4.1 Introduction to web services
• A web service is any piece of software that makes itself available over the internet and uses a standardized XML messaging system. XML is used to encode all communications to a web service. For example, a client invokes a web service by sending an XML message, then waits for a corresponding XML response. As all communication is in XML, web services are not tied to any one operating system or programming language—Java can talk with Perl; Windows applications can talk with Unix applications.
• Web services are self-contained, modular, distributed, dynamic applications that can be described, published, located, or invoked over the network to create products, processes, and supply chains. These applications can be local, distributed, or web-based. Web services are built on top of open standards such as TCP/IP, HTTP, Java, HTML, and XML.
• Web services are XML-based information exchange systems that use the Internet for direct application-to-application interaction. These systems can include programs, objects, messages, or documents.
• A web service is a collection of open protocols and standards used for exchanging data between applications or systems. Software applications written in various programming languages and running on various platforms can use web services to exchange data over computer networks like the Internet in a manner similar to inter-process communication on a single computer. This interoperability (e.g., between Java and Python, or Windows and Linux applications) is due to the use of open standards.
To summarize, a complete web service is, therefore, any service that –
• Is available over the Internet or private (intranet) networks
• Uses a standardized XML messaging system
• Is not tied to any one operating system or programming language
• Is self-describing via a common XML grammar
• Is discoverable via a simple find mechanism
4.2 Restfull Web Service
REST stands for REpresentational State Transfer. REST is web standards based architecture and uses HTTP Protocol. It revolves around resource where every component is a resource and a resource is accessed by a common interface using HTTP standard methods. REST was first introduced by Roy Fielding in 2000.
In REST architecture, a REST Server simply provides access to resources and REST client accesses and modifies the resources. Here each resource is identified by URIs/ global IDs. REST uses various representations to represent a resource like text, JSON, XML. JSON is the most popular one.
HTTP methods
Following four HTTP methods are commonly used in REST based architecture.
• GET − Provides a read only access to a resource.
• POST − Used to create a new resource.
• DELETE − Used to remove a resource.
• PUT − Used to update a existing resource or create a new resource.
A web service is a collection of open protocols and standards used for exchanging data between applications or systems. Software applications written in various programming languages and running on various platforms can use web services to exchange data over computer networks like the Internet in a manner similar to inter-process communication on a single computer.
This interoperability (e.g., between Java and Python, or Windows and Linux applications) is due to the use of open standards.
Web services based on REST Architecture are known as RESTful web services. These webservices uses HTTP methods to implement the concept of REST architecture. A RESTful web service usually defines a URI, Uniform Resource Identifier a service, provides resource representation such as JSON and set of HTTP Methods.
Creating RESTFull Webservice
Sr.No. URI HTTP Method POST body Result
1 /UserService/users GET empty Show list of all the users.
2 /UserService/addUser POST JSON String Add details of new user.
3 /UserService/getUser/:id GET empty Show details of a user.
4.3 Soap Web Service
SOAP is an XML-based protocol for accessing web services over HTTP. It has some specification which could be used across all applications.
SOAP is known as the Simple Object Access Protocol, but in later times was just shortened to SOAP v1.2. SOAP is a protocol or in other words is a definition of how web services talk to each other or talk to client applications that invoke them.
SOAP was developed as an intermediate language so that applications built on various programming languages could talk easily to each other and avoid the extreme development effort.
In today's world, there is huge number of applications which are built on different programming languages. For example, there could be a web application designed in Java, another in .Net and another in PHP.
Exchanging data between applications is crucial in today's networked world. But data exchange between these heterogeneous applications would be complex. So will be the complexity of the code to accomplish this data exchange.
One of the methods used to combat this complexity is to use XML (Extensible Markup Language) as the intermediate language for exchanging data between applications.
Every programming language can understand the XML markup language. Hence, XML was used as the underlying medium for data exchange.
But there are no standard specifications on use of XML across all programming languages for data exchange. That is where SOAP comes in.
SOAP was designed to work with XML over HTTP and have some sort of specification which could be used across all applications.
4.4 JSON parsing
JSON stands for JavaScript Object Notation. It is an independent data exchange format and is the best alternative for XML. This chapter explains how to parse the JSON file and extract necessary information from it.
Android provides four different classes to manipulate JSON data. These classes are JSONArray,JSONObject,JSONStringer and JSONTokenizer.
The first step is to identify the fields in the JSON data in which you are interested in.
Advantage of JSON over XML
1) JSON is faster and easier than xml for AJAX applications.
2) Unlike XML, it is shorter and quicker to read and write.
3) It uses array.
json object
A JSON object contains key/value pairs like map. The keys are strings and the values are the JSON types. Keys and values are separated by comma. The { (curly brace) represents the json object.
{
"employee": {
"name": "sachin",
"salary": 56000,
"married": true
}
}
json array
The [ (square bracket) represents the json array.
["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]
Let's take another example of json array.
{ "Employee" :
[
{"id":"101","name":"Sonoo Jaiswal","salary":"50000"},
{"id":"102","name":"Vimal Jaiswal","salary":"60000"}
]
}
4.5 XML parsing
XML stands for Extensible Mark-up Language.XML is a very popular format and commonly used for sharing data on the internet. This chapter explains how to parse the XML file and extract necessary information from it.
Android provides three types of XML parsers which are DOM,SAX and XMLPullParser. Among all of them android recommend XMLPullParser because it is efficient and easy to use. So we are going to use XMLPullParser for parsing XML.
The first step is to identify the fields in the XML data in which you are interested in. For example. In the XML given below we interested in getting temperature only.
<?xml version="1.0"?>
<current>
<city id="2643743" name="London">
<coord lon="-0.12574" lat="51.50853"/>
<country>GB</country>
<sun rise="2013-10-08T06:13:56" set="2013-10-08T17:21:45"/>
</city>
<temperature value="289.54" min="289.15" max="290.15" unit="kelvin"/>
<humidity value="77" unit="%"/>
<pressure value="1025" unit="hPa"/>
</current>
XML - Elements
1Prolog
An XML file starts with a prolog. The first line that contains the information about a file is prolog
2Events
An XML file has many events. Event could be like this. Document starts , Document ends, Tag start , Tag end and Text e.t.c
3Text
Apart from tags and events, and xml file also contains simple text. Such as GB is a text in the country tag.
4.Attributes
Attributes are the additional properties of a tag such as value e.t.c.
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