Java Serialization interview questions

Q1) What is Serialization?

Ans) Serializable is a marker interface. When an object has to be transferred over a network ( typically through rmi or EJB) or to persist the state of an object to a file, the object Class needs to implement Serializable interface. Implementing this interface will allow the object converted into bytestream and transfer over a network.

Q2) What is use of serialVersionUID?

Ans) During object serialization, the default Java serialization mechanism writes the metadata about the object, which includes the class name, field names and types, and superclass. This class definition is stored as a part of the serialized object. This stored metadata enables the deserialization process to reconstitute the objects and map the stream data into the class attributes with the appropriate type
Everytime an object is serialized the java serialization mechanism automatically computes a hash value. ObjectStreamClass's computeSerialVersionUID() method passes the class name, sorted member names, modifiers, and interfaces to the secure hash algorithm (SHA), which returns a hash value.The serialVersionUID is also called suid.
So when the serilaize object is retrieved , the JVM first evaluates the suid of the serialized class and compares the suid value with the one of the object. If the suid values match then the object is said to be compatible with the class and hence it is de-serialized. If not InvalidClassException exception is thrown.

Changes to a serializable class can be compatible or incompatible. Following is the list of changes which are compatible:

  • Add fields
  • Change a field from static to non-static
  • Change a field from transient to non-transient
  • Add classes to the object tree

List of incompatible changes:

  • Delete fields
  • Change class hierarchy
  • Change non-static to static
  • Change non-transient to transient
  • Change type of a primitive field

So, if no suid is present, inspite of making compatible changes, jvm generates new suid thus resulting in an exception if prior release version object is used .

The only way to get rid of the exception is to recompile and deploy the application again.

If we explicitly mention the sUid using the statement:
private final static long serialVersionUID = <integer value>

then if any of the metioned compatible changes are made the class need not to be recompiled. But for incompatible changes there is no other way than to compile again.

Q3) What is the need of Serialization?

Ans) The serialization is used :-

  • To send state of one or more object’s state over the network through a socket.
  • To save the state of an object in a file.
  • An object’s state needs to be manipulated as a stream of bytes.

Q4) Other than Serialization what are the different approach to make object Serializable?

Ans) Besides the Serializable interface, at least three alternate approaches can serialize Java objects:

  • For object serialization, instead of implementing the Serializable interface, a developer can implement the Externalizable interface, which extends Serializable. By implementing Externalizable, a developer is responsible for implementing the writeExternal() and readExternal() methods. As a result, a developer has sole control over reading and writing the serialized objects.
  • XML serialization is an often-used approach for data interchange. This approach lags runtime performance when compared with Java serialization, both in terms of the size of the object and the processing time. With a speedier XML parser, the performance gap with respect to the processing time narrows. Nonetheless, XML serialization provides a more malleable solution when faced with changes in the serializable object.
  • Finally, consider a "roll-your-own" serialization approach. You can write an object's content directly via either the ObjectOutputStream or the DataOutputStream. While this approach is more involved in its initial implementation, it offers the greatest flexibility and extensibility. In addition, this approach provides a performance advantage over Java serialization.

Q5) Do we need to implement any method of Serializable interface to make an object serializable?

Ans) No. Serializable is a Marker Interface. It does not have any methods.

Q6) What happens if the object to be serialized includes the references to other serializable objects?

Ans) If the object to be serialized includes references to the other objects, then all those object’s state also will be saved as the part of the serialized state of the object in question. The whole object graph of the object to be serialized will be saved during serialization automatically provided all the objects included in the object’s graph are serializable.

Q7) What happens if an object is serializable but it includes a reference to a non-serializable object?

Ans- If you try to serialize an object of a class which implements serializable, but the object includes a reference to an non-serializable class then a ‘NotSerializableException’ will be thrown at runtime.


  public class NonSerial { 
      //This is a non-serializable  class 
  }
  public class MyClass implements Serializable { 
    private static final long serialVersionUID = 1L; 
    private NonSerial nonSerial; 
    MyClass(NonSerial  nonSerial){ 
      this.nonSerial = nonSerial; 
    } 
    public static void main(String [] args)  { 
      NonSerial nonSer = new NonSerial(); 
      MyClass c = new MyClass(nonSer); 
      try { 
       FileOutputStream fs = new FileOutputStream("test1.ser"); 
       ObjectOutputStream os = new ObjectOutputStream(fs); 
       os.writeObject(c); 
       os.close(); 
      } catch (Exception e) {  e.printStackTrace(); } 
      try { 
        FileInputStream fis = new FileInputStream("test1.ser"); 
        ObjectInputStream ois = new ObjectInputStream(fis); 
        c = (MyClass)  ois.readObject(); 
        ois.close(); 
      } catch (Exception e) { 
        e.printStackTrace(); 
      } 
     } 
    }     
On execution of above code following exception will be thrown;
java.io.NotSerializableException: NonSerial 
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java) 

Q8) Are the static variables saved as the part of serialization?

Ans) No. The static variables belong to the class are not the part of the state of the object so they are not saved as the part of serialized object.

