Distributed Programming in Java

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Distributed Programming in Java

• Web Services (4)

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Properties of Web Services

• Web services assume that services are
  loosely-coupled
• they have minimal dependencies and
• they are typically defined, developed, and
  deployed by different companies.
• This assumption leads to a world view that
  everything is a service and helps to realize
  the vision of service-oriented architectures in
  which software applications are created by
  combining multiple services.

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Web Services Architecture

• Two aspects internal and external
• Internal how do we connect a Web service to a
  company's information systems
• External how do we connect a client with a Web
  service

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Internal view

• Web services are viewed as wrappers of existing
  information systems. We need a middleware layer
  that can connect Web services to a company's
  existing middleware platform.

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External View

• Web services assume the existence of a
  third-party directory service that is used for
  service discovery. Once such a service is in
  place, then the external architecture looks like
  this

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• Since it would be difficult to scale this
  approach (assuming the existence of a third-party
  manager) for other middleware characteristics
  (such as support for transactions), it is assumed
  that in the future, individual Web services will
  each provide these managers (in some fashion) and
  that these individual managers will coordinate in
  a peer-to-peer fashion to achieve the desired
  effects.

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UDDI

• Universal Description, Discovery a mechanism for
  registering and discovering Web services.
• From simple yellow pages directory to a framework
  for brokering collaboration between Web services

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How to find Web Services

• Static discovery programmer manually find and
  then hardwire it into the application.
• Human intelligence and
• public registries
• Dynamic discovery the application itself at
  runtime finds a required Web service selected on
  the basis of some criteria, eg. price,
  availability, or efficiency.
• private registries where the semantics and
  validity of entries are guaranteed by trusted
  organizations.

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Descriptions UDDI

• A hierarchy of five levels
• publisherAssertion
  (describes relations between businesses
  eg one is a subsidiary of another)
• businessEntity
  (describes a concrete business
  contact info and UDDI key)
• businessService
  (describes a Web service with an
  description, UDDI key, and key references)
• bindingTemplate
  (describes how to invoke a Web service with
  an address and a reference to tModel)
• tModel
  (technical details for invoking a Web
  service, reference to WSDL description)

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Business Entity for Recipe Server (1/2)
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Business Entity for Recipe Server (2/2)
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tModel for Recipe Server
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Discovery UDDI
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Summary

• SOAP a transport neutral protocol for XML data
• interchange (but focusing on HTTP)
• WSDL description of Web service interfaces
• UDDI registries and discovery of Web services

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Essential Online Resources

• SOAP http//www.w3.org/TR/soap/
• WSDL http//www.w3.org/2002/
• UDDI http//www.uddi.org
• XML-RPC http//www.xmlrpc.com/
• The Web Service Interoperability
  http//www.ws-i.org/

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Architecture

• Structure(s) of a system, comprising
• components (core blocks)
• externally visible properties of these components
  (constraints)
• their relationships (connectors)
• Architectural style is a specialization of
  component and connector types with a set of
  constraints on how they can be used

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Interaction of Java and WS

• WS architecture has its root in component-based
  architecture, and supports different
  communication styles messages, call-return
• Java platform embodies object-oriented
  architecture with call-return communication
• Need to convert ...
• between Javas call-return style to the dominant
  message-based communication style of WS
• between rich (objects) and flat data structures

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Architecture Adapter (Take 1)

• Architecture adapter as a component that mediates
  between two architectures

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Architecture Adapter (Take 1)

• Architecture A Component implements functionality
  in style A, eg OO, call-return
• Architecture B Component implements functionality
  in style B, eg message, flat
• Architecture Adapter mediates between style A and
  style B, which means that is must offer a natural
  interface to both styles
• Needs to convert component architecture, ie
  communication style and data style

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Architecture Adapter (Take 2)

• Often the Architecture Adapter is split into two
  halves, which communicate with one another using
  their own architectural style

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Architecture Adapters in WS

• WS frameworks such as Axis allow you to be fully
  WS-agnostic to access WS from Java use method
  calls via architecture adapters

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Customer Database

• Core classes of a customer database

Adhere to JavaBeansconventions
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XML to Java Data Mapping

• Basic mapping defined by JAX-RPC spec
• Mapping of primitive types
• Complex types that follow the JavaBeans
  convention (Bean Serializer)
• Arrays and some Collections
• Exceptions
• Key consideration is the interoperability between
  SOAP implementations !

