Java Thread Serialization , why serialized Thread Object can be started

Question

A thread in java can't be restarted in Java, so I implemented a java Thread , and then tried to restart the thread after getting the serialized object of Thread.

import java.io.Serializable;

public class ThreadSerialization extends Thread implements Serializable {

    int iCheck = 10;
    @Override
    public void run() {
        System.out.println("STARTING");
        for(int i=0;i<10;i++){
            iCheck+=i;
        }
    }

}

and Serializing algo-

public class CallingThreadSerializable {

    public static void main(String[] args) {
        ThreadSerialization ser = new ThreadSerialization();
        ser.start();
        FileOutputStream fos = null;
        ObjectOutputStream out = null;
        FileInputStream fis = null;
        ObjectInputStream ois = null;
        try {
            fos = new FileOutputStream("thread.ser");
            out = new ObjectOutputStream(fos);
            out.writeObject(ser);
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } finally {
            try {
                out.close();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

        try {
            fis = new FileInputStream("thread.ser");
            ois = new ObjectInputStream(fis);
            ThreadSerialization ser1 = (ThreadSerialization) ois.readObject();
            System.out.println("---> " + ser1.iCheck);
            ser1.start();
            System.out.println("---> " + ser1.iCheck);
            ois.close();
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

}

OUTPUT-

STARTING
---> 55
---> 55
STARTING

Why the ser1 object is starting again ?

Answer 1

There are two points:

First: Thread is NOT Serializable and hence the following excerpt from the serializable JavaDoc applies:

To allow subtypes of non-serializable classes to be serialized, the subtype may assume responsibility for saving and restoring the state of the supertype's public, protected, and (if accessible) package fields.

Which mean, your class ThreadSerialization would have the responsibility to store and restore the state of Thread. But you cannot do that due to many private fields in Thread. Therefore all private field in Thread are default initialized. Now look at implementation of Thread.start() :

    //...
    if (threadStatus != 0)
        throw new IllegalThreadStateException();
    // ...
    start0();
    //...

Because threadStatus has not been stored/restored properly you can start a second time.

Second: Don't confuse the actual operating system thread and the "manager" object java.lang.Thread - they are only loosely coupled. In your example you only serialize the manager but not the OS-thread which has no representation in Java. After deserialization you have a second manager instance which no attached OS-thread. So telling the manager to start will succeed.