Access of Instance variable in servlet

hi,
If i declare a instance variable in servlet , can i access that variable throughout my application??.
please let me know.
thanks in advance.

Servlets are Java classes. They follow all the rules of Java.

Similar Messages

Instance variables in Servlets.

Hi
Is it a good practice to use instance variable in servlet.
Please refer code snippet below.
public class ExcelAction extends HttpServlet {
private int var1 =0;
doGet () {
var1++;
fun1();
fun1 (){
var1++;
}Like in above code I have used an instance variable var1 because I wish the same to be accessible in all servlet functions.
I guess it is not a good practice, as different requests to this servlet at same time will result in multiple threads of this servlet. Which will make the state of var1 unsafe.
If this is correct then what are the alternates if I wish to share variables between functions in a servlet.
Thanks

money321 wrote:
Because in beginning I was not sure that Servlets instances are also actually threads.They are not not threads. Each call to doGet(), doPost() etc is called in a Thread and since there can be many simultaneous calls each having it's own thread a Servlet has to be thread safe.
My other question was specific to threads as I was trying to implement threads.Why?
>
But in this one I rather used servlets.
But apologies, as instance of servlet are also threads.As I said, they are not threads.
>
But still one last doubt.
Sabre if you could please clarify the same as well :How about you do some reading [http://java.sun.com/developer/onlineTraining/Servlets/Fundamentals/|http://java.sun.com/developer/onlineTraining/Servlets/Fundamentals/] and use Google.
>
in your option 2 and three I will have to pass request object to function fun().No. You will need access to the request object for 2 and the servlet context for 3. You need to pass something
If i am correct on this then is this too a better approach ?My preferred approach is 1 since it makes much of your code testable outside of a Servlet.

Scope of instance variables in servlets..

Hi all,
          Sorry for asking a dumb question..
          What is the scope of instance variables in a servlet? Is the scope is
          limited to thread?
          In other words, in the following example, i am setting "testStr", in one
          request and when i tried to access the same instance variable from another
          request, i am getting it as null. Does it mean instance variable is limited
          to that particular thread/request?
          thanks in advance..
          -ramu


