CHAPTER

38   Introducing Servlets

This chapter presents an introduction to servlets. Servlets are small programs that execute on the server side of a web connection. The topic of servlets is quite large, and it is beyond the scope of this chapter to cover it all. Instead, we will focus on the core concepts, interfaces, and classes, and develop several examples.

Background

In order to understand the advantages of servlets, you must have a basic understanding of how web browsers and servers cooperate to provide content to a user. Consider a request for a static web page. A user enters a Uniform Resource Locator (URL) into a browser. The browser generates an HTTP request to the appropriate web server. The web server maps this request to a specific file. That file is returned in an HTTP response to the browser. The HTTP header in the response indicates the type of the content. The Multipurpose Internet Mail Extensions (MIME) are used for this purpose. For example, ordinary ASCII text has a MIME type of text/plain. The Hypertext Markup Language (HTML) source code of a web page has a MIME type of text/html.

Now consider dynamic content. Assume that an online store uses a database to store information about its business. This would include items for sale, prices, availability, orders, and so forth. It wishes to make this information accessible to customers via web pages. The contents of those web pages must be dynamically generated to reflect the latest information in the database.

In the early days of the Web, a server could dynamically construct a page by creating a separate process to handle each client request. The process would open connections to one or more databases in order to obtain the necessary information. It communicated with the web server via an interface known as the Common Gateway Interface (CGI). CGI allowed the separate process to read data from the HTTP request and write data to the HTTP response. A variety of different languages were used to build CGI programs. These included C, C++, and Perl.

However, CGI suffered serious performance problems. It was expensive in terms of processor and memory resources to create a separate process for each client request. It was also expensive to open and close database connections for each client request. In addition, the CGI programs were not platform-independent. Therefore, other techniques were introduced. Among these are servlets.

Servlets offer several advantages in comparison with CGI. First, performance is significantly better. Servlets execute within the address space of a web server. It is not necessary to create a separate process to handle each client request. Second, servlets are platform-independent because they are written in Java. Third, the Java security manager on the server enforces a set of restrictions to protect the resources on a server machine. Finally, the full functionality of the Java class libraries is available to a servlet. It can communicate with other software via the sockets and RMI mechanisms that you have seen already.

The Life Cycle of a Servlet

Three methods are central to the life cycle of a servlet. These are init( ), service( ), and destroy( ). They are implemented by every servlet and are invoked at specific times by the server. Let us consider a typical user scenario to understand when these methods are called.

First, assume that a user enters a Uniform Resource Locator (URL) to a web browser. The browser then generates an HTTP request for this URL. This request is then sent to the appropriate server.

Second, this HTTP request is received by the web server. The server maps this request to a particular servlet. The servlet is dynamically retrieved and loaded into the address space of the server.

Third, the server invokes the init( ) method of the servlet. This method is invoked only when the servlet is first loaded into memory. It is possible to pass initialization parameters to the servlet so it may configure itself.

Fourth, the server invokes the service( ) method of the servlet. This method is called to process the HTTP request. You will see that it is possible for the servlet to read data that has been provided in the HTTP request. It may also formulate an HTTP response for the client.

The servlet remains in the server’s address space and is available to process any other HTTP requests received from clients. The service( ) method is called for each HTTP request.

Finally, the server may decide to unload the servlet from its memory. The algorithms by which this determination is made are specific to each server. The server calls the destroy( ) method to relinquish any resources such as file handles that are allocated for the servlet. Important data may be saved to a persistent store. The memory allocated for the servlet and its objects can then be garbage collected.

Servlet Development Options

To experiment with servlets, you will need access to a servlet container/server. Two popular ones are Glassfish and Apache Tomcat. Glassfish is an open source project sponsored by Oracle and is provided by the Java EE SDK. It is supported by NetBeans. Apache Tomcat is an open-source product maintained by the Apache Software Foundation. It can also be used by NetBeans. Both Tomcat and Glassfish can also be used with other IDEs, such as Eclipse.

Although IDEs such as NetBeans and Eclipse are very useful and can streamline the creation of servlets, they are not used in this chapter. The way you develop and deploy servlets differs among IDEs, and it is simply not possible for this book to address each environment. Furthermore, many readers will be using the command-line tools rather than an IDE. Therefore, if you are using an IDE, you must refer to the instructions for that environment for information concerning the development and deployment of servlets. For this reason, the instructions given here and elsewhere in this chapter assume that only the command-line tools are employed. Thus, they will work for nearly any reader.

