[Course Notes | Magercises | Module Intro]
A servlet is a JavaTM component that can be plugged into a Java-enabled web server to provide custom services. These services can include:
Servlets are designed to work within a request/response processing model. In a request/response model, a client sends a request message to a server and the server responds by sending back a reply message. Requests can come in the form of an
HTTP URL, FTP, URL,or a custom protocol.
The request and the corresponding response reflect the state of the client and the server at the time of the request. Normally, the state of the client/server connection cannot be maintained across different request/response pairs. However, session information is maintainable with servlets through means to be described later.
The Java Servlet API includes several Java interfaces and fully defines
the link between a hosting server and servlets. The Servlet API is defined
as an extension to the standard JDK. JDK extensions are packaged under
javax--the root of the Java extension library tree. The Java
Servlet API contains the following packages:
javax.servlet
javax.servlet.http
Servlets are a powerful addition to the Java environment. They are fast, safe, reliable, and 100% pure Java. Because servlets plug into an existing server, they leverage a lot of existing code and technology. The server handles the network connections, protocol negotiation, class loading, and more; all of this work does not need to be replicated! And, because servlets are located at the middle tier, they are positioned to add a lot of value and flexibility to a system.
In this course you will learn about the Servlet API and you will get a brief tour of the types of features servlets can implement.
Because of their power and flexibility, servlets can play a significant role in a system architecture. They can perform the application processing assigned to the middle tier, act as a proxy for a client, and even augment the features of the middle tier by adding support for new protocols or other features. A middle tier acts as the application server in so called three-tier client/server systems, positioning itself between a lightweight client like a web browser and a data source.
In many systems a middle tier serves as a link between clients and back-end services. By using a middle tier a lot of processing can be off-loaded from both clients (making them lighter and faster) and servers (allowing them to focus on their mission).
One advantage of middle tier processing is simply connection management. A set of servlets could handle connections with hundreds of clients, if not thousands, while recycling a pool of expensive connections to database servers.
Other middle tier roles include:
When used to support applets, servlets can act as their proxies. This can be important because Java security allows applets only to make connections back to the server from which they were loaded.
If an applet needs to connect to a database server located on a different machine, a servlet can make this connection on behalf of the applet.
The Servlet API provides a tight link between a server and servlets. This allows servlets to add new protocol support to a server. (You will see how HTTP support is provided for you in the API packages.) Essentially, any protocol that follows a request/response computing model can be implemented by a servlet. This could include:
Servlet support is currently available in several web servers, and will probably start appearing in other types of application servers in the near future. You will use a web server to host the servlets in this class and only deal with the HTTP protocol.
Because HTTP is one of the most common protocols, and because HTML can provide such a rich presentation of information, servlets probably contribute the most to building HTTP based systems.
HTML can provide a rich presentation of information because of its
flexibility and the range of content that it can support. Servlets can
play a role in creating HTML content. In fact, servlet support for HTML
is so common, the
javax.servlet.http
package is dedicated to supporting HTTP protocol and HTML generation.
Complex web sites often need to provide HTML pages that are tailored for each visitor, or even for each hit. Servlets can be written to process HTML pages and customize them as they are sent to a client. This can be as simple as on the fly substitutions or it can be as complex as compiling a grammar-based description of a page and generating custom HTML.
Some web servers, such as the Java Web ServerTM (JWS), allow servlet tags to be embedded directly into HTML files. When the server encounters such a tag, it calls the servlet while it is sending the HTML file to the client. This allows a servlet to insert its contribution directly into the outgoing HTML stream.
Another example is on the fly tag processing known as server-side
includes (SSI). With SSI, an HTML page can contain special commands
that are processed each time a page is requested. Usually a web server
requires HTML files that incorporate SSI to use a unique extension, such
as .shtml. As an example, if an HTML page (with an .shtml
extension) includes the following:
<!--#include virtual="/includes/page.html"-->
it would be detected by the web server as a request to perform an inline file include. While server side includes are supported by most web servers, the SSI tags are not standardized.
Servlets are a great way to add server side include processing to a web server. With more and more web servers supporting servlets, it would be possible to write a standard SSI processing servlet and use it on different web servers.
An HTTP servlet is a direct replacement for Common Gateway Interface (CGI)
scripts. HTTP servlets are accessed by the user entering a URL in a browser
or as the target of an HTML form action. For example, if a user enters the
following URL into a browser address field, the browser requests a servlet
to send an HTML page with the current time:
http://localhost/servlet/DateTimeServlet
The DateTimeServlet responds to this request by sending an
HTML page to the browser.
Note that these servlets are not restricted to generating web pages; they can perform any other function, such as storing and fetching database information, or opening a socket to another machine.
Servlets are not run in the same sense as applets and applications. Servlets provide functionality that extends a server. In order to test a servlet, two steps are required:
There are many web servers that support servlets. It is beyond the scope of
this course to cover the different ways to install servlets in each server.
This course examines the JSDK's servletrunner utility and the JWS.
