DISTRIBUTED SYSTEMS Practical Lab WebServices 2 A pragmatic - - PowerPoint PPT Presentation

DISTRIBUTED SYSTEMS

Practical Lab

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A pragmatic Introduction

WebServices

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WS - Overview

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 Problem

 Sockets / RMI / CORBA requires open ports for

communication

 Active firewalls may prohibit proprietary

communication protocol

 Idea

 Use http based messages for remote method

invocations

 Messages, Interfaces and the Communication are

defined/realized using standardized protocols

 SOAP / WSDL and HTTP (defined by W3C)

WS - Overall Architecture

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 Requester / Provider and Service Broker

 Architecture – Programmers Perspective  Server offers its service via a Web Container (not stand alone)

 E.g. Apache Tomcat, WebSphere, …

 Messages are encoded using SOAP (XML file format)  Interfaces are defined using WSDL (XML based description)  Communication is realized using HTTP (requires a webserver)

WebServices - Overview

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JVM - B Client

• Gen. Artefacts

SOAP/HTTP Service Web Container JVM - A

SOAP – The message protocol

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 Former: Simple Object Access Protocol  XML based message exchange format  See: http://de.wikipedia.org/wiki/SOAP

SOAP-ENV: Header SOAP-ENV: Body SOAP-ENV: Envelope

WSDL – Interface Specification

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 Web Services Description Language  XML base description language of service interfaces  Defines (examples):

 Endpoints – connection information  Ports/Interfaces – the service interface (collection of operations)  Operations – one “method” offered by the interface  Messages – used to encode arguments for operations

 See:

http://en.wikipedia.org/wiki/Web_Services_Description_Language

WebServices in Java

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Building a WebService using Java

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 Requires WebService Framework  Various implementations available

 Apache Axis - http://ws.apache.org/axis/  Apache CXF - http://cxf.apache.org/  JAX-WS – part of SDK since Java 6  J2EE Container specific implementations

 We will use JAX-WS (requires Java 6)

Defining an Interface (JAX-WS)

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 Can be an interface or an actual implementation  Definition marked using @WebSphere annotation

 => see DemoWS.java in WS Demo  Additional markup options offered via annotations

 See package javax.jws in Java API  Potential types for parameters and return types

are heavily restricted

 See

http://download.oracle.com/javaee/5/tutorial/doc/bnaz q.html#bnazs

Implement a Server

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 Server instantiates implementation of Service  Publishes service using an Endpoint  See: Server.java in WS demo

Endpoint.publish("http://localhost:8080/services", new DemoWS());

 JAX-WS is automatically generating WSDL for Service  Can be accessed via the URL

http://localhost:8080/services?wsdl while the server is running.

Implementing a Client

12  To access the server, code has to be generated based on the

WSDL

 Generation tool: wsimport (part of JDK)

wsimport -d ../../ -keep -p demo.ws.gen <URL>

 -d … root for code generation  -p … package name  -keep … preserves generated files  Note:

 files are compiled after generating them

 May lead to problems with eclipse => delete *.class

 Generates code for “complex” classes, independent of original classes  The tool does not support all kind of WSDL features

Implementing a Client (2)

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 Use generated classes to access Service

 XYZService … to obtain remote reference (Port)  Use generated interface to invoke methods  See: Client.java

 Attention:

 generated classes are incompatible to original

classes

WS – RMI comparison

 RMI  Uses “native” Java

means (Serialization)

 Supported by the JVM  Full support of user-

defined data types

 Synchronous

invocations only

 Only for Java  WebService  Uses XML files for

communication

 Requires additional

tools (part of SDK)

 Complexity for

parameters / results is limited

 Supports synchronous

and asynchronous calls

 Independent of Java

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