MELJUN CORTES JEDI Slides Web Programming Chapter02 Basic Servlets

Basic Servlets

Web Programming

Servlet ●

●

●

Java programming language class used to extend the capabilities of servers that host applications accessed via a request-response programming model Java class that implements the Servlet interface and accepts requests that can come from Java classes, Web clients, or other Servlets and generates responses Also called "HTTP Servlet"

Web Programming

2

Servlet ●

●

To start making servlets, you'll need to have knowledge about: –

Java programming

–

Client-server concepts

–

Basic HTML and HTTP

To create a Servlet, you'll need to import in your java program the standard extension classes in the packages javax.servlet and javax.servlet.http. –

javax.servlet contains the basic servlet framework

–

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

Web Programming

3

Servlet Architecture Overview ●

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 a new process on the server ●

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

Web Programming

4

Servlet Architecture Overview ●

●

●

Servlets are designed to be able to bypass the CGI problem and 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 to run. Servlets are also loaded into memory only once: either the container loads them into memory on server startup, or the first time the servlet is required to service a client.

Web Programming

5

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

Web Programming

6

Servlet First Look ●

●

HelloServlet shows the structure of a basic servlet that handles 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 provide interfaces and classes for wrtiting servlets (for HttpServlet, HttpServletRequest and HttpServletResponse).

Web Programming

7

Servlet First Look ●

By extending HttpServlet, this class inherits methods that are automatically called by the server depending on certain conditions. 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 they used, etc.). HttpServletResponse object: contains all the necessary methods needed by developers to produce a response that will be sent back to the client (methods to 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 produce output). Web Programming

8

Servlet First Look ●

Aside from the comments, there are only several lines that we 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.

Web Programming

9

Servlet First Look ●

Output of HelloServlet

Web Programming

10

Testing HelloServlet ●

●

●

At this point we need to be able to display the output of HelloServlet. To initially abstract the details of servlet deployment and configuration, we will be making use of the automated tools provided 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 well as a host of additional features.

Web Programming

11

Testing HelloServlet ●

●

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 default values for the other fields.

Web Programming

12

Testing HelloServlet

Web Programming

13

Testing HelloServlet

Web Programming

14

Testing HelloServlet ●

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.

Web Programming

15

Testing HelloServlet

Web Programming

16

Testing HelloServlet ●

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

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

–

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

Web Programming

17

Testing HelloServlet

Web Programming

18

Testing HelloServlet

Web Programming

19

Testing HelloServlet ●

The IDE has created and partially implemented for us a method 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 listing for 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 the output.

Web Programming

20

Testing HelloServlet

Web Programming

21

Servlet Lifecycle ●

●

●

A servlet is managed through a well-defined lifecycle described in the servlet specification. The servlet lifecycle describes how a servlet is loaded, instantiated, initialized, services requests, destroyed and finally garbage collected The life cycle of a servlet is controlled by the container in which the servlet has been deployed.

Web Programming

22

Servlet Lifecycle ●

The servlet life cycle allows servlet engines to address both the performance and resource problems of CGI and the security 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's private data. Servlets may also be allowed to persist between requests as object instances, taking up far less memory than full-fledged processes.

Web Programming

23

Servlet Lifecycle ●

The major events in a servlet's life: –

Instantiation

–

Initialization

–

Service

–

Destruction

–

Garbage Collection

Web Programming

24

Servlet Lifecycle ●

Instantiation –

The servlet class is loaded into memory, and an instance is created by 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 a request 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 each servlet.

–

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.

Web Programming

25

Servlet Lifecycle ●

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 a servlet.

–

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 as provides a way for the servlet to access application-wide information and facilities. Any configuration or initialization code should be placed in the init method.

Web Programming

26

Servlet Lifecycle (Initialization continued) –

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

–

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

Web Programming

27

Servlet Lifecycle ●

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 its functionality.

–

The method invoked by the servlet container during this phase is the service() method. public void service(ServletRequest req, ServletResponse res) ●

●

ServletRequest and ServletResponse objects: provides methods to extract information from the user's request and methods to generate the response. One important thing to note is that the servlet container makes these repeated calls to the service method using separate threads. That is, mostly there is only one active servlet instance taking up space in memory, handling multiple requests. That is one advantage of a Java servlet. That is also one of the reasons why a servlet (and its service method) should always be designed to be thread-safe. Web Programming

28

Servlet Lifecycle (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 GET or POST method calls, servlets can implement only doGet, doPost, or both.

Web Programming

29

Servlet Lifecycle ●

Destruction –

When a servlet is to be removed from a container's management, it is 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 this happens, the container will attempt to free up memory by destroying one or more servlet instances. What servlet is removed is determined by the servlet container and 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 is the destroy() method. ●

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

Web Programming

30

Servlet Lifecycle ●

Garbage Collection –

This phase in the servlet lifecycle is equivalent to that in any other Java 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.

Web Programming

31

Review ●

●

The main purpose of a servlet is to provide dynamic content to the user. By definition, dynamic content is content that changes in response to various conditions. Examples of which are the particulars of a user request, time of day, etc.

