Web Services In Java- An Introduction


As the name implies, a web service is a "service available on web".

The main difference between a web application (which may also be called as a service available on web) and a Web service is that the website/web application is meant for humans as end users whereas a web service is meant for another application. A web service basically services the needs of an application running on another server which is network accessible. 

The main advantage of Web Service is Interoperability, which means it is not any technology or platform specific.

This means that a C# application running on an Application Server A on a Windows machine can request for a web service written in Java and running on an Application Server B on a Linux machine.

It may also happen that the web service host is an Application Server whereas the web service client is a mobile device having a network access to the host.

Web Services are mainly of two types:

SOAP based (JAX-WS specification) – uses standardized XML messaging system where client sends SOAP messages to request for a service and the host responds back again with SOAP messages. The transport mainly happens over HTTP though FTP and SMTP are other protocols supported.

REST Style (JAX-RS specification) – uses HTTP for both transporting the messages as well as an API for forming the service requests and response.

We will first start with learning the SOAP based web services and then move on to REST style Web services.

Architecture:

 The SOAP based web architecture looks something like shown in the below diagram:




















The main components of a SOAP-based web service can be defined as below:

     a) WSDL (Web Service Description Language) - As the name implies, this is a description of the Web Service. It describes the exposed methods, their parameters, return types etc. It is an XML document.

      b)UDDI (Universal Description, Discovery and Integration) - This acts as a directory of the published web services. The web service hosts can publish their services and the clients can find the services with this layer.

 c)SOAP (Simple Object Access Protocol) – It is an XML based protocol for sending and receiving the data to and from the web services.

d) HTTP/FTP/SMTP/BEEP – These are the protocols responsible for transporting the messages between the applications.

That's all for the introduction part. In the next post in this series, we will look at writing a Web Service Client for an available web service.

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