Custom Serialization using ReadObject and WriteObject

Question

I am trying to write a instance of pojo class by using WriteObject method. when i write code like this :

    private void writeObject(ObjectOutputStream oos) throws IOException,ClassNotFoundException{ 
    oos.defaultWriteObject();    
    oos.writeObject(this);
}

It works fine but when I try and create a new local object and pass it to writeObject method it fails with

Exception in thread "main" java.lang.StackOverflowError

can some one please explain why it keeps calling writeObject method again and again recursively?

class Employee implements Serializable{
    private String name;
    private int age;
    private void readObject(ObjectInputStream ois) throws IOException,ClassNotFoundException{
        ois.defaultReadObject();
        Employee emp = (Employee)ois.readObject();
        emp.toString();
    }
    private void writeObject(ObjectOutputStream oos) throws IOException,ClassNotFoundException{
        oos.defaultWriteObject();
        Employee emp = new Employee("sumit",10);
        oos.writeObject(emp);
    }
    public Employee(){

    }
    public Employee(String name, int age){
        this.name = name;
        this.age = age;
    }   
}

Answer 1

It is because of the fact that you are overriding writeObject method in your Employee class. So, when you create the Employee object and try to write it using writeObject method, it is called recursively leading to StackOverflow error.

But, when you do not write Employee object the code executes properly.

---Edit as per the clarification asked in comment

In your Employee class, you are overriding the writeObject method , so , whenever, you try to invoke ObjectOutputStream.writeObject with Employee as parameter, your overridden method will be invoked. Now in your overridden writeObject in Employee class, you are again calling ObjectOutputStream.writeObject( oos.writeObject(emp);) with Employee as parameter, thus, writeObject method of Employee class gets recursively called (with new Employee object everytime)and you get stackoverflow error.

Now in case when you try to call recursively this keyword, it is because of the fact that you try to invoke ObjectOutputStream.writeObject with the same instance of Employee class. As per the ObjectOutputStream.writeObject documentation at below mentioned link :

https://docs.oracle.com/javase/7/docs/api/java/io/ObjectOutputStream.html

Multiple references to a single object are encoded using a reference sharing mechanism so that graphs of objects can be restored to the same shape as when the original was written.

Infact, if you try the below code in your main method :

Employee emp = new Employee("sumit",10);
oos.writeObject(emp);
oos.writeObject(emp);

i.e if you invoke writeObject multiple times on same object, it is invoked only once.

Answer 2

What is happening here is 'Employee' class is serializable. And you have overridden writeObject method in it.

Now in this overridden method, you are again creating an instance of Employee object and calling writeObject method of oos.

Serialization logic checks if the object being added is serializable. If it is, then serialize it or else throw NotSerializable exception.

Here, the object being added is Serializable. Hence it goes to serialize it and found that this Object has writeObject method overridden. So it calls that method.

Now again it goes to your overridden method logic and does the same method call again

oos.defaultWriteObject(); 
Employee emp = new Employee("sumit",10); 
oos.writeObject(emp);

and the loops continue, leading to StackOverflow error as it recursively calls it over and over again without breaking