SSC2: Web Services

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

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

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

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

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

XML example Heller Joseph Catch 22 Macmillan Preece Jenny Rogers Yvonne Sharp Helen Human Computer Interaction Addison Wesley

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

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

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

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

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

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

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

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 doesn’t allow the use of other Java packages

Data types xsd:base64Binarybyte[] xsd:booleanBoolean xsd:byteByte xsd:dateTimejava.util.Calendar xsd:decimaljava.math.BigDecimal xsd:doubleDouble xsd:floatFloat xsd:hexBinarybyte[] xsd:intInt xsd:integerjava.math.BigInteger xsd:longLong xsd:Qnamejavax.xml.namespace.Qname xsd:shortShort xsd:stringjava.lang.String

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

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

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

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

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 user’s session Application: a new instance is created each time the application is started

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