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.

Similar Messages

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.

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
          > > > > > > >
          > > > > > > >
          > > > > > > >
          > > > > > >
          > > > > > >
          > > > >
          > > >
          > > >
          > > >
          > >
          > >
          >
          >

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

Stateless Bean - scope of instance variable in EJB Timer call back function

Hi,
I would like to know on the scope of an instance variable of a Stateless Bean object,
when used in a EJB Timer call back.Let me explain this in more detail below.
I have a requirement to use a EJB Timer.
For this, I have created a stateless object since Timer creation needs to be done
from a stateless bean. I have a member variable "count" of the stateless bean class.
In the timer call back(ejbTimeout), I am able to use this count variable during
each time of the call back, and the value of this variable is also updated properly.
I have a few queries with respect to the above behaviour:
1) Does stateless bean object not get destroyed once the Timer is created from the Bean?
2) If the Bean object is not destroyed, then when does the bean object get destroyed?
3) If both (1) and (2) are not true, then can anyone explain on how the above behaviour is possible?
Thanks in advance,
Ulrich

Hi Ulrich,
The ejb timer is associated with the stateless session bean component, not with a particular bean instance. There is no formal relationship between the bean instance that called createTimer() and the bean instance on which the timer callback happens. If they're the same in your test run that's just a coincidence and not something your application should be depending on.
In the stateless session bean model, the container can create and destroy stateless session bean instances at any time. The container is free to pick any stateless session bean instance to service any client invocation or timer callback. If you need to pass context into a timer callback, one way to do it is via the timer "info" object. However, the info object is immutable so it wouldn't be a good match for a counter. You could of course always just use a database for any necessary coordinated state.
--ken

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.

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());
}

Scope of instance variables clarification requested

under the topic "Scope of instance variables" in the class notes for the course i'm doing it is stated:
inside the class definition, the name can be prefixed by the keyword "this".
e.g. available = this.balance + overdraft;
in simple words, please explain what it means
or:
what is a class definition? and what does using "this" mean?
i'll gladly take internet links on where i can find the answer to my own question if you don't have time.
thanks a million!

this can be regarded simply as a reference variable which points to the intance of the class it's used in.
So this.fieldOne referer to the instance field fieldOne withing the current instance just as theOther.fieldOne refers to the instance field in another object.

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.

Servlet Instances

How can we differentiate between Servlet instances and threads.
Actuaaly i want to know how a servlet handles multiple client requests at the same time.
what i know is like:::
a client sends a request to the server.
A new thread is generated which will call the instance of the serlvlet on the server for processing..
Is this flow is correct???
am bit confused between threads and instances.
Please help!!!!

If you make that your servlets implement the
SingleThreadModel interface (definition below), then
you can configure Tomcat with the size of the pool. I
think that even you can have only one instance, but
this is not very advisable, unless you want your users
to wait...Thnx for reply, Do you know how I can tell Tomcat to start multiple instances or alway stick with single instance. I cannot find this.
"public interface SingleThreadModel
Ensures that servlets handle only one request at a
time. This interface has no methods. Yes I am aware of this.
If a servlet implements this interface, you are
guaranteed that no two threads will execute
concurrently in the servlet's service method. The
servlet container can make this guarantee by
synchronizing access to a single instance of the
servlet, or by maintaining a pool of servlet instances
and dispatching each new request to a free servlet.
This interface does not prevent synchronization
problems that result from servlets accessing shared
resources such as static class variables or classes
outside the scope of the servlet."If only one user can access the servlet using this interface then why would you want syncrhonization??? There shouldn't be synchornization issues with a servlet that implements SingleThreadModel right??

