Download Multi-Threaded Programming in Java

Servlets and the Java Web Server
Server-Side Programming Made Easy
Written by Alex Chaffee (alexc@purpletech.com)
Contents Copyright (c) 1998 Purple Technology, Inc.
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
1
Servlets and the Java Web Server
Course Outline
 Servlet Overview
 Using Servlets
 Writing Servlets
 Saving State
 Java Web Server Features
 Appendix: CGI Tutorial
 Appendix: FAQ
 Inside the Exercises Handout
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
2
Section I
Servlet Overview
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
3
Servlets and the Java Web Server
What Is A Servlet
 A Java object
 Plug-in for a web server
 Replacement for CGI scripts
 Can also be used to extend server as a plug-in
 Full power of Java
 Platform-independent
 Database access
 Fun to write
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
4
Servlets and the Java Web Server
Server/Service/Servlet
 server - a process running on a host
machine
 Apache, Java Web Server
 service - a protocol running on a port
 HTTP, FTP
 servlet - a module running inside a service
 PhoneServlet
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
5
Servlets and the Java Web Server
Servlet/Service/Server Diagram
 (diagram from Java Web Server tutorial)
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
6
Servlets and the Java Web Server
Servlets vs. Applets
 Servlets have no GUI
 Server-side, not client-side
 Different security model
 Installed, not downloaded
 But you can download remote servlets too
 Consistent server-side VM
 Much easier to test
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
7
Servlets and the Java Web Server
Servlets vs. CGI
 "performance, flexibility, portability, and
security" (whitepaper)
 Faster and Leaner




No fork-process like Perl
No need to initialize for each request
Only lightweight thread context switching
Built-in multithreading
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
8
Servlets and the Java Web Server
Servlets vs. CGI (Cont.)
 Easy to manage state
 share data across successive requests
 share data between concurrent requests
 use hidden fields, cookies, or sessions
 Write once, run anywhere
 It's easy to write unportable Perl
 Servlets have standard API
 Supports all methods
 GET, POST, PUT, DELETE, et al.
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
9
Servlets and the Java Web Server
Servlets vs. FastCGI
 FastCGI sends multiple requests to a single
separate process
 requires process context switch
 Servlets send multiple requests to multiple threads
in same process
 requires lightweight thread context switch
 (Also applies to ISAPI)
 Nice diagram in White Paper
 Servlets also automatically take advantage of
multiprocessors
 if the underlying JVM does
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
10
Servlets and the Java Web Server
Supported Servers
 Java Web Server
 Apache
 Netscape
 Many others (see web site)
 Servlet Engines
 IBM's ServletExpress
 Live Software’s JRun
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
11
Servlets and the Java Web Server
Servlet Security
 Trusted Servlets (full access)
 JWS Internal
 Local (in the "servlets" directory)
 Servlet Sandbox
 Signed Network Servlets (full access)
 Unsigned Network Servlets (limited access)
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
12
Servlets and the Java Web Server
Servlet Security: Implications
 IT managers can sign servlets for use in
their organization
 ISPs can allow users to run servlets
 less of a security hole than CGI scripts, since Java is
safe and secure (at least more so than C or Perl)
 still allows denial-of-service attacks
 Network servlets are possible
 chaining / proxying
 allows agents
 common servlet repository for multiple servers
 one place to install updates
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
13
Servlets and the Java Web Server
Servlet Security: Problems
 Too simplistic
 All or nothing
 Should allow ACLs for particular signers
 They claim it will in a future version
 Should get better with 1.2 security model
 Finer-grained access control
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
14
Servlets and the Java Web Server
Servlet Client Security
 Java Web Server
 Allows Access Control Lists for clients
 Supports HTTP authentication
 Supports Digest Authentication
 Other Web Servers
 Usually support HTTP authentication
 May have other security features
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
15
Servlets and the Java Web Server
SSL in JWS
 It works
 Extra $$
 https: supported
 Digest Authentication supported
 SSL 3 (client certificates) required
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
16
Servlets and the Java Web Server
Authenticating the user’s identity
 HTTP Authentication
 Username/password sent to server on every request
(like cookies)
 Very light encryption (uuencode)
 Digest Authentication
 Cryptographic handshaking between client and
server
 Very good encryption
 Not supported by all servers/browsers
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
17
Servlets and the Java Web Server
User Authentication Methods
 request.getRemoteUser()
 returns username
 request.getAuthType()
 HTTP or Digest
 request.getScheme()
 “http” or “https”
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
18
Servlets and the Java Web Server
API Availability
 Standard Java Extension API
 From white paper: "This means that while it is not
part of the core Java framework which must always
be part of all products bearing the Java brand, it will
be made available with such products by their
vendors as an add-on package."
