MELJUN_CORTES_JEDI Slides Web Programming Chapter02 Basic Servlets

8/3/2019 MELJUN_CORTES_JEDI Slides Web Programming Chapter02 Basic Servlets

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Basic Servlets

Web Programming


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Servlet

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Java programming language class used to extend thecapabilities of servers that host applications accessed via arequest-response programming model

Java class that implements the Servlet interface andaccepts requests that can come from Java classes, Webclients, or other Servlets and generates responses

Also called "HTTP Servlet"


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Servlet

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To start making servlets, you'll need to have knowledgeabout:

Java programming

Client-server concepts Basic HTML and HTTP

To create a Servlet, you'll need to import in your javaprogram the standard extension classes in the packagesjavax.servlet and javax.servlet.http.

javax.servlet contains the basic servlet framework

javax.servlet.http is used as an extension of the Servlet frameworkfor Servlets that answer HTTP requests


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Servlet Architecture

Overview

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Common Gateway Interface (CGI)

One of the most common ways of adding functionality for a web

server, prior to Servlets Provides the server an interface to an external program, allowing it

to be called by the server to handle client requests

Designed in such a way that each call for a CGI resource creates anew process on the server

This approach imposed a heavy requirement on system resources, limiting thenumber of users that the application can handle at the same time.


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Servlet Architecture

Overview

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Servlets are designed to be able to bypass the CGI problemand provide developers a robust Java solution to creating

applications for the Web. With servlets, there is only one process that handles ALL

requests: the process required by the servlet container torun.

Servlets are also loaded into memory only once: either the

container loads them into memory on server startup, or thefirst time the servlet is required to service a client.


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import java.io.*;import javax.servlet.*;import javax.servlet.http.*;

public class HelloServlet extends HttpServlet {public void doGet(HttpServletRequest request,

HttpServletResponse response)throws ServletException, IOException {

//"request" is used for reading incoming HTTP headers// HTML form data (e.g. data the user entered and submitted)// and other data that can be inferred from the client request.

// "response" is for specifying the HTTP response line and// headers(e.g. specifying the content type, setting cookies).// It also contains methods that allow the servlet to generate// responses for the client.

PrintWriter out = response.getWriter();out.println(" Hello Page
");

out.println("Hello World!");out.println("");

//"out" for sending content to browser}

}


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Servlet First Look

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HelloServlet shows the structure of a basic servlet thathandles GET requests, as well as displaying the traditional'Hello World'.

The first part is simply importing classes:import java.io.*;

java.io (for PrintWriter, etc.)

import javax.servlet.*;

import javax.servlet.http.*;

javax.servlet and javax.servlet.http are packages that provideinterfaces and classes for wrtiting servlets (for HttpServlet,HttpServletRequest and HttpServletResponse).


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Servlet First Look

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By extending HttpServlet, this class inherits methods thatare automatically called by the server depending on certainconditions.

public class HelloServlet extends HttpServlet {

doGet method is inherited and overriden

public void doGet(HttpServletRequest request,

HttpServletResponse response)

throws ServletException, IOException { HttpServletRequest object: provides access to all information regarding the

client's request (HTTP request headers, the HTTP request method that theyused, etc.).

HttpServletResponse object: contains all the necessary methods needed bydevelopers to produce a response that will be sent back to the client (methodsto set the HTTP response headers, to declare the MIME type of the response,to retrieve instances of Java I/O classes that we can directly use to produceoutput).


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Servlet First Look

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Aside from the comments, there are only several lines thatwe use to display "Hello World!"

PrintWriter out = response.getWriter();

out.println(" Hello Page
");

out.println("Hello World!");

out.println("");

PrintWriter is an object that lets us output text to the client browser.


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Servlet First Look

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Output of HelloServlet


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Testing HelloServlet

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At this point we need to be able to display the output ofHelloServlet.

To initially abstract the details of servlet deployment andconfiguration, we will be making use of the automated toolsprovided by IDEs

Sun Studio Enterprise 8

The IDE we will use.

Available for free for members of the Sun Developer Network.

Has complete support for the servlet and JSP specifications, as wellas a host of additional features.


