Enum singleton serialization and deserialization

[bassfero]

I've found a nice Singleton implementation in the book "Effective Java - 2nd edition (Joshua Bloch)". I understand how to implement singleton via enum but I can't find the way how to serialize and deserialize it.

Here is the code of my enum:

public enum Parameters implements Serializable{

   INSTANCE;

   public String playerString;
   private short Collumns;                 
   private short Lines;                    
   private short Interval;                 

   private Parameters() {
       File f = new File("config.cfg");
       if (f.exists()){
           if(!load()){
               SetDefaultParameters();
           }
       }
       else{
           SetDefaultParameters();
       }
   }

   private void SSetDefaultParameters(){
      //fill the parameters with default values
      save();
   }
   
   private void save(){
      // save to file
   }

   private boolean load(){
      try{
         //load from file
      }catch (Exception ex){
         return false;
      }
      return true;
   }
}

All I need is to know how to write save and load function.

thanks...

[Tolls]

Why do you need to write save and load methods?

ETA: Scratch that...since it's serialisable you should be able to simply create an ObjectStream (Output for save, Input for load) to the file, and stream this.

ETA2: On load, since you will already have INSTANCE, I'm not too sure what'll happen when you read a new Parameters object in via serilisation, but assuming it doesn't throw a wobbly you'll then need to assign the sttributes from the read in one to the "real" enum one.

[bassfero]

this singleton supose to be a configuration, so i want to be able to save it in file

[Tolls]

See my edits...

[bassfero]

Why do you need to write save and load methods?

ETA: Scratch that...since it's serialisable you should be able to simply create an ObjectStream (Output for save, Input for load) to the file, and stream this.

ETA2: On load, since you will already have INSTANCE, I'm not too sure what'll happen when you read a new Parameters object in via serilisation, but assuming it doesn't throw a wobbly you'll then need to assign the sttributes from the read in one to the "real" enum one.

Well ObjectOutputStream.writeObject(INSTANCE) doesnt throw an exception and create a file, but this file has only 5bytes, so i don´t think that all enum attributes are inside (currently 16 attributes). ObjectInputStream.readObject() is not usable. I cannot assign object to INSTANCE or this.

[Tolls]

You don't assing the read object to this, you assign it to another instance of Parameters (which is why I'm not convinced this'll work), and then assign the individual attributes of this temporary Parameters object to this.

I assume from your description then that the above isn't your real code.

[bassfero]

...then assign the individual attributes of this temporary Parameters object to this

that is exactly what I am trying to avoid

I assume from your description then that the above isn't your real code.

whole enum has 288 lines, i pasted only part of it


Anyway, if there's not way how to save/load enum within few lines I am going to implement this singleton another way. I expected a simple solution (due to book).

Thank you for your time Tolls

[Tolls]

Does the book serialise them?

I ask because I've just done what you're trying to do and this:

TestEnum temp = (TestEnum)ois.readObject();

ETA: note ois is an ObjectInputStream.

throws this:

java.io.InvalidObjectException: enum constant INSTANCE does not exist in class test.TestEnum
	at java.io.ObjectInputStream.readEnum(ObjectInputStream.java:1704)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
	at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
	at test.TestEnum.<init>(TestEnum.java:26)
	at test.TestEnum.<clinit>(TestEnum.java:14)
	at Scratch.main(Scratch.java:27)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
0
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:110)
Caused by: java.lang.IllegalArgumentException: test.TestEnum is not an enum type
	at java.lang.Class.enumConstantDirectory(Class.java:2965)
	at java.lang.Enum.valueOf(Enum.java:191)
	at java.io.ObjectInputStream.readEnum(ObjectInputStream.java:1702)
	... 10 more

which is why I talked about it throwing a wobbly.

[bassfero]

Does the book serialise them?

I ask because I've just done what you're trying...

All I was able to do was

oos.writeObject(INSTANCE);

but as I mentioned, there were no attributes values in file.

Maybe I should write to Joshua Bloch :D

[Tolls]

But does he serialise them in the book?

Does he have code reading them in using an Object stream?

What does the book say about what you're trying to do?

[bassfero]

But does he serialise them in the book?

This is quote from book

This approach is functionally equivalent to the public field approach, except that it
is more concise, provides the serialization machinery for free, and provides an
ironclad guarantee against multiple instantiation, even in the face of sophisticated
serialization or reflection attacks. While this approach has yet to be widely
adopted, a single-element enum type is the best way to implement a singleton.

There are some sample codes in the book but not about serialization.

[Tolls]

I don't think he's talking about using serialisation to store the singleton, but about using the nature of an enum to prevent using serialisation to bypass the standard singleton mechanism. Can't guarantee that since I don't have the book, but a "normal" singleton pattern can be bypassed via serialisation, allowing the creation of multiple instances.

In addition, look here. That is how enum constants are serialised. Which makes sense. If you want to save an load parameters into your enum you'll have to have specific code for that.

[JosAH]

Well ObjectOutputStream.writeObject(INSTANCE) doesnt throw an exception and create a file, but this file has only 5bytes, so i don´t think that all enum attributes are inside (currently 16 attributes). ObjectInputStream.readObject() is not usable. I cannot assign object to INSTANCE or this.

(De)serialization of enums is different from (de)serialization of ordinary class objects. An enum in serialized form only contains the enum name and a notion of the particular enum (class). All attributes you have added yourself are not (de)serialized. This is to preserve the singleton behaviour of a particular enum instantiation.

kind regards,

Jos

[bassfero]

I don't think he's talking about using serialisation to store the singleton...

possible

I don't have the book...

just google for it, this topic is on page 17-19

...
In addition, look here...

I read this before posting on this forum.



Probably I did't understand item in book.

[Tolls]

I didn't think second edition was free?
I thought only the first edition was?

[bassfero]

I didn't think second edition was free?
I thought only the first edition was?

I download first 2 chapters of a book for free(which include also this topic), in application for my iPod, but I am sure that is possible to find a full PDF for free (legality depends on the law of your country).

[Tolls]

Since it's not a free book I'll pass.