A Little More Java Servlets

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A Little More Java Servlets

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Title: A Little More Java Servlets

1
A Little More JavaServlets

Based on lectures by Walter Makovoz, Ph.D.

2
NETWORK INTERFACES

Java provides a full range of network interfaces
for client / server communications
Two types of sockets are supported
connection-oriented (TCP)
connection-less (UDP)

3
Servlets

Servlets are modules that run inside
request/response-oriented servers, such as
Java-enabled web servers, and extend them in some
manner. For example, a servlet might be
responsible for taking data in an HTML
order-entry form and applying the business logic
used to update a company's order database.
Servlets are to servers what applets are to
browsers.
The Servlet API, which you use to write servlets,
assumes nothing about how a servlet is loaded,
the server environment in which the servlet runs,
or the protocol used to transmit data to and from
the user. This allows servlets to be embedded in
many different web servers.
Servlets are an effective substitute for CGI
scripts they provide a way to generate dynamic
documents that is both easier to write and faster
to run. They also address the problem of doing
server-side programming with platform-specific
APIs. Servlets are developed with the Java
Servlet API, a standard Java extension. 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.
It is already supported by many popular web
servers.

4
Servlets

In some ways, a servlet is similar to an applet.
An applet is a chunk of Java code that executes
under control of a browser. A servlet is a chunk
of Java code that executes under control of a
server program.

5
Servlets

According to JavaSoft
The servletrunner is a small utility, intended
for testing. It is multithreaded, so it can run
more than one servlet. It can be used therefore,
to run multiple servlets simultaneously, or to
test one servlet that calls other servlets in
order to satisfy client requests. Unlike some web
servers, it does not automatically reload
servlets when they are updated. Because it is
small, however, there is very little overhead
associated with stopping and restarting it in
order to use a new version of a servlet.

6
Servlets

The purpose of this program is to illustrate a
very simple servlet.
The servlet was tested using the JDK 1.2 download
package
from JavaSoft along with the Java Servlet
Development Kit
(JSDK) 2.0 from JavaSoft. All tests were
performed under Win95.
The servlet was tested using the servletrunner
program that
is included in the JSDK with the servletrunner
program
running on the same machine and also with the
servletrunner
program running on a different machine on the
network.
The servlet was also tested using the JavaSoft
Java Web
Server 1.1.1 program running on a different
machine on the network.
The servlet produces the following text in the
browser
window in large red letters.
Hello Big Red World

7
Servlets

import java.io.
import javax.servlet.import javax.servlet.http.
public class Servlet01 extends HttpServlet
public void doGet(HttpServletRequest req,
HttpServletResponse res)
throws ServletException,
IOException
//Establish the type of output
res.setContentType("text/html")
//Get an output stream PrintWriter out
res.getWriter()
//Construct an HTML page to return to the
client out.println("ltHTMLgt")
out.println("ltHEADgtltTITLEServlet01lt/TITLEgtlt/H
EADgt")
out.println("ltBODYgt")
out.println("lth1 align\"center\"gt"
"ltfont color\"FF0000\"gt")
out.println("Hello Big Red World")
out.println("lt/fontgtlt/h1gt")
out.println("lt/BODYgtlt/HTMLgt") //end
doGet()//end class Servlet01

8
Servlets

All servlets must implement the Servlet
interface. You can implement it directly.
However, it is more common to implement it
indirectly by extending a class that implements
the interface (such as HttpServlet).
The Servlet interface declares methods for
managing the servlet and its communications with
clients. You will need to provide some or all of
these methods when you write your servlet.

9
Servlets

Two objects are passed to a servlet when it
called by a client
ServletRequest - encapsulates the communication
from the client to the server.
ServletResponse - encapsulates the communication
from the servlet back to the client

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Figure 1. Servlet invocation.
An attractive alternative to CGI and proprietary
server APIs
Supported accross different HTTP servers - Apache
and most commercial servers support Servlet API
Third party Servlet API implementations
Java VMs
Integration with the host servers via their
native APIs
Don't have to be HTTP-specific - may be used to
implement complex multi-tier applications
See the javax.servlet package

Familiar model of accepting requests and
submitting responses
Extend HTTPServet or GenericServlet classes
Access to servlet-specific configuration data at
initialization time
Interact with their environment through a
ServletContext object
Usage modes
Basic request/response mode
Servlet chaining for filtering
Specialized for HTTP - CGI replacement With SSI

Basic example
import javax.servlet.
public class MyServlet
extends GenericServlet
public void service (
ServletRequest request,
ServletResponse response
) throws ServletException, IOException
ServletInputStream
in request.getInputStream ()
ServletOutputStream
out response.getOutputStream ()
...