 package javax.servlet.*, javax.servlet.http.*
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
19
Servlets and the Java Web Server
Servlet Architectures:Three-tier system
 Tier 1: Client
 HTML browser
 Java client
 Tier 2: Servlets
 embody business logic
 secure, robust
 Tier 3: Data Sources
 Java can talk to SQL, CORBA, OODB, File system,
etc. etc.
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
20
Servlets and the Java Web Server
Servlet Architectures: N-tier system
 Tier 1: HTML Browser
 Tier 2: Servlet
 User interface
 Tier 3: EJB/CORBA/RMI Objects
 Business logic
 Tier 4: Other Servers (e.g. RDBMS)
 Data storage
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
21
Servlets and the Java Web Server
Servlet Architectures: Web Publishing
 SSI Servlets
 JSP Servlets
 Best to keep business logic inside Java objects
 Keep the JSP light so designers don’t get scared
 Chaining servlets
 Multiple servers
 data gathering, collecting, serving, load balancing,
etc.
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
22
Section II
Using Servlets
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
23
Servlets and the Java Web Server
Loading Servlets
 From CLASSPATH
 includes <root>/classes/ on JWS
 From <root>/servlets/ directory
 not in classpath
 servlets can be added or recompiled inside a running
server
 class.initArgs file
 From remote codebase
 specified by URL
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
24
Servlets and the Java Web Server
Remote Servlets
 Three ways to configure
 configure with Administration Tool
 invoke inside a server-side include
 configure inside a servlet chain
 Loaded in a Servlet Sandbox
 more later
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
25
Servlets and the Java Web Server
What's In A Name
 A servlet's name is its class name
 if it's in the servlets directory
 Or, you can assign it a name in the "Add
Servlet" admin tool
 maps code word to servlet class
 Name is usually a single word
 possibly with a package name and dots
 no other punctuation
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
26
Servlets and the Java Web Server
Standard Servlets
 DateServlet
 echoes current date/time
 EchoServlet
 echoes CGI parameters (good for testing)
 MailServlet
 sends email in response to a CGI form
 RedirectServlet
 used by server to manage HTTP redirects
 SessionServlet
 used by server to manage sessions
 Many more...
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
27
Servlets and the Java Web Server
Server-side Includes (SSI)
 Must be in a file named .shtml or .jsp
 can change this with Admin Tool
 Normal SSI
 <!--#include file="foo.txt" -->
 Servlet SSI
 <servlet code=DateServlet.class>
 </servlet>
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
28
Servlets and the Java Web Server
SSI Details
 pass init parameters in servlet tag
 pass servlet parameters in param tags
 can specify codebase in servlet tag
 e.g.
<servlet code=DateServlet.class
codebase=http://servlets.foo.com/
initParam1=val1 initParam2=val2>
<param name=serviceParam1 value=val3>
<param name=serviceParam2 value=val4>
</servlet>
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
29
Servlets and the Java Web Server
URL invocation
 Directly from browser as URL
 http://www.myserver.com/servlet/MyServlet
 From inside FORM tag as script
<FORM METHOD=POST ACTION=”/servlet/MyServlet”>
...
</FORM>
 From inside JHTML or JSP page
 Uses “Page Compilation”
 Compiles the jsp file into a servlet on the fly, then
executes it
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
30
Servlets and the Java Web Server
A Note on CLASSPATH and JWS
 JWS uses its own JRE
 Three ways to add classes
 Put the class files into the “classes” subdirectory
 Jar them, and put the jar files into the “lib”
subdirectory
 Start the server with the -classpath option
httpd -classpath c:\projects\utils
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
31
Section III
Writing Servlets
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
32
Servlets and the Java Web Server
The Servlet API
 Independent of
 web protocol
 server brand or platform
 whether it's local or remote
 Simple, small, easy
 Base class provides core functionality; just
extend it
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
33
Servlets and the Java Web Server
CGI, or not, whichever
 Fairly generic interface
 Accepts query, returns response
 Used for plugins, etc.
