Singleton? Don't want to keep reading database

Question

Hi there,
I have a java web application, when it starts up I want it to load some information into a singleton so that all requests for this information come from memory not via a database call. The information will change very infrequently but I don't want to hard-code it (I guess I could use a property file). Anyway if I use a singleton will it not be garbage collected as soon as no other objects reference it? If it's garbage collected then the next time some accesses it another database call with be made and I don't want that to happen. I want this object to reside in memory always (it's not large so the hit is not that great). The singleton will basically hold a Vector of "objects" and one of this "objects" attributes is another Vector. To populate it requires 3 trips to the database each time. I would prefer 3 trips up front upon application loading and then no more until a restart occurs. Any suggestions?
Cheers, Max

Answer 1

The Singleton pattern is a fairly straightforward pattern. I have used it succesfully several times. Here is an example.
public class GetInfo {
  private static GetInfo singleton;
  private static Object dataSavedInMemory;
  //Prevent anyone from instantiating this object
  private GetInfo() {
  private static synchronized void initialize() {
    singleton = new GetInfo();
    //One time database processing to save data in memory would go here
    dataSavedInMemory = new String("Hello World");  //just for illustration purposes
  public static GetInfo instance() {
    if (singleton == null)
      initialize();
    return singleton;
  public Object getData() {
    return dataSavedInMemory;
  //Example usage
  public static void main(String[] args) {
    //Get data saved in memory
    Object data = GetInfo.instance().getData();
    System.out.println(data);
    //or
    GetInfo inst = GetInfo.instance();
    Object data2 = inst.getData();
    System.out.println(data2);
}