Servlets can be started and stopped for each client request, or they can be started as the web server is started and kept alive until the server is shut down. Temporary servlets are loaded on demand and offer a good way to conserve resources in the server for less-used functions.
Permanent servlets are loaded when the server is started, and live until the server is shutdown. Servlets are installed as permanent extensions to a server when their start-up costs are very high (such as establishing a connection with a DBMS), when they offer permanent server-side functionality (such as an RMI service), or when they must respond as fast as possible to client requests.
There is no special code necessary to make a servlet temporary or permanent; this is a function of the server configuration.
Because servlets can be loaded when a web server starts, they can use this
auto-loading mechanism to provide easier loading of server-side Java programs.
These programs can then provide functionality that is totally unique and
independent of the web server. For example, a servlet could provide R-based
services (rlogin, rsh, ...) through TCP/IP ports while
using the servlet request/response protocol to present and process HTML
pages used to manage the servlet.
servletrunner
For both JDK 1.1 and the Java 2 platform, you need to install the Java
Servlet Development Kit (JSDK). To use servletrunner, make sure
your PATH environment variable points to its directory. For the
JSDK 2.0 installed with all default options, that location is:
c:\jsdk2.0\bin on a Windows platform.
To make sure that servletrunner has access to the Java servlet
packages, check that your CLASSPATH environment variable is
pointing to the correct JAR file, c:\jsdk2.0\lib\jsdk.jar on
a Windows platform. With the Java 2 platform, instead of modifying the
CLASSPATH, it is easier to just copy the JAR file to the ext
directory under the Java runtine environment. This treats the servlet
packages as standard extensions.
With the environment set up, run the servletrunner program from
the command line. The parameters are:
Usage: servletrunner [options] Options: -p port the port number to listen on -b backlog the listen backlog -m max maximum number of connection handlers -t timeout connection timeout in milliseconds -d dir servlet directory -s filename servlet property file name
The most common way to run this utility is to move to the directory that
contains your servlets and run servletrunner from that location.
However, that doesn't automatically configure the tool to load the servlets
from the current directory.
Sun's Java Web Server (JWS) is a full featured product. For servlet developers, a nice feature is its ability to detect when a servlet has been updated. It detects when new class files have been copied to the appropriate servlet directory and, if necessary, automatically reloads any running servlets.
The JWS can be installed as a service under Windows NT. While this makes it convenient for running a production server, it is not recommended for servlet development work. Under Windows 95, there are no OS services, so the command line start-up is your only option.
To run JWS from the c:\JavaWebServer1.1\bin directory, type in
the httpd command. This starts the server in a console window.
No further display is shown in the console unless a servlet executes a
System.out.println() statement.
Servlets are installed by moving them to the
c:\JavaWebServer1.1\servlets directory. As mentioned, JWS detects
when servlets have been added to this directory. Although you can use the
JWS management applet to tailor the servlet installation, this is generally
not advised except for production server installations.
To shut down the JWS, press <Control>+C in the command window. The server prints a message to the console when it has finished shutting down.
The Java Servlet API defines the interface between servlets and servers.
This API is packaged as a standard extension to the JDK under javax:
javax.servlet
javax.servlet.http
The API provides support in four categories:
Servlets run on the web server platform as part of the same process as the web server itself. The web server is responsible for initializing, invoking, and destroying each servlet instance.
A web server communicates with a servlet through a simple interface,
javax.servlet.Servlet.
This interface consists of three main methods:
You may notice a similarity between this interface and that of Java applets. This is by design! Servlets are to web servers what applets are to web browsers.An applet runs in a web browser, performing actions it requests through a specific interface. A servlet does the same, running in the web server.
init() Method
When a servlet is first loaded, its
init()
method is invoked. This allows the servlet to per form any setup processing
such as opening files or establishing connections to their servers. If a
servlet has been permanently installed in a server, it loads when the server
starts to run. Otherwise, the server activates a servlet when it receives the
first client request for the services provided by the servlet.
The
init()
method is guaranteed to finish before any other calls are made to the
servlet--such as a call to the
service()
method. Note that
init()
will only be called once; it will not be called again unless the servlet
has been unloaded and then reloaded by the server.
The init()
method takes one argument, a reference to a
ServletConfig
object which provides initialization arguments for the servlet. This
object has a method
getServletContext()
that returns a
ServletContext
object containing information about the servlet's environment (see the
discussion on Servlet
Initialization Context below).
service() Method
The
service()
method is the heart of the servlet. Each request message from a client
results in a single call to the servlet's service() method.
The service() method reads the request and produces the response
message from its two parameters:
ServletRequest
object with data from the client. The data consists of name/value pairs of
parameters and an InputStream. Several methods are provided that
return the client's parameter information. The InputStream from
the client can be obtained via the
getInputStream()
method. This method returns a
ServletInputStream,
which can be used to get additional data from the client. If you are
interested in processing character-level data instead of byte-level data,
you can get a
BufferedReader
instead with
getReader().
