Interview question: about Java serialization and singletons [closed]

Question

In an interview the interviewer asked me the following question: is it possible to serialize a singleton object? I said yes, but in which scenario should we serialize a singleton?

And is it possible to design a class whose object can not be serialized?

Answer 1

The question should probably be better phrased as "is it possible to use serialization and deserialization with a singleton-pattern class C in a way that does not break the singleton pattern?"

The answer is basically yes:

import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectStreamException; import java.io.Serializable;  public class AppState implements Serializable {     private static AppState s_instance = null;      public static synchronized AppState getInstance() {         if (s_instance == null) {             s_instance = new AppState();         }         return s_instance;     }      private AppState() {         // initialize     }      private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {         ois.defaultReadObject();         synchronized (AppState.class) {             if (s_instance == null) {                 // re-initialize if needed                  s_instance = this; // only if everything succeeds             }         }     }      // this function must not be called other than by the deserialization runtime     private Object readResolve() throws ObjectStreamException {         assert(s_instance != null);         return s_instance;     }      public static void main(String[] args) throws Throwable {         assert(getInstance() == getInstance());              java.io.ByteArrayOutputStream baos = new java.io.ByteArrayOutputStream();             java.io.ObjectOutputStream oos = new java.io.ObjectOutputStream(baos);             oos.writeObject(getInstance());             oos.close();              java.io.InputStream is = new java.io.ByteArrayInputStream(baos.toByteArray());             ObjectInputStream ois = new ObjectInputStream(is);             AppState s = (AppState)ois.readObject();             assert(s == getInstance());     } } 

but note that it is possible for multiple instances of AppState to exist using this code. However, only one is referenced. The others are eligible for garbage collection, created only by the deserialization runtime, so they don't exist for practical purposes.

For answers to your other two questions (In which scenario should we serialize a singleton? Is it possible to design a class whose object can not be serialized?), see @Michael Borgwardt's answer.