SSC2: Web Services

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SSC2 Web Services
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Web Services

• Web Services offer interoperability using the web
• Web Services provide information on the
  operations they can perform
• Clients can be built using this information, and
  coupled to any interface
• Natural extension of OOP

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Uses of Web Services

• To glue together components of a system,
  especially distributed computing
• MOBIlearn
• mobile gaming
• To provide standardised access to databases,
  services etc
• Amazon

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Important elements

• SOAP
• The XML messaging protocol that all web services
  use
• UDDI
• A directory service for locating Web Services
• Built in to .NET
• WSDL
• XML document describing what a web service can do

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Linux Java
Web Service HTTP XML
Windows C
PDA .NET
Server PHP
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XML

• Plain text mark-up
• Strict syntax
• Flexible vocabulary
• Supports schemas

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XML example
ltbooks source"Pete's Bookshelf"gt ltbookgt
ltauthor type"single author"gt
ltlastnamegtHellerlt/lastnamegt
ltfirstnamegtJosephlt/firstnamegt lt/authorgt
lttitlegtCatch 22lt/titlegt
ltpublishergtMacmillanlt/publishergt ltisbn
number"010176483933" /gt lt/bookgt ltbookgt
ltauthor type"primary author"gt
ltlastnamegtPreecelt/lastnamegt
ltfirstnamegtJennylt/firstnamegt lt/authorgt
ltauthor type"secondary author"gt
ltlastnamegtRogerslt/lastnamegt
ltfirstnamegtYvonnelt/firstnamegt lt/authorgt
ltauthor type"secondary author"gt
ltlastnamegtSharplt/lastnamegt
ltfirstnamegtHelenlt/firstnamegt lt/authorgt
lttitlegtHuman Computer Interactionlt/titlegt
ltpublishergtAddison Wesleylt/publishergt
ltisbn number"8575689937334" /gt
lt/bookgt lt/booksgt
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XML schemas

• Describe the expected structure of an XML
  document
• Books example

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XML and Web Services

• Web Services were envisaged as a way to allow
  different platforms, languages, systems etc to
  communicate using a standardised language
• Not just communicate, but perform actions
• XML schemas provide this language

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SOAP

• SOAP Simple Object Access Protocol
• Plain text XML transmitted using HTTP
• Sent using GET and POST, instead of HTML
• Allows sending of text/XML data
• workaround allows sending of binary files e.g.
  images, by sending byte array
• can also use SOAP with Attachments

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Summary

• A Web Service is a web application that has some
  of its methods accessible over the web
• Clients can retrieve a structured document giving
  details about how those methods should be called,
  and what they will return
• All done over HTTP/XML/SOAP

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Axis

• Apache Axis is a web application for deploying
  and consuming web services using Java
• Runs as an application under Tomcat
• Allows Java methods to be exposed as web
  service methods

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Axis tools

• Axis comes with a number of useful tools for
  working with WSDL, XML etc
• We can call them from the command line, but we
  need all the required libraries in our classpath
• So we can call them from a project inside
  NetBeans instead

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Setting up Axis

• Axis is a web application that runs under Tomcat
• Copy the axis folder from J to your work
  folder
• Create a new Web Application within NetBeans,
  using existing sources, with Axis as the source
• Add new classes to your Axis application

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Deployment

• Axis lets us deploy web services in 2 ways
• instantly, using .jws files that are deployed at
  runtime
• using a deployment config file to specify which
  classes and methods to expose
• Instant method is great for quick easy web
  services, but doesnt allow the use of other Java
  packages

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Data types

• xsdbase64Binary byte
• xsdboolean Boolean
• xsdbyte Byte
• xsddateTime java.util.Calendar
• xsddecimal java.math.BigDecimal
• xsddouble Double
• xsdfloat Float
• xsdhexBinary byte
• xsdint Int
• xsdinteger java.math.BigInteger
• xsdlong Long
• xsdQname javax.xml.namespace.Qname
• xsdshort Short
• xsdstring java.lang.String

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Returning sets of results

• Often we want to send/receive sets of data of
  with varying number of elements
• 3 basic methods
• concatenate the results into a single string
• return an array (of strings, integers etc)
• return a structured XML document

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Consuming Web Services

• Any platform that can parse generate SOAP
  requests can communicate with a Web Service
• Tools that generate code stubs from WSDL
  documents make it easy

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Consuming with Axis

• Axis tool WSDL2Java generates code stubs from
  WSDL document
• Can use stubs to create proxy classes that act
  as local versions of the web service, making
  calls to the actual service when necessary

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Consuming with Axis


Server side
Client side
User Interface
Web Service classes
Local classes
Axis web app
Proxy classes
Tomcat server
HTTP SOAP
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Understanding Scope

• Scope determines how often a servlet or web
  service gets loaded
• Request a new instance is created for every
  request that is received
• Session a new instance is created for each
  users session
• Application a new instance is created each time
  the application is started

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Issues

• Web Services only offer content pull
• Even with WSDL, ambiguity can arise because of
  interpretation
• XML might be difficult to parse, depending on
  methods used
• Not all implementations of SOAP are the same
• eg Infinity vs Inf
• Date/Time formats
• Trailing zeroes in decimals 0.70 vs 0.7