The API consists of 2 packages, 12 interfaces,
and 9 classes
javax.servlet
Interface RequestDispatcher - defines an object
that receives client requests and forwards them
to a proper resource.
Interface Servlet - defines and object that
extends functionalioty of a Web server.

Interface ServletConfig - defines an object that
controls servlet configuration (unique for every
servlet).
Interface ServletContext - defines an object that
provides a servlet with information about the
environment.
Interface ServletRequest - defines an object that
enables the servlet to get data about a client
request.
Interface ServletResponse - defines an object
that enables the servlet to respond to the client
request.
Interface SingleThreadModel - empty interface,
the purpose is to flag servlets for sequential
execution.

Class GenericServlet - holds a reference to the
ServletConfig and ServletContext objects (created
at initialization time), provides simple
implementations of the servlet lifecycle methods.
Class ServletInputStream - input stream for
reading clienbt requests.
Class ServletOutputStream - abstract class,
implemented by all servlet engines, utilized to
retrun data to clients.
Class ServletException - thrown to indicate a
servlet problem.
Class UnavailableException - thrown to indicate
servlet's unavailability.
java.servlet.http

Interface HttpServletRequest extends
ServletRequest - defines an object that enables
the servlet to access http-specific request
information from the client request (e.g.,
getAuthType(), getCookies(), getHeader(String
name), getSession(), etc.)
Interface HttpServletResponse extends
ServletResponse - defines an object that
represents an http response back to the client.
Interface HttpSession - provides an association
between an HTTP client and an HTTP session
(implemented through cookies or URL rewriting).

Interface HttpSessionBindingListener - objects
implement this interface if they wish to be
notified about being bound to or unbound from a
session.
Interface HttpSessionContext - deprecated
(originally introduced to define common context
for multiple sessions).
Class Cookie - represents a Cookie as defined in
RFC 2109.

Class HttpServlet extends GenericSevlet -
provides convenience in iumplementing HTTP
servlets (e.g., doGet(HttpServletRequest request,
HttpServletResponse response), doPost(HttpServletR
equest request, HttpServletResponse response),
doPut(HttpServletRequest request,
HttpServletResponse response), etc.).
Class HttpSessionBindingEvent - this event is
communicated to a HttpSessionBindingListener.
Class HttpUtils - a collection of static utility
methods useful to HTTP servlets (e.g.,
parseQueryString(String s), etc.).

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Figure 2. Servlet Lifecycle.
Dynamic loading (using normal Java class loading
facilities) and loading on server startup
Servlet initialization - provide your own
implementation of the init() method to perform
i/o-intensive setup once vs. per request (e.g.,
initializing access to other network services or
retrieving the state)
One service() call per request, class-static data
may be used to share info between requests
Servlet remains active until shut down using the
destroy() method, after which the class may
become eligible for garbage collection

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Figure 3. Servlet security.
Access to client information, HTTP-specific
servlets - access to HTTP authentication data
Faulty servlets don't crash servers - not true
for proprietary APIs
Java VM becomes part of the server - Java
Security Manager - control over network and file
access
Unique feature of Java
Fundamental difference from proprietary C and C
server APIs and scripting interfaces
Untrusted and trusted (e.g., built-in and those
that live in a designated local directory, signed
servlets in JAR files) - different access
priviliges
Possible for ISPs to accept servlets from their
customers (native API server extensions are
disallowed because of security)

Servlet Roles
HTTP request processing - CGI-style
Same functionality may be achieved with CGIs
Support for GET, POST, HEAD, etc.
Superior performance and security
Servlets responsible for HTML generation
Server-side includes via the SERVLET tag
Syntax
ltSERVLET NAMEServletNamegt
ltPARAM NAMEparam1 VALUEval1gt
ltPARAM NAMEparam2 VALUEval2gt
Alternative text
lt/SERVLETgt
Example to support a database search

Discussed so far
Introduction to Java
Learned how to use the tools in the JDK
Application programming in Java using OO concepts
Simple Java GUIs using AWT
Writing Applets in Java
Multi-threading
Networking
URL Networking
Socket Networking
Missing one critical component
Database access!