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
34
Servlets and the Java Web Server
Servlet Architecture Overview
 Servlet Interface
 methods to manage servlet
 GenericServlet
 implements Servlet interface
 HttpServlet
 extends GenericServlet
 exposes HTTP-specific functionality
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
35
Servlets and the Java Web Server
Servlet Architecture Overview
 ServletRequest
 What the client says to the server
 Access to information like protocol, client IP#,
parameters, and body
 ServletResponse
 What the servlet says to the client
 HttpServletRequest, HttpServletResponse
 HTTP-specific communication and information
 State-tracking and session management
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
36
Servlets and the Java Web Server
Servlet Lifecycle Overview
 Server loads and instantiates servlet
 Server calls init()
 Loop
 Server receives request from client
 Server calls service()
 service() calls doGet() or doPost()
 Server calls destroy()
 More detail to come later...
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
37
Servlets and the Java Web Server
ServletRequest
 passed to the service() method
 contains lots of useful goodies…





Client info
URL info
Content info
Content itself
User-entered parameters
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
38
Servlets and the Java Web Server
ServletRequest - Client Info
 getRemoteAddr()
 Returns the IP address of the agent that sent the request
 getRemoteHost()
 Returns the fully qualified host name of the agent that
sent the request
 getProtocol()
 Returns the protocol and version of the request as a string
of the form <protocol>/<major version>.<minor
version>.
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
39
Servlets and the Java Web Server
ServletRequest - URL Info
 getScheme()
 Returns the scheme of the URL used in this request, for
example "http", "https", or "ftp".
 getServerName()
 Returns the host name of the server that received the
request
 getServerPort()
 Returns the port number on which this request was
received
 getServletPath()
 Returns the URI path that got to this script, e.g.
“/servlet/com.foo.MyServlet”
 Useful for putting in a <FORM> tag
40
 See also getRequestURI()
(in Technology
HttpServletRequest)
5/24/2017
Copyright © 1997-8, Purple
Inc.
Servlets and the Java Web Server
ServletRequest - Content Info
 getContentLength()
 Returns the size of the request data
 getContentType()
 Returns the MIME type of the request data
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
41
Servlets and the Java Web Server
ServletRequest - Content
 getInputStream()
 Returns an input stream for reading binary data in
the request body.
 getReader()
 Returns a buffered reader for reading text in the
request body.
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
42
Servlets and the Java Web Server
ServletRequest - Parameters
 String getParameter(String)
 Returns a string containing the lone value of the specified parameter,
or null if the parameter does not exist.
 Was deprecated, but due to popular demand, it'll be undeprecated
 String[ ] getParameterValues(String)
 Returns the values of the specified parameter for the request as an
array of strings, or null if the named parameter does not exist.
 For parameters with multiple values, like lists
 Enumeration getParameterNames()
 Returns the parameter names for this request as an enumeration of
strings, or an empty enumeration if there are no parameters or the
input stream is empty.
