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

1 DISTRIBUTED SYSTEMS Practical Lab

WebServices 2 A pragmatic Introduction

WS - Overview 3  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 4  Requester / Provider and Service Broker

WebServices - Overview 5  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) JVM - A JVM - B Client Service Gen. Artefacts Web Container SOAP/HTTP

SOAP – The message protocol 6  Former: Simple Object Access Protocol  XML based message exchange format SOAP-ENV: Envelope SOAP-ENV: Header SOAP-ENV: Body  See: http://de.wikipedia.org/wiki/SOAP

WSDL – Interface Specification 7  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 8

Building a WebService using Java 9  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) 10  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 11  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) 13  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 14  RMI  WebService  Uses “native” Java  Uses XML files for means (Serialization) communication  Supported by the JVM  Requires additional tools (part of SDK)  Full support of user- defined data types  Complexity for parameters / results is limited  Supports synchronous  Synchronous and asynchronous calls invocations only  Independent of Java  Only for Java