Servlet API (Part II)

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IBM Software Group

© 2007 IBM Corporation

Servlet API (Part II)

4.1.02

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After completing this unit, you should be able to: Discuss servlets as controllers (Model-View-Controller

architecture) Look at the processing of request and response headers Understand how servlets handle redirection Discuss object sharing Look at how servlets can forward to or include content from

other servlets Understand the requirements for special characters in HTML

documents Discuss multithreading and thread safety issues Describe some of the internationalization support for servlets Understand the use of and need for the servlet context

After completing this unit, you should be able to: Discuss servlets as controllers (Model-View-Controller




Unit objectives

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Overview of Model-View-Controller (MVC) Servlet-only applications – servlet acts as model, view, and

controllerPoor separation of concerns

Servlet and JavaServer Pages (JSP) applications – the servlet is the controller, the JSP page is responsible for presentation, and other Java classes are the modelServlets (controller)JSP page (view)Business logic (model)

JavaBeans Enterprise JavaBeans (EJBs)

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Display Page (JSP)

Request

Response

Fo

rward

Interaction Controller

(Servlet, JSP)

Enterprise Logic and

Data

Enterprise Servers

Third-tier PlatformsApplication Server

Browser

Model-View-Controller (MVC)

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Controller Organization The Controller establishes the overall control flow in response

to a request At the highest level, the controller must:

Perform precondition checks Delegate business tasksEstablish state management tasks

The delegate business tasks step typically involves:Location of appropriate business objectsDelegate computation to business objectsBased on responses, select response agent

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Processing Request Headers The request header is built by the Web browser and sent to the

Web server The servlet accesses the request header values from the

HttpServletRequest object passed to the doGet or doPost methods

Headers supported differ based on HTTP levelHTTP 1.1 headers are a superset of HTTP 1.0 headersQuery the HTTP level via the

HttpServletRequest.getProtocol method Returns HTTP/1.1 for HTTP 1.1

There are HttpServletRequest methods for standard headersThe getHeaders and getHeaderNames methods return

Enumeration objects which provide access to all the header values associated with a particular header name

The getHeader method returns the first (or only) value for the named header

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Common Headers and HttpServletRequest Methods

Input data type and lengthHeaders: Content-type and Content-length Methods: getContentType and getContentLength

CookiesHeader: CookieMethod: getCookies

Identification for authorization purposesHeader: AuthorizationMethods: getAuthType and getRemoteUser

ProtocolMethod: getMethod

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// called by doGet and doPost methods to process requestprivate void processRequest( HttpServletRequest request, HttpServletResponse response) throws ... { ... // input parameters have been processed String method = request.getMethod(); if (method.equals("GET")) { // special "GET" processing String lang = request.getHeader("Accept-Language"); // lang has the client's language // now set the status and resp. headers // and build the output document

}}

Processing Request Headers (Example)

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Setting the Response Headers Can be set via the HttpServletResponse methods

setHeader, setDateHeader, and setIntHeader method.Some headers have their own methods: setContentType,

setContentLength, addCookie and sendRedirect

Response headers are required for some status codes:Document-moved status codes (range 300 to 307) require

a Location headerStatus code of 401 must have an associated WWW-

Authenticate header

Add additional support for your servlet.Cache-Control (HTTP 1.1) and Pragma (HTTP 1.0) -

cache optionsRefresh - how soon (in seconds) the browser should ask for

an updated page

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Main type / subtype Document type

application/pdf Acrobat file (.PDF)

application/postscript PostScript file

application/vnd.lotus-notes Lotus Notes file

application/x-gzip Gzip archive

application/x-java-archive JAR file

application/zip Zip Archive

audio/x-wav Windows sound file

text/html HTML document

text/xml XML document

image/gif GIF image

Content-Type The Content-Type header is set via the setContentType

method of the HttpServletResponse object Specifies the Multipurpose Internet Mail Extension (MIME)

type of the returned document Form is main_type/sub_type

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// process input parms and request headers...// set the return valueresponse.setStatus(HttpServletResponse.SC_OK);

// set the document typeresponse.setContentType("text/html");

// turn off cachingif (request.getProtocol().equals("HTTP/1.0")) {

// HTTP 1.0response.setHeader("Pragma","no-cache");

} else {// HTTP 1.1 or laterresponse.setHeader("Cache-Control","no-cache");

}response.setDateHeader("Expires", 0);// build the output document...

Response Header (Example)

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Response Redirection and Error Sending Response Redirect: sendRedirect()

Sends redirection response to client URL specifies the redirection location

May be absolute or relative

Error Sending: sendError() Sets a response status codeServer-specific error page describing the error sent as

responseCustom pages may be defined for specific codes in Web

deployment descriptor

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private void processRequest( HttpServletRequest request, HttpServletResponse response) ... { // process request headers & query data ... // redirect to another URL String url = "/YourResults.html"; if (test.equals("Error")) response.sendError(HttpServletResponse.SC_BAD_REQUEST); else response.sendRedirect (response.encodeRedirectURL(url)); return;}

Redirection and Send Error (Example)

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Request Dispatcher The RequestDispatcher allows you to forward a request to

another servlet or to include the output from another servlet Used to support both forwarding processing to and including

response from a variety of local Web resourcesFor example, JSP pages and HTML files

If a reference to the RequestDispatcher is acquired from the ServletContextPath information is relative to the ServletContext

