Loading an object using a different classloader

Question

I have 2 web-apps which share a common object. I want to store that
common object in a common cache accessible to both web-apps. The problem
is the 2 web-apps have a different classloader, so how do I return an
object that was stored by web-app1 to be used with web-app2 without getting a ClassCastException
-

Answer 1

First question is whether the Web-Apps are on the same JVM - if you're using Tomcat, it is likely that they are. The next question is whether the Web-Apps' classloaders descend from a common parent classloader - again in Tomcat this is the case. If they do, then you should use the common parent classloader to instantiate the classes that need to be shared.
To do this, you need to add the common classes to either the system classpath or to the classpath configured for your servlet/jsp engine. If the Web-Apps' classloaders are standard they will defer any requests initially to their parent classloader which will provide them with the same class definition and make it so that these classes can be shared between the Web-Apps.
Now comes the nitty-gritty of implementation. The best way to share a common object is via a static instance in one of the classes loaded by the parent classloader, using the singleton pattern or the like.
A trivial example would be if you wanted to hold a map of common (thread-safe) objects:public class Shared{
    private static java.util.HashMap sharedMap;
    static{
        // Thread safe, assuming all shared objects are initialised
        // in this static block.
        // If objects are to be created and placed in the
        // sharedMap later, use the synchronized map wrapping
        // methods from the Collections utility class.
        sharedMap = new java.util.HashMap();
        ... //instantiate and store the shared objecs here.
    public static Object getCommonObject(String key){
        return sharedMap.get(key);
}As long as the Shared class comes from an ancestral classloader, both Web-Apps will see the same map of the same objects.
Hope this helps,
Bob B.