Apache Axis:

1
Apache Axis

• A Set of Java Tools for SOAP Web Services

2
Whats a SOAP web service ?

• A remote resource that accepts and returns SOAP
  messages.
• SOAP messages are XML.
• The XML sent to the resource describes an
  invocation of a method call parameters and
  (optionally) their types and method name, and
  some metadata about the message itself.
• The XML received from the resource describes the
  return value and the type, or exceptions, and
  some metadata about the message itself.
• A programming languages do the same thing with
  their syntax, but XML is weaker in readability
  and economy of expression.
• XML can be described in a schema. Web service
  schemas are called WSDL and also define protocols
  ( e.g. http ) and a usually a URI. Schemas must
  define the types but the SOAP XML instance does
  not have too.
• Anything that is optional is implementation
  dependent and varies among resource providers.
• Web services do not have to use SOAP, they do not
  have to use HTTP even. A SOAP web service does
  not require the HTTP protocol, but in practice
  its almost always used.
• SOAP is the standard that major vendors have
  agreed to, and can be expected to be supported by
  external systems.

3
Apache Axis

• Hide implementation details of web services that
  talk SOAP
• Wrap Java methods and deploy them as web
  services.
• Create web service clients that accept and return
  Java objects.
• In the simplest case, the developer need never
  know that SOAP/XML/HTTP is being used. Axis does
  all the serialization and protocol handling.
• It is possible to customize for more complex
  cases with hooks provided by Axis.
• SOAP and WSDL are complicated, because they use
  XML schemas with namespaces, and do serialization
  of java objects and primitives to and from mapped
  XML representations. Best let the tools do it.
• This is on purpose SOAP was developed by MS to
  sell their tools.
• Fortunately a free set of tools was developed
  Axis.

4
Create a Web Service in Java

• Set up server add Axis servlet mapping to
  web.xml make sure Axis in classpath.
• Write your classes and methods in POJ.
• Use types that have built-in type mappings.
• Can also generate a skeleton from a Java
  interface.
• Write a deployment descriptor (.wsdd).
• Use the Axis admin client to deploy.
• Cest tout !

5
Create a Web Service Client

• Two simplest options
• Use the Service and Call classes.
• Generate code for proxies and stubs from a WSDL.
• If its not a web service you have the source
  code for, you have to use the WSDL. Therefore
  always generate and make available a WSDL for
  your own web services. It defines the contract
  with your clients, and they can use their tools
  to create the client code.

6
Code Generation

• Conveniently generate WSDL, client and server
  code from one Java interface.
• Generated code is a skeleton for an
  implementation. You can focus on your business
  logic in your implementation without worrying
  about protocol details.
• All the usual pluses and minuses of code
  generation.

7
Why Web Services ?

• Easier distributed computing.
• Move toward systems of software components with
  well defined interfaces that are able to easily
  plug-in ( and out ) modular, not monolithic,
  software systems without unnecessary coupling (
  SOA). Tight / inappropriate coupling makes
  brittle systems that cannot be evolved with
  changing conditions.
• Disparate systems need access to a particular
  piece of functionality. Creating it as a service
  means it can be coded and maintained in one place
  but still be available everywhere its needed.
• The SOAP and Web Service specifications represent
  a consensus for allowing software components to
  be included in a computing environment and to be
  used, irrespective of physical location,
  operating system, programming language, etc. HTTP
  XML removes barriers.
• May seem to introduce complexity but the tools
  can hide that. Performance may be an issue ( due
  to serialization / de-serialization to XML,
  network latency ) depending on how its used.

8
Why Web Services ? cont.

• Identify candidate services.
• Discrete unit of work.
• More than one client.
• Clients cant be constrained to one
  implementation language, platform, or server.
• Does something tricky that we only want to
  maintain in one place.
• Does something that many clients need to do.
• Need flexibility to change other system
  components, uncouple them from implementations
  and use generic interface instead.
• Develop a service catalog.
• Get buy in by development teams overcome I can
  just hack it myself much easier than dealing with
  all this stuff.
• Good communication required. Silos wont use
  services from other silos.
• Problems
• Co-variant development and dependency loops.
• Additional testing, development environments.
  Integration environment needed.
• Deployment issues between environments must
  configure to use the right service for the given
  environment. Use a good configuration
  deployment process.
• Monitoring and troubleshooting may have to
  debug a process across different servers.