This chapter uses Tomcat in the examples. It provides a simple, yet effective way to experiment with servlets using only the command line tools. It is also widely available in various programming environments. Furthermore, since only command-line tools are used, you don’t need to download and install an IDE just to experiment with servlets. Understand, however, that even if you are developing in an environment that uses a different servlet container, the concepts presented here still apply. It is just that the mechanics of preparing a servlet for testing will be slightly different.

REMEMBER The instructions for developing and deploying servlets in this chapter are based on Tomcat and use only command-line tools. If you are using an IDE and/or a different servlet container/server, consult the documentation for your environment.

Using Tomcat

Tomcat contains the class libraries, documentation, and run-time support that you will need to create and test servlets. At the time of this writing, several versions of Tomcat are available. The instructions that follow use 8.5.5. You can download Tomcat from tomcat.apache.org. You should choose a version appropriate to your environment.

The examples in this chapter assume a 64-bit Windows environment. Assuming that a 64-bit version of Tomcat 8.5.5 was unpacked from the root directly, the default location is

This is the location assumed by the examples in this book. If you load Tomcat in a different location (or use a different version of Tomcat), you will need to make appropriate changes to the examples. You may need to set the environmental variable JAVA_HOME to the top-level directory in which the Java Development Kit is installed.

NOTE All of the directories shown in this section assume Tomcat 8.5.5. If you install a different version of Tomcat, then you will need to adjust the directory names and paths to match those used by the version you installed.

Once installed, you start Tomcat by selecting startup.bat from the bin directly under the apache-tomcat-8.5.5 directory. To stop Tomcat, execute shutdown.bat, also in the bin directory.

The classes and interfaces needed to build servlets are contained in servlet-api.jar, which is in the following directory:

To make servlet-api.jar accessible, update your CLASSPATH environment variable so that it includes

Alternatively, you can specify this file when you compile the servlets. For example, the following command compiles the first servlet example:

Once you have compiled a servlet, you must enable Tomcat to find it. For our purposes, this means putting it into a directory under Tomcat’s webapps directory and entering its name into a web.xml file. To keep things simple, the examples in this chapter use the directory and web.xml file that Tomcat supplies for its own example servlets. This way, you won’t have to create any files or directories just to experiment with the sample servlets. Here is the procedure that you will follow.

First, copy the servlet’s class file into the following directory:

Next, add the servlet’s name and mapping to the web.xml file in the following directory:

For instance, assuming the first example, called HelloServlet, you will add the following lines in the section that defines the servlets:

Next, you will add the following lines to the section that defines the servlet mappings:

Follow this same general procedure for all of the examples.

A Simple Servlet

To become familiar with the key servlet concepts, we will begin by building and testing a simple servlet. The basic steps are the following:

1.  Create and compile the servlet source code. Then, copy the servlet’s class file to the proper directory, and add the servlet’s name and mappings to the proper web.xml file.

2.  Start Tomcat.

3.  Start a web browser and request the servlet.

Let us examine each of these steps in detail.

Create and Compile the Servlet Source Code

To begin, create a file named HelloServlet.java that contains the following program:

Let’s look closely at this program. First, note that it imports the javax.servlet package. This package contains the classes and interfaces required to build servlets. You will learn more about these later in this chapter. Next, the program defines HelloServlet as a subclass of GenericServlet. The GenericServlet class provides functionality that simplifies the creation of a servlet. For example, it provides versions of init( ) and destroy( ), which may be used as is. You need supply only the service( ) method.

Inside HelloServlet, the service( ) method (which is inherited from GenericServlet) is overridden. This method handles requests from a client. Notice that the first argument is a ServletRequest object. This enables the servlet to read data that is provided via the client request. The second argument is a ServletResponse object. This enables the servlet to formulate a response for the client.

The call to setContentType( ) establishes the MIME type of the HTTP response. In this program, the MIME type is text/html. This indicates that the browser should interpret the content as HTML source code.

Next, the getWriter() method obtains a PrintWriter. Anything written to this stream is sent to the client as part of the HTTP response. Then println( ) is used to write some simple HTML source code as the HTTP response.

Compile this source code and place the HelloServlet.class file in the proper Tomcat directory as described in the previous section. Also, add HelloServlet to the web.xml file, as described earlier.

Start Tomcat

Start Tomcat as explained earlier. Tomcat must be running before you try to execute a servlet.

Start a Web Browser and Request the Servlet

Start a web browser and enter the URL shown here:

Alternatively, you may enter the URL shown here:

This can be done because 127.0.0.1 is defined as the IP address of the local machine.