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First, we need to create a new web application project:

To do this select New->Project in the menubar.

In the screen that appears next, select the Web category.

On the right, choose New Web Application. The next screen will prompt you for details about the project:

For HelloServlet, our first test servlet, let's make use of"FirstServletProject" as our project name, and make use of defaultvalues for the other fields.


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To add our servlet to the application:

Right click on Source Packages, select New->Servlet.

If Servlet does not appear in the New context menu, select New->File/Folder instead. In the next screen, select the Web category,then Servlet.


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The IDE will then launch a series of screens which will askfor details about the servlet to be created.

In the first screen, name the servlet FirstServlet. Under packagename, use jedi.servlet. (see next slide)

In the screen that follows, leave the default values untouched. Then,click the Finish button.


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The IDE has created and partially implemented for us amethod named processRequest.

ProcessRequest

Simply a method that will be called by both doGet and doPost.

Its content forms the basis of the functionality of our servlet.

First, remove all contents of the processRequest method. Then,copy / paste the contents of the doGet method from the code listingfor the test servlet into it. (see next slide)

To run, press Shift + F6. The IDE will then package, deploy,

and invoke the servlet automatically for us, producing theoutput.


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Servlet Lifecycle

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A servlet is managed through a well-defined lifecycledescribed in the servlet specification.

The servlet lifecycle describes how a servlet is loaded,instantiated, initialized, services requests, destroyed andfinally garbage collected

The life cycle of a servlet is controlled by the container inwhich the servlet has been deployed.


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Servlet Lifecycle

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The servlet life cycle allows servlet engines to address boththe performance and resource problems of CGI and thesecurity concerns of low-level server API programming.

A servlet engine may execute all its servlets in a single Java virtual

machine (JVM): Because they are in the same JVM, servlets can efficiently share data with each

other, yet they are prevented by the Java language from accessing one another'sprivate data.

Servlets may also be allowed to persist between requests as object instances,taking up far less memory than full-fledged processes.


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Servlet Lifecycle

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The major events in a servlet's life:

Instantiation

Initialization

Service Destruction

Garbage Collection


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Servlet Lifecycle

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Instantiation

The servlet class is loaded into memory, and an instance is createdby the servlet container.

By default, a servlet container practices what is called lazy loading.

A servlet class is loaded into memory, instantiated, and initialized only after arequest has been made for it.

Advantage: makes for a faster startup time for the application.

Disadvantage: there is a little bit of overhead associated for the first call to eachservlet.

A servlet goes through the instantiation phase only once per lifetime. The relevant method that the container will call at this stage will be

the servlet's no-argument constructor.


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Servlet Lifecycle

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Initialization

The servlet is primed for use in the application.

A servlet also goes through this stage only once.

It is only after this phase that our object instance starts to be called aservlet.

The method that is called by the container at this point is the init()method.

public void init(ServletConfig config)

ServletConfig object: contains the servlet's configuration information, as well asprovides a way for the servlet to access application-wide information andfacilities.

Any configuration or initialization code should be placed in the init method.


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Servlet Lifecycle

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(Initialization continued)

After initialization, the servlet is ready to handle client requests.

This method can only be called again when the server reloads theservlet . The server cannot reload a servlet until after the server has

destroyed the servlet by calling the destroy method.


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Servlet Lifecycle

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Service

The phase that a servlet is in for the bulk of its lifetime.

The servlet can be repeatedly called by the container to provide itsfunctionality.

The method invoked by the servlet container during this phase isthe service() method.

public void service(ServletRequest req, ServletResponse res)

ServletRequest and ServletResponse objects: provides methods to extractinformation from the user's request and methods to generate the response.

One important thing to note is that the servlet container makes these repeatedcalls to the service method using separate threads. That is, mostly there is onlyone active servlet instance taking up space in memory, handling multiplerequests. That is one advantage of a Java servlet. That is also one of thereasons why a servlet (and its service method) should always be designed tobe thread-safe.