Oops ... I had misunderstood and had the problem backwards ;-)
          > Is it known behavior? With registered servlet its working fine (a
          > instance variable is shared by all requests)
          I believe so; I typically deploy in a WAR so have not seen the problem that
          you describe. Servlets can be reloaded, so what you saw could have been
          caused bye a date/time mismatch between the .class file and the server's
          current time. On the other hand, that could be how WebLogic works.
          Peace,
          Cameron Purdy
          Tangosol, Inc.
          http://www.tangosol.com
          +1.617.623.5782
          WebLogic Consulting Available
          "Ramu" <[email protected]> wrote in message
          news:[email protected]...
          > Hi Purdy,
          >
          > I got it. I am testing the servlet as a unregistered servlet, which is
          > creating instance for every new request!!!, which created this whole
          > confusion. Is it known behavior? With registered servlet its working fine
          (a
          > instance variable is shared by all requests)
          >
          > > What theory? ;-) Instance variables are on the object, and the object
          can
          > > be used by multiple threads, thus allowing multiple threads to access
          the
          > > same instance variables.
          > what i mean by theory here is, all instance variables in servlet should
          be
          > shared by all requests. Now i got it sir.
          >
          > Thank you..
          > -ramu
          >
          >
          > >
          > > Peace,
          > >
          > > --
          > > Cameron Purdy
          > > Tangosol, Inc.
          > > http://www.tangosol.com
          > > +1.617.623.5782
          > > WebLogic Consulting Available
          > >
          > >
          > > "Ramu" <[email protected]> wrote in message
          > > news:[email protected]...
          > > > Hi,
          > > >
          > > > > No, an instance variable in a servlet class is not limited to a
          > thread.
          > > > There
          > > > > is exactly one copy of the servlet created in memory and all
          requests
          > > pass
          > > > > through the same instance, thus sharing any instance variables. A
          > > couple
          > > > of
          > > > > things to remember:
          > > > I totally agree with you, But i am wondering why my sample servlet(i
          am
          > > > attaching the file) is not sharing the instance variables across
          > > > threads/reqs. (in 1st request set some string to "testStr" instance
          > > > variable, in next request if you read the same instance variable, you
          > will
          > > > get null!!)
          > > > Our current code is having instance variables. But right now we are
          not
          > > > getting any problems. But as per theory, instance variables should be
          > > shared
          > > > across threads/requests..
          > > >
          > > > Any how we are changing our code to remove all instance variables.
          > > >
          > > > thanks,
          > > > -ramu
          > > >
          > > > >
          > > > > 1.) Using instance or class variables in servlets that are not
          > read-only
          > > > is not
          > > > > a good thing to do. This makes the servlet not thread-safe. To
          > > maintain
          > > > > variables across requests, sessions, etc., use the servlet-defined
          > > objects
          > > > to
          > > > > store the state (e.g., request, session, etc.).
          > > > >
          > > > > 2.) If you modify the servlet class on disk, WebLogic will reload
          the
          > > > servlet
          > > > > class thus discarding the old instance and creating a new one (of
          > > course,
          > > > this
          > > > > depends on some configuration parameters for servlet reloading).
          > > > >
          > > > > Hope this helps,
          > > > > Robert
          > > > >
          > > > > Ramu wrote:
          > > > >
          > > > > > Hi,
          > > > > > thanks for quick reply.
          > > > > > I am not at all using SingleThreadModel. See the following code.
          > > > > > In first request i am passing "abc" value to testStr. But in
          second
          > > > request,
          > > > > > I am getting null for testStr in second request. It looks like
          (for
          > > me)
          > > > for
          > > > > > non single threaded model also, scope of instance variables is
          > > limited
          > > > to
          > > > > > Thread/Request.. But as per theory, because its not single
          threaded
          > > > model,
          > > > > > only one instance should be there AND testStr(instace variables)
          > > should
          > > > be
          > > > > > shared across the all threads. But i am not able see that
          behaviour
          > > > here.
          > > > > > But if declare instance variable as static, o am able to share
          it
          > > > across
          > > > > > all threads/requests.
          > > > > >
          > > > > > note:
          > > > > > From browser i am setting testStr to "abc" with URL like
          > > > > > "localhost/test1?testStr=abc"
          > > > > > And 2nd req from browser is like "localhost/test1" (on server
          > output
          > > i
          > > > am
          > > > > > getting null for testStr)
          > > > > >
          > > > > > Any ideas?
          > > > > >
          > > > > > thanks,
          > > > > > -ravi
          > > > > >
          > > > > > import java.io.*;
          > > > > > import javax.servlet.*;
          > > > > > import javax.servlet.http.*;
          > > > > >
          > > > > > public class test1 extends HttpServlet
          > > > > > {
          > > > > > public String testStr;
          > > > > >
          > > > > > public void doGet (HttpServletRequest req, HttpServletResponse
          res)
          > > > > > throws ServletException, IOException
          > > > > > {
          > > > > > doPost(req, res);
          > > > > > }
          > > > > >
          > > > > > public void doPost (HttpServletRequest req,
          HttpServletResponse
          > > res)
          > > > > > throws ServletException, IOException
          > > > > > {
          > > > > > try {
          > > > > > res.setContentType("text/html");
          > > > > > PrintWriter out = res.getWriter();
          > > > > > if(req.getParameter("testStr") != null)
          > > > > > {
          > > > > > testStr = req.getParameter("testStr");
          > > > > > System.out.println("set testStr = " + testStr);
          > > > > > }
          > > > > > else{
          > > > > > System.out.println("get testStr = " + testStr);
          > > > > > }
          > > > > >
          > > > > > }
          > > > > > catch(Exception e){
          > > > > > e.printStackTrace();
          > > > > > }
          > > > > >
          > > > > > }
          > > > > > }
          > > > > >
          > > > > > "Cameron Purdy" <[email protected]> wrote in message
          > > > > > news:[email protected]...
          > > > > > > Yes or no, depending on if your servlet implements
          > > SingleThreadModel.
          > > > > > >
          > > > > > > See the servlet 2.2 spec for documentation on how many instances
          > of
          > > a
          > > > > > > servlet will be created. See the doc for the SingleThreadModel
          > > > interface.
          > > > > > >
          > > > > > > Peace,
          > > > > > >
          > > > > > > --
          > > > > > > Cameron Purdy
          > > > > > > Tangosol, Inc.
          > > > > > > http://www.tangosol.com
          > > > > > > +1.617.623.5782
          > > > > > > WebLogic Consulting Available
          > > > > > >
          > > > > > >
          > > > > > > "Ramu" <[email protected]> wrote in message
          > > > > > > news:[email protected]...
          > > > > > > > Hi all,
          > > > > > > >
          > > > > > > > Sorry for asking a dumb question..
          > > > > > > >
          > > > > > > > What is the scope of instance variables in a servlet? Is the
          > scope
          > > > is
          > > > > > > > limited to thread?
          > > > > > > >
          > > > > > > > In other words, in the following example, i am setting
          > "testStr",
          > > in
          > > > one
          > > > > > > > request and when i tried to access the same instance variable
          > from
          > > > > > another
          > > > > > > > request, i am getting it as null. Does it mean instance
          variable
          > > is
          > > > > > > limited
          > > > > > > > to that particular thread/request?
          > > > > > > >
          > > > > > > > thanks in advance..
          > > > > > > >
          > > > > > > > -ramu
          > > > > > > >
          > > > > > > >
          > > > > > > >
          > > > > > >
          > > > > > >
          > > > >
          > > >
          > > >
          > > >
          > >
          > >
          >
          >