Database Access in Java
SQL and Relational DBs
Using SQL in Java JDBC
Problems with Relational DBs
Some approaches to resolving those problems
Outline
Motivation
Introduction to JDBC
JDBC Drivers
Using JDBC in applets
Middleware
Steps in accessing DBs via JDBC
Sample JDBC application

What is the foundation of Computing?
Data, Information
Computers are noting more than number crunchers
at the grass-roots level!
What proof do we have that Data is a critical
component of the IT infrastructure?
Oracle is the 2 software company in the world
their primary product is Database
It is impossible to write a serious application
without requiring some form of persistence.
How useful would Microsoft Word be if you
couldnt Save!?
Data is essential
CRUD cycle Create, Read, Update, Write

Were going to ASSUME you all know what a
database is!
This is not a Database class!
Just a refresher
Types of databases
Hierarchical or Flat-file
Relational
Object
Some products
Oracle 8
SQL Server 7
Sybase, Informix
and believe it or not Microsoft Access

Commonalities between DBs
Most databases today
are built as Relational Databases
support SQL
SQL
Structured Query Language
A REALLY simple language with a total of about
15-20 keywords which make it possible to perform
CRUD operations on databases.
Assumption
You should all be familiar with SQL!
If not, then please pick up a book

Database Access in Java
Since MOST Databases support SQL
simplest way to have Java access a database is to
be able to use SQL!
JDBC
Java DataBase Connectivity API
Provides an SQL interface to Databases
JDBC allows us to
load an appropriate JDBC driver for the
database we are using
open a connection to the database using URL
notation
embed SQL calls within Java code!
Create a new SQL Query or Update statement
Execute the query or update on the Database
Process results

JDBC/ODBC
ODBC
Microsofts Open Database Connectivity Standard
Proprietary standard from Microsoft!
Already exists in every Windows 95 and Windows NT
machine!
JDBC
similar in design to ODBC
made a little bit more generic than ODBC
a LOT easier to use!
supports the ANSI SQL92 Entry Level
Accessing a JDBC/ODBC data source
Each database vendor MUST provide a driver
(API) which conforms to the JDBC or ODBC
specification!

JDBC Drivers
Type 1 JDBC-ODBC bridge
provides access to a the database by translating
JDBC calls into ODBC calls understood by the
Database
Type 2 native-API partly-Java driver
converts JDBC calls into native calls supported
by the underlying database
Type 3 net-protocol all-Java driver
translates JDBC calls into a DBMS-independent net
protocol which is then translated to a DBMS
protocol by a server
Type 4 native-protocol all-Java driver
converts JDBC calls into the network protocol
used by DBMSs directly

Using JDBC in Applets
Remember
Applets can ONLY communicate back to their
CODEBASE!
Applets can only load pure Java classes from
their CODEBASE
Implications
Only Type 3 and Type 4 drivers may be used in
Java Applets
Type 1 and Type 2 contain native code which may
not be executed within a Java applet!
Workarounds
Three-Tier architectures for allowing database
access via applets
Signed/Trusted Applets which can break out of the
Applet Sandbox

Middleware products
Provide a server on the CODEBASE machine which
listens for Database requests and forwards the
requests to the correct Database!
Example
dbAnywhere from Symantec included in their Visual
Caf? for Java product
Lots of other Middle ware products out there!

Getting ready to use JDBC
For our Demo
We will use JDBC in an application since an
application has no security restrictions
We will use the JDBC-ODBC bridge (Type 1 driver)
since the only database we can have here is
Microsoft Access
Other pre-requisites
Database must be installed
ODBC driver must be installed in Control Panels
JDBC is only included in JDK1.1!
JDBC-ODBC bridge driver must be installed
included in JDK1.1

Setting up the Database
Create a new Database
AddressBookDB
ID, firstName, lastName, middleInitial,
addressLine1, addressLine2, city, state, zip,
country, phone, fax, email
Create a DSN for the Database
DSN Data Source Name
tells your program which database to use!
Done in the ODBC control panel
Well use the AddressBookDSN
Remember the DSN since that is what will be
required in the URL to the Database for ODBC!!