How can I use evaluate to get the instance variable in customized tag

1.
At first , I create a class called bean,and declared several params in it and do not define any getter function for the param.
class bean{
String param = "test";
SomeClass scObj = new SomeClass();
2.
The second ,I use
request.setAttribute("beanObj",new bean());
3.
And then I wanna use the customized tag to show a text box , then initialize it's value.
<salt:text name="param" value="beanObj.param">
<salt:text name="obj" value="beanObj.scObj.func()">
4.
I tried the evaluator provided by JexlContext ,Struts, JSTL and it seems that if I do not define the getter for the variable ,I can not get the bean's instance variable's value.
Expression e = ExpressionFactory.createExpression( value );
JexlContext jc = JexlHelper.createContext();
jc.getVars().put(strInitBeanName, request.getAttribute("beanObj"));
Object obj = e.evaluate(jc);
the result of the obj is null....
Can anybody recommand some other evaluator can get the value of a instance variable from an object?

do you have any other suggestion ? Nops, somebody else may have though. AFAIK, all lookups of the type
beanName.propertyNameuse reflection on the getXXX() methods to access the property.
Having said that, I guess you could write one though in a custom tag, using the same - reflection (you will ahve to rely on the java.lang.reflect.Field class quite heavily) - but that would be reinventing the wheel for most other functionality that you would have to include (like looking up the bean in scope etc)
cheers,
ram.

Binding: Instance variable loses value.

Hi all,
Just making my first steps into Objective-C, I've done a lot of C, C++ and C# on win & linux. Anyhow, I've been hacking away happily and discovered a peculiar behaviour, and I'm not sure if it's my code or some obscure bug in Xcode.
1. I have a Check Box(NSButton) directly bound to a BOOL instance variable called "checkValue" using KVC. The containing class is a custom NSView subclass.
2. I manually implemented the setter according to KVC naming rules "setCheckValue".
3. When debugging the UI (check box) calls the setter perfectly with the correct value, which shows up in the NSLog output.
4. I hooked up the action for the check box as well, and inside the action handler the instance variable reports the value correctly. Everything looks fine.
5. Now here's the rub. I put a mousedown event handler into the class as well, and it is called flawlessly when I click on the custom view. However, "checkValue" does not report the correctly set value.
So, how can a class instance variable, which is set and reports correctly elsewhere in the same scope, all of the sudden take on a nonsense value in a event handler? If a variable has a value set, it should be the same everywhere within the same scope!
Below is simplified code extracted from the original project for clarity. It produces exactly the same behaviour as the more complex project without the distracting code.
#import "CheckBoxHandler.h"
@implementation CheckBoxHandler
@synthesize checkValue;
//Apparently this is required for binding in NSView based classes
//The same behaviour occurs even if you remove this.
+(void)initialize
[self exposeBinding:@"checkValue"];
-(id)initWithFrame:(NSRect)frameRect
self = [super initWithFrame:frameRect];
if(!self)
return nil;
[self setCheckValue:YES];
return self;
//manual impelementation of KVC setter
-(void)setCheckValue:(BOOL)v
checkValue = v;
NSLog(@"Setter Called: checkValue is %d", checkValue);
//The action works fine as evidenced by Log output
//Check and uncheck the box a few times.
-(IBAction)checkBoxAction:(id)sender
NSLog(@"UI State:%d Ivar State: %d", [checkBox state], checkValue);
//Mouse clicking on the view calls into this event.
//The problem is the value reported by [self checkValue] OR checkValue directly
//do not match the UI state. In fact it always reports some nonsense value.
-(void)mouseDown:(NSEvent *)event
NSLog(@"Inside Mousedown: checkValue is: %d", [self checkValue]);
@end
I have a workaround, but it really bends my head when something doesn't behave as expected!

I have synthesized against "checkValue", although I manually implemented the setter so I could set a breakpoint and observe the value. I thought [self checkValue] and self.checkValue both simply call the same getter, just different syntax.
The setting of the value using the UI checkbox happens outside of the event call chain. I can check and uncheck the checkbox and the value changes correctly. The checkbox is in a separate space on the window, so clicking it does not activate mousedown on the view, this is correct behaviour. Clicking on the view does activate the mousedown event, this is done after I've set the checkbox so the value has been set well before the mousedown event. Thus it should already have the correct value in the event.
For completeness this is the interface file:
#import <Cocoa/Cocoa.h>
@interface CheckBoxHandler : NSView {
IBOutlet NSButton *checkBox;
BOOL checkValue;
-(IBAction)checkBoxAction:(id)sender;
@property (assign, readwrite) BOOL checkValue;
@end
I'll change everything to self.checkValue just for the excercise. Ok, done. checkValue still reports a value of 1 in the mousedown event regardless of the value set elsewhere.