ServletResponse
represents the servlet's reply back to the client. When preparing a
response, the method
setContentType()
is called first to set the MIME type of the reply. Next, the method
getOutputStream()
or
getWriter()
can be used to obtain a
ServletOutputStream
or
PrintWriter,
respectively, to send data back to the client.
As you can see, there are two ways for a client to send information to a
servlet. The first is to send parameter values and the second is to send
information via the InputStream (or Reader).
Parameter values can be embedded into a URL.
How this is done is discussed below. How the
parameter values are read by the servlet is discussed
later.
The service() method's job is conceptually simple--it creates a
response for each client request sent to it from the host server. However,
it is important to realize that there can be multiple service requests
being processed at once. If your service method requires any outside
resources, such as files, databases, or some external data, you must
ensure that resource access is thread-safe. Making your servlets thread-safe
is discussed in a later section of this
course.
destroy() Method
The destroy()
method is called to allow your servlet to clean up any resources (such as
open files or database connections) before the servlet is unloaded. If you
do not require any clean-up operations, this can be an empty method.
The server waits to call the destroy() method until either all
service calls are complete, or a certain amount of time has passed. This
means that the
destroy()
method can be called while some longer-running
service()
methods are still running. It is important that you write your
destroy() method to avoid closing any necessary resources until all
service()
calls have completed.
The code below implements a simple servlet that returns a static HTML page
to a browser. This example fully implements the Servlet interface.
import java.io.*;
import javax.servlet.*;
public SampleServlet implements Servlet {
private ServletConfig config;
public void init (ServletConfig config)
throws ServletException {
this.config = config;
}
public void destroy() {} // do nothing
public ServletConfig getServletConfig() {
return config;
}
public String getServletInfo() {
return "A Simple Servlet";
}
public void service (ServletRequest req,
ServletResponse res
) throws ServletException, IOException {
res.setContentType( "text/html" );
PrintWriter out = res.getWriter();
out.println( "<html>" );
out.println( "<head> );
out.println( "<title>A Sample Servlet</title>" );
out.println( "</head>" );
out.println( "<body>" );
out.println( "<h1>A Sample Servlet</h1>" );
out.println( "</body>" );
out.println( "</html>" );
out.close();
}
}
A servlet lives and dies within the bounds of the server process. To understand its operating environment, a servlet can get information about its environment at different times. Servlet initialization information is available during servlet start-up; information about the hosting server is available at any time; and each service request can contain specific contextual information.
Initialization information is passed to the servlet via the
ServletConfig
parameter of the
init()
method. Each web server provides its own way to pass initialization
information to a servlet. With the JWS, if a servlet class
DatePrintServlet takes an initialization argument timezone,
you would define the following properties in a servlets.properties
file:
servlet.dateprinter.code=DatePrinterServlet servlet.dateprinter.timezone=PSTor this information could be supplied through a GUI administration tool.
The timezone information would be accessed by the servlet with the following code:
String timezone;
public void init(ServletConfig config) {
timeZone = config.getInitParameter("timezone");
}
An Enumeration of all initialization parameters is available to
the servlet via the
getInitParameterNames()
method.
Server context information is available at any time through the
ServletContext
object. A servlet can obtain this object by calling the
getServletContext()
method on the
ServletConfig
object. Remember that this was passed to the servlet during the
initialization phase. A well written
init()
method saves the reference in a private variable.
The ServletContext interface defines several methods. These
are outlined below.
getAttribute() |
An extensible way to get information about a server via attribute name/value pairs. This is server specific. |
getMimeType() |
Returns the MIME type of a given file. |
getRealPath() |
This method translates a relative or virtual path to a new path relative to the server's HTML documentation root location. |
getServerInfo() |
Returns the name and version of the network service under which the servlet is running. |
getServlet()
|
Returns a Servlet
object of a given name. Useful when you want to access the services of
other servlets. |
getServletNames() |
Returns an enumeration of servlet names available in the current namespace. |
log() |
Writes information to a servlet log file. The log file name and format are server specific. |
The following example code shows how a servlet uses the host server to write a message to a servlet log when it initializes:
private ServletConfig config;
public void init(ServletConfig config) {
// Store config in an instance variable
this.config = config;
ServletContext sc = config.getServletContext();
sc.log( "Started OK!" );
}
Each service request can contain information in the form of name/value
parameter pairs, as a ServletInputStream, or a BufferedReader.
This information is available from the
ServletRequest
object that is passed to the
service()
method.