You will observe the output of the servlet in the browser display area. It will contain the string Hello! in bold type.

The Servlet API

Two packages contain the classes and interfaces that are required to build the servlets described in this chapter. These are javax.servlet and javax.servlet.http. They constitute the core of the Servlet API. Keep in mind that these packages are not part of the Java core packages. Therefore, they are not included with Java SE. Instead, they are provided by Tomcat. They are also provided by Java EE.

The Servlet API has been in a process of ongoing development and enhancement. The servlet specification supported by Tomcat 8.5.5 is version 3.1. (Subsequent versions of Tomcat are expected to support servlet specification 4.) This chapter discusses the core of the Servlet API, which will be available to most readers and works with all modern versions of the servlet specification.

The javax.servlet Package

The javax.servlet package contains a number of interfaces and classes that establish the framework in which servlets operate. The following table summarizes several key interfaces that are provided in this package. The most significant of these is Servlet. All servlets must implement this interface or extend a class that implements the interface. The ServletRequest and ServletResponse interfaces are also very important.

The following table summarizes the core classes that are provided in the javax.servlet package:

Let us examine these interfaces and classes in more detail.

The Servlet Interface

All servlets must implement the Servlet interface. It declares the init( ), service( ), and destroy( ) methods that are called by the server during the life cycle of a servlet. A method is also provided that allows a servlet to obtain any initialization parameters. The methods defined by Servlet are shown in Table 38-1.

The init( ), service( ), and destroy( ) methods are the life cycle methods of the servlet. These are invoked by the server. The getServletConfig( ) method is called by the servlet to obtain initialization parameters. A servlet developer overrides the getServletInfo( ) method to provide a string with useful information (for example, the version number). This method is also invoked by the server.

Table 38-1   The Methods Defined by Servlet

The ServletConfig Interface

The ServletConfig interface allows a servlet to obtain configuration data when it is loaded. The methods declared by this interface are summarized here:

The ServletContext Interface

The ServletContext interface enables servlets to obtain information about their environment. Several of its methods are summarized in Table 38-2.

Table 38-2   Various Methods Defined by ServletContext

The ServletRequest Interface

The ServletRequest interface enables a servlet to obtain information about a client request. Several of its methods are summarized in Table 38-3.

Table 38-3   Various Methods Defined by ServletRequest

The ServletResponse Interface

The ServletResponse interface enables a servlet to formulate a response for a client. Several of its methods are summarized in Table 38-4.

Table 38-4   Various Methods Defined by ServletResponse

The GenericServlet Class

The GenericServlet class provides implementations of the basic life cycle methods for a servlet. GenericServlet implements the Servlet and ServletConfig interfaces. In addition, a method to append a string to the server log file is available. The signatures of this method are shown here:

void log(String s)

void log(String s, Throwable e)

Here, s is the string to be appended to the log, and e is an exception that occurred.

The ServletInputStream Class

The ServletInputStream class extends InputStream. It is implemented by the servlet container and provides an input stream that a servlet developer can use to read the data from a client request. In addition to the input methods inherited from InputStream, a method is provided to read bytes from the stream. It is shown here:

int readLine(byte[ ] buffer, int offset, int size) throws IOException

Here, buffer is the array into which size bytes are placed starting at offset. The method returns the actual number of bytes read or –1 if an end-of-stream condition is encountered.

The ServletOutputStream Class

The ServletOutputStream class extends OutputStream. It is implemented by the servlet container and provides an output stream that a servlet developer can use to write data to a client response. In addition to the output methods provided by OutputStream, it also defines the print( ) and println( ) methods, which output data to the stream.

The Servlet Exception Classes

javax.servlet defines two exceptions. The first is ServletException, which indicates that a servlet problem has occurred. The second is UnavailableException, which extends ServletException. It indicates that a servlet is unavailable.

Reading Servlet Parameters

The ServletRequest interface includes methods that allow you to read the names and values of parameters that are included in a client request. We will develop a servlet that illustrates their use. The example contains two files. A web page is defined in PostParameters.html, and a servlet is defined in PostParametersServlet.java.

The HTML source code for PostParameters.html is shown in the following listing. It defines a table that contains two labels and two text fields. One of the labels is Employee and the other is Phone. There is also a submit button. Notice that the action parameter of the form tag specifies a URL. The URL identifies the servlet to process the HTTP POST request.