Web Programming

32

Handling Requests and Responses ●

●

To give the servlet access to the particulars of a user's request, it is provided an instance of a ServletRequest object that encapsulates those details. HTTP-based servlets are given a subclass called HTTPServletRequest, that provides additional methods for retrieving HTTP-specific info, such as cookie information, header details, etc.

Web Programming

33

Handling Requests and Responses ●

Form Data and Parameters

●

Retrieving Info from the Request URL

●

Header Information

●

Output Generation

Web Programming

34

Form Data and Parameters ●

request.getParameter

●

request.getParameterValues

●

request.getParameterNames

Web Programming

35

request.getParameter ●

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

–

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

Web Programming

36

Halloa! Enter user name:

Web Programming

37

request.getParameter ●

For this and other similar scenarios, Java provides the getParameter 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 the form.

Web Programming

38

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 PrintWriter out = response.getWriter(); out.println(""); out.println("HELLO AND WELCOME, " + userName + "!"); out.println(""); out.close(); } }

Web Programming

39

request.getParameter ●

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

Web Programming

40

request.getParameterValues

Web Programming

41

Choice selection What sports activities do you perform? Biking Table Tennis Swimming Basketball Others

Web Programming

42

request.getParameterValues ●

There are times when we need to retrieve data from a form with multiple elements of the same name. In this case, using the getParameter method will return only the value of the first 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 to retrieve.

–

Returns a String array.

Sample code and HTML (see next slides)

Web Programming

43

public class GetParameterValuesServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletExceptionm IOException{ String paramValues[] = request.getParameterValues("sports"); StringBuffer myResponse = new StringBuffer(); PrintWriter out = response.getWriter(); out.println("Your choices"); out.println("Your choices were : "); for (int i = 0; i < paramValues.length; i++) { out.println(""); out.println(paramValues[i]); } } }

Web Programming

44

request.getParameterValues ●

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

Web Programming

45

request.getParameterNames ●

There are times when we want our servlet to be aware of the name of one or more of the parameters within the form without having to hardcode them into the servlet. In this case, 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.

Web Programming

46

Retrieving Info from the Request URL ●

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's request, 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 invoking request.getContextPath(). Path info – the rest of the request after the context name. Can be retrieved by invoking request.getPathInfo().

Query String – request.getQueryString() Web Programming

47

Retrieving Info from the Request URL ●

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 on the aforementioned methods: request.getServerName() request.getServerPort() request.getContextPath() request.getPathInfo() request.getQueryString()

www.myjedi.net 8888 HelloApp greetUser name=Jedi

Web Programming

48

Header Information ●

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

getHeader(String name) ●

–

getHeaders(String name) ●

–

Returns all values for the specified header as an Enumeration object.

getHeaderNames() ●

–

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

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

getIntHeader(String name) ●

Returns the value of the specified header as an int; -1, if it does not exist.

Web Programming

49

Header Information (Methods in HttpServletRequest continued) –

getDateHeader(String name) ●

●

Returns the value of the specified header as a long value that represents a Date object; -1, if it does not exist. Throws an IllegalArgumentException if the header cannot be converted to a Date.

Web Programming

50

Output Generation ●

In all the previous examples, we are able to generate dynamic output for the user by using methods exposed in the 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)

Web Programming

51

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

Web Programming

52

Output Generation ●

Other notable methods in the HttpServletResponse object are: –

–

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 response to the user. Using this OutputStream, we can then use standard Java I/O objects and methods to produce all kinds of output.

Web Programming

53

public JPEGOutputServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) { // define a byte array to hold data byte bufferArray[] = new byte[1024]; // retrieve the ServletContext which we will use to retrieve the resource ServletContext ctxt = getServletContext(); // inform the browser that the resource we are sending is a GIF file response.setContentType("image/gif"); // retrieve the output stream we will use to produce response ServletOutputStream os = response.getOutputStream(); // retrieve the resource as an input stream InputStream is = ctxt.getResource("/WEB-INF/images/logo.gif").openStream();

}

}

// read the contents of the resource and write it afterwards into the outputstream int read = is.read(bufferArray); while (read != -1) { os.write(bufferArray); read = is.read(bufferArray); } // close the streams used is.close(); os.close();

Web Programming

54

Configuration, Packaging, and Deployment ●

In all the examples we have done so far, we have used the tools inherent in the Enterprise IDE to abstract the details of web application configuration, packaging, and deployment. We will take a look at those details now.

Web Programming

55

Web Application Configuration ●

●

The servlet specification defines an XML file named web.xml that acts as a configuration file for our web applications. This file is also called the deployment descriptor. We will use our FirstServlet example as our starting point in exploring web.xml. –

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

Web Programming

56

FirstServlet jedi.servlet.FirstServlet FirstServlet /FirstServlet 30 index.jsp

Web Programming

57

web.xml ●

–

●

Serves as both the root element of the configuration file as well as declaring the necessary info for the servlet container to recognize the 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 used for 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 on container/application startup, bypassing the normal lazy loading rule. The value for this element is a number which dictates the order of its loading compared to other servlets. Web Programming

58

web.xml ●

–

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 before adding any servlet mappings.

