How can I best initialize constants from a database or servlet context?

Question

We have constants declared in an interface in our application like this.

public interface IConstants
{
    public static final String FEVER="6";
    public static final String HEADACHE="8";
}

We now want to populate these constants values (6 and 8) from the database (or application servlet context).

The database values stored in a look up table are already available in the application session (in servlet context attribute) and hence I don't have to make a database call every time.

How do we accomplish this?

Answer 1

As constants, you can't readily.

Create a class with those members, protect the setters (or set them from the constructor) and provide getter methods.

More detail:

Okay, to start with, you can't readily initialize constants from a database because they must be initialized at the time they're defined.

Instead, create a class something like this:

public class Constants {
    private static final String FEVER ;
    private static final String HEADACHE ;

    public Constants(String fever, String headache){
        if(FEVER == null){
            FEVER = fever;
            HEADACHE = headache;
         } else {
            // Do something; shouldn't be resetting these.
         }
    }

    public String getFever(){ return FEVER; }
    public String getHeadache(){ return HEADACHE; }
}

Now, when you construct the class, you set FEVER and HEADACHE. Since they're static, there's only one value; since you have only getters and they're private, no one can reset them; and since they're final, no one can sneak behind your back with inheritance.

Answer 2

Since the constant values are persisted in a database, the best approach would be to create an enum type and make them available through it.

Answer 3

To make them vary based on a data source, you'd need some kind of data structure for them.

You could define a bean to hold the values (fields with getters/setters), or use a map for the values.

You should probably think of this as more like a configuration problem than a constants problem. Constants are really intended from a programming point of view.

Answer 4

Are you sure you need to?!

If you need to add new code when adding a new disease anyways, there is little point in making the "constants" to be data driven static (non-final) globals. (It will just complicate things.)

• If you are worried about a mismatch between the database and the code, you can verify that database and the constants match when starting up the application. You have the diseases defined in a table and are using some kind of referential integrity right?

• If you DO think You need a data driven approach, you probably should not need any fields for "known diseases" at all, as the code shouldn't really depend on those. In that case each disease should be a proper object with a identity and other properties.

• If you need special handling for certain types you probably should go back to an enum again...

• If you instead have many diseases (that might be added dynamically) and only a few types - several diseases are handled in the same way by the same code; add a type in the disease-table and in the code as an enum (or constant) for the type-of-disease and use that to run different logic.

• If each disease actually have a lot of complexity, it might be a good idea to try to write one class for each disease. (You can then do a proper O/R thing with sub-classes and all...)

The interface enum cludge is not neccesary since java 5, as you can get both enums and do a static import if you need to.

(And when using enum, validating the database is simpler as you get an enumeration for free.)

Your fields should probably be located in a class called Disease or SickDude instead of a gigantic global constants class. (Like, for instance, the fields in Calendar). Possibly as an inner public enum in the class where they "belong."

Answer 5

Given that you don't want to change the existing code too much, the simplest way would be:

public interface IConstants {
    public static final String FEVER = getConstFromDatabase("FEVER");
    public static final String HEADACHE = getConstFromDatabase("HEADACHE");
}