The following code shows how to get service-time information:
BufferedReader reader;
String param1;
String param2;
public void service (
ServletRequest req,
ServletResponse res) {
reader = req.getReader();
param1 = req.getParameter("First");
param2 = req.getParameter("Second");
}
There are additional pieces of information available to the servlet through
ServletRequest. These are shown in the following table.
getAttribute() |
Returns value of a named attribute for this request. |
getContentLength() |
Size of request, if known. |
getContentType() |
Returns MIME type of the request message body. |
getInputStream() |
Returns an InputStream for reading binary data from the
body of the request message. |
getParameterNames() |
Returns an array of strings with the names of all parameters. |
getParameterValues() |
Returns an array of strings for a specific parameter name. |
getProtocol() |
Returns the protocol and version for the request as a string of the form <protocol>/<major version>.<minor version>. |
getReader() |
Returns a BufferedReader to get the text from the body
of the request message. |
getRealPath() |
Returns actual path for a specified virtual path. |
getRemoteAddr() |
IP address of the client machine sending this request. |
getRemoteHost() |
Host name of the client machine that sent this request. |
getScheme() |
Returns the scheme used in the URL for this request (for example, https, http, ftp, etc.). |
getServerName() |
Name of the host server that received this request. |
getServerPort() |
Returns the port number used to receive this request. |
The following Magercise shows you how to extract parameters from a service request.
There are several utilities provided in the Servlet API. The first is the
interface
javax.servlet.SingleThreadModel
that can make it easier to write simple servlets. If a servlet implements
this marker interface, the hosting server knows that it should never call
the servlet's
service()
method while it is processing a request. That is, the server processes all
service requests within a single thread.
While this makes it easier to write a servlet, this can impede performance. A full discussion of this issue is located later in this course.
Two exception classes are included in the Servlet API. The exception
javax.servlet.ServletException
can be used when there is a general failure in the servlet. This notifies
the hosting server that there is a problem.
The exception
javax.servlet.UnavailableException
indicates that a servlet is unavailable. Servlets can report this exception
at any time. There are two types of unavailability:
Servlets that use the HTTP protocol are very common. It should not be a
surprise that there is specific help for servlet developers who write them.
Support for handling the HTTP protocol is provided in the package
javax.servlet.http.
Before looking at this package, take a look at the HTTP protocol itself.
HTTP stands for the HyperText Transfer Protocol. It defines a protocol used by web browsers and servers to communicate with each other. The protocol defines a set of text-based request messages called HTTP methods. (Note: The HTTP specification calls these HTTP methods; do not confuse this term with Java methods. Think of HTTP methods as messages requesting a certain type of response). The HTTP methods include:
GET
HEAD
POST
PUT
DELETE
TRACE
CONNECT
OPTIONS
For this course, you will only need to look at only three of these methods:
GET, HEAD, and POST.
GET Method
The HTTP GET method requests information from a web server. This information
could be a file, output from a device on the server, or output from a program
(such as a servlet or CGI script).
An HTTP GET request takes the form:
GET URL <http version> Host: <target host>
in addition to several other lines of information.
For example, the following HTTP GET message is requesting the home page from
the MageLang web site:
GET / HTTP/1.1 Connection: Keep-Alive User-Agent: Mozilla/4.0 ( compatible; MSIE 4.01; Windows NT) Host: www.magelang.com Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg
On most web servers, servlets are accessed via URLs that start with
/servlet/. The following HTTP GET method is requesting the servlet
MyServlet on the host www.magelang.com:
GET /servlet/MyServlet?name=Scott&
company=MageLang%20Institute HTTP/1.1
Connection: Keep-Alive
User-Agent: Mozilla/4.0 (
compatible;
MSIE 4.01;
Windows NT)
Host: www.magelang.com
Accept: image/gif, image/x-xbitmap,
image/jpeg, image/pjpeg
The URL in this GET request invokes the servlet called MyServlet
and contains two parameters, name and company. Each parameter
is a name/value pair following the format name=value. The parameters
are specified by following the servlet name with a question mark ('?'), with
each parameter separated by an ampersand ('&').
Note the use of %20 in the company's value. A space would signal
the end of the URL in the GET request line, so it must be "URL encoded", or
replaced with %20 instead. As you will see later, servlet developers
do not need to worry about this encoding as it will be automatically decoded
by the
HttpServletRequest
class.
HTTP GET requests have an important limitation. Most web servers limit
how much data can be passed as part of the URL name (usually a few hundred
bytes.) If more data must be passed between the client and the server, the
HTTP POST method should be used instead.
It is important to note that the server's handling of a GET method is
expected to be safe and idempotent. This means that a
GET method will not cause any side effects and that it can be executed
repeatedly.
When a server replies to an HTTP GET request, it sends an HTTP response
message back. The header of an HTTP response looks like the following:
HTTP/1.1 200 Document follows Date: Tue, 14 Apr 1997 09:25:19 PST Server: JWS/1.1 Last-modified: Mon, 17 Jun 1996 21:53:08 GMT Content-type: text/html Content-length: 4435 <4435 bytes worth of data -- the document body>
HEAD Method
The HTTP HEAD method is very similar to the HTTP GET method. The request
looks exactly the same as the GET request (except the word HEAD is used
instead of GET), but the server only returns the header information.
HEAD is often used to check the following:
Note that HEAD, like GET, is expected to be safe and
idempotent.
POST Method
An HTTP POST request allows a client to send data to the server. This can
be used for several purposes, such as
GET request allows
Pay special attention to the third bullet above. The HTTP GET request
passes all its arguments as part of the URL. Many web servers have a
limit to how much data they can accept as part of the URL. The POST
method passes all of its parameter data in an input stream, removing
this limit.
