proper usage of synchronized singleton?

Question

So I am thinking about building a hobby project, one off kind of thing, just to brush up on my programming/design.

It\'s basically a multi threaded web spider, upda

Answer 1

Using lazy initialization for the database in a web crawler is probably not worthwhile. Lazy initialization adds complexity and an ongoing speed hit. One case where it is justified is when there is a good chance the data will never be needed. Also, in an interactive application, it can be used to reduce startup time and give the illusion of speed.

For a non-interactive application like a web-crawler, which will surely need its database to exist right away, lazy initialization is a poor fit.

On the other hand, a web-crawler is easily parallelizable, and will benefit greatly from being multi-threaded. Using it as an exercise to master the java.util.concurrent library would be extremely worthwhile. Specifically, look at ConcurrentHashMap and ConcurrentSkipListMap, which will allow multiple threads to read and update a shared map.

When you get rid of lazy initialization, the simplest Singleton pattern is something like this:

class Singleton {

  static final Singleton INSTANCE = new Singleton();

  private Singleton() { }

  ...

}

The keyword final is the key here. Even if you provide a static "getter" for the singleton rather than allowing direct field access, making the singleton final helps to ensure correctness and allows more aggressive optimization by the JIT compiler.