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Mapping of Primitive Types

• Primitive typesdefined instandard SOAPencoding

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Complex Types

• Axis can serialize and deserialize any classes
  that follow the JavaBeans convention ...
• ... without requiring you to write any code !
• Simple properties
• setAddress and getAddress
• Indexed properties (arrays of values)
• Customer
• Business Object Pattern PM

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Bean Serializers

• To configure a bean mapping, add a ltbeanMappinggt
  tag to the WSDD
• Maps a Java bean to an XML QName

ltbeanMapping qname"nslocal" xmlnsns"someNamesp
ace" languageSpecificType"javamy.java.SomeEnt
ity"/gt
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BeanService
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Order (simple)
public class Order private String
customerName private String shippingAddress p
ublic String getCustomerName() return
customerName public void setCustomerName(Stri
ng name) customerName name public
String getShippingAddress() return
shippingAddress public void
setShippingAddress(String address)
shippingAddress address // ...
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Arrays and Collections

• Some Java collections (Vector, Hashtable, ...)
  have serializers, but interoperability between
  SOAP implementations is not guaranteed
• eg .NET does not support Hashtables
• Most reliable way is to use Arrays

public CustomerImpl getCustomers() public void
setCustomers(CustomerImpl customers)
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Order (indexed)
// ... private String itemCodes private
int quantities public String getItemCodes()
return itemCodes public void
setItemCodes(String items)
itemCodes items public int
getQuantities() return quantities public
void setQuantities(int quants) quantities
quants
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BeanService
public class BeanService public String
processOrder(Order order) String sep
System.getProperty("line.separator")
String response "Hi, " order.getCustomerName()
response sep "You have ordered
these items" sep String items
order.getItemCodes() int quantities
order.getQuantities() for (int i 0 i
lt items.length i) response
sep quantitiesi " of item "
itemsi return response

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WSDD for BeanService

• Add data for the WS architecture that does not
  exist in the Java platform
• Namespace (urnBeanService)
• Serializers (beanMapping to nsorder)

ltdeployment xmlns"http//xml.apache.org/axis/wsdd
/" xmlnsjava"http//xml.apache.org/axis/wsdd/pro
viders/java"gt ltservice name"OrderProcessor"
provider"javaRPC"gt ltparameter
name"className" value"samples.userguide.example5
.BeanService"/gt ltparameter
name"allowedMethods" value"processOrder"/gt
ltbeanMapping qname"nsOrder"
xmlnsns"urnBeanService" languageSpecificType"j
avasamples.userguide.example5.Order"/gt
lt/servicegt lt/deploymentgt
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Exceptions

• java.rmi.Exceptions are mapped to SOAP Faults
  (recap request, response, fault)
• Fault code contains class name of fault
• Other exceptions treated differently
• Mapped to wsdlfault in WSDL of the method
• Need to follow JavaBeans convention (need to have
  accessors for all fields of the exception and a
  constructor with all fields as its arguments)

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Using Axis Step by Step

• Provide a Java interface or class that describes
  the service interface
• Create WSDL using Java2WSDL tool
• Create bindings through WSDL2Java tool
• Implement the service interface
• Deploy the service
• Implement clients using generated stubs

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Provide Java Interface

• Define the functionality you want to expose as a
  web service using an interface

package dpj.ws.steps public interface
OrderProcessor public void processOrder(Order
order)
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Create WSDL from Java