Problem in recognising instance variables declared in the class

Hi all,
I am trying to deploy an Servlet desiged in WebObjects5.1 application server on application on Iplanet Application server 6.5
Back ground: All of you might be aware that WebObjects5.1 is an J2ee compliant application server. When an application is designed in WebObjects, it creates a WAR file which can be deployed on any J2ee complaint application server.
I tried to create such application in WebObjects and tried to deploy on IplanetApplication server6.5(running on Windows using IIS web server).
I am able to successfully deploy the application. But inorder to do this i had to manually add ias-Web.xml file to the war file generated( gave some arbitrary global id). The aplication just consists some string displaying hello world.
Now when i try to deploy an application which has some instance variables, the instance variables are not getting recognised. This application works fine on WebObjects5.1. But variables are not getting recognised when deployed iplanet application server
Has any one faced such kind of problem earlier? If yes please help me out
Thanks
Venkatesh

Hi,
Please send me your servlet code and war file.
Thanks

BufferedReader bf; is this also treated as instance variable?

hi i m just now finished my assignment and doing documentation
and i got a single line here
BufferedReader bf; //instance variable
constructor{
bf = new BufferedReader(new InputStreamReader(System.in));
}and now i m doing documentation suddenly i got confused whether i should
write this as instance variable or not. should i?? or shouldn't i
i think this is obejct,,,,,,,but..i think it is instance variable as well..coz
it is located in the class scope..confusing.....
or not..

hi i m just now finished my assignment and doing
documentation
and i got a single line here
BufferedReader bf; //instance variable
constructor{
bf = new BufferedReader(new
w InputStreamReader(System.in));
}and now i m doing documentation suddenly i got
confused whether i should
write this as instance variable or not. should i?? or
shouldn't i
i think this is obejct,,,,,,,but..i think it is
instance variable as well..coz
it is located in the class scope..confusing.....
or not..Okay, first of all constructor {} isn't valid Java syntax, I'm assuming you just used it for brevity. Second, yes bf is an instance variable because it's not a) static (class-) variable or b) local variable.
If you thought instance variables refer only to primitives (int, byte etc.) you're wrong. Object references can also be instance variables.
So yes, it is both an object reference and an instance variable.
Sincerely,
Jussi

Javascript discussion: Variable scopes and functions

Hi,
I'm wondering how people proceed regarding variable scope, and calling
functions and things. For instance, it's finally sunk in that almost
always a variable should be declared with var to keep it local.
But along similar lines, how should one deal with functions? That is,
should every function be completely self contained -- i.e. must any
variables outside the scope of the function be passed to the function,
and the function may not alter any variables but those that are local to
it? Or is that not necessary to be so strict?
Functions also seem to be limited in that they can only return a single
variable. But what if I want to create a function that alters a bunch of
variables?
So I guess that's two questions:
(1) Should all functions be self-contained?
(2) What if the function needs to return a whole bunch of variables?
Thanks,
Ariel