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
43
Servlets and the Java Web Server
ServletResponse
 Embodies the response
 Basic use:
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println(
"<HTML><BODY>Hello</BODY></HTML>");
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
44
Servlets and the Java Web Server
ServletResponse - Output
 getWriter()
 for writing text data
 getOutputStream()
 for writing binary data
 or for writing multipart MIME
 you must call setContentType() before
calling getWriter() or getOutputStream()
 by default it's text/plain, which you don't want
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
45
Servlets and the Java Web Server
The GenericServlet class
 implements Servlet
 also implements Serializable, ServletConfig
 implements all Servlet methods
 so you don't have to
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
46
Servlets and the Java Web Server
The HelloWorld Servlet
import javax.servlet.*;
import java.io.*;
public class HelloServlet extends GenericServlet
{
public void service(ServletRequest req, ServletResponse res)
throws IOException, ServletException
{
res.setContentType("text/plain");
ServletOutputStream out = res.getOutputStream();
out.println("Hello, World!");
}
}
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
47
Servlets and the Java Web Server
The HttpServlet class
 extends the GenericServlet base class
 provides a framework for handling the
HTTP protocol
 has its own subclasses of ServletRequest
and ServletResponse that do HTTP things
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
48
Servlets and the Java Web Server
HttpServlet methods
 provides helper methods for HTTP methods




doGet (GET and HEAD)
doPost (POST)
doPut, doDelete (rare)
doTrace, doOptions (not overridden)
 the service() method dispatches requests
to the do* methods
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
49
Servlets and the Java Web Server
HttpServlet: Receiving Data
 getParameter / getParameterValues /
getParameterNames
 process the data and return you the parameters
 getQueryString
 for GET method
 returns a single string in url-encoded format
 getReader / getInputStream
 for POST, PUT, DELETE
 returns a stream of characters / bytes
 mutually exclusive
 use EITHER getParameter* OR one of the others (never
both)
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
50
Servlets and the Java Web Server
SimpleServlet (GET)
public class SimpleServlet extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException {
// set header field first
res.setContentType("text/html");
// then get the writer and write the response data
PrintWriter out = res.getWriter();
out.println(
"<HEAD><TITLE> SimpleServlet Output</TITLE></HEAD><BODY>");
out.println("<h1> SimpleServlet Output </h1>");
out.println("<P>This is output is from SimpleServlet.");
out.println("</BODY>");
out.close();
}
public String getServletInfo() { return "A simple servlet"; }
}
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
51
Servlets and the Java Web Server
DateServlet
public class DateServlet extends HttpServlet {
public void service(
HttpServletRequest req,
HttpServletResponse res)
throws ServletException, IOException
{
Date today = new Date();
res.setContentType("text/plain");
ServletOutputStream out = res.getOutputStream();
out.println(today.toString());
}
public String getServletInfo() {
return "Returns a string representation of the current
time";
}
}
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
52
From Java Web Server Tutorial by Sun Microsystems
Servlets and the Java Web Server
HelloHttpServlet
 Reads in a parameter
 Can use a form
 <FORM METHOD=GET
ACTION=”/servlet/HelloHttpServlet”>
 <INPUT NAME=name>
 </FORM>
 Can use right in a URL
http://localhost:8080/servlet/HelloHttpServlet?name=Fred
 Outputs it as HTML
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
53
Servlets and the Java Web Server
HelloHttpServlet
public class HelloHttpServlet extends HttpServlet
{
public void doGet(HttpServletRequest req, HttpServletResponse
res) throws IOException, ServletException
{
String name = req.getParameter("name");
if (name == null) name = "Joe";
res.setContentType("text/plain");
ServletOutputStream out = res.getOutputStream();
out.println("Hello, " + name + "!");
}
}
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
54
Servlets and the Java Web Server
More Advanced Servlets
 See Post Servlet
 from Servlet Tutorial
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
55
Servlets and the Java Web Server
HttpServletRequest
 Cookie[ ] getCookies()
 returns list of cookies sent by client
 String getMethod()
 GET, POST, etc.
 String getRequestURI()
 returns the URI or URL that was invoked
 useful for putting inside <FORM> tag
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
56
Servlets and the Java Web Server
HttpServletRequest (Cont.)
 CGI Variable Methods
 getServletPath(), getPathInfo(), getPathTranslated(),
getQueryString(), getRemoteUser(), getAuthType()
 String getHeader(String name)
 Session Management Methods
 HttpSession getSession(boolean create)
 More later...
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
57
Servlets and the Java Web Server
HttpServletResponse
 Contains HTTP status codes as constants
 int HttpServletResponse.SC_NOT_FOUND = 404;
 Can send Error or Status codes to client
 Deals with Cookies
 Deals with HTTP Headers
 Can send HTTP Redirect to client
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
58
Servlets and the Java Web Server
Servlet Lifecycle: Init()
 public void init(ServerConfig cfg)
 called once, when servlet loads
 don't worry about synchronization
 perform costly setup here, rather than once
per request
 open database connection(s)
 load in persistent data
 spawn background threads
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
59
Servlets and the Java Web Server
Init Details
 if you fail, throw an UnavailableException
 must call super.init(cfg), which saves off
cfg
 if you like, you can save it yourself and override
getServletConfig, but why bother?