A typical POST request might be as follows:
POST /servlet/MyServlet HTTP/1.1 User-Agent: Mozilla/4.0 ( compatible; MSIE 4.01; Windows NT) Host: www.magelang.com Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, */ Content-type: application/x-www-form-urlencoded Content-length: 39 name=Scott&company=MageLang%20Institute
Note the blank line--this signals the end of the POST request header
and the beginning of the extended information.
Unlike the GET method, POST is not expected to be safe
nor idempotent; it can perform modifications to data, and it is
not required to be repeatable.
Now that you have been introduced to the HTTP protocol, consider how the
javax.servlet.http
package helps you write HTTP servlets. The abstract class
javax.servlet.http.HttpServlet
provides an implementation of the
javax.servlet.Servlet
interface and includes a lot of helpful default functionality. The easiest
way to write an HTTP servlet is to extend
HttpServlet
and add your own custom processing.
The class HttpServlet
provides an implementation of the
service()
method that dispatches the HTTP messages to one of several special methods.
These methods are:
and correspond directly with the HTTP protocol methods.
As shown in the following diagram, the service() method interprets
each HTTP method and determines if it is an HTTP GET, HTTP
POST, HTTP HEAD, or other HTTP protocol method:
The class
HttpServlet
is actually rather intelligent. Not only does it dispatch HTTP requests,
it detects which methods are overridden in a subclass and can report back
to a client on the capabilities of the server. (Simply by overriding the
doGet()
method causes the class to respond to an HTTP OPTIONS method with
information that GET, HEAD, TRACE, and OPTIONS are all supported.
These capabilities are in fact all supported by the class's code).
In another example of the support
HttpServlet
provides, if the
doGet()
method is overridden, there is an automatic response generated for
the HTTP HEAD method. (Since the response to an HTTP HEAD method is
identical to an HTTP GET method--minus the body of the message--the
HttpServlet
class can generate an appropriate response to an HTTP HEAD request from
the reply sent back from the
doGet()
method). As you might expect, if you need more precise control, you can
always override the
doHead()
method and provide a custom response.
When using the HTTP support classes, you generally create a new servlet that
extends
HttpServlet
and overrides either
doGet()
or
doPost(),
or possibly both. Other methods can be overridden to get more fine-grained
control.
The HTTP processing methods are passed two parameters, an
HttpServletRequest
object and an
HttpServletResponse
object. The
HttpServletRequest
class has several convenience methods to help parse the request,
or you can parse it yourself by simply reading the text of the request.
A servlet's
doGet()
method should
It is important to note that the handling of a GET method is expected to
be safe and idempotent.
GET request without penalty.
Think of it this way: GET should be "looking without touching." If
you require processing that has side effects, you should use another
HTTP method, such as POST.
A servlet's
doPost()
method should be overridden when you need to process an HTML form posting
or to handle a large amount of data being sent by a client.
HTTP POST method handling is discussed in detail
later.
HEAD requests are processed by using the
doGet()
method of an
HttpServlet.
You could simply implement
doGet()
and be done with it; any document data that you write to the response
output stream will not be returned to the client. A more efficient
implementation, however, would check to see if the request was a GET or
HEAD request, and if a HEAD request, not write the data to the response
output stream.
The Java Servlet API is a standard extension. This means that there is an explicit definition of servlet interfaces, but it is not part of the Java Development Kit (JDK) 1.1 or the Java 2 platform. Instead, the servlet classes are delivered with the Java Servlet Development Kit (JSDK) version 2.0 from Sun (http://java.sun.com/products/servlet/). This JSDK version is intended for use with both JDK 1.1 and the Java 2 platform. There are a few significant differences between JSDK 2.0 and JSDK 1.0. See below for details. If you are using a version of JSDK earlier than 2.0, it is recommended that you upgrade to JSDK 2.0.
Servlet support currently spans two packages:
Servlet |
An interface that defines communication between a web server and
a servlet. This interface defines the
init(),
service(),
and
destroy()
methods (and a few others). |
ServletConfig |
An interface that describes the configuration parameters for a servlet.
This is passed to the servlet when the web server calls its
init()
method. Note that the servlet should save the reference to the
ServletConfig
object, and define a
getServletConfig()
method to return it when asked. This interface defines how to get the
initialization parameters for the servlet and the context under which
the servlet is running. |
ServletContext |
An interface that describes how a servlet can get information
about the server in which it is running. It can be retrieved via the
getServletContext()
method of the ServletConfig
object. |
ServletRequest |
An interface that describes how to get information about a client request. |
ServletResponse |
An interface that describes how to pass information back to the client. |
GenericServlet |
A base servlet implementation. It takes care of saving the
ServletConfig object reference, and provides several methods
that delegate their functionality to the ServletConfig object.
