Object serialization

[sky]

Hi,

I have one question about object serialization: an object can write itself into a file or, in other words, both methods writeObject and readObject can be called from the same object we want to serialize?

[camickr]

Try it and see what happens.

[Fubarable]

AFAIK, you don't call those methods on the objects themselves, but on the ObjectStreams that read and write the objects.

[sky]

AFAIK, you don't call those methods on the objects themselves, but on the ObjectStreams that read and write the objects.

I know, but my question was about the word "this" in this code, because in all the examples I've seen on the Internet they serialize an object different from the one which containes the code to serialize:

ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(fileName));
out.writeObject(this);
out.close();

[camickr]

So did it work? Write a complete program that writes it out and reads it back in. If it works great, if not then you have a real question to post along with demo code.

[sky]

I follow this steps:
- Make a new object of class Serialization, with the name of the file and the string "write"

I get this result:
- Exception IOException caught when I try to write the object.

Conclusion:
-You cannot write an object like this or am I missing something?

public class Test implements Serializable
{
   public int number;
   private Serialization s;

   public Test(Serialization s) 
   {
      this.s = s;
      number = 4;
   }

   public void write(String fileName) {
      
       try {
            ObjectOutputStream out = new ObjectOutputStream(new    FileOutputStream(fileName));
            out.writeObject(this);
            out.close();
        } catch (IOException e) {
            System.out.println("Error!");
        }
    }

   public Test read(String fileName) {

       Test object=null;
       
       try {
           ObjectInputStream in = new ObjectInputStream(new FileInputStream(fileName));
           object=(Test)in.readObject();
           in.close();
        } catch (IOException e) {
            System.out.println("Error!");
        } catch (ClassNotFoundException ex) {
            System.out.println("Error!");
        }
        
        return object;
    }
}

public class Serialization
{
   Test test;

   public Serialization(String fileName, String mode) {

       if (mode.equals("write")) {
            test = new Test(this);
            test.write(fileName);
        } else if (mode.equals("read")) {
            test=test.read(fileName);
            System.out.println(test.number);
        }
    }

}

[Fubarable]

There's no main method here.

[JosAH]

Better change this:

} catch (IOException e) {
   System.out.println("Error!");
}

... to this:

} catch (IOException e) {
   e.printStackTrace();
}

... and tell us what the stack trace was.

kind regards,

Jos

[sky]

There are not main method because I am executing the project with BlueJ, but a new class like this can be added:

public class Main
{
    public static void main (String[] args) {
    
        Serialization s = new Serialization(args[0], args[1]);
       
    }

}

Stack trace:

java.io.NotSerializableException: Serialization
	at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1156)
	at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1509)
	at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1474)
	at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1392)
	at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1150)
	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:326)
	at Test.write(Test.java:18)
	at Serialization.<init>(Serialization.java:10)
	at __SHELL4.run(__SHELL4.java:14)
	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)
	at bluej.runtime.ExecServer$3.run(ExecServer.java:814)

[JosAH]

Stack trace:

java.io.NotSerializableException: Serialization
	at java.io.ObjectOutputStream.writeObject0

Well, that first line says it all. Fix it and you're in business.

kind regards,

Jos

[sky]

What exactly those lines mean? The object Test is not serializable? In that case I don't understand why, because it's implemented as "serializable".

[JosAH]

What exactly those lines mean? The object Test is not serializable? In that case I don't understand why, because it's implemented as "serializable".

True, but your Serialization class isn't Serializable and you have a member variable of that class in your Test class; the Serializable framework wants to serialize that variable but it can't.

In order to make a class Serializable it has to implement that interface plus all types of the member variables also have to be serializable or have to be declared transient.

kind regards,

Jos

[sky]

EDIT: Everything seems to work fine with the following code:

import java.io.*;

public class Test implements Serializable
{
   public int number;

   public Test() 
   {
      number=0;
   }

   public void write(String fileName) {
      
       number = 4;
       
       try {
            ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(fileName));
            out.writeObject(this);
            out.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

   public Test read(String fileName) {

       Test object=null;
       
       try {
           ObjectInputStream in = new ObjectInputStream(new FileInputStream(fileName));
           object=(Test)in.readObject();
           in.close();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException ex) {
            ex.printStackTrace();
        }
        
        return object;
    }
}

public class Serialization
{
    private Test test;
   
    public Serialization(String fileName, String mode) {
        
        test = new Test();
        
        if (mode.equals("write")) {
            test.write(fileName);
        } else if (mode.equals("read")) {
            test=test.read(fileName);;
            System.out.println(test.number);
        }
    }

}

public class Main
{

    public static void main (String[] args) {
    
        Serialization s = new Serialization(args[0], args[1]);
       
    }

}

[JosAH]

EDIT: Everything seems to work fine with the following code:

Sure that works because you removed the Serialization object from your Test object (the Serialization class isn't Serializable).

kind regards,

Jos

[Tolls]

Put simply, for something to be serializable, all it's non-transient fields must be serializable...that is must "implement Serializable". If they are't then you get that exception.

Think about it. You've said "I want Test to be serializable", which implies that you want all the fields (non-transient ones that is) to be serialised. Which means that they themselves have to be serializable.

[sky]

Yes, that's right. I get a little confusing at the beginning because Java classes as ArrayList are already serializable, and I thought that I only have to write it in the class I'm serializing directly. However, there was also another problem with the code: the line test = new Test(this) should be out of the if, otherwise a NullPointerException is thrown when trying to access the object Test. Thank you for the quick help.