 Can call getInitParameter(paramName) to
read from the server-side config file
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
60
Servlets and the Java Web Server
Servlet Lifecycle: Service
 public void service(ServletRequest req,
ServletResponse res)
 takes Request and Response objects
 called many times, once per request
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
61
Servlets and the Java Web Server
service() and Concurrency
 Might be called simultaneously in several
threads
 it is your responsibility to handle synchronized
access to shared resources
 It is possible to declare a servlet as singlethreaded
 implement SingleThreadModel (empty interface)
 performance will suffer (if there are multiple
simultaneous requests)
 You can use class-static data to share data
across successive or concurrent requests
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
62
Servlets and the Java Web Server
Servlet Lifecycle: Destroy
 public void destroy()
 takes no parameters
 you must clean up
 close database connections
 stop threads
 Afterwards, servlet may be garbage collected
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
63
Servlets and the Java Web Server
Servlet Lifecycle: Destroy Details
 The server calls destroy after all service calls have
been completed, or after a certain number of
seconds have passed, whichever comes first.
 Warning: other threads might be running service
requests, so be sure to synchronize, and/or wait
for them to quit
 Sun's Servlet Tutorial has an example of how to do this
with reference counting
 Destroy can not throw an exception, so if
something bad happens, call log() with a helpful
message (like the exception)
 See “closing a JDBC connection” example in Tutorial
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
64
Servlets and the Java Web Server
Init Parameters
 ServletConfig
 String getInitParameter()
 Enumeration getInitParameterNames()
 There are convenience methods of the
same name inside GenericServlet
 Init Parameters are set by the server
administrator
 Servlet Parameters are set by the web page
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
65
Servlets and the Java Web Server
ServletContext
 call GenericServlet.getServletContext()
 getServlets()
 returns list of all installed Servlets
 getServlet(String name)
 returns the named Servlet
 log()
 see next slide
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
66
Servlets and the Java Web Server
Logging
 GenericServlet.log(String message)
 Writes the name of your servlet, plus the message,
to the server log file
 Location of log file is server-specific
 on JWS, you can check in the Admin Tool
 "If a servlet will have multiple instances (for example, if the
network service runs the servlet for multiple virtual hosts),
the servlet writer should override this method. The
specialized method should log an instance identifier, along
with the requested message." - Javadoc for GenericServlet
 But usually, there is only one instance of each
servlet, called reentrantly by the web server
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
67
Servlets and the Java Web Server
Servlet.getServletInfo()
 You should override this method
 Returns a string containing author, version,
copyright, etc.
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
68
Servlets and the Java Web Server
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
69
Servlets and the Java Web Server
HTTP Servlet Efficiency
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
70
Servlets and the Java Web Server
Efficiency: KeepAlive
 HTTP keepalive improves performance
 Keeps connection alive across multiple HTTP
requests
 Servlet must set content-length
 You can write to a ByteArray or StringBuffer, then
get its length before writing it
 res.setContentLength(sb.length());
 out.print(sb);
 KeepAlive should be enabled by default if all you
do is write short strings, then close the output
stream
 but maybe not
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
71
Servlets and the Java Web Server
Efficiency: getLastModified
 long HttpServlet.getLastModified(
HttpServletRequest req )
 Returns the time the requested entity was
last modified
 difference in milliseconds between that time and
midnight, January 1, 1970
 negative = unknown (or dynamic)
 Improves performance on browser/proxy
caching
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
72
Section IV
Saving State
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
73
Servlets and the Java Web Server
Saving State: Why
 Shopping Cart
 User Preferences
 “Wizard” interfaces
 i.e., successive linked dialog boxes / form entry
pages
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
74
Servlets and the Java Web Server
Saving State: How
 Client-side storage
 Hidden fields
 URL Rewriting
 Cookies
 Server-side storage
 Instance variables
 Database Access
 JWS Session Management
 Best possible solution (but still flawed)
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
75
Servlets and the Java Web Server
Hidden Fields
 Save data inside the servlet, keyed to a
handle
 Store a handle inside each successive
FORM
 Use that handle to retrieve data each query
 Of course, you could always store all the
data in hidden fields, instead
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
76
Servlets and the Java Web Server
Hidden Fields: Example
private Dictionary cache = new Hashtable();
public void doGet(...) {
String handle = getParameter(“handle”);
UserData data;
if (handle == null) {
data = new UserData();
handle = makeNewHandle(); // defined elsewhere
cache.put( handle, data );
}
else
data = (UserData)cache.get(handle);
77
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
...
