Per Web Application singleton pattern

Hello
I have a application (ear) file that look something like this:
App.ear
-- Web_1.war
----- WEB-INF/lib/helper.jar
-- Web_2.war
----- WEB-INF/lib/helper.jar
-- Web_3.war
----- WEB-INF/lib/helper.jar
inside helper.jar there is a singleton class., which get initialised differently depending on which Web_<X> it is loaded in. This currently works because each lib directory get loaded by each own classloader.
I would really like to move the helper.jar up to the <ear file>/lib directory, but that means it is only loaded by the classloaders once anf thus the 3 singletons break as there is now only one.
I would like to have some sort of "Per Web application" globally reachable "singleton".
I have thought about using ServletContext, but it appears that there is no easy ways for helper classes to look it up, unless it is passed as a parameter.
A second idea would be to use ThreadLocals, but that would rely on the Web container not reusing threads accross web applications. I am not sure if this is guranteed not to happen ...
In any case, what is the best way to handle this ? is there any standard way or a design pattern to follow...

I suppose another way of asking this question is this:
is there a way to use the singleton pattern on a
per-web-application basis without storing the
singleton in the ServletContext? If I can find a way
to do that, I can solve my initial problem.Some web application servers run each webapp in a separate JVM, or at least a separate classloader. If yours does either of those, then each webapp will have its own instance of the singleton. Try it.

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Please remember to mark your question as answered if this solves your problem

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Lin,
Forget about the strange behavior. It seems that the amount of memory that appears next to java.exe in the Windows task manager has nothing to do with reality.
So, it seems that singletons are bad after all. I did the test, and here are the results
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Timestamp: Tue Apr 11 08:43:38 GST 2006
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Version: 2
Timestamp: Tue Apr 11 08:53:10 GST 2006
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Runtime.totalMemory() = 407539712
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Parent Application (Default) Restarted:
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Timestamp: Tue Apr 11 08:55:52 GST 2006
Value = 23709168
Runtime.maxMemory() = 532742144
Runtime.totalMemory() = 532742144
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     instance = null;
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Initial:
Version: 1
Timestamp: Tue Apr 11 09:07:27 GST 2006
Value = 25392555
Runtime.maxMemory() = 532742144
Runtime.totalMemory() = 211107840
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Timestamp: Tue Apr 11 09:08:24 GST 2006
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Timestamp: Tue Apr 11 09:09:21 GST 2006
Value = 7051219
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Runtime.freeMemory() = 76432112
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Version: 1
Timestamp: Tue Apr 11 09:12:21 GST 2006
Value = 16319980
Runtime.maxMemory() = 532742144
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Runtime.freeMemory() = 10034600
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Version: 1
Timestamp: Tue Apr 11 09:13:45 GST 2006
Value = 21348648
Runtime.maxMemory() = 532742144
Runtime.totalMemory() = 178896896
Runtime.freeMemory() = 53236696
Which indicates that everything is working fine :)
Well, I must say I'm happy I learned this the easy way, instead of having to deal with a production issue.
Thanks Lin for your help.

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####<Dec 26, 2012 8:27:17 AM GMT> <Warning> <netuix> <soa-app-dev-01.am.health.ge.com> <AdminServer> <[ACTIVE] ExecuteThread: '21' for queue: 'weblogic.kernel.Default (self-tuning)'> <501226496> <> <9b375c7445d8c121:79ee4f70:13bc880be7a:-8000-0000000000000209> <1356510437306> <BEA-423420> <Redirect is executed in begin or refresh action. Redirect url is /console/console.portal?_nfpb=true&_pageLabel=WebAppApplicationOverviewPage&WebAppApplicationOverviewPortlethandle=com.bea.console.handles.AppDeploymentHandle%28%22com.bea%3AName%3D*Weblogs*%2CType%3DAppDeployment%22%29.>
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Web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
<display-name>Weblogs</display-name>
<welcome-file-list>
<welcome-file>index.html</welcome-file>
</welcome-file-list>
<servlet>
<description></description>
<display-name>SOA11gLogServlet</display-name>
<servlet-name>SOA11gLogServlet</servlet-name>
<servlet-class>com.ge.healthcare.soa.logs.SOA11gLogServlet</servlet-class>
</servlet>
</web-app>
==================================================================
weblogic.xml
<?xml version="1.0" encoding="UTF-8"?>
<wls:weblogic-web-app xmlns:wls="http://xmlns.oracle.com/weblogic/weblogic-web-app" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd http://xmlns.oracle.com/weblogic/weblogic-web-app http://xmlns.oracle.com/weblogic/weblogic-web-app/1.3/weblogic-web-app.xsd">
<wls:weblogic-version>10.3.6</wls:weblogic-version>
<wls:context-root>/Weblogs</wls:context-root>
</wls:weblogic-web-app>
==================================================================
Please help me to resolve the issue,its bit urgent.Let me know in case of anything else required to check and verify.
Thanks in advance.
Regards,
Rahul Aahir
Edited by: 978673 on Dec 26, 2012 4:22 PM
Edited by: 978673 on Dec 26, 2012 4:32 PM