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Servlet Lifecycle

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(Service continued)

For HTTP-specific servlets (servlets extending HttpServlet),developers should not override the service method directly. Instead,developers should override any of the following methods:

public void doGet(HttpServletRequest req, HttpServletResponse res)

public void doPost(HttpServletRequest req, HttpServletResponse res)

public void doPut(HttpServletRequest req, HttpServletResponse res)

public void doTrace(HttpServletRequest req, HttpServletResponse res)

Each of these methods correspond to a specific HTTP method (GET, POST, ...).

Since most HTTP calls that developers concern themselves with are either GETor POST method calls, servlets can implement only doGet, doPost, or both.


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Servlet Lifecycle

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Destruction

When a servlet is to be removed from a container's management, itis said to be in its destruction phase.

There are times when a servlet container will run out of memory, or detect that

the amount of free memory it has is within a certain threshold. When thishappens, the container will attempt to free up memory by destroying one or moreservlet instances. What servlet is removed is determined by the servlet containerand is not something a developer has direct control over.

A container will also free up a servlet instance as part of its shutdown process.

The method called by the container before this is accomplished isthe destroy() method.

Servlet should be coded to explicitly free up resources that it handles, such asdatabase connections, etc.


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Servlet Lifecycle

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Garbage Collection

This phase in the servlet lifecycle is equivalent to that in any otherJava object.

Occurs just before an object instance is removed from memory.

Developers have no direct control as to when this will occur.

The object method called in this phase is the finalize() method.


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Review

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The main purpose of a servlet is to provide dynamic contentto the user.

By definition, dynamic content is content that changes inresponse to various conditions. Examples of which are the

particulars of a user request, time of day, etc.


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Handling Requests and

Responses

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To give the servlet access to the particulars of a user'srequest, it is provided an instance of a ServletRequestobject that encapsulates those details.

HTTP-based servlets are given a subclass calledHTTPServletRequest, that provides additional methods forretrieving HTTP-specific info, such as cookie information,header details, etc.


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Handling Requests and

Responses

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Form Data and Parameters

Retrieving Info from the Request URL

Header Information Output Generation


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Form Data and Parameters

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request.getParameter

request.getParameterValues

request.getParameterNames


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One of the most often encountered scenarios requiringdynamic content is when we want our application to respondto user data as presented in a form.

Given the form above which takes in the user's name, we want to beable to produce a response customized to that user's name.


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Halloa!

Enter user name:


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For this and other similar scenarios, Java provides thegetParameter method in the HttpServletRequest object.

public String getParameter(String parameterName)

This method takes in the name of the parameter we wish to retrieve

and returns the String value of that parameter. Now, let's create the code that takes in the user's name and

outputs a simple greeting, and the HTML used to display theform.


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public class GetParameterServlet extends HttpServlet {

public void doPost(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

// retrieve the value supplied by the user

String userName = request.getParameter("userName");

// retrieve the PrintWriter object and use it to output the greeting


out.println("");

out.println("HELLO AND WELCOME, " + userName + "!");

out.println("");

out.close();

}

}


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If we were to supply "JEDI" as the value for the form wecreated, the following result would be generated:


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Choice selection

What sports activities do you perform?

Biking

Table Tennis

Swimming

Basketball

Others


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There are times when we need to retrieve data from a formwith multiple elements of the same name. In this case, usingthe getParameter method will return only the value of thefirst element with the given name. To retrieve all of the

values, we use the getParameterValues method:public String[] getParameterValues(String parameterName)

Also takes in the name of the parameter whose values we wish toretrieve.

Returns a String array.

Sample code and HTML (see next slides)


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public class GetParameterValuesServlet extends HttpServlet {

public void doGet(HttpServletRequest request,

HttpServletResponse response) throws ServletExceptionm IOException{

String paramValues[] = request.getParameterValues("sports");

StringBuffer myResponse = new StringBuffer();


out.println("Your choices");

out.println("Your choices were : ");

for (int i = 0; i < paramValues.length; i++) {

out.println("
");

out.println(paramValues[i]);

}

}

}


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If the entries "Biking", "Table Tennis", and "Swimming" areto be chosen, the output would be:


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request.getParameterNames

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There are times when we want our servlet to be aware ofthe name of one or more of the parameters within the formwithout having to hardcode them into the servlet. In thiscase, we can use the getParameterNames method:

public Enumeration getParameterNames() The Enumeration object that this method returns contains all of the

names of the parameters embedded in the user's request.