The Database URL
Syntax
jdbcltsub-protocolgtltsubnamegt
Example
jdbcodbcAddressBookDSN
Network addressing
jdbcdbnet//wombat.eng.sun.com356/fred
For details please see the JDBC Specification at
http//java.sun.com80/products/jdbc/index.html

Steps in using JDBC
Load the appropriate JDBC driver
Done using dynamic class loading in Java
We havent explained this, so treat this as an
idiom
Open a Connection to the Database
Create a new query as an SQL Statement
Execute the query
Process the ResultSet
for database meta-data and the records
Close the Statement
Close the ResultSet
Close the database connection

Sample Application
Simple SELECT call into a DB
First a quick look at the JDBC API
java.sql.
Lets go to the code for
SelectDB.java

Analysis of Example
We only did a simple Select call
This application will work with ANY database!
To update the database or add to the DB
use an INSERT SQL statement
Statement.executeUpdate()
You now have the building blocks for doing more
esoteric JDBC applications!!

Problems with Relational DBs
Remember Assignment Two
Person, Student, Instructor, Lecturer, Course,
Grade etc?
What would it take to take all of that data and
STORE and RETRIEVE it from a database!!
Many classes contained One to Many or even Many
to Many mappings!
Each student had many Grade objects keyed by
Course
Need to convert from Object world into
Relational world
Object-Relational Mapping
by far the most time consuming and error prone
part of doing persistence in Java using JDBC
Need to provide method which can convert
Relational -gt Object data
Object -gt Relational data

More Esoteric Databases
Object Databases
Store each object in the database
Do not have the concept of storing tables
OQL
Object Query Language
analog of SQL for relational databases
Some products
POET
ObjectStore
Versant
For more information
Object Database Management Group (www.odmg.org)

Other solutions
JavaBlend
automates the process of mapping from Object to
Relational world and vice versa
Still in Beta
early access release is out
Several other vendors provide their own tools to
create relational to object mappings
how useful they are depends on how convoluted
your data model is!

Status
Seen a brief introduction to JDBC
You have all the building blocks to build more
esoteric JDBC applications!

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Servlet performance.
Performance advantages over both CGI and FastCGI
CGI - heavy startup and initialization code on
each request
FastCGI - heavy process context switching per
request
Servlets
Light-weight process context switching per
request, initalization costs spread over multiple
requests
Automatically takes advantage of JVM permance
features (running threads on different
processors, etc.)

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Interfaces

Similar to a class but the methods are only
declared (not defined) and the data attributes
must be initialized and are treated as "static
final" (constants)
All attributes in an interface must be public
Any class that implements an interface is
responsible for defining all of the methods
declared in that interface

class Client
Socket socket
public PrintStream out
public Client(String host, int port)
try
socket new Socket(host, port)
out new PrintStream(socket.getOutputStream())
catch (IOException e)
public void close()
try
socket.close()

53
Server (Receive.java)

import java.io.
import java.net.
public class Receive
public static void main(String args)
Server server new Server(1234)
try
String data
while ((data server.in.readLine()) ! null)
System.out.println(data)
catch (IOException e)

class Server
ServerSocket listen
Socket socket
public DataInputStream in
public Server(int port)
try
listen new ServerSocket(port, 30)
socket listen.accept()
in new DataInputStream(socket.getInputStream())
catch (IOException e)

Classes can inherit multiple interfaces (but can
inherit only one super class)
Interfaces can inherit multiple interfaces
Unlike classes, interface inheritance is not
based on a single inheritance tree

Defining an interface
package Employee
interface EmpInterface
float weeklyPay()
String toString()
Implementing an interface
package Employee
public class Emp implements EmpInterface
// must define public versions of weeklyPay() and
toString()

57
ABSTRACT CLASSES

An "abstract class" is an intermediate step
between an interface and a class
An abstract class can contain both implemented
and unimplemented (abstract) methods
No objects can be created from an abstract class

58
Interfaces

Regular classes that inherit from an abstract
class must provide definitions for all inherited
abstract methods
Abstract classes that inherit from an abstract
class may either provide definitions for
inherited abstract methods or leave them
"abstract"