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You have to add a <servlet> element to this file.
Something like this:
<servlet>
<servlet-name>blabla</servlet-name>
<servlet-class>blablapackage.Blablaclass</servlet-class>
<init-param>...</init-param>
</servlet>
Now this may not solve your problem. Make sure you refer to your servlets using their full qualified names.btw, just to be sure, what is your definition of "servlet"? (i mean: any java class or only javax.servlet.Servlet)

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<web-app>
<resource-ref>
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<res-type>javax.sql.DataSource</res-type>
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<Listener className="org.apache.catalina.mbeans.ServerLifecycleListener" />
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<Resource name="UserDatabase" auth="Container"
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My first page in the project is called Homepage.html. To start my project I give http://localhost:5050/tdm/homepage.html
in a browser. Here I accept a username and password from the user and then do the validation in
a valid.jsp file, where I connect to the database and check and use jsp:forward to go to next pages
accordingly. However when I enter the username and password and click Go in the homepage, nothing is
displayed on the next page. The URL in the browser says valid.jsp but a blank screen appears.
WHY DOES IT HAPPEN SO? DOES IT MEAN THAT TOMCAT IS NOT RECOGNIZING JAVA IN MY SYSTEM OR IS IT A PROBLEM
WITH THE DATABASE CONNECTION OR SOMETHING ELSE? I FEEL THAT TOMCAT IS NOT EXECUTING JSP COMMANDS?
IS IT POSSIBLE?WHY WILL THIS HAPPEN?
I set the JAVA_HOME and CATALINA_HOME environment to the jdk and tomcat folders resp.
Is there any other thing that I need to set in classpath? Should I have my project as a
WAR file in the webapps of TOMCAT or just a folder i.e. directory structure will fine?

I am using Tomcat 5.5 and Jdk 1.5.0_12 and Oracle 10g. I am using jdbc-odbc bridge connection
to connect to the database. I have placed my project folder called
tdm under the webapps folder in Tomcat. This 'tdm' folder consists of
a collection of html pages,jsp pages and images of my project. Also I created a
WEB-INF folderand in that I have lib folder which contains catalina-root.jar
, classes12.jar and nls_charset.jar files. And also in the WEB-INF folder I have the web.xml
file which looks like this
<?xml version="1.0" encoding="ISO-8859-1"?>

<web-app>
<resource-ref>
<description>Oracle Datasource example</description>
<res-ref-name>jdbc/gdn</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
</resource-ref>
</web-app>
My Server.xml file in Tomcat\conf folder is as follows



<Server port="8005" shutdown="SHUTDOWN">

<Listener className="org.apache.catalina.mbeans.ServerLifecycleListener" />
<Listener className="org.apache.catalina.mbeans.GlobalResourcesLifecycleListener" />

<GlobalNamingResources>

<Environment name="simpleValue" type="java.lang.Integer" value="30"/>

<Resource name="UserDatabase" auth="Container"
type="org.apache.catalina.UserDatabase"
description="User database that can be updated and saved"
factory="org.apache.catalina.users.MemoryUserDatabaseFactory"
pathname="conf/tomcat-users.xml" />
<Resource name="jdbc/gdn" auth="Container"
type="javax.sql.DataSource" driverClassName="sun.jdbc.odbc.JdbcOdbcDriver"
url="jdbc:odbc:gdn"
username="system" password="tiger" maxActive="20" maxIdle="10"
maxWait="-1"/>
</GlobalNamingResources>