Retrieving Info from the


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Retrieving Info from theRequest URL

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A request URL is composed of the following parts:

http://[host]:[port]/[requestPath]?[queryString]

To retrieve the curent values for each part from the user'srequest, call on methods in the HttpServletRequest object:

Host request.getServerName()

Port request.getServerPort()

Request Path divided into 2 logical components in Java:

Context the context of the web application. Can be retrieved by invokingrequest.getContextPath().

Path info the rest of the request after the context name. Can be retrieved byinvoking request.getPathInfo().

Query String request.getQueryString()

Retrieving Info from the


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Retrieving Info from theRequest URL

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So, with a request URL of:

http://www.myjedi.net:8080/HelloApp/greetUser?name=Jedi

The following table presents the results yielded if we call onthe aforementioned methods:

request.getServerName() www.myjedi.net

request.getServerPort() 8080

request.getContextPath() HelloApp

request.getPathInfo() greetUser

request.getQueryString() name=Jedi


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Header Information

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Can be retrieved from within the servlet by calling on thefollowing methods in HttpServletRequest:

getHeader(String name)

Returns the value of the specified header as a String; null, if it does not exist.

getHeaders(String name) Returns all values for the specified header as an Enumeration object.

getHeaderNames()

Returns as an Enumeration object the names of all headers included in the HTTPrequest.

getIntHeader(String name) Returns the value of the specified header as an int; -1, if it does not exist.


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Header Information

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(Methods in HttpServletRequest continued)

getDateHeader(String name)

Returns the value of the specified header as a long value that represents a Dateobject; -1, if it does not exist.

Throws an IllegalArgumentException if the header cannot be converted to aDate.


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Output Generation

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In all the previous examples, we are able to generatedynamic output for the user by using methods exposed inthe HttpServletResponse object.

So far, we have been making use mostly of the getWriter method.

getWriter returns a PrintWriter object associated with our response to the user.

Can you still recall HelloServlet? (see next slide)

import java.io.*;import javax servlet *;


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import javax.servlet. ;import javax.servlet.http.*;

public class HelloServlet extends HttpServlet {public void doGet(HttpServletRequest request,

HttpServletResponse response)throws ServletException, IOException {

//"request" is used for reading incoming HTTP headers// HTML form data (e.g. data the user entered and submitted)// and other data that can be inferred from the client request.

// "response" is for specifying the HTTP response line and// headers(e.g. specifying the content type, setting cookies).// It also contains methods that allow the servlet to generate// responses for the client.

PrintWriter out = response.getWriter();out.println(" Hello Page
");out.println("Hello World!");

out.println("");

//"out" for sending content to browser}

}


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Output Generation

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Other notable methods in the HttpServletResponse objectare:

SetContentType

Informs the client's browser of the MIME type of the output it is about to receive.

Limited to text outputs: JPEG, PDF, DOC, XLS, etc.

getOutputStream

Retrieves an instance of the OutputStream object associated with our responseto the user.

Using this OutputStream, we can then use standard Java I/O objects andmethods to produce all kinds of output.

public JPEGOutputServlet extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response) {

// define a byte array to hold data


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// define a byte array to hold databyte bufferArray[] = new byte[1024];

// retrieve the ServletContext which we will use to retrieve the resourceServletContext ctxt = getServletContext();

// inform the browser that the resource we are sending is a GIF fileresponse.setContentType("image/gif");

// retrieve the output stream we will use to produce responseServletOutputStream os = response.getOutputStream();

// retrieve the resource as an input streamInputStream is = ctxt.getResource("/WEB-INF/images/logo.gif").openStream();

// read the contents of the resource and write it afterwards into the outputstreamint read = is.read(bufferArray);while (read != -1) {

os.write(bufferArray);read = is.read(bufferArray);

}// close the streams usedis.close();os.close();

}}

Configuration Packaging


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Configuration, Packaging,and Deployment

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In all the examples we have done so far, we have used thetools inherent in the Enterprise IDE to abstract the details ofweb application configuration, packaging, and deployment.

We will take a look at those details now.