Servlets and the Java Web Server
Hidden Fields: Example
out.println(“<FORM ACTION=/servlet/Whatever>”);
out.println(
“<INPUT TYPE=hidden NAME=handle VALUE=” +
handle + “>”);
out.println( ... rest of form ... );
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
78
Servlets and the Java Web Server
Hidden Fields: Example
 Survey.java
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
79
Servlets and the Java Web Server
Hidden Fields: Pros and Cons
 Pros
 Well understood
 You have control
 Can use your own caching mechanism
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
80
Servlets and the Java Web Server
Hidden Fields: Pros and Cons
 Cons
 Need to use FORMs
 hidden fields do not persist across normal links
 Sessions are not persistent across server restarts
 unless you write code to do it
 Sessions do not expire
 unless you write code to do it
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
81
Servlets and the Java Web Server
URL Rewriting
 Change HREF and ACTION URLs on the fly
 Change “/servlet/catalog” into
“/servlet/catalog?user=1234”
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
82
Servlets and the Java Web Server
URL Rewriting
 Pro:
 Don’t need to use FORMs
 Con
 Lose user if he/she travels outside your web site
 Need to use Servlet for all accesses -- can’t access a
raw HTML page
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
83
Servlets and the Java Web Server
Using Instance Variables for State
 Session data stored in instance variables
 directly is bad - not valid for multiple users
 indirectly is better - in a hashtable or vector, keyed
off a unique handle
 Pro: Quick, easy
 Con: Not persistent, memory can fill up
easily
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
84
Servlets and the Java Web Server
Database State
 Session data stored in a database
 You should open a connection to the
database in your init() method, and close it
in your destroy() method
 You can still use the hidden field technique
 When you get a handle, you pull in the user
data via a DB query
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
85
Servlets and the Java Web Server
Database State: Pros and Cons
 Pro:
 persistent
 high capacity
 Con:
 more complicated
 have to write more code
 still doesn’t automatically expire old sessions
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
86
Servlets and the Java Web Server
C is for Cookie
Cookie Monster is a trademark of
Children’s Television Workshop
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
87
Servlets and the Java Web Server
What’s A Cookie?
 Client-side storage
 Server can drop arbitrary data on browser
 Sent back to server on EVERY successive
request
 Automatically expires
 Cookies should be neither large nor
numerous
 Browsers should support twenty cookies per host, of
at least four kilobytes each
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
88
Servlets and the Java Web Server
Cookie Uses
 save session data
 save handle to session data
 store user preferences for next session
 store user login information
 not very secure, but appropriate for some
applications
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
89
Servlets and the Java Web Server
Cookies and Servlets
 Servlets can easily use Cookies
 HttpServletRequest.getCookies() method
 HttpServletResponse.addCookie() method
 Cookie object
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
90
Servlets and the Java Web Server
javax.servlet.http.Cookie
 get/setName()
 get/setValue()
 Attributes
 Comment, Domain, MaxAge, Path, Secure, Version
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
91
Servlets and the Java Web Server
Cookie Example
 Cookie Counter Servlet
 Counter.java
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
92
Servlets and the Java Web Server
Cookie Pros and Cons
 Pro:
 No server-side storage requirements
 Survive server restarts
 Automatically expire
 Con:




Not supported by all browsers
Bandwidth limitations
Not good for large amount of data
User can disable them
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
93
Servlets and the Java Web Server
Detecting Cookie Acceptance
 CookieDetector.java
 Drops a cookie on the client
 Sends a redirect back to CookieDetector,
with a flag saying “this is the test phase”
 The test phase detects whether
 The client accepted the cookie
 The client rejected the cookie (or the browser
doesn’t support cookies)
 Sends another redirect to appropriate page
 You can tell the user “pretty please” here
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
94
Servlets and the Java Web Server
JWS Session Management
 Flexible
 Lightweight
 General
 Automatic
 Uses cookies if it can, URL rewriting if it
can’t
 Based on technology from ATG
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
95
Servlets and the Java Web Server
Session Objects
 Server-side
 One per client (not one per servlet)
 Preserved automatically
 even in browsers that don’t support cookies