<Service name="Catalina">


<Connector
port="5050" maxThreads="150" minSpareThreads="25" maxSpareThreads="75"
enableLookups="false" redirectPort="8443" acceptCount="100"
connectionTimeout="20000" disableUploadTimeout="true" />

     



<Connector port="8009"
enableLookups="false" redirectPort="8443" protocol="AJP/1.3" />






<Engine name="Catalina" defaultHost="localhost">




<Realm className="org.apache.catalina.realm.UserDatabaseRealm"
resourceName="UserDatabase"/>







<Host name="localhost" appBase="webapps"
unpackWARs="true" autoDeploy="true"
xmlValidation="false" xmlNamespaceAware="false">









<Context path="/tdm" docBase="tdm" debug="0" reloadable="true" />
</Host>
</Engine>
</Service>
</Server>
I have set the context path to /tdm in the server.xml file. Should this be placed in context.xml?
My first page in the project is called Homepage.html. To start my project I give http://localhost:5050/tdm/homepage.html
in a browser. Here I accept a username and password from the user and then do the validation in
a valid.jsp file, where I connect to the database and check and use jsp:forward to go to next pages
accordingly. However when I enter the username and password and click Go in the homepage, nothing is
displayed on the next page. The URL in the browser says valid.jsp but a blank screen appears.
WHY DOES IT HAPPEN SO? DOES IT MEAN THAT TOMCAT IS NOT RECOGNIZING JAVA IN MY SYSTEM OR IS IT A PROBLEM
WITH THE DATABASE CONNECTION OR SOMETHING ELSE? I FEEL THAT TOMCAT IS NOT EXECUTING JSP COMMANDS?
IS IT POSSIBLE?WHY WILL THIS HAPPEN?
I set the JAVA_HOME and CATALINA_HOME environment to the jdk and tomcat folders resp.
Is there any other thing that I need to set in classpath? Should I have my project as a
WAR file in the webapps of TOMCAT or just a folder i.e. directory structure will fine?

Backup/Recovery from web application

Hello guys,
I am using Oracle 9i as DB and Oracle 9iAS for web application server. I want to provide Backup and Recovery functionality to the user via web. I don't know any thing in this regard.
Is it possible that we can take backup and recovery from web application?
Is there any alternative for this function.
any other comments will be appreciated.
Thank you,
Jawed Nazar Ali

Read this article in order to get an idea about Java Stored Procedures.
Oracle Developer JAVA STORED PROCEDURES
Simplify with Java Stored Procedures
By Kuassi Mensah
Use Java stored procedures to bridge SQL, XML, Java, and J2EE and Web Services.
Stored procedures allow a clean separation of persistence logic that runs in the database tier from business logic that runs in the middle tier. This separation reduces overall application complexity and increases reuse, security, performance, and scalability.
A major obstacle, however, for widespread adoption of stored procedures is the set of various proprietary, database-dependent implementation languages that different database vendors use. The use of Java-based stored procedures fixes this concern. Oracle has implemented ANSI standards that specify the ability to invoke static Java methods from SQL as procedures or functions. This implementation is called simply "Java stored procedures."
In this article, you will learn how Java stored procedures help simplify and increase the performance of your business logic and extend database functionality. I'll show how Oracle enables the use of Java stored procedures within the database. I'll also look at how Java stored procedures access data, and show how to create a basic Java stored procedure.
PL/SQL or Java
When you think of Oracle stored procedures, you probably think of PL/SQL. Oracle, however, has provided Java support in the database since Oracle8i, to offer an open and portable alternative to PL/SQL for stored procedures. I can hear the $64,000 question: "How do I choose between PL/SQL and Java? Should I forget all the things I've been told about PL/SQL and move on to the greener Java pastures?"
Both languages are suitable for database programming, and each has its strengths and weaknesses. In deciding which language to use, here's a general rule of thumb:
Use PL/SQL for database-centric logic that requires seamless integration with SQL and therefore complete access to database objects, types, and features.
Use Java as an open alternative to PL/SQL for database independence, but also for integrating and bridging the worlds of SQL, XML, J2EE, and Web services.
OracleJVM Lets You Run Java within the Database
Since Oracle8i, Release 1 (Oracle 8.1.5), Oracle has offered a tightly integrated Java virtual machine (JVM) that supports Oracle's database session architecture. Any database session may activate a virtually dedicated JVM during the first Java code invocation; subsequent users then benefit from this already Java-enabled session. In reality, all sessions share the same JVM code and staticsonly private states are kept and garbage collected in an individual session space, to provide Java sessions the same session isolation and data integrity capabilities as SQL operations. There is no need for a separate Java-enabled process for data integrity. This session-based architecture provides a small memory footprint and gives OracleJVM the same linear SMP scalability as the Oracle database.
Creating Java Stored Procedures
There are a few steps involved in turning a Java method into a Java stored procedure. These include loading the Java class into the database using the loadjava utility, and publishing the Java methods using a call specification (Call Spec) to map Java methods, parameter types, and return types to their SQL counterparts. The following section shows how to do this.
I'll use a simple Hello class, with one method, Hello.world(), that returns the string "Hello world":
public class Hello
public static String world ()
return "Hello world";
The Loadjava Utility
Loadjava is a utility for loading Java source files, Java class files, and Java resource files; verifying bytecodes; and deploying Java classes and JAR files into the database. It is invoked either from the command line or through the loadjava() method contained within the DBMS_JAVA class. To load our Hello.class example, type:
loadjava -user scott/tiger Hello.class
As of Oracle9i Release 2, loadjava allows you to automatically publish Java classes as stored procedures by creating the corresponding Call Specs for methods contained in the processed classes. Oracle provides Oracle9i JDeveloper for developing, testing, debugging, and deploying Java stored procedures.
The Resolver Spec
The JDK-based JVM looks for and resolves class references within the directories listed in the CLASSPATH. Because Oracle database classes live in the database schema, the OracleJVM uses a database resolver to look for and resolve class references through the schemas listed in the Resolver Spec. Unlike the CLASSPATH, which applies to all classes, the Resolver Spec is applied on a per-class basis. The default resolver looks for classes first in the schema in which the class is loaded and then for classes with public synonyms.
loadjava -resolve <myclass>
You may need to specify different resolvers, and you can force resolution to occur when you use loadjava, to determine at deployment time any problems that may occur later at runtime.
loadjava -resolve -resolver "((* SCOTT) (foo/bar/* OTHERS)
(* PUBLIC))"
Call Spec and Stored Procedures Invocation
To invoke a Java method from SQL (as well as from PL/SQL and JDBC), you must first publish the public static method through a Call Spec, which defines for SQL the arguments the method takes and the SQL types it returns.
In our example, we'll use SQL*Plus to connect to the database and define a top-level Call Spec for Hello.world():
SQL> connect scott/tiger
SQL> create or replace function helloworld return
VARCHAR2 as language java name 'Hello.world () return
java.lang.String';
Function created.
You can then invoke the Java stored procedure as shown below:
SQL> variable myString varchar2[20];
SQL> call helloworld() into :myString;
Call completed.
SQL> print myString;
MYSTRING
Hello world
Java stored procedures are callable, through their Call Spec, from SQL DML statements (INSERT, UPDATE, DELETE, SELECT, CALL, EXPLAIN PLAN, LOCK TABLE, and MERGE), PL/SQL blocks, subprograms, and packages, as well as database triggers. The beauty of Call Spec is that stored procedure implementations can change over time from PL/SQL to Java or vice versa, transparently to the requesters.
Call Spec abstracts the call interface from the implementation language (PL/SQL or Java) and therefore enables sharing business logic between legacy applications and newer Java/J2EE-based applications. At times, however, when invoking a database-resident Java class from a Java client, you may not want to go through the PL/SQL wrapper. In a future release, Oracle plans to provide a mechanism that will allow developers to bypass the Call Spec.
Advanced Data-Access Control
Java stored procedures can be used to control and restrict access to Oracle data by allowing users to manipulate the data only through stored procedures that execute under their invoker's privileges while denying access to the table itself. For example, you can disable updates during certain hours or give managers the ability to query salary data but not update it, or log all access and notify a security service.
Sharing Data Logic Between Legacy and J2EE Applications
Because legacy applications and J2EE applications both invoke stored procedures through the Call Spec, the same data logic can be shared between J2EE and non-J2EE worlds. Thanks to Call Spec, this data logic can be shared regardless of the implementation language used (whether PL/SQL or Java).
Autogeneration of Primary Keys for BMP Entity Beans
When using BMP for EJB entity beans, a bean instance can be uniquely identified by the auto-generated primary key associated with the newly inserted data as a return value for ejbCreate(). You can retrieve this value within ejbCreate() in one database operation by using a stored procedure that inserts the corresponding data and retrieves or computes the primary key. Alternatively, you could insert the data and retrieve the corresponding key (or ROWID) in one SQL statement, using the RETURN_GENERATED_KEYS feature in JDBC 3.0. However, the stored procedure approach is more portable across JDBC driver versions and databases.
You can implement this pattern with these three steps:
Create the Java stored procedure, defining a public static Java method insertAccount() within a public GenPK class. This method will insert data, compute a unique key (by passing out a sequence number), and return the computed key as primary key.
Define the Call Spec.
CREATE OR REPLACE PROCEDURE insertAccount(owner IN
varchar, bal IN number, newid OUT number)
AS LANGUAGE JAVA NAME 'GenPK.insertAccount(
java.lang.String [])';
Invoke the stored procedure within ejbCreate().
Public AccountPK ejbCreate(String ownerName, int balance) throws CreateException
try {
CallableStatement call = conn.prepareCall{
"{call insertAccount(?, ?, ?)}"};
return new AccountPK(accountID);
Custom Primary Key Finders for CMP Entity Beans
Finder methods are used for retrieving existing EJB entity bean instances. Primary key finders allow you to retrieve a uniquely identified EJB instance. For CMP entity beans, the EJB container automatically generates the primary key finder findByPrimaryKey() method, based on declarative description. In some situations, however, you might need more control; for example, you may need a specialized finder such as findByStoredProcKey(). In these situations, you can use Java stored procedures in conjunction with an object relational framework (such as Oracle9i Application Server [Oracle9iAS] TopLink) to implement a custom primary key finder method. After you define the EJB finder as a REDIRECT or NAMED finder, TopLink will generate the SQL query for retrieving the bean instance.
Data-Driven EJB Invocation
In a data-driven architecture, business logic invocation can be triggered as a result of database operations (such as inserts, updates, or deletes). A Java stored procedure implementing the data logic can be declared as a database trigger to invoke EJBs running in a middle-tier J2EE application server. You can make EJB calls by using either standard remote method invocation (RMI) over Interoperable Inter-ORB Protocol (IIOP), using a J2EE 1.3 compatible server, or RMI over a vendor-specific transport protocol (such as ORMI with Oracle9iAS/OC4J or RMI over T3 with BEA WebLogic). Each application server vendor has its own optimized protocol while providing RMI over IIOP for interoperability. Oracle9iAS supports both RMI calls over IIOP and ORMI protocols.
