HttpServlet

HttpServlet

• What is Servlet’s Job?
• Servlets are Java programs that run on Web or Application servers.
• It is acting as a middle layer between requests coming from Web browsers or other HTTP clients and databases or applications on the HTTP server.
• Figure:

• Jobs are:
• Read the explicit data sent by the client.
• Read the implicit HTTP request data sent by the browser.
• Generate the results.
• Send the explicit data (i.e., the document) to the client.
• Send the implicit HTTP response data.
• The HttpServlet class extends the GenericServlet class and implements Serializable interface. 
• It provides http specific methods such as doGet, doPost, doHead, doTrace etc.

Methods:

• public void service(ServletRequest req,ServletResponse res):dispatches the request to the protected service method by converting the request and response object into http type.
• protected void service(HttpServletRequest req, HttpServletResponse resp):receives the request from the service method, and dispatches the request to the doXXX() method depending on the incoming http request type.
• protected void doGet(HttpServletRequest req, HttpServletResponse resp):handles the GET request. It is invoked by the web container.
• protected void doPost(HttpServletRequest req, HttpServletResponse resp):handles the POST request. It is invoked by the web container.
• protected void doHead(HttpServletRequest req, HttpServletResponse resp):handles the HEAD request. It is invoked by the web container.
• protected void doOptions(HttpServletRequest req, HttpServletResponse resp):handles the OPTIONS request. It is invoked by the web container.
• protected void doPut(HttpServletRequest req, HttpServletResponse resp):handles the PUT request. It is invoked by the web container.
• protected void doTrace(HttpServletRequest req, HttpServletResponse resp):handles the TRACE request. It is invoked by the web container.
• protected void doDelete(HttpServletRequest req, HttpServletResponse resp):handles the DELETE request. It is invoked by the web container.
• protected long getLastModified(HttpServletRequest req):returns the time when HttpServletRequest was last modified since midnight January 1, 1970 GMT.

• Servlet methods has main two types of arguments:
• HttpServletRequest Vs. HttpServletResponse
• The HttpServletRequest lets you get at all of the incoming data; 
• It has methods by which you can find out about information such as form (query) data, HTTP request headers, and the client’s hostname. 
• The HttpServletResponse lets you specify outgoing information such as HTTP status codes (200, 404, etc.) and response headers (Content-Type, Set-Cookie, etc.). 
• Most importantly, HttpServletResponse lets you obtain a PrintWriter that you use to send document content back to the client. 
• For simple servlets, most of the effort is spent in println statements that generate the desired page. 
• Generating plain text using Servlet
• Exmple: 
  
• How it works?
  
• Generating HTML using Servlet
• Most servlets generate HTML, not plain text. 
• To generate HTML, you add three steps: 
• 1. Tell the browser that you’re sending it HTML. 
• You accomplish the first step by setting the HTTP Content-Type response header to text/html. 
• There is also a special setContentType method just for this purpose.
• Example:
• response.setContentType("text/html"); // for HTML output
• response.setContentType("application/vnd.ms-excel"); // for Excel spreadsheet output
• 2. Modify the println statements to build a legal Web page. 
• 3. Check your HTML with a formal syntax validator.
• PrintWriter is an abstract class for writing to character streams. 
• Print formatted representations of objects to a text-output stream. 
• This class implements all of the print methods found in PrintStream. 
• Example: