Question

About project:
I’m trying to write CMS-like web-site on Java. Previously I used PHP for such purposes, but now I need tight integration with existing server-side solution, written on java, so decided to use Java for web-site too. Currently, for easier developing, I made some “hierarchy” of my project: top-level “Server” class (in closest future it’ll be filled with content and functionality, but now it’s empty), next Subsystems, Extensions, and as a part of any of this – Modules and Blocks. Each of them have some restrictions and possibilities, not available to one another… Maybe there is other way, but currently I don’t know it.

I’m making some kind of caching, depending on the fact, that mine Dynamic Web Project’s vars can be kept in the memory all the time. For example, ‘Tree’, that represents all this elements, loaded from database and kept in memory, and all next requests (for example, what Extension has this Module as a child – for managing privileges) made without asking database.

Of course, for this purposes, I need to have some flag, that will tell elements, that their data was changed and they need to refresh it from database. Single global flag (in context, for example) is bad, while in almost all situations I’ll need to refresh single element or group. Single element’s flag is done, but…

About realization:
The problem itself. Lets say I have class Module:

public abstract class Module{
    private final static boolean Changed;
    public static boolean getChanged(){
        return Changed;}
    public static void setChanged(boolean newState){
        Changed = newState;}
    public abstract String getName();
    public abstract String getTitle(HttpServletRequest request,
        HttpServletResponse response, ServletContext context);}

And I have some module:

public final class Articles extends Module{
    public class Article{...}
    private Tree<Article> ArticleList;
    public final ArrayList<Article> getArticlesInGroup(int groupID){
        if(getChanged())
            refreshFromDatabase();
        ...
    }
}

Here, for example, this flag used as in case of changing some global preferences, that may change Articles work – for example, some operations on database, that performed through Query browser or some else – class ‘caching’ a tree of articles and groups (it doesn’t keeping whole article – only Title, ID, ParentGroup or maybe some else…

The problem itself: in servlet Main in doGet(…) I have next code executed:

if(request.getParameter("SetModify") && UsersManager.inGroup(currentUser,"Administrator")
    Module.setChanged(true);

And, I checked, “Module.setChanged(true)” executed successfully… But when it comes to “getChanged()” in class Article, I got false…

Does there exists any way to create static variable, that will be static among all class, that extends it, or I simply doesn’t understand something? Case “Module.getChanged()” in class Articles tested as well – the result is the same…

Answer 1

I’m surprised that your code compiles since you use the ‘final’ keyword here:

private final static boolean Changed;

Generally you shouldn’t be able to modify members declared as final. so the setter shouldn’t be allowed:

public static void setChanged(boolean newState){
     Changed = newState;}

Protip: you might want to look into a more full fledged system like EhCache with Spring to decouple all this ‘optimization’ of data base access from the actual code.