Data-Driven Messaging
Oracle9i Database embeds Advanced Queuing (AQ), which is an integrated, persistent, reliable, secure, scalable, and transactional message-queuing framework. Oracle exposes AQ features to Java developers through the standard Java Messaging System (JMS) API. Java stored procedures can invoke AQ operations through the JMS interface to allow fast, intra-session, scalable, data-driven messaging.
Java stored procedures can use JMS to invoke AQ operations. You can implement this pattern in four steps:
Create and start the JMS Queue (to do so, embed the following operations within a SQL script):
execute dbms_aqadm.create_queue_table(queue_table =>
'queue1', queue_payload_type =>
'SYS.AQ$_JMS_TEXT_MESSAGE', comment => 'a test queue',
multiple_consumers => false, compatible => '8.1.0');
execute dbms_aqadm.create_queue( queue_name => 'queue1',
queue_table => 'queue1' );
execute dbms_aqadm.start_queue(queue_name => 'queue1');
Create the Java stored procedure (a code snippet is shown):
public static void runTest(String msgBody)
try
// get database connection
ora_drv = new OracleDriver();
db_conn = ora_drv.defaultConnection();
// setup sender (cf online code sample)
// create message
s_msg = s_session.createTextMessage(msgBody);
// send message
sender.send(s_msg);
s_session.commit();
// receive message
r_msg = (TextMessage) receiver.receive();
r_session.commit();
// output message text
String body = r_msg.getText();
System.out.println("message was '"+body+"'");
Create the Call Spec:
create or replace procedure jmsproc (t1 IN VARCHAR)
as language java name 'jmsSample.main (java.lang.String[])';
Invoke the stored procedure:
call jmsproc('hello');
Database-Assisted Web Publishing (Cache Invalidation)
One of the common issues application architects must face is how to cache database information reliably to increase overall system performance. JCACHE is an upcoming standard specification (JSR 107) that addresses this problem. It specifies an approach for temporary, in-memory caching of Java objects, including object creation, shared access, spooling, invalidation, and consistency across JVMs. It can be used to cache read-mostly data such as product catalogs and price lists within JSP. Using JCACHE, most queries will have response times an order of magnitude faster because of cached data (in-house testing showed response times about 15 times faster).
In order to track all the changes to the origin data and refresh the cached data, a Java stored procedure is attached to a table as a trigger. Any change to this table will result in the automatic invocation of this stored procedure, which in turn will call out a defined JSP to invalidate the JCACHE object that maps its state to the database table. Upon invalidation, the very next query will force the cache to be refreshed from the database. Next Steps
READ MORE about Java Stored Procedures
This article is adapted from the white paper "Unleash the Power of Java Stored Procedures." You can find the white paper at:
/tech/java/java_db/pdf/
OW_30820_JAVA_STORED_PROC_paper.PDF
New PL/SQL features in Oracle9i Database, Release 2
/tech/pl_sql/pdf/
Paper_30720_Doc.pdf
Resolver Spec
/docs/products/oracle9i/
doc_library/release2/java.920/a96659.pdf
OracleJVM and Java 2 Security
/docs/products/oracle9i/
doc_library/release2/java.920/a96656.pdf
DOWNLOAD Code
Exercise code examples from this article:
/sample_code/tech/
java/jsp/Oracle9iJSPSamples.html
LEARN about stored procedures as Web services
/tech/webservices
Extending Database Functionality
One of the great things about running Java code directly in the database is the ability to implement new functionality by simply loading the code or library and using the Call Spec to make the entry points (public static methods) available to SQL, PL/SQL, Java, J2EE, and non-Java APIs. Oracle9i Database customers can easily extend database functionality. Oracle itself leverages this capability for new utilities and packages such as the XML Developer Kits (XDKs).
Bridging SQL, PL/SQL, Java, J2EE, .NET, and XML
The Oracle XDK is written in Java and exposes its public methods as Java stored procedures, extending the database's XML programmability. SQL, PL/SQL, Java, J2EE, and non-Java (.NET) business logic all have access to the XML parser, the XSLT processor, the XPath engine, and XML SQL Utility (XSU).
The XML parser is accessible through the xmlparser and xmldom packages. XSU is a Java utility that generates an XML document from SQL queries or a JDBC ResultSet, and writes data from an XML document into a database table or view. Using XSU, XML output can be produced as Text, DOM trees, or DTDs. XSU is exposed to PL/SQL through the dbms_xmlquery and dbms_xmlsave packages.
Conclusion
The integration of the Oracle database with a Java VM enables the creation of portable, powerful, database-independent data logic and persistence logic. The loose coupling of business logic that runs in the middle tier with data logic that runs in the database tier improves application scalability, performance, flexibility, and maintenance.
Kuassi Mensah ([email protected]) is a product manager in the Server Technologies division at Oracle.
http://otn.oracle.com/oramag/oracle/03-jan/o13java.html
Joel Pérez