• Create the WSDL (OrderProcessor.wsdl) from the
  Java interface defined earlier
• -o name of output file
• -l location of web service
• -n target namespace of the WSDL document
• -p mapping of name space to packages

java org.apache.axis.wsdl.Java2WSDL -o
"dpj/ws/steps/OrderProcessor.wsdl" -l
"http//localhost8080/axis/services/OrderProcesso
r" -n "urnSteps" -p"dpj.ws.steps"
"urnSteps" dpj.ws.steps.OrderProcessor
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Create WSDL from Java

• Conversion to WSDL will also generate XML types
  (using XML Schema) for all non-primitive types
  references in the interface
• Supports bean classes, enumeration classes,
  arrays, and holder classes (inout)

ltcomplexType name"Order"gt ltsequencegt ltelement
name"customerName" nillable"true"
type"soapencstring"/gt ltelement
name"itemCodes" nillable"true"
type"implArrayOf_soapenc_string"/gt ltelement
name"quantities" nillable"true"
type"implArrayOf_xsd_int"/gt ltelement
name"shippingAddress" nillable"true"
type"soapencstring"/gt lt/sequencegt lt/complexType
gt
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Create Bindings from WSDL

• Earlier we had handcraft SOAP messages to invoke
  a web service (too low-level)
• Help is on its way with WSDL2Java you can
  convert a WSDL file into Java stubs

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Create Bindings from WSDL

• Create a client-side architecture adapter using
  WSDL2Java
• -o name of output file
• -N mapping of name space to target package
• Adapter classes created
• public interface OrderProcessorService
• public class OrderProcessorServiceLocator

java org.apache.axis.wsdl.WSDL2Java -o .
\ -NurnSteps dpj.ws.steps.client
dpj/ws/steps/OrderProcessor.wsdl
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Implement Service and Clients

• Implement the service by implementing the
  OrderProcessing interface
• Implement the client

public class OrderProcessorImpl implements
OrderProcessor
OrderProcessorService service new
OrderProcessorServiceLocator() OrderProcessor
port service.getOrderProcessor() Order order
... // construct the order port.processOrder(or
der)
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Tips

• Read the Axis User Guide
• If you can deploy it, you can undeploy it!

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If You Can Deploy It, ...

• Deploying is the act of making a web service
  available to the Axis engine
• Send to Axis engine with AdminClient

ltdeployment xmlns"http//xml.apache.org/axis/wsdd
/" xmlnsjava"http//xml.apache.org/a
xis/wsdd/providers/java"gt ltservice
name"MyService" provider"javaRPC"gt
ltparameter name"scope" value"Application"/gt
ltparameter name"className" value"samples.usergui
de.example3.MyService"/gt ltparameter
name"allowedMethods" value""/gt
lt/servicegt lt/deploymentgt
gt java org.apache.axis.client.AdminClient -p 9090
deploy.wsddltAdmingtDone processinglt/Admingt
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... You Can Undeploy It!

• A service removed from the Axis engine by
  undeploying it (eg to install a new one)
• As an aside, you can get a list of installed
  services from the AdminClient

ltundeployment xmlns"http//xml.apache.org/axis/ws
dd/"gt ltservice name"MyService"/gt lt/undeploymentgt
gt java org.apache.axis.client.AdminClient -p 9900
list ... ltns1service name"MyService"
provider"javaRPC"gt ltns1parameter
name"allowedMethods" value""/gt
ltns1parameter name"className"
value"samples.userguide.example3.MyService"/gt
lt/ns1servicegt
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Deployment Step by Step

• Assuming that you have started your Axis engine
  with the SimpleAxisServer ...
• Deploy your service with the AdminClient
• After using the service, undeploy

gt cd workspace/Axis gt java org.apache.axis.transpo
rt.http.SimpleAxisServer -p 9090
gt java org.apache.axis.client.AdminClient -p 9090
deploy.wsddltAdmingtDone processinglt/Admingt
gt java org.apache.axis.client.AdminClient -p 9090
undeploy.wsddltAdmingtDone processinglt/Admingt
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• http//developer.apple.com/documentation/WebObject
  s/Web_Services/Web_Services/chapter_4_section_7.ht
  ml