Web Programming

59

web.xml ●

–

●

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 a particular resource.

–

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

Web Programming

60

Packaging the Web Application 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

●

Our application can be deployed to a server by making use of 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.

Web Programming

61

Generating WAR Files ●

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

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

Web Programming

62

Generating WAR Files (WAR file... continued) –

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

Web Programming

63

Packaging the Web Application ●

Using an Ant build file to package the application –

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

–

Ant ●

●

●

A build tool that has wide industry acceptance. An open-source project of the Apache Software Foundation, and can be downloaded from: http://ant.apache.org Reads in a build file (traditionally named build.xml) made up of targets, which essentially define logical activities that can be carried out by the build file. These targets are composed of one or more tasks that define the details of how the targets perform their activities.

Web Programming

64

Requirements of the Build File ●

●

It must be located in the project root of the directory structure recommended by the Apache Software Foundation for web application development. Additionally, there must also exist a lib directory in the project root that will contain all JAR dependencies of the application. –

There must exist a file named build.properties in the same directory as the build script and must contain values for the following properties: ●

●

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

Web Programming

65

Using an Ant Build File ●

The following is the directory structure recommended for web application development: After build script has executed, will contain the application in a directory structure recognized by servlet 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 document root of your project. Directory containing configuration files such as the deployment descriptor (web.xml), tag library descriptors, etc.

–

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

Web Programming

66

Using an Ant Build File (Directory Structure continued) –

Apache lists the following advantages for having such a separate directory structure: ●

●

●

Contents of source directory are more easily administered, moved, or backed up if the deployment version is not intermixed. Source code control is easier to manage on directories that contain only source files (no compiled classes, etc.). The files that make up an installable distribution of the application are easier to select when the deployment hierarchy is separate.

Web Programming

67

Using an Ant Build File (Directory Structure continued) –

To perform the packaging of an app using this structure, run the following on the command-line (in the same directory containing the build file): ant dist ●

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

Web Programming

68

Deployment into Server ●

●

Servlet containers generally contain administrative tools that can be used to deploy web applications. Steps required to deploy the generated WAR file into Sun Application Server 8.1: –

First step, log-in into the administrative console. This can be accessed by entering the following URL in your browser: http://localhost:[ADMIN_PORT] ●

ADMIN_PORT is the port configured during installation to handle administrative requests.

Web Programming

69

Deployment into Server (Steps continued) –

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

Web Programming

70

Deployment into Server (Steps continued) –

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

–

Click on the Finish button in the next screen.

–

Congratulations, your application is now deployed.

Web Programming

71

Servlet and Application Parameters ●

ServletConfig and Servlet Initialization Parameters

●

ServletContext and Application Parameters

Web Programming

72

ServletConfig and Servlet Initialization Parameters ●

●

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 deployment of the application.

–

Allows deployers the ability to change servlet behaviour without having to recompile the code.

Web Programming

73

ServletConfig and Servlet Initialization Parameters ●

We can add initialization parameters to the servlet by specifying them in the servlet's definition in the deployment descriptor. ... FirstServlet jedi.servlet.FirstServlet debugEnabled true ...

Web Programming

74

ServletConfig and Servlet Initialization Parameters (Adding Initialization Parameters continued) –

The and tags tell the container that we are starting and ending parameter definition, respectively: ●

defines the name of the parameter

●

defines its value

Web Programming

75

ServletConfig and Servlet Initialization Parameters ●

To gain access to the servlet parameters, a servlet must first get a handle on its ServletConfig object, which can be done by calling on the getServletConfig() method. Afterwards, the parameter value can be retrieved as a String by calling the getInitParameter method and supplying the value of as the parameter. public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ServletConfig config = getServletConfig(); String isDebugEnabled = config.getInitParameter("debugEnabled"); if (isDebugEnabled.equals("true") { ... } Web Programming

76

ServletContext and Application Parameters ●

The ServletContext object gives the servlet access to the application 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 the context 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 the application.

Web Programming

77

ServletContext and Application Parameters ●

●

In much the same way that initialization parameters can be supplied for individual servlets, they can also be supplied for use by the entire application. Example of how to add application-wide parameters: databaseURL jdbc:postgresql://localhost:5432/jedidb

Web Programming

78

ServletContext and Application Parameters (Adding Application-wide Parameters continued) –

Each instance of defines a parameter for use by the whole application. ●

–

and works the same way as their counterparts

NOTE: Again, be reminded that the specification is strict about the ordering of elements inside the deployment descriptor. To keep your web.xml valid, all entries must be located BEFORE any entries.

Web Programming

79

ServletContext and Application Parameters ●

Retrieving the parameter values is very similar to that used for retrieving servlet-specific parameters. Only this time, it is an instance of ServletContext that the servlet must have a handle to. This can be retrieved by calling the getServletContext() method from an instance of the servlet's ServletConfig object. public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ServletContext ctxt = getServletConfig().getServletContext(); String jdbcURL = ctxt.getInitParameter("databaseURL"); // custom code setURL(jdbcURL); .... } Web Programming

80