It also provides a dummy implementation for init() and
destroy(). |
ServletInputStream |
A subclass of
InputStream
used for reading the data part of a client's request. It adds a
readLine()
method for convenience. |
ServletOutputStream |
An
OutputStream
to which responses for the client are written. |
ServletException |
Should be thrown when a servlet problem is encountered. |
UnavailableException |
Should be thrown when the servlet is unavailable for some reason. |
HttpServletRequest |
A subclass of ServletRequest
that defines several methods that parse HTTP request headers. |
HttpServletResponse |
A subclass of ServletResponse
that provides access and interpretation of HTTP status codes and
header information. |
HttpServlet |
A subclass of GenericServlet
that provides automatic separation of HTTP request by method type.
For example, an HTTP GET request will be processed by the
service()
method and passed to a
doGet()
method. |
HttpUtils |
A class that provides assistance for parsing HTTP GET and POST requests. |
Now for an in-depth look at several servlets. These examples include:
Sometimes a web page needs only a small piece of information that is customized at runtime. The remainder of a page can be static information. To substitute only small amounts of information, some web servers support a concept known as server-side includes, or SSI.
If it supports SSI, the web server designates a special file extension
(usually .shtml) which tells the server that it should look
for SSI tags in the requested file. The JWS defines a special SSI tag
called the <servlet> tag, for example:
<servlet code="DatePrintServlet">
<param name=timezone value=pst>
</servlet>
This tag causes the invoking of a servlet named DatePrintServlet
to generate some in-line content.
SSI and servlets allow an HTML page designer to write a skeleton for a page, using servlets to fill in sections of it, rather than require the servlet to generate the entire page. This is very useful for features like page-hit counters and other small pieces of functionality.
The DatePrintServlet servlet works just like a regular servlet
except that it is designed to provide a very small response and not a
complete HTML page. The output MIME type gets set to "text/plain"
and not "text/html".
Keep in mind that the syntax of server-side includes, if they are even supported, may vary greatly from one web server to another.
In the following Magercise you create the DatePrintServlet and
see how to use it in an HTML page.
HTTP POST method processing differs from HTTP GET method processing in
several ways. First, because POST is expected to modify data on the server,
there can be a need to safely handle updates coming from multiple clients
at the same time. Second, because the size of the information stream sent
by the client can be very large, the
doPost()
method must open an InputStream (or Reader) from the
client to get any of the information. HTTP POST does not support
sending parameters encoded inside of the URL as does the HTTP GET method.
The problem of supporting simultaneous updates from multiple clients has been solved by database systems (DBMSs); unfortunately the HTTP protocol does not work well with database systems. This is because DBMSs need to maintain a persistent connection between a client and the DBMS to determine which client is trying to update the data.
The HTTP protocol does not support this type of a connection as it is a message based, stateless protocol. Solving this problem is not easy and never elegant! Fortunately, the Servlet API defines a means to track client/server sessions. This is covered in the Maintaining Session Information section later in this course. Without session management tracking, one can resort to several different strategies. They all involve writing data to the client in hidden fields which is then sent back to the server. The simplest way to handle updates is to use an optimistic locking scheme based on date/time stamps. One can use a single date/time stamp for a whole form of data, or one could use separate date/time stamps for each "row" of information.
Once the update strategy has been selected, capturing the data sent to the
server via an HTTP POST method is straightforward. Information from the HTML
form is sent as a series of parameters (name/value pairs) in the
InputStream object. The
HttpUtils
class contains a method
parsePostData()
that accepts the raw InputStream from the client and return a
Hashtable with the parameter information already processed. A
really nice feature is that if a parameter of a given name has multiple
values (such is the case for a column name with multiple rows), then this
information can be retrieved from the Hashtable as an array of
type String.
In the following Magercise, you will be given skeleton code that implements a pair of servlets that display data in a browser as an editable HTML form. The structure of the data is kept separate from the actual data. This makes it easy to modify this code to run against arbitrary tables from a JDBC-connected database.
For those unfamiliar with cookies, a cookie is a named piece of data
maintained by a browser, normally for session management. Since HTTP
connections are stateless, you can use a cookie to store persistent
information accross multiple HTTP connections. The
Cookie class is
where all the "magic" is done. The HttpSession class, described
next, is actually easier
to use. However, it doesn't support retaining the information across
multiple browser sessions.
To save cookie information you need to create a Cookie, set the
content type of the HttpServletResponse response, add the cookie
to the response, and then send the output. You must add the cookie after
setting the content type, but before sending the output, as the cookie is
sent back as part of the HTTP response header.
private static final String SUM_KEY = "sum";
...
int sum = ...; // get old value and add to it
Cookie theCookie =
new Cookie (SUM_KEY, Integer.toString(sum));
response.setContentType("text/html");
response.addCookie(theCookie);
It is necessary to remember that all cookie data are strings. You must
convert information like int data to a String object. By default,
the cookie lives for the life of the browser session. To enable a cookie
to live longer, you must call the
setMaxAge(interval)
method. When positive, this allows you to set the number of seconds a
cookie exists. A negative setting is the default and destroys the cookie
when the browser exits. A zero setting immediately deletes the cookie.