Web Application


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Web ApplicationConfiguration

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The servlet specification defines an XML file named web.xmlthat acts as a configuration file for our web applications. Thisfile is also called the deployment descriptor.

We will use our FirstServlet example as our starting point inexploring web.xml.

NOTE: The web.xml for Enterprise web application projects can befound by expanding the Configuration Files tab in the project view.


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FirstServletjedi.servlet.FirstServlet

FirstServlet/FirstServlet

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index.jsp

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web.xml

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Serves as both the root element of the configuration file as well asdeclaring the necessary info for the servlet container to recognizethe file as a valid deployment descriptor file.

Each instance defines a servlet to be used by the application.

Child nodes:

A logical name supplied by the developer which will be usedfor all future references to this servlet.

The fully qualified class name of the servlet. (Optional) Having an entry and value for this element tells

the container that the servlet should be instantiated and initialized oncontainer/application startup, bypassing the normal lazy loading rule. The valuefor this element is a number which dictates the order of its loading compared toother servlets.

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web.xml

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Each instance defines a mapping to a servlet.

Child nodes:

the logical name of the servlet to be mapped. Must be defined

previously in the descriptor file. the URL pattern to which this servlet will be mapped. Having a

value of /* will make all requests to your application redirected to your servlet.

Take note that all servlet definitions must first be supplied beforeadding any servlet mappings.

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web.xml

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Defines configuration details for session management.

Defines a web component that will be automatically loaded if the

user enters a request for the application without specifying aparticular resource.

More than one file can be specified but the first one visible to theweb container will be loaded.

Packaging the Web


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Packaging the WebApplication

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Our application can be deployed to a server by making useof what is called a WAR file.

WAR files are the same as JARs: they contain Java application code

compressed using the ZIP format.

Informally, WAR stands for Web Archive.

Contains HTML, images, other static content, plus JSPs

Contains meta-information about your application (optional)

All contents of this folder cannot be seen from the web browser

Contains class files of Java classes created for this application (optional)

Contains JAR files of any third-party libraries used by your app (optional)

XML file storing the configuration entries for your application

Generating WAR Files


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To produce the WAR file containing our web applicationfrom an existing project in Sun Studio Enterprise 8:

Right click on the project name in the Project view, and select BuildProject. The IDE will then proceed to package your application.



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(WAR file... continued)

The IDE will inform you if the build operation is successful. It willinform you of the location of the created WAR file, as well.

Packaging the Web


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g gApplication

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Using an Ant build file to package the application

Aside from using an IDE to package the web application, we canalso make use of a build tool that can automate for us thecompilation and packaging process.

Ant A build tool that has wide industry acceptance.

An open-source project of the Apache Software Foundation, and can bedownloaded from: http://ant.apache.org

Reads in a build file (traditionally named build.xml) made up of targets, whichessentially define logical activities that can be carried out by the build file. Thesetargets are composed of one or more tasks that define the details of how the

targets perform their activities.

Requirements of the Build File
http://ant.apache.org/http://ant.apache.org/


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Requirements of the Build File

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It must be located in the project root of the directorystructure recommended by the Apache Software Foundationfor web application development.

Additionally, there must also exist a lib directory in the

project root that will contain all JAR dependencies of theapplication.

There must exist a file named build.properties in the same directoryas the build script and must contain values for the followingproperties:

app.name the name of the application / project. appserver.home the installation directory of a Sun Application Server 8.1

instance.

Using an Ant Build File


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The following is the directory structure recommended forweb application development:

This directory structure was designed to be separate from thedirectory structure required by the servlet specification.

After build script has executed, will contain the application in a directory structure recognized byservlet containers.

After build script has executed, will contain the WAR file.

Used to contain whatever documentation is used by the dev team.

Directory under which all Java source files must be placed.

Directory containing all static content of the app (HTML, JSP), will be the basis of the documentroot of your project.

Directory containing configuration files such as the deployment descriptor (web.xml),tag library descriptors, etc.



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(Directory Structure continued)

Apache lists the following advantages for having such a separatedirectory structure:

Contents of source directory are more easily administered, moved, or backed upif the deployment version is not intermixed.

