serialVersionUId - ayushmathur94/DirectQuesAns_Prep GitHub Wiki

Add serialVersionUId to singleton objects So far so good. Untill now, we have solved both of the problems of synchronization and serialization. Now, we are just one step away from a correct and complete implementation. The only missing part is a serial version id.

This is required in cases where your class structure changes between serialization and deserialization. A changed class structure will cause the JVM to give an exception in the de-serializing process.

java.io.InvalidClassException: singleton.DemoSingleton; local class incompatible: stream classdesc serialVersionUID = 5026910492258526905, local class serialVersionUID = 3597984220566440782
at java.io.ObjectStreamClass.initNonProxy(Unknown Source)
at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
at java.io.ObjectInputStream.readClassDesc(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readObject(Unknown Source)
at singleton.SerializationTest.main(SerializationTest.java:24)

This problem can be solved only by adding a unique serial version id to the class. It will prevent the compiler from throwing the exception by telling it that both classes are same, and will load the available instance variables only.

public class DemoSingleton implements Serializable {
    private static final long serialVersionUID = 1L;
 
    private DemoSingleton() {
        // private constructor
    }
 
    private static class DemoSingletonHolder {
        public static final DemoSingleton INSTANCE = new DemoSingleton();
    }
 
    public static DemoSingleton getInstance() {
        return DemoSingletonHolder.INSTANCE;
    }
 
    protected Object readResolve() {
        return getInstance();
    }
}