Retrieving cookie data is a little awkward. You cannot ask for the cookie with a specific key. You must ask for all cookies and find the specific one you are interested in. And, it is possible that multiple cookies could have the same name, so just finding the first setting is not always sufficient. The following code finds the setting of a single-valued cookie:
int sum = 0;
Cookie theCookie = null;
Cookie cookies[] = request.getCookies();
if (cookies != null) {
for(int i=0, n=cookies.length; i < n; i++) {
theCookie = cookies[i];
if (theCookie.getName().equals(SUM_KEY)) {
try {
sum = Integer.parseInt(theCookie.getValue());
} catch (NumberFormatException ignored) {
sum = 0;
}
break;
}
}
}
The complete code example shown above is available for testing.
A session is a continuous connection from the same browser over a fixed
period of time. (This time is usually configurable from the web server. For
the JWS, the default is 30 minutes.) Through the implicit use of browser
cookies, HTTP servlets allow you to maintain session information with the
HttpSession class.
The
HttpServletRequest
provides the current session with the
getSession(boolean)
method. If the boolean parameter is true, a new session will be
created when a new session is detected. This is, normally, the desired
behavior. In the event the parameter is false, then the method
returns null if a new session is detected.
public void doGet (HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
HttpSession session = request.getSession(true);
// ...
Once you have access to an HttpSession, you can maintain a
collection of key-value-paired information, for storing any sort of
session-specific data. You automatically have access to the creation
time of the session with getCreationTime() and the last accessed
time with getLastAccessedTime(), which describes the time the last
servlet request was sent for this session.
To store session-specific information, you use the
putValue(key,
value) method. To retrieve the information, you ask the session
with
getValue(key).
The following example demonstrates this, by continually summing up the
integer value of the Addend parameter. In the event the value
is not an integer, the number of errors are also counted.
private static final String SUM_KEY =
"session.sum";
private static final String ERROR_KEY =
"session.errors";
Integer sum = (Integer) session.getValue(SUM_KEY);
int ival = 0;
if (sum != null) {
ival = sum.intValue();
}
try {
String addendString =
request.getParameter("Addend");
int addend = Integer.parseInt (addendString);
sum = new Integer(ival + addend);
session.putValue (SUM_KEY, sum);
} catch (NumberFormatException e) {
Integer errorCount =
(Integer)session.getValue(ERROR_KEY);
if (errorCount == null) {
errorCount = new Integer(1);
} else {
errorCount = new Integer(errorCount.intValue()+1);
}
session.putValue (ERROR_KEY, errorCount);
}
As with all servlets, once you've performed the necessary operations,
you need to generate some output. If you are using sessions, it is
necessary to request the session with HttpServletRequest.getSession()
before generating any output.
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<html>" +
"<head><title>Session Information</title></head>" +
"<body bgcolor=\"#FFFFFF\">" +
"<h1>Session Information</h1><table>");
out.println ("<tr><td>Identifier</td>");
out.println ("<td>" + session.getId() + "</td></tr>");
out.println ("<tr><td>Created</td>");
out.println ("<td>" + new Date(
session.getCreationTime()) + "</td></tr>");
out.println ("<tr><td>Last Accessed</td>");
out.println ("<td>" + new Date(
session.getLastAccessedTime()) + "</td></tr>");
out.println ("<tr><td>New Session?</td>");
out.println ("<td>" + session.isNew() + "</td></tr>");
String names[] = session.getValueNames();
for (int i=0, n=names.length; i<n; i++) {
out.println ("<tr><td>" + names[i] + "</td>");
out.println ("<td>" + session.getValue (names[i])
+ "</td></tr>");
}
out.println("</table></center></body></html>");
out.close();
The complete code example shown above is
available for testing. One thing
not demonstrated in the example is the ability to end a session,
where the next call to request.getSession(true) returns a different
session. This is done with a call to
invalidate().
In the event a user has browser cookies disabled, you can encode the
session ID within the
HttpServletResponse
by calling its
encodeUrl()
method.
It is very common to have servlets connect to databases through JDBC. This allows you to better control access to the database by only permitting the middle-tier to communicate with the database. If your database server includes sufficient simultanious connection licenses, you can even setup database connections once, when the servlet is initialized, and pool the connections between all the different service requests.
The following demonstrates sharing a single Connection between all
service requests. To find out how many simultaneous connections the driver
supports, you can ask its DatabaseMetaData and then create a pool
of Connection objects to share between service requests.
init() method connect to the database.