Not able to access parent instance variable in outside of methods in child

Hi,
I am not getting why i am not able to access parent class instance variable outside the child class intance methods.
class Parent
     int a;
class Child extends Parent
     a = 1; // Here i am getting a compilation error that Syntax error on token "a", VariableDeclaratorId expected after this token
     void someMethod()
          a = 1; // Here i am not getting any compilation error while accessing parent class variable
}Can any one please let me know exact reason for this and what is the error talks about?
Thanks,
Uday
Edited by: Udaya Shankara Gandhi on Jun 13, 2012 3:30 AM

You can only put assignments or expressions inside methods, constructors or class initializors, or when declaring a variable.
It has nothing to the with Child extending Parent.
class Parent {
    int a = 1;
    { a = 1; }
    public Parent() {
        a = 1;
   public void method() {
       a = 1;
}

I want to access that BEA instance variable inside of a external java class

Hi,
This is N.pradeep. I am a java programmer, and I am new to BEA AquaLogic BPM studio5.7. I have a question regarding how to retrieve an instance variable of BEA Aqua Logic to an external Java Program. i.e. I want to access that BEA instance variable inside of a external java class, and vice versa.

You can use PAPI or PAPI-WS for accessing BPM instance variable from a Java code, thougt I am not sure how the reverse will be done. Assuming that you are aware aof PAPI-WS and have created the necessary stubs out of the PAPI-WS WSDL using Axis, you can access instance variables using the API's or operations defined therein.
Thanks and Regards
Vivek Nandey
BEA Certified Developer for Integration Solutions
[email protected]

Accessing instance variables within frames.

I have 2 jsf within frames that is having one managed bean.
i would like to access the instance variable defined in one jsf from another jsf. Since the managed bean is getting instantiated for the second jsf, the values in first jsf is getting reintialized.
Is it possible to access like this? or do we need to have a separate backing bean and call the first from second.

<script language="JavaScript">
var scriptVar = "<%= jspVar %>";
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Thread cannot access the class variables.

Hi
I have below code snippet : (Only section of which I have copied below)
public class ProcessAppendAction extends HttpServlet implements Runnable{
     public ProcessAppendAction ()
MI_OS_APPEND port ;
protected void doGet(
          HttpServletRequest request,
          HttpServletResponse response)
          throws ServletException, IOException {
          //TODO : Implement
port =
                    (MI_OS_APPEND) obj.getLogicalPort(
                         "MI_OS_APPENDPort",
                         MI_OS_APPEND.class);
Thread[] threads = new Thread[noOfThreads];
ProcessAppendAction run = new ProcessAppendAction(req);
                                        Thread th = new Thread(run);
                                        threads[no] = th;
                              threads[no].start();
                              threads[no].join();
public ProcessAppendAction(DT_REQUEST_APPEND req) {
          this.requestObj = req;
          // TODO Auto-generated constructor stub
public void run()
          try
               DT_RESPONSE res = this.port.MI_OS_APPEND(requestObj);
                              catch(Exception e)
                                   int ch=0;
                                   ch++;
          }     In above code I am successfully creating an object in line :
port =
                    (MI_OS_APPEND) obj.getLogicalPort(
                         "MI_OS_APPENDPort",
                         MI_OS_APPEND.class);But when I am trying to access this port variable in run method it is giving null.
Port in my code is a class variable.
Is it the case that thread cannot access class variable !!