If a reference to the RequestDispatcher is acquired from the HttpServletRequestPath information is relative to the path of the current request

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ServletA

ServletB

Content returned to Browser

forward

Content returned to Browser

include

Request Dispatcher Flow

ServletA

ServletB

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Sample Use of Request Dispatcher Forward to a JSP

getServletContext().

getRequestDispatcher("/pages/showBalance.jsp").

forward(request, response);

Include static HTML

getServletContext().

getRequestDispatcher("/pages/navigation_bar.html").include(request, response);

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Involving other Resources: Forwarding To have another resource build the response, use the

RequestDispatcher's forward method getRequestDispatcher(resourceName).

forward(request,response)

IllegalStateException is thrown if the source servlet accesses the ServletOutputStream or PrintWriter object

Request Dispatcher

showBalance.jsp

getServletContext() .getRequestDispatcher("/pages/showBalance.jsp")

.forward(request, response);

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private void processRequest( HttpServletRequest request, HttpServletResponse response) ... { // process request headers & query data ... // Forward the request if (errorFound) { String res = "/ErrorFound.html"; getServletContext().getRequestDispatcher(res). forward(request, response); return; }}

Forward Method (Example)

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Involving other Resources: Including To have another resource be included in the response, use the

RequestDispatcher's include method

getRequestDispatcher(resourceName).include(request,response)

The target resource should not set the response headers. If attempted, there is no guarantee that the target values will be used

nav_bar.html

getServletContext() .getRequestDispatcher("/pages/nav_bar.html") .include(req, res); Request

Dispatcher

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private void processRequest( HttpServletRequest request, HttpServletResponse response) ... { // process request headers & query data ... // include the request response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("<HTML><BODY>Start of INCLUDED request"); out.println("<P>Hi " + request.getParameter("name")); out.flush(); getServletContext().getRequestDispatcher(

"/ILSCS01/DispatcherInclude").include(request, response);

out.println("<P>End of request</BODY></HTML>");}

Include Method (Example)

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Sharing Objects There are several ways to share objects between servlets and

JSPs:ServletContext

getServletContext().setAttribute("objectName",anObject);

getServletContext().getAttribute("objectName");

HttpServletRequest

request.setAttribute("objectName",anObject);

request.getAttribute("objectName");

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HttpServletRequest

"CUSTOMER"Customer

Sharing Objects Example

// Servlet "A"public void doGet (HttpServletRequest request, HttpServletResponse resp)... { // process request headers & query data Customer cust; ... request.setAttribute("CUSTOMER", cust); String res = "/Internal/ServletB"; getServletContext().getRequestDispatcher(res).forward(request, resp);}

// Servlet "B"public void doGet (HttpServletRequest req, HttpServletResponse resp) ... { Customer aCust = (Customer) req.getAttribute("CUSTOMER");...}

getAttribute()

1

2

setAttribute()

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Servlet Context A Servlet Context defines a group of related servlets

Allows for relative pathsRooted at a particular point in the URI namespaceScope for data sharing among servlets

Programmatically accessed via javax.servlet.ServletContext Servlet Context Attributes

Allows for simple application scoped data sharing between servlets

getAttribute() and setAttribute() methods ServletContext.getResource

Allows a servlet to load resources specified via a relative path without assuming an absolute directory structure on the server

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www.hotel.ibm.com

/Rooms/HRApps/Personnel

/Hire /DisplayRooms/Login/Retire

ServletContext

/HRApps/Personnel

ServletContext/Rooms

Web container

servlet

Web Containers and Servlet Context

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Client Sue

Client Bob

ServletRequestServletResponse

ServletConfig

ServletContext

ServletResponseServletRequest

Thread 1

Thread 2

Servlet A

Servlet Objects (1 of 2)

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Client Sue

Client Bob

ServletRequestServletResponse

ServletConfig

ServletContext

ServletResponseServletRequest

Servlet AThread 1

Servlet BThread 2

ServletConfig

Servlet Objects (2 of 2)

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Internationalization Accepting Locale

Communicated from the client using the Accept-Language header

Use getLocale() and getLocales() methods of the ServletRequest interface to get the locales client will accept content in

Setting LocaleUse setLocale() method of the ServletResponse interface

to set the language attributes of a responsesetLocale() method should be called before the getWriter()

method of the ServletResponse interface is called Default encoding of a response is ISO-8859-1 if none has

been specified

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Checkpoint

1. How do you forward a request to another servlet?2. How can you pass an object to a servlet you are forwarding

to?3. How can you include dynamic content generated by

another servlet?4. What is the servlet context?5. What servlets share a servlet context?

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Checkpoint solutions

1. Use the request dispatcher to invoke the target servlet:getServletContext().getRequestDispatcher

(“/servlet/MyServlet”).forward(req, res);

2. Set the object as an attribute to the HttpServletRequest. The target servlet can then getAttribute() and cast it to the proper type. You call also use the session or the servlet context.

3. Use an include() to temporarily give control to another servlet.

4. The servlet context is an execution context of a group of related servlets. It allows for the servlets to use relative paths to refer to each other, and provides another way to share objects between servlets. Servlets within the same servlet context are defined in the same Web application.

5. Servlets defined within the same Web application share a servlet context.

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Having completed this unit, you should be able to: Discuss servlets as controllers (Model-View-Controller




Having completed this unit, you should be able to: Discuss servlets as controllers (Model-View-Controller




Unit summary

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Servlet API (Part II)

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