Re: implement writeObject how? serializable singleton how?

Tom Hawtin <>
Wed, 17 Jan 2007 21:59:06 +0000
Frank Fredstone wrote:

Tom Hawtin <> writes:

Perhaps you are seeing the construction during static (class)
initialisation. When an object is deserialised the constructor of the
most derived non-serialisable class (possibly Object) is invoked by
the serialisation mechanism.

So, you and Chris Uppal's post as well, are agreeing that the
constructor is called during deserialization.

The constructor is not called for the deserialised copy. It is called
for the singleton copy.

I want an object that has a state that is initialized only once, as a
result of an external side-effect. I also want it to be possible for
that state to be serialized, without the state being "reinitialized"
(in the sense of being intitialized again by a specific block of code)
during deserialization. I want the state (the field values) restored,
without the code that initializes them during construction being

    private static MySingleton instance = null;

    public static MySingleton instance() {
        if (instance == null) {
            instance = new MySingleton(true);
        return instance;

This is not thread safe.

    private void readObject(ObjectInputStream s) throws IOException {
        try {
            ObjectInputStream.GetField fields = s.readFields();
            instance = (MySingleton) fields.get("instance", null);

But that doesn't change any current instance of MySingleton to the new
singleton. I suspect that a singleton is really not what you want (it
almost never is).

Anyway, proceeding on with deserialisation mutates the singleton, I
suggest just updating the actual singleton instance, and replacing
references to it in other deserialised object to the real version.

public class MySingleton implements {
     private static final MySingleton INSTANCE = new MyInstance();
     public static getInstance() {
         return INSTANCE;

     private String value;
     private MySingleton() {
         System.out.println("Constructing MySingleton");
     public synchronized String getValue() {
         return value;
     public synchronized void setValue(String value) {
         this.value = value;
     private Object readResolve( s) {
         // Copy data into real instance.
         synchronized (INSTANCE) {
             INSTANCE.value = this.value;

         // Replace serial instance with the real one.
         return INSTANCE;

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