money321 wrote:
ok, I have removed join() from just after start().So that now the Servlet can return before the new Thread has finished. Is this what you want?
First I did invoked all threads and then in second loop i invoked Join on all threads.I don't understand. Why do you need to join() all the threads since you only start one thread in this request. What am I missing?
>
Now, my problem.
It was solved when I substituted the line
ProcessAppendAction run = new ProcessAppendAction(req)
with
ProcessAppendAction run = new ProcessAppendAction(req,port);Of course. Instance variables in Servlet instances are a no-no so passing the 'port' though an instance variable is just wrong.
>
That is passes port alongwith req while creating thread object.
But still if class variables are not thread safe, then it means if I pass object 1 to thread 1.
And then try to pass object 2 to thread 2, it might be possibility that object 1 of thread 1 might be replaced by object 2.
So, thread 1 will get an updated value object 2.Yep - that is why you should not use instance variables in Servlets and why they are not thread safe.
>
Which is not what I intend to do...:-)

Doubt about scope of instance variable

Hi All,
i have a doubt about scope of instance variable.
if i declare a instance variable in servlet , i want to know whether it can be shared(means everywhere ) or not.
thanks in advance
Gopal

The servlet container will create one servlet object, and run multiple threads through its service() / doGet() / doPost() methods.
That means that any instance variables of the servlet are shared between multiple requests, and can cause problems with threads accessing the same variable..
To make your servlets thread-safe
1 - have no instance variables - only use local variables in the doGet/doPost method.
2 - use the session scope for storing variables you need over multiple calls.
Cheers,
evnafets

Inheritance - instance variable initialization

Hi all.
I have a question regarding inherited instance variables.
class Animal {
   int weight;
   int age = 10;
class Dog extends Animal {
   String name;
class Test {
   public static void main(String[] args) {
      new Dog();
}This is just an arbitrary code example.
When new Dog() is invoked, I understand that it's instance variable "name" is give the default
type value of null before Dog's default constructor's call to super() is invoked.But Im a little perplexed about
Animals instance variables. I assume that like Dog, Animals instance variables are given their default type values
of 0 & 0 respectively, before Animals invocation of super(); What im unclear about is that at the point that weight and age are given their default type values, are Dog's inherited weight and age variables set to 0 aswell at that exact moment? Or are Dog's inherited weight and age variables only assigned their values after Animals constructor fully completes because initialization values are only assigned to a classes instance variables after it's call to super().
Many thanks & best regards.

Boeing-737 wrote:
calvino_ind wrote:
Boeing-737 wrote:
newark wrote:
why not throw in some print statements and find out?Super() or this() have to be the very first statement in a constructor..
Also you cannot access any instance variables until after super or this.
:-S
Kind regardsbut if you add the "print" statement in animal constructor, you can easily see what happened to the attributes of Dog ; that s the trick to print "before" super()You can never add a print before super(). It's a rule of thumb, super() and this() must always be the first statements
in a constructor, whether added implicitly by the compiler or not. In this case there are 2 default constructors (inserted by the compiler), each with a super() invocation. Even if i added them myself and tried to print before super(), the compiler would complain.
Thanks for the help & regards.you didn't understand what i meant ; take a look at that:
class Animal {
   int weight;
   int age = 10;
   public Animal() {
       Dog thisAsADog = (Dog) this;
      System.out.println(thisAsADog.name);
      System.out.println(thisAsADog.x);
class Dog extends Animal {
   String name;
   int x = 10;
public class Test {
   public static void main(String[] args) {
      new Dog();
}this way you will know what really does happen

When can I make instance variables public?

I noticed that some of the in-built Java classes have public instance variables, for example in Rectangle the variables x, y, width and height are all directly accessible from outside. When is it okay to make public instance variables? Is it when they are primitive types? I thought maybe it is when they are final variables - but that can't be the case because you can directly modify the Rectangle's public instance variables.
I've always used getter and setter methods in my objects because I was told "making instance variables public is bad". But obviously this isn't necessarily the case (unless the guys at Java are bad programmers). I think directly accessing the instance variables makes for much neater and more readable code. Is there any convention or best-practise guidelines on when I can or can not make them public?
P.S. I also just "don't get" why you can't make them public if you're just going to provide a getter and setter method anyway... what's the difference? I suppose the theory is that the setter method should do some checking to make sure the input is valid. But what if the setter method just accepts -anything- of the appropriate type (accepts any Rectangle, for example)? What's the use of hiding the instance variables?
teach me.
Thanks. ;-)

For the most part, it is true that making instance member variables other than private is not a good idea. This concept, known in OO circles as "information hiding", typically allows the greatest flexibility in internal implementation of a class, since you can change the internal structure of the class without affecting the clients that use the class.
Let me add that Sun's programmers are a team, and teams have members of differing capabilities.
It is also true that there may be cases where exposing instance variables is unlikely to cause problems. Simple classes, or very stable classes that will not change are some examples, but not all.
Following a guideline blindly is not a good idea either. You need to evaluate each instance in which you decide you might want to deviate from "good programming practices". Still, best practices are best practices for a reason. It is best to understand them, and then decide if you can violate them.
For my own part, I don't think it is "neater" to expose such variables, and I have personally had to deal with situations where a change in a class's internal structure was extremely painful because the original developer chose to make all his instance variables public.
{?

