Singleton pattern

[Sugreev Naik]

I have a question regarding singleton pattern. I created a singleton object in my application and wrote it to a file and read it into another object. Here am I not loosing the singleton beaviour? All singleton objects are vulnerable to this problem right ?

[Pradeep bhatt]

Are you using java serialization for writing it to a file. You can overide the writeObject and readObject methods so that there is only one instance.

[Greg Reinl]

To ensure there remains only one instance of the singleton object after deserialization you will need to override the readResolve() method. For one example, see Simply Singleton

[Nicholas Cheung]

You may add a flag in your singleton object, or create a constructor like this:

ObjectX x = null;
...
public ObjectX getInstance() {
  if (x == null) {
    x = new ObjectX();
  }
  return x;
}
...

Then, for any readObject() and writeObject(), you first create the object, and call its corresponding method to create the object. Thus, you can always has the single object.
Nick

[Stan James]

Just a note: The code shown above for creating a singleton is not safe in multi-threaded multi-processor environments. See ...
http://www.javaworld.com/jw-02-2001/jw-0209-double.html
http://www.javaworld.com/javaworld/jw-05-2001/jw-0525-double.html

[Nicholas Cheung]

The code I shown is just an idea.
In other to cater for other issues, like threading, multiple access from distributed JVM, etc, you will need extra effort to fulfill those requirements.
Nick