Question

I know this is a fundamental question on Java’s static variables, but I would like an official answer, maybe a pointer to the right documentation.

I am using a static variable to cache the results from the database. Question is, will these results be shared among all users of the web application?

Here’s my code:

private static TreeMap<String,String> cachedOwners = null;
public static TreeMap<String,String> fetchOwners( ) 
        throws Exception
{
    if ( Owners.cachedOwners != null )
    {
        return Owners.cachedOwners;
    }

    DBHandler db = DBHandler.getInstance( );
    Owners.cachedOwners = db.fetchCAROwners( );
    return Owners.cachedOwners;
}

Now, based on my tests, caching works and the static variable “cachedOwners” is shared among all users of the application.

How come this happens? I thought a static variable is only tied to a user’s session.

Answer 1

Yes!

There is not only one official documentation because there are two sides together

1. classloader inheritation
2. java-webserver classloading order

Static means the same address in the classloader-inheritation, so, if you ever have two webapplications using the same inherted “classloader”.

i.e. if you have an singeton.jar with an

static int org.me.Test.born=System.currentTimeMillis();

an the singleton.jar is located in

• tomcat/lib,
• tomcat/webapps/webapp1/WEB-INF/libs/
• and tomcat/webapps/webapp2/WEB-INF/libs/

only the “tomcat/lib” will be define the born-field because the /lib-classloader will be load first.