The source code for PostParametersServlet.java is shown in the following listing. The service( ) method is overridden to process client requests. The getParameterNames( ) method returns an enumeration of the parameter names. These are processed in a loop. You can see that the parameter name and value are output to the client. The parameter value is obtained via the getParameter( ) method.

Compile the servlet. Next, copy it to the appropriate directory, and update the web.xml file, as previously described. Then, perform these steps to test this example:

1.  Start Tomcat (if it is not already running).

2.  Display the web page in a browser.

3.  Enter an employee name and phone number in the text fields.

4.  Submit the web page.

After following these steps, the browser will display a response that is dynamically generated by the servlet.

The javax.servlet.http Package

The preceding examples have used the classes and interfaces defined in javax.servlet, such as ServletRequest, ServletResponse, and GenericServlet, to illustrate the basic functionality of servlets. However, when working with HTTP, you will normally use the interfaces and classes in javax.servlet.http. As you will see, its functionality makes it easy to build servlets that work with HTTP requests and responses.

The following table summarizes the interfaces used in this chapter:

The following table summarizes the classes used in this chapter. The most important of these is HttpServlet. Servlet developers typically extend this class in order to process HTTP requests.

The HttpServletRequest Interface

The HttpServletRequest interface enables a servlet to obtain information about a client request. Several of its methods are shown in Table 38-5.

The HttpServletResponse Interface

The HttpServletResponse interface enables a servlet to formulate an HTTP response to a client. Several constants are defined. These correspond to the different status codes that can be assigned to an HTTP response. For example, SC_OK indicates that the HTTP request succeeded, and SC_NOT_FOUND indicates that the requested resource is not available. Several methods of this interface are summarized in Table 38-6.

Table 38-5   Various Methods Defined by HttpServletRequest

The HttpSession Interface

The HttpSession interface enables a servlet to read and write the state information that is associated with an HTTP session. Several of its methods are summarized in Table 38-7. All of these methods throw an IllegalStateException if the session has already been invalidated.

Table 38-6   Various Methods Defined by HttpServletResponse

The Cookie Class

The Cookie class encapsulates a cookie. A cookie is stored on a client and contains state information. Cookies are valuable for tracking user activities. For example, assume that a user visits an online store. A cookie can save the user’s name, address, and other information. The user does not need to enter this data each time he or she visits the store.

A servlet can write a cookie to a user’s machine via the addCookie( ) method of the HttpServletResponse interface. The data for that cookie is then included in the header of the HTTP response that is sent to the browser.

The names and values of cookies are stored on the user’s machine. Some of the information that can be saved for each cookie includes the following:

•   The name of the cookie

•   The value of the cookie

•   The expiration date of the cookie

•   The domain and path of the cookie

Table 38-7   Various Methods Defined by HttpSession

The expiration date determines when this cookie is deleted from the user’s machine. If an expiration date is not explicitly assigned to a cookie, it is deleted when the current browser session ends.

The domain and path of the cookie determine when it is included in the header of an HTTP request. If the user enters a URL whose domain and path match these values, the cookie is then supplied to the web server. Otherwise, it is not.

There is one constructor for Cookie. It has the signature shown here:

Cookie(String name, String value)

Here, the name and value of the cookie are supplied as arguments to the constructor. The methods of the Cookie class are summarized in Table 38-8.

The HttpServlet Class

The HttpServlet class extends GenericServlet. It is commonly used when developing servlets that receive and process HTTP requests. The methods defined by the HttpServlet class are summarized in Table 38-9.

Table 38-8   The Methods Defined by Cookie

Table 38-9   The Methods Defined by HttpServlet

Handling HTTP Requests and Responses

The HttpServlet class provides specialized methods that handle the various types of HTTP requests. A servlet developer typically overrides one of these methods. These methods are doDelete( ), doGet( ), doHead( ), doOptions( ), doPost( ), doPut( ), and doTrace( ). A complete description of the different types of HTTP requests is beyond the scope of this book. However, the GET and POST requests are commonly used when handling form input. Therefore, this section presents examples of these cases.

Handling HTTP GET Requests

Here we will develop a servlet that handles an HTTP GET request. The servlet is invoked when a form on a web page is submitted. The example contains two files. A web page is defined in ColorGet.html, and a servlet is defined in ColorGetServlet.java. The HTML source code for ColorGet.html is shown in the following listing. It defines a form that contains a select element and a submit button. Notice that the action parameter of the form tag specifies a URL. The URL identifies a servlet to process the HTTP GET request.

