How do we use global variables in servlets?

[harke baj]

Hi all,

How can I use global variables in servlets? Like if I want some variables that can be accessed by all the servlets within the application. The variables can be read or written.

I looked into the forums and found two methods: using servletcontext and static.

They had their own disadvantages:

servletcontext: have problem with synchronization

static: they belong to a class and not to an application. ( I didn't get their exact disadvantages. Could someone tell me?)


Are there other methods for using global variables?

How about using database? Will I have synchronization problem?

thanks,
harke

[Bear Bibeault]

Any read-write data accessible across threads will need to be synchronized, regardless of how it is stored.

[Seetharaman Venkatasamy]

harke baj wrote:servletcontext: have problem with synchronization

what about *session* ? Ofcourse, Session has concurrency issue if, two browsers(same type) are open in a client machine. but it is a rare scenario

[harke baj]

I want the variables to be accessible throughout the application for all users. So httpsession wont be useful, i think, as it is only for one user.

So i need synchronization even if I use database?


thanks,
harke

[Bear Bibeault]

Seetharaman Venkatasamy wrote:but it is a rare scenario

Actually not rare at all. Multiple threads can be accessing the same session not only from multiple browser instances, but from different frames, iframes, and Ajax requests.

[Ankur Goel Goel]

I think you can use beans. You can use static variable in any bean and that variable will use its value till the server is reset.

[Bear Bibeault]

Beans or not, the issue is the same. The point is where the data is stored and if it needs to be synchronized.

[Seetharaman Venkatasamy]

harke baj wrote:I want the variables to be accessible throughout the application for all users. So httpsession wont be useful, i think, as it is only for one user.

Cool . you need to mention requirement clearly .

this is from your post

harke baj wrote:How can I use global variables in servlets? Like if I want some variables that can be accessed by all the servlets within the application. The variables can be read or written.

[Seetharaman Venkatasamy]

Bear Bibeault wrote: Multiple threads can be accessing the same session not only from multiple browser instances, but from different frames, iframes, and Ajax requests.

Ohh. thanks bear

[parampreet sethi]

You can use application level variables as well. We have three levels of variables depending upon their scope in any web application.

1. Page level - specific to that page or request
2. Session level - specific to a session
3. Application level - specific to the application

To store variables at application level, below mentioned is the sample code:

  ServletContext application = getServletConfig().getServletContext();

String data = "test";
application.setAttribute("variable", data);

String data_rtrvd= (String) application.getAttribute("variable");  

The other way would be to use the below mentioned code:

In the JSP, you should have

  <jsp:useBean id="obj" class="my.package.name.MyClass" scope="application" />  

In the servlet, you would have the following equivalent code:

Object obj = getServletContext().getAttribute("obj");
if(obj==null || (!obj instanceof my.package.name.MyClass)) { // If object is null reinitialize it
   obj = new MyClass();
   getServletContext().setAttribute("obj", obj);
}

[Bear Bibeault]

parampreet sethi wrote:We have three levels of variables depending upon their scope in any web application.

Four. You left request scope off the list.