Servlets and a little bit of Web Services Russell Beale.
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Servlets and a little bit of Web Services Russell Beale
Overview In general Provide remote access to applications Servlets What are servlets How can we use them Web Services What are web services…
Objectives Learn about using servlets as one way of providing web based interfaces to databases and other applications. Learn how to create and deploy servlets using the NetBeans IDE and Tomcat server Learn about Web Services and their advantages in relation to providing web based interfaces to databases and other applications See how to create and deploy Web Services using Java, Apache Tomcat, and Apache Axis Be aware of other tools for developing, deploying, and consuming web services
Access over the Web Web Application Web Server Web Application Web Browser HTTP Web Service Client HTTP Web Pages Application Interface
Servlets and Web Services Servlets providing generic access to an application, using a web interface we need to build both client and server Web Services providing generic access with a defined API allows custom interface at the client we can just build the server
Using servlets A user (1) requests some information by filling out a form containing a link to a servlet and clicking the Submit button (2). The server (3) locates the requested servlet (4). The servlet then gathers the information needed to satisfy the user's request and constructs a Web page (5) containing the information. That Web page is then displayed on the user's browser (6). (bit like CGI scripts, bit like applets) (from Sun)
Servlets Servlets are server-side resources Servlets are Java objects that act as compact web servers Can support all protocols, but are not as flexible/powerful as full servers Need to run inside a web server that supports servlets Take in requests re-directed from the web-server, write HTML back to the client
Advantages of servlets Based on Java: convenient & powerful, can talk directly to the server Efficient – lightweight Java processes, servlet code loads only once Free/very cheap
Typical uses Processing and/or storing data submitted by an HTML form. Providing dynamic content from, for example, a database query Managing state information on top of HTTP (which is stateless) e.g. an online shopping cart which manages baskets for many concurrent customers and maps every request to the right customer.
Servlets Servlets are part of J2EE All servlets implement interface javax.servlet.Servlet javax.servlet.Servlet We will be using javax.servlet.http.HttpServlet javax.servlet.http.HttpServlet
HTTP protocol 8 request methods: GET – retrieve content POST – send data, retrieve content HEAD – retrieve headers only PUT – upload content DELETE – remove content TRACE – echos the request, showing servers etc OPTIONS – returns list of supported methods CONNECT – used with SSL proxy tunnels
Lifecycle init() set up the servlet service() respond to requests, after init() destroy() shutdown the servlet
Using HttpServlet By extending HttpServlet, we only have to over-ride the methods we need to E.g., doGet(), doPost()
HelloWorld servlet Using NetBeans, we can easily create servlets under Tomcat Tomcat is a Java server that supports servlets Tomcat is bundled with NetBeans IDE HelloWorld servletservlet
POST and GET GET and POST allow information to be sent back to the webserver from a browser (or other HTTP client for that matter) Imagine that you have a form on a HTML page and clicking the "submit" button sends the data in the form back to the server, as "name=value" pairs.
GET… Choosing GET as the "method" will append all of the data to the URL and it will show up in the URL bar of your browser. The amount of information you can send back using a GET is restricted as URLs can only be 1024 characters.
POST… A POST will send the information through a socket back to the webserver and it won't show up in the URL bar. It is stored on the request object You can send much more information to the server this way not restricted to textual data - you can send files and even binary data such as serialized Java objects
Handling GET requests GET requests call the doGet() method on your servlet Put code in that method to handle GET, or call another method to do it GET can pass in data through URL encoding
Handling POST requests POST requests call the doPost() method Put code in this method, or call another one Post data is stored on the request object PostExample.htm
Storing Data We often want to store some data about the user and their requests We can do this in 2 ways: Client-side - cookies Server-side – session data, database etc
What are cookies? HTTP protocol is stateless Browser contacts server ata URL, requests a page, provides its capabilities Server sends info to client Connection closed So to mark one visitor to track visit to site, need to store a piece of information on the client side This is the cookie HTTP header that contains text string
Two sorts Session Temporary, erased when you close browser Often used by e-commerce sites for shopping carts Persistent Written to hard drive Remain until erased or expire Used to store user preferences
Sessions Live on the server Actually built on top of cookies or URL rewritin, but you don’t have to bother with this HttpSession object Stores all the information for a session Saves you having to access the cookies yourself
Servlets and JSP Putting large amounts of HTML into servlets is a bit cumbersome JSP pages let you use Java code directly in a HTML document The Java code is then executed as a servlet at runtime