The source code for ColorGetServlet.java is shown in the following listing. The doGet( ) method is overridden to process any HTTP GET requests that are sent to this servlet. It uses the getParameter( ) method of HttpServletRequest to obtain the selection that was made by the user. A response is then formulated.

Compile the servlet. Next, copy it to the appropriate directory, and update the web.xml file, as previously described. Then, perform these steps to test this example:

1.  Start Tomcat, if it is not already running.

2.  Display the web page in a browser.

3.  Select a color.

4.  Submit the web page.

After completing these steps, the browser will display the response that is dynamically generated by the servlet.

One other point: Parameters for an HTTP GET request are included as part of the URL that is sent to the web server. Assume that the user selects the red option and submits the form. The URL sent from the browser to the server is

The characters to the right of the question mark are known as the query string.

Handling HTTP POST Requests

Here we will develop a servlet that handles an HTTP POST request. The servlet is invoked when a form on a web page is submitted. The example contains two files. A web page is defined in ColorPost.html, and a servlet is defined in ColorPostServlet.java.

The HTML source code for ColorPost.html is shown in the following listing. It is identical to ColorGet.html except that the method parameter for the form tag explicitly specifies that the POST method should be used, and the action parameter for the form tag specifies a different servlet.

The source code for ColorPostServlet.java is shown in the following listing. The doPost( ) method is overridden to process any HTTP POST requests that are sent to this servlet. It uses the getParameter( ) method of HttpServletRequest to obtain the selection that was made by the user. A response is then formulated.

Compile the servlet and perform the same steps as described in the previous section to test it.

NOTE Parameters for an HTTP POST request are not included as part of the URL that is sent to the web server. In this example, the URL sent from the browser to the server is http://localhost:8080/examples/servlets/servlet/ColorPostServlet. The parameter names and values are sent in the body of the HTTP request.

Using Cookies

Now, let’s develop a servlet that illustrates how to use cookies. The servlet is invoked when a form on a web page is submitted. The example contains three files as summarized here:

The HTML source code for AddCookie.html is shown in the following listing. This page contains a text field in which a value can be entered. There is also a submit button on the page. When this button is pressed, the value in the text field is sent to AddCookieServlet via an HTTP POST request.

The source code for AddCookieServlet.java is shown in the following listing. It gets the value of the parameter named "data". It then creates a Cookie object that has the name "MyCookie" and contains the value of the "data" parameter. The cookie is then added to the header of the HTTP response via the addCookie( ) method. A feedback message is then written to the browser.

The source code for GetCookiesServlet.java is shown in the following listing. It invokes the getCookies( ) method to read any cookies that are included in the HTTP GET request. The names and values of these cookies are then written to the HTTP response. Observe that the getName( ) and getValue( ) methods are called to obtain this information.

Compile the servlets. Next, copy them to the appropriate directory, and update the web.xml file, as previously described. Then, perform these steps to test this example:

1.  Start Tomcat, if it is not already running.

2.  Display AddCookie.html in a browser.

3.  Enter a value for MyCookie.

4.  Submit the web page.

After completing these steps, you will observe that a feedback message is displayed by the browser.

Next, request the following URL via the browser:

Observe that the name and value of the cookie are displayed in the browser.

In this example, an expiration date is not explicitly assigned to the cookie via the setMaxAge( ) method of Cookie. Therefore, the cookie expires when the browser session ends. You can experiment by using setMaxAge( ) and observe that the cookie is then saved on the client machine.

Session Tracking

HTTP is a stateless protocol. Each request is independent of the previous one. However, in some applications, it is necessary to save state information so that information can be collected from several interactions between a browser and a server. Sessions provide such a mechanism.

A session can be created via the getSession( ) method of HttpServletRequest. An HttpSession object is returned. This object can store a set of bindings that associate names with objects. The setAttribute( ), getAttribute( ), getAttributeNames( ), and removeAttribute( ) methods of HttpSession manage these bindings. Session state is shared by all servlets that are associated with a client.

The following servlet illustrates how to use session state. The getSession( ) method gets the current session. A new session is created if one does not already exist. The getAttribute( ) method is called to obtain the object that is bound to the name "date". That object is a Date object that encapsulates the date and time when this page was last accessed. (Of course, there is no such binding when the page is first accessed.) A Date object encapsulating the current date and time is then created. The setAttribute( ) method is called to bind the name "date" to this object.

When you first request this servlet, the browser displays one line with the current date and time information. On subsequent invocations, two lines are displayed. The first line shows the date and time when the servlet was last accessed. The second line shows the current date and time.