 Expire after 30 minutes (by default)
 Saved to disk if server dies; restored if
server restarts
 “Loosely speaking, a session corresponds
to a single sitting of a single anonymous
user” - JWS Tutorial
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
96
Servlets and the Java Web Server
Using Sessions
HttpSession session = request.getSession (true);
String info = (String)session.getValue(“foo.info”);
// assume getNewInfo defined elsewhere
String newinfo = getNewInfo();
session.putValue(“foo.info”, newinfo);
// then output page
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
97
Servlets and the Java Web Server
URL Rewriting
 Preserves sessions on non-cookie browsers
 Changes
<a href="/store/catalog">
 into
<a href="/store/catalog;$sessionid$DA32242SSGE2">
 You must actively call res.encodeUrl(“/store/catalog”)
 see next slide
 Does not work if user merely disables cookies
 Has to actually BE a non-cookie browser
 Lame
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
98
Servlets and the Java Web Server
HttpServletResponse - Encoding
 Has methods to process URLs to splice in the session ID if
appropriate
 Not the same as URLEncode / URLDecode
 the server deals with that
 String encodeUrl(String url)
 rewrites the given URL if necessary
 if the browser supports cookies, returns URL unchanged
 All URLs emitted by a session-using Servlet should be run through
this method
 e.g.
 out.println("<A HREF=\"" + resp.encodeUrl("next.html") + "\">");
 also String encodeRedirectUrl(String url)
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
99
Servlets and the Java Web Server
Session Persistence
 Sessions swap to disk
 When server shuts down
 When memory fills up
 Uses Java Serialization
 Only works for Serializable or Externalizable objects
 “Note: Session persistence is intended to
be used as a means for preserving
Sessions across server restarts. It is not
meant to be used as a general long-term
session persistence mechanism.”
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
100
Servlets and the Java Web Server
Example
 VectorSessionServlet.java
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
101
Servlets and the Java Web Server
Bugs
 Can’t use custom classes inside session
data
 Doesn’t really detect whether client
supports cookies
 Instead, detects whether browser can potentially
support cookies
 Lame - they should use my CookieDetector
technique
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
102
Section V
Java Web Server Features
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
103
Servlets and the Java Web Server
Administration Tools
 Play with Admin Tool
 http://localhost:9090/
 Click on a service, click “Manage” button
 To shut down server, click “Shut Down”
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
104
Servlets and the Java Web Server
Manage Servlets
 Add
 Properties
 Load on Startup
 Unload
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
105
Servlets and the Java Web Server
Servlet Aliases
 Specify a partial URL
 Map it to a particular servlet
 e.g.
 you want http://foo.com/lunch to execute
/servlets/meal?type=lunch
 set alias = /lunch
 set servlet invoked = meal?type=lunch
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
106
Servlets and the Java Web Server
Servlet Chains (Filters)
 specify a comma-separated list of servlets
 the first servlet gets the user input
 each servlet in turn will get the previous
output
 the final servlet will return to the user
 all servlets in chain must use same ACL
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
107
Servlets and the Java Web Server
HTML Templates
 Define standard look for all (or some)
pages
 Template Servlet
 A tag inside template page inserts section
from original page
 <subst data="HEAD"></subst>
 <subst data="BODY"></subst>
 Specify which files are templated via
Servlet Aliases in Admin Tool
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
108
Servlets and the Java Web Server
Page Compilation (JSP)
 Embed Java code in static HTML pages
then compile those pages into individual
Java servlets to create a dynamic web site
 Based on JHTML technology from Art
Technology Group (http://www.atg.com/)
 Product: Dynamo, a Java Web Application Server
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
109
Servlets and the Java Web Server
Session Tracking
 See above
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
110
Servlets and the Java Web Server
Servlet Beans
 Using Servlets That are Beans
 Changes to config file are instantly updated
 Servlet itself is persistent across server restarts
 instance variables, like counters or caches, are
preserved
 Calling JavaBeans from Servlets
 Invisible Beans
 Installed inside “lib” subdirectory
 Calling JavaBeans in JHTML/JSP Files
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
111
Servlets and the Java Web Server
FAQ
 Answers in the Exercises book
 How do I develop using the servlet classes without
installing JDK1.2?