Connection con = null;
public void init (ServletConfig cfg)
throws ServletException {
super.init (cfg);
// Load driver
String name = cfg.getInitParameter("driver");
Class.forName(name);
// Get Connection
con = DriverManager.getConnection (urlString);
}
doGet() method retrieve database information.
public void doGet (HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
// Have browser ignore cache - force reload
response.setHeader ("Expires",
"Mon, 01 Jan 1990 00:00:00 GMT");
Statement stmt = null;
ResultSet result = null;
try {
// Submit query
stmt = con.createStatement();
result = stmt.executeQuery (
"SELECT programmer, cups " +
"FROM JoltData ORDER BY cups DESC;");
// Create output
PrintWriter out = response.getWriter();
while(result.next()) {
// Generate output from ResultSet
}
} finally {
if (result != null) {
result.close();
}
if (stmt != null) {
stmt.close();
}
}
out.flush();
out.close();
}
destroy() method disconnect from the database.
public void destroy() {
super.destroy();
con.close();
}
It is not good practice to leave a database connection permanently open, so this servlet should not be installed as a permanent servlet. Having it as a temporary servlet that closes itself down after a predefined period of inactivity allows the sharing of the database connection with requests that coincide, reducing the cost of each request.
You can also save some information in the HttpSession to possible
page through the result set.
As with Java applets, Java servlets have security issues to worry about, too.
A servlet can originate from several sources. A webmaster may have written it; a user may have written it; it may have been bought as part of a third-party package or downloaded from another web site.
Based on the source of the servlet, a certain level of trust should be associated with that servlet. Some web servers provide a means to associate different levels of trust with different servlets. This concept is similar to how web browsers control applets, and is known as "sandboxing".
A servlet sandbox is an area where servlets are given restricted authority
on the server. They may not have access to the file system or network, or
they may have been granted a more trusted status. It is up to the
web server administrator to decide which servlets are granted this status.
Note that a fully trusted servlet has full access to the server's file
system and networking capabilities. It could even perform a
System.exit(),
stopping the web server...
Many web servers allow you to restrict access to certain web pages and servlets via access control lists (ACLs). An ACL is a list of users who are allowed to perform a specific function in the server. The list specifies:
Each web server has its own means of specifying an ACL, but in general, a list of users is registered on the server, and those user names are used in an ACL. Some servers also allow you to add users to logical groups, so you can grant access to a group of users without specifying all of them explicitly in the ACL.
ACLs are extremely important, as some servlets can present or modify sensitive data and should be tightly controlled, while others only present public knowledge and do not need to be controlled.
A web server can call a servlet's
service() method
for several requests at once. This brings up the issue of thread safety in
servlets.
But first consider what you do not need to worry about: a servlet's
init() method. The
init() method will
only be called once for the duration of the time that a
servlet is loaded. The web server calls
init() when loading,
and will not call it again unless the servlet has been unloaded and reloaded.
In addition, the
service() method or
destroy() method
will not be called until the
init() method has
completed its processing.
Things get more interesting when you consider the
service() method.
The service()
method can be called by the web server for multiple clients at the same
time. (With the JSDK 2.0, you can tag a servlet with the
SingleThreadModel
interface. This results in each call to
service() being
handled serially. Shared resources, such as files and databases, can still
have concurrency issues to handle.)
If your service()
method uses outside resources, such as instance data from the servlet object,
files, or databases, you need to carefully examine what might happen if
multiple calls are made to
service() at the
same time. For example, suppose you had defined a counter in your servlet
class that keeps track of how many service() method invocations are
currently running:
private int counter = 0;
Next, suppose that your
service() method
contained the following code:
int myNumber = counter + 1; // line 1 counter = myNumber; // line 2 // rest of the code in the service() method counter = counter - 1;
What would happen if two
service() methods
were running at the same time, and both executed line 1 before either executed
line 2? Both would have the same value for myNumber, and the
counter would not be properly updated.
For this situation, the answer might be to synchronize the access to the counter variable:
synchronized(this) {
myNumber = counter + 1;
counter = myNumber;
}
// rest of code in the service() method
synchronized(this) {
counter = counter - 1 ;
}
This ensures that the counter access code is executed only one thread at a time.
There are several issues that can arise with multi-threaded execution, such as deadlocks and coordinated interactions. There are several good sources of information on threads, including Doug Lea's book Concurrent Programming in Java.
The Java Servlet Development Kit (JSDK) provides servlet support for JDK 1.1 and Java 2 platform developers.
JSDK 1.0 was the initial release of the development kit. Everything worked fine, but there were some minor areas that needed improvement. The JSDK 2.0 release incorporates these improvements. The changes between JSDK 1.0 and JSDK 2.0 are primarily the addition of new classes. In addition, there is also one deprecated methods. Because some web servers still provide servlet support that complies with the JSDK 1.0 API definitions, you need to be careful about upgrading to the new JSDK.
JSDK 2.0 adds the following servlet support:
SingleThreadModel
indicates to the server that only one thread can call the
service()
method at a time.
Reader
and
Writer
access from
ServletRequest
and ServletResponse
HttpServletResponse
DELETE, OPTIONS, PUT, and TRACE to appropriate
methods in
HttpServlet
JSDK 2.0 deprecated one method:
getServlets()
--you should use getServletNames()
instead
Interest and support for servlets is exploding. Here are some links to help you keep up to date:
Copyright © 1998 MageLang Institute. All Rights Reserved.