Ariel:
(Incidentally, I couldn't find any way of marking answers correct when I visited the web forums a few days ago, so I gave up.)
It's there...as long as you're logged in at least, and are the poster of the thread.
What I want is to write code that I can easily come back to a few months later
and make changes/add features -- so it's only me that sees the code, but
after long intervals it can almost be as though someone else has written
it! So I was wondering if I would be doing myself a favour by being more
strict about make functions independent etc. Also, I was noticing that
in the sample scripts that accompany InDesign (written by Olav Kvern?)
the functions seem always to require all variables to be passed to it.
Where it is not impractical to do so, you should make functions independent and reusable. But there are plenty of cases where it is impractical, or at least very annoying to do so.
The sample scripts for InDesign are written to be in parallel between three different languages, and have a certain lowest-common-denominator effect. They also make use of some practices I would consider Not Very Good. I would not recommend them as an example for how to learn to write a large Javascript project.
I'm not 100% sure what you mean by persistent session. Most of my
scripts are run once and then quit. However, some do create a modeless
dialog (ie where you can interface with the UI while running it), which
is the only time I need to use #targetengine.
Any script that specifies a #targetengine other than "main" is in a persistent session. It means that variables (and functions) will persist from script invokation to invokation. If you have two scripts that run in #targetengine session, for instance, because of their user interfaces, they can have conficting global variables. (Some people will suggest you should give each script its own #targetengine. I am not convinced this is a good idea, but my reasons against it are mostly speculation about performance and memory issues, which are things I will later tell you to not worry about.)
But I think you've answered one of my questions when you say that the
thing to avoid is the "v1" scope. Although I don't really see what the
problem is in the context of InDesign scripting (unless someone else is
going to using your script as function in one of theirs). Probably in
Web design it's more of an issue, because a web page could be running
several scripts at the same time?
It's more of an issue in web browsers, certainly (which I have ~no experience writing complex Javascript for, by the way), but it matters in ID, too. See above. It also complicates code reuse across projects.
Regarding functions altering variables: for example, I have a catalog
script. myMasterPage is a variable that keeps track of which masterpage
is being used. A function addPage() will add a page, but will need to
update myMasterPage because many other functions in the script refer to
that. However, addPage() also needs to update the total page count
variable, the database-line-number-index-variable and several others,
which are all used in most other functions. It seems laborious and
unnecessary to pass them all to each function, then have the function
alter them and return an array that would then need to be deciphered and
applied back to the main variables. So in such a case I let the function
alter these "global" (though not v1) variables. You're saying that's okay.
Yes, that is OK. It's not a good idea to call that scope "global," though, since you'll promote confusion. You could call it...outer function scope, maybe? Not sure; that assumes addPage() is nested within some other function whose scope is containing myMasterPage.
It is definitely true that you should not individually pass them to the function and return them as an array and reassign them to the outer function's variables.
I think it is OK for addPage() to change them, yes.
Another approach would be something like:
(function() {
var MPstate = {
    totalPages: 0,
    dbline: -1
function addPage(state) {
    state.totalPages++;
    state.dbline=0;
    return state;
MPstate = addPage(MPstate);
I don't think this is a particularly good approach, though. It is clunky and also doesn't permit an easy way for addPage() to return success or failure.
Of course it could instead do something like:
    return { success: true, state: state };
var returnVal = addPage(MPstate);
if (returnVal.success) { MPstate = returnVal.state; }
but that's not very comforting either. Letting addPage() access it's parent functions variables works much much better, as you surmised.
However, the down-side is that intuitively I feel this makes the script
more "messy" -- less legible and professional. (On the other hand, I
recall reading that passing a lot of variables to functions comes with a
performance penalty.)
I think that as long as you sufficiently clearly comment your code it is fine.
Remember this sort of thing is part-and-parcel for a language that has classes and method functions inside those classes (e.g. Java, Python, ActionScript3, C++, etc.). It's totally reasonable for a class to define a bunch of variables that are scoped to that class and then implement a bunch of methods to modify those class variables. You should not sweat it.
Passing lots of variables to functions does not incur any meaningful performance penalty at the level you should be worrying about. Premature optimization is almost always a bad idea. On the other hand, you should avoid doing so for a different reason -- it is hard to read and confusing to remember when the number of arguments to something is more than three or so. For instance, compare:
addPage("iv", 3, "The rain in spain", 4, loremIpsumText);
addPage({ name: "iv", insertAfter: 3, headingText: "The rain in spain",
numberColumns: 4, bodyText: loremIpsumText});
The latter is more verbose, but immensely more readable. And the order of parameters no longer matters.
You should, in general, use Objects in this way when the number of parameters exceeds about three.
I knew a function could return an array. I'll have to read up on it
returing an object. (I mean, I guess I intuitively knew that too -- I'm
sure I've had functions return textFrames or what-have-you),
Remember that in Javascript, when we say Object we mean something like an associative array or dictionary in other languages. An arbitrary set of name/value pairs. This is confusing because it also means other kinds of objects, like DOM objects (textFrames, etc.), which are technically Javascript Objects too, because everything inherits from Object.prototype. But...that's not what I mean.
So yes, read up on Objects. They are incredibly handy.

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