Source code control is easier to manage on directories that contain only sourcefiles (no compiled classes, etc.).

The files that make up an installable distribution of the application are easier toselect when the deployment hierarchy is separate.



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(Directory Structure continued)

To perform the packaging of an app using this structure, run thefollowing on the command-line (in the same directory containing thebuild file):

ant dist

This will call the dist target in the build file which will generate the WAR file andplace it into the dist directory. This WAR file can then be loaded into the targetservlet container using the admin tools provided by the container.

Deployment into Server


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Servlet containers generally contain administrative tools thatcan be used to deploy web applications.

Steps required to deploy the generated WAR file into SunApplication Server 8.1:

First step, log-in into the administrative console. This can beaccessed by entering the following URL in your browser:

http://localhost:[ADMIN_PORT]

ADMIN_PORT is the port configured during installation to handle administrativerequests.



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(Steps continued)

Second, left click on the Web Applications tab on the panel to theleft, then click on the Deploy button found in the panel to the right.



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(Steps continued)

In the screen that appears next, click on the Browse button to selectthe WAR file to upload. Click on the Next button found in the upperright.

Click on the Finish button in the next screen.

Congratulations, your application is now deployed.

Servlet and Application


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Parameters

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ServletConfig and Servlet Initialization Parameters

ServletContext and Application Parameters

ServletConfig and Servlet


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Initialization Parameters

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ServletConfig object

Passed to a specific servlet during its initialization phase.

Using this, a servlet can:

Retrieve information specific to itself, such as initialization parameters.

Gain access to an instance of the ServletContext object.

Initialization parameters are of great use:

When dealing with information that may vary with each deploymentof the application.

Allows deployers the ability to change servlet behaviour withouthaving to recompile the code.

ServletConfig and ServletI i i li i P


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We can add initialization parameters to the servlet byspecifying them in the servlet's definition in the deploymentdescriptor.

...

FirstServlet

jedi.servlet.FirstServlet

debugEnabled

true

...

ServletConfig and ServletI iti li ti P t


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(Adding Initialization Parameters continued)

The and tags tell the container that weare starting and ending parameter definition, respectively:

defines the name of the parameter defines its value

ServletConfig and ServletI iti li ti P t


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To gain access to the servlet parameters, a servlet must firstget a handle on its ServletConfig object, which can be doneby calling on the getServletConfig() method. Afterwards, the

parameter value can be retrieved as a String by calling thegetInitParameter method and supplying the value of as the parameter.

public void doGet(HttpServletRequest request, HttpServletResponse response) throwsServletException, IOException {

ServletConfig config = getServletConfig();String isDebugEnabled = config.getInitParameter("debugEnabled");if (isDebugEnabled.equals("true") {

...}

ServletContext andA li ti P t


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Application Parameters

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The ServletContext object gives the servlet access to theapplication context.

Application context

Area which an application moves in. Provided by the container for each web application with each application's

context being separate from each other an application may not access thecontext of another app.

Having access to this context is important:

Servlet can retrieve application-wide parameters and data.

Servlet can also store data that can be retrieved by any components in theapplication.



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In much the same way that initialization parameters can besupplied for individual servlets, they can also be supplied foruse by the entire application.

Example of how to add application-wide parameters:

databaseURLjdbc:postgresql://localhost:5432/jedidb



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(Adding Application-wide Parameters continued)

Each instance of defines a parameter for use bythe whole application.

and works the same way as their counterparts

NOTE: Again, be reminded that the specification is strict about theordering of elements inside the deployment descriptor. To keep yourweb.xml valid, all entries must be located BEFOREany entries.

ServletContext andApplication Parameters


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Retrieving the parameter values is very similar to that usedfor retrieving servlet-specific parameters. Only this time, it isan instance of ServletContext that the servlet must have a

handle to. This can be retrieved by calling thegetServletContext() method from an instance of the servlet'sServletConfig object.

public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {

ServletContext ctxt = getServletConfig().getServletContext();String jdbcURL = ctxt.getInitParameter("databaseURL");// custom codesetURL(jdbcURL);

....

}

Documents

MELJUN_CORTES_JEDI Slides Web Programming Chapter02 Basic Servlets