 Is it the “servlets” directory or the “servlet” directory
 Why doesn’t my servlet work inside a <SERVLET>
tag?
 How do I support both GET and POST protocol
from the same Servlet?
 How do I fully shut down the server?
 My browser says “the server returned an invalid or
unrecognized response” – what gives?
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
112
Servlets and the Java Web Server
References
 Java Server 1.1
 http://java.sun.com/javastore/jserv/buy_try.html
 http://java.sun.com/products/java-server/index.html
 be sure to download the JWS documentation
 The home for servlets and the Java Web Server.
 http://jserv.javasoft.com
 The Java Web Server 1.1 is available for trial or purchase.
 http://java.sun.com/javastore/jserv/buy_try.html
 The Java Web Server 1.1.1 upgrade pack is available for free.
 http://java.sun.com/products/java-server/webserver/jws111.html
 The Java Server Pages preview pack is available for free.
 http://developer.javasoft.com/developer/earlyAccess/jwspreview.html
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
113
Servlets and the Java Web Server
References
 RFC2045 - MIME
 http://info.internet.isi.edu/in-notes/rfc/files/rfc2045.txt
 RFC 2109 - Cookies
 http://info.internet.isi.edu/in-notes/rfc/files/rfc2109.txt
 Live Software
 http://www.livesoftware.com/
 JRun, many commercial servlets
 ATG - Dynamo Web Application Server
 http://www.atg.com/
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
114
Servlets and the Java Web Server
References
 Advanced Web Technologies
 http://www.javatrain.com/
 Purple Technology
 http://www.purpletech.com/
 Gamelan
 http://java.developer.com/
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
115
Servlets and the Java Web Server
Appendix: CGI Tutorial
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
116
Servlets and the Java Web Server
What Is CGI?
 Common Gateway Interface
 Allows web pages to send parameters to
web server
 Use HTML forms on client side
 Can also use Java – it's just a protocol!
 Use scripts on server side
 Can use Servlets!
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
117
Servlets and the Java Web Server
Example CGI HTML
<FORM ACTION="/servlets/GuestBook"
METHOD=POST>
Name: <INPUT TYPE=text
NAME="name"><BR>
Message: <INPUT TYPE=textarea
NAME="message"><BR>
<INPUT TYPE=submit>
</FORM>
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
118
Servlets and the Java Web Server
CGI Flow
 Browser downloads HTML page containing
FORM tag
 Browser lays out input widgets
 User fills out form and clicks "Submit"
 Browser takes parameters and sends them
in CGI format
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
119
Servlets and the Java Web Server
CGI Flow (Cont.)
 Server receives parameters and sends
them to CGI script
 CGI script returns MIME document
 usually it's "text/html"
 can be any MIME type
 Browser receives response document and
displays it
 If response contains FORM tag, whole thing
can happen again
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
120
Servlets and the Java Web Server
The FORM tag
 Opens a form
 ACTION
 the URL of the script to execute
 METHOD
 GET or POST
 Usually use POST
 closed with </FORM>
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
121
Servlets and the Java Web Server
INPUT TYPE=text
 Specifies a text field
 NAME
 names parameter to be passed to script
 VALUE (optional)
 initial value for text
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
122
Servlets and the Java Web Server
INPUT TYPE=textarea
 Specifies a multi-line text area
 NAME
 names parameter to be passed to script
…
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
123
Servlets and the Java Web Server
INPUT TYPE=checkbox
 Specifies a check box (duh)
 NAME
 names parameter to be passed to script
 ISCHECKED=true
 default value on
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
124
Servlets and the Java Web Server
INPUT TYPE=radio
 Specifies a radio button (or grouped
checkbox)
 NAME
 names group of buttons
 VALUE
 specifies the value for the group
 e.g.
<INPUT TYPE=radio NAME="gender"
VALUE="male">Male
<INPUT TYPE=radio NAME="gender"
VALUE="female">Female
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
125
Servlets and the Java Web Server
INPUT TYPE=submit
 A push button that submits the form
 NAME
 specifies name of variable
 VALUE
 specifies name of button
 yes, "value" specifies the name
 hey, I didn't write the spec
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
126
Servlets and the Java Web Server
INPUT TYPE=reset
 A push button that clears the form
 Does not submit it
5/24/2017
Copyright © 1997-8, Purple Technology Inc.
127