Servlet instance variables

This might be a dumb question, but I'll ask it anyway...
I have methods in my servlet to handle registration, specifically creating address, customer and order objects.
All the information needed for creating my objects is coming from the registration form.
Right now if I were to call the createAddress() method, I have to pass the request object.... createAddress(request);
Would it be "thread safe" to make an instance variable of the request and refer to it using getters/setters? Or would it be wiser (and best practice) to make instance variables for the request attributes (like firstName, lastName, etc.)? Or am I way off and shouldn't be doing any of the above?
Thanks for the help.

GB_java_guy wrote:
To answer you question, I think it's different, from what I've read, because request attributes are supposed to be thread safe and I'd be using the request attributes instead of the request object.You said something about "making instance variables for the request attributes." I don't know what you mean by that, but if it's instance variable in the servlet class, don't do it.
I have no clue what you mean by "request attirbutes intead of request object."
So are you saying I should just use the request object and pass that to my methods instead? In general, yes. In some cases you may want to pass individual pieces of it--e.g. if there's a method that doesn't need to know or care about the fact that you're operating in a servlet context, and just needs a couple values to do its job.

Access a instance level variable from activity in a subflow

I have a flow such that it calls a number of subflows chained together.
I have defined a instance variable to store a flag when a perticuler activity has been executed.
in one of the subflows, there is an activity which i need it to update the parent level instance variable when it has done its task.
but, i'm not sure how to get access to the parent level instance variables from an activity with in a subflow.
I have PAPI code polling on the instance wating for this variable to change.
any ideas?

in a clip on the main timeline:
var mc = this.parent;
in a clip on the main timeline that was added with a loader:
var mc = this.parent.parent;
the first parent would be the loader
the second parent is the loader's parent, or, the main timeline

Accessing Instance variables from within javascript

Is it possible to access Instance variables of a JSP using
javascript? If so, an example would be very much appreciated.
LaMont

<script language="JavaScript">
var scriptVar = "<%= jspVar %>";
</script>

Servlet Instance Variable scope

Just want to clarify my understanding of instance variables in regards to Servlets.
public class TestServlet extends HttpServlet
    public String instVariable = "";
public void processRequest(HttpServletRequest request, HttpServletResponse response)
      instVariable = request.getParameter("param");
      response.sendRedirect( instVariable ) ;
}If two users both enter the processRequest logic at the same time, is it possible that they will both get the same
result? My understanding of this was that if they each execute the first line, then each execute the second
line, each user would be redirected to the same instVariable that was set by the second request.
What would be the easiest way to stop this from happening? syncronized? ThreadSafe?
I just want a simple clarification.
Thanks for the insight.

If two users both enter the processRequest logic at
the same time, is it possible that they will both get
the same
result? My understanding of this was that if they
each execute the first line, then each execute the
second
line, each user would be redirected to the same
instVariable that was set by the second request. Yes, it is possible, since there is only one instVariable.
What would be the easiest way to stop this from
happening? syncronized? ThreadSafe? The easiest way to avoid that is to simply not use an instance variable. Use a local variable, declared within your processRequest method. Each thread (i.e. each user of the servlet) has its own set of local variables.

Servlets and their instance variables

I understand that for every request to a servlet, a new thread handles those requests. So it's one request, one thread. But how about instance variables of a servlet class? Are they also one instance variable per thread/request or just like the servlet, one instance?

hi, its exactly as you expect - one instance at all. all threads are working with one intance. you can indeed finetune the number of instances of a servlet using the
<load-on-startup/>tag in the web.xml file, but its up to container if multiple thread will be using many instances.
you can also implement the (deprecated) SingleThreadedModel interface which flags the container not to share that servlet instance to other threads. But this shall not be used in productive environments.
so its always a good idea to put your business impleementation to another class and only use the servlets to connect your business implementation to http like
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException{
new MyBusinessLogic().perform(request.getInputStream(), response.getOutputStream());
}

Access of Instance variable in servlet

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