Data Binding

XML data binding refers to the process of representing the information in an XML document as an object in computer memory. This allows applications to access the data in the XML from the object rather than using the DOM or SAX to retrieve the data from a direct representation of the XML itself.

Java Architecture for XML Binding (JAXB)
JABX allows Java developers to map Java classes to XML representations. JAXB provides two main features: the ability to marshal Java objects into XML and the inverse, i.e. to unmarshal XML back into Java objects. In other words, JAXB allows storing and retrieving data in memory in any XML format, without the need to implement a specific set of XML loading and saving routines for the program's class structure.

Usage of JAXB
The tool "xjc" can be used to convert XML Schema and other schema file types (as of Java 1.6, RELAX NG, XML DTD, and WSDL are supported experimentally) to class representations. Classes are marked up using annotations from javax.xml.bind.annotation.* namespace, for example, @XmlRootElement and @XmlElement. XML list sequences are represented by attributes of type java.util.List. Marshallers and Unmarshallers are created through an instance of JAXBContext.
In addition, JAXB includes a "schemagen" tool which can essentially perform the inverse of "xjc", creating an XML Schema from a set of annotated classes.

It is a tool that allows access to the full power of XML in a Java friendly way. The idea is to take advantage of the richness and features of XML and XML Schema and have these features mapped as naturally as possible to the equivalent Java language and typing constructs. XMLBeans uses XML Schema to compile Java interfaces and classes that can then be used to access and modify XML instance data. Using XMLBeans is similar to using any other Java interface/class: with methods like getFoo or setFoo, just as when working with Java. While a major use of XMLBeans is to access XML instance data with strongly typed Java classes there are also APIs that allow access to the full XML infoset (XMLBeans keeps XML Infoset fidelity) as well as to allow reflection into the XML schema itself through an XML Schema Object model.

Usage of XMLBean
To accomplish the above objectives, XMLBeans provides three major APIs:
  • XmlObject
  • XmlCursor
  • SchemaType
    XmlObject: The java classes that are generated from an XML Schema are all derived from XmlObject. These provide strongly typed getters and setters for each of the elements within the defined XML. Complex types are in turn XmlObjects. For example getCustomer might return a CustomerType (which is an XmlObject). Simple types turn into simple getters and setters with the correct java type. For example getName might return a String.
    XmlCursor: From any XmlObject the developer can get an XmlCursor. This provides efficient, low level access to the XML Infoset. A cursor represents a position in the XML instance. The cursor can be moved around the XML instance at any level of granularity needed from individual characters to Tokens.
    SchemaType: XMLBeans provides a full XML Schema object model that can be used to reflect on the underlying schema meta information. For example, the developer might generate a sample XML instance for an XML schema or perhaps find the enumerations for an element so that they may be displayed.
  • XML Parser

    Java API for XML Processing - JAXP
    is one of the Java XML programming APIs. It provides the capability of validating and parsing XML documents. The three basic parsing interfaces are:
  • the Document Object Model parsing interface or DOM interface
  • the Simple API for XML parsing interface or SAX interface
  • the Streaming API for XML or StAX interface (added in JDK 6; separate jar available for JDK 5)

    XML Parsing
    Traditionally, XML APIs are either:
  • tree based - the entire document is read into memory as a tree structure for random access by the calling application
  • event based - the application registers to receive events as entities are encountered within the source document.
    Both have advantages; the former (for example, DOM) allows for random access to the document, the latter (e.g. SAX) requires a small memory footprint and is typically much faster.

    These two access metaphors can be thought of as polar opposites. A tree based API allows unlimited, random, access and manipulation, while an event based API is a 'one shot' pass through the source document.

    Event-based XML Parsing Libs
    SAX is the Simple API for XML, originally a Java-only API. SAX was the first widely adopted API for XML in Java, and is a "de facto" standard.

    A parser which implements SAX (ie, a SAX Parser) functions as a stream parser, with an event-driven API. The user defines a number of callback methods that will be called when events occur during parsing. The SAX events include:
  • XML Text nodes
  • XML Element nodes
  • XML Processing Instructions
  • XML Comments

    Events are fired when each of these XML features are encountered, and again when the end of them is encountered. XML attributes are provided as part of the data passed to element events. SAX parsing is unidirectional; previously parsed data cannot be re-read without starting the parsing operation again.

    DOM-based XML Parsing Libs
    An open source Java-based document object model for XML that was designed specifically for the Java platform so that it can take advantage of its language features. JDOM integrates with Document Object Model (DOM) and Simple API for XML (SAX), supports XPath and XSLT. It uses external parsers to build documents.

    An open source Java library for working with XML, XPath and XSLT. It is compatible with DOM, SAX and JAXP standards.

    Streaming API for XML (StAX)
    An application programming interface (API) to read and write XML documents, originating from the Java programming language community.

    StAX was designed as a median between these two opposites. In the StAX metaphor, the programmatic entry point is a cursor that represents a point within the document. The application moves the cursor forward - 'pulling' the information from the parser as it needs. This is different from an event based API - such as SAX - which 'pushes' data to the application - requiring the application to maintain state between events as necessary to keep track of location within the document.

    StAX XML Parsing Libs
    Apache Axiom
    A a light weight XML object model based on top of Stax and also provides lazy object building.