Authentication in clustered web application without sticky session

I have built JSP/Servlet/Struts application in the past on a cluster of app servers. Each app server has its own JVM running the Servlel Container. All of the HTTP requests come into a hardware load balancer, which directs the requests to one of the app servers in the cluster.
I have wanted to use the Java HttpSession management without any kind of distributed session provided by the app server. We have used "sticky" sessions. The application writes a cookie to the client on the first request. The load balancer looks for that cookie on subsequent requests and directs the request to the server that originally wrote the cookie. This ensures that all requests within the same session are handled by the same application server. This also means that if I do request.getSession().setAttribute("authenticated",true) on one request, when I do request.getSession().getAttribute(authenticated) on subsequent requests in the same session, I can be sure the value will not be null. This allows me to create a filter that checks for that session attribute on each request, and if it is false or null, redirect the request to some sort of login page. Otherwise I can be sure the user has logged in.
I want to build a stateless/non-session based application that can still handle authentication. What I mean by that is that I don't want the load balancer to have to send requests for the same session all to the same server. I would like the load balancer to send each request where ever it wants. That means the simple authentication example I explained in the last paragraph would not work. The user could login on server A, but then on a subsequent request during the same "session", the user's request could be handled by server B. In that case, the session attribute would be null, and the app would think that the user has not logged in.
My application can require that users have cookies enabled, so therefore I can assume the user is accepting cookies (I would have something to check that and redirect the user to an error page saying "turn cookies on" if cookies weren't on). I think one thing that I could do is use encrpytion with a key that is shared between all the servers in the cluster. For example, user logins in on server A, server A writes a cookie with the contents "username,1109272102009". The first part being the username that the user successfully authenticated as and the second part being a timestamp for when the cookie was created. The contents of the actual cookie would be encrypted and I would send the ciphertext as the value of the cookie. When server B gets the cookie, it can decrypt the ciphertext (using the same key as was used to encrypt the data on server A), and check that the username is valid and that the timestamp does not exceed some timeout. The timestamp in the cookie would then have to be updated for the next request.
So my question is (thanks for sticking with me and reading this really long post), has anyone done anything like this before? Is what I have described totally ridiculous or insecure? Are there any books or articles that describe a pattern similar to this that has been know to work well?

I have worked on a web site that did exactly that.
The cookie contained a little bit more information - there was a small amount of user data that were needed on heavily accessed pages.
You'll have a problem if your web application uses attributes. We solved this by keeping most stuff in hidden inputs (backed up by hidden input cryptographic checksums in places where forgery was a concern.) HttpSession attributes have some problems and gotchas.
A few possible fine tunings:
Add a random number to the cookie. Should make known plaintext attacks harder.
Add some extra stuff to the cookie, so that any random hex string that happens to decode to "xZoiyqw,15" isn't accepted. It's easy to try a million cookies until you get "<something>,<integer>" but getting "<something>,<integer>,HelloHowAreYou" is a lot harder.
Be paranoid in checking the format of the cookie. If you add a random number, check that it is all digits etc. Belt and suspenders: also check that the time stamp isn't in the future (allow e.g. 15 seconds future time, in case different servers' clocks are a bit off.)
Don't update the cookie at every hit, only if the time stamp is older than a couple of minutes. Saves encryption CPU power.
After encrypting, prepend a short version number to the cookie. E.g. if the hex cookie is ABCDEF, make it 1ABCDEF. If you later e.g. change the encryption algorithm, change version to 2 and you can easily skip any obviously non-decipherable cookies. A second version number within the cookie might or might not be useful.
Even though you can make random load balancing, consider not doing that. E.g. a server might pull the user's name from the database into memory cache. You get less database traffic and smaller caches if the user still goes to the same server. If a server goes down, only then switch him elsewhere. Downside though: if one server is "half alive" (doesn't respond to requests but alive enough so the load balancer doesn't notice the malfunction), all users bound to that server see a 100% failure.
Benchmark cookie decryption time when selecting the crypto algorithm. How many hits per second you can get and how many you need.
Guard your crypto keys like the crown jewels. Change them periodically and whenever someone in your company (especially IT department) gets the pink slip.

Use of Singleton pattern in Distributed environment

Can somebody say why it is not advisable to use Singleton pattern for developing client server applications.

Client-server does not imply distributed environment, IMHO. There are pretty simple C/S architectures where Singleton pattern is not generally a bad thing.
Distributed environments usually have multiple VMs running, while the singleton pattern typically implemented is a per-VM singleton.

Per Web Application singleton pattern

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