Q9)What is a transient variable?

Ans) These variables are not included in the process of serialization and are not the part of the object’s serialized state.

Q10) What will be the value of transient variable after de-serialization?

Ans) It’s default value.
e.g. if the transient variable in question is an int, it’s value after deserialization will be zero.

public class TestTransientVal implements Serializable { 
  private static final long serialVersionUID =  -22L; 
  private String name; 
  transient private int age; 
TestTransientVal(int age, String  name) { 
  this.age = age; 
  this.name = name; 
} 
public static void main(String [] args)  { 
 TestTransientVal c = new TestTransientVal(1,"ONE"); 
 System.out.println("Before serialization:" + c.name + " "+ c.age); 
 try { 
   FileOutputStream fs =new FileOutputStream("testTransient.ser");
   ObjectOutputStream os = new ObjectOutputStream(fs); 
   os.writeObject(c); 
   os.close(); 
 } catch (Exception e) {  e.printStackTrace(); } 
   try { 
     FileInputStream fis =new FileInputStream("testTransient.ser");
     ObjectInputStream ois =new ObjectInputStream(fis); 
     c = (TestTransientVal)  ois.readObject(); 
     ois.close(); 
   } catch (Exception e) {  e.printStackTrace(); } 
    System.out.println("After  de-serialization:" + c.name +" "+ c.age); 
 } 
}

Result of executing above piece of code –
Before serialization: - Value of non-transient variable ONE Value of transient variable 1
After de-serialization:- Value of non-transient variable ONE Value of transient variable 0

Explanation –
The transient variable is not saved as the part of the state of the serailized variable, it’s value after de-serialization is it’s default value.

Q11) Does the order in which the value of the transient variables and the state of the object using the defaultWriteObject() method are saved during serialization matter?

Ans) Yes, while restoring the object’s state the transient variables and the serializable variables that are stored must be restored in the same order in which they were saved.

Q12) How can one customize the Serialization process? or What is the purpose of implementing the writeObject() and readObject() method?

Ans) When you want to store the transient variables state as a part of the serialized object at the time of serialization the class must implement the following methods –

private void wrtiteObject(ObjectOutputStream outStream) {
  //code to save the transient variables state 
  //as a part of serialized  object
}
private void readObject(ObjectInputStream inStream) {
 //code to read the transient variables state 
//and assign it to the  de-serialized object
}

public class TestCustomizedSerialization implements Serializable { 
 private static final long serialVersionUID =-22L; 
 private String noOfSerVar; 
 transient private int noOfTranVar; 
 TestCustomizedSerialization(int noOfTranVar, String  noOfSerVar) { 
    this.noOfTranVar = noOfTranVar; 
    this.noOfSerVar = noOfSerVar; 
  } 
  private void writeObject(ObjectOutputStream os) { 
    try { 
      os.defaultWriteObject(); 
      os.writeInt(noOfTranVar); 
    } catch (Exception e) {  e.printStackTrace(); } 
  } 
private void readObject(ObjectInputStream is) { 
 try { 
  is.defaultReadObject(); 
   int noOfTransients =  (is.readInt()); 
 } catch (Exception e) { 
   e.printStackTrace(); } 
 } 
public int getNoOfTranVar() { 
return noOfTranVar; 
}

The value of transient variable ‘noOfTranVar’ is saved as part of the serialized object manually by implementing writeObject() and restored by implementing readObject().
The normal serializable variables are saved and restored by calling defaultWriteObject() and defaultReadObject()respectively. These methods perform the normal serialization and de-sirialization process for the object to be saved or restored respectively.

Q13) If a class is serializable but its superclass in not, what will be the state of the instance variables inherited from super class after deserialization?

Ans) The values of the instance variables inherited from superclass will be reset to the values they were given during the original construction of the object as the non-serializable super-class constructor will run.

E.g.
public class ChildSerializable extends ParentNonSerializable implements Serializable {
  private static final long serialVersionUID = 1L; 
  String color; 
  ChildSerializable() { 
    this.noOfWheels = 8; 
    this.color = "blue"; 
  } 
}

public class SubSerialSuperNotSerial { 
  public static void main(String [] args)  { 
    ChildSerializable c = new ChildSerializable(); 
      System.out.println("Before : -  " +  c.noOfWheels + " "+ c.color); 
    try { 
      FileOutputStream fs = new FileOutputStream("superNotSerail.ser"); 
      ObjectOutputStream os = new ObjectOutputStream(fs); 
      os.writeObject(c); 
      os.close(); 
    } catch (Exception e) {  e.printStackTrace(); } 
    try { 
      FileInputStream fis = new FileInputStream("superNotSerail.ser"); 
      ObjectInputStream ois = new ObjectInputStream(fis); 
      c = (ChildSerializable) ois.readObject(); 
      ois.close(); 
    } catch (Exception e) {  e.printStackTrace(); } 
      System.out.println("After :-  " +  c.noOfWheels + " "+ c.color); 
  } 
} 
  Result  on executing above code –
  Before : - 8 blue 
  After :- 4 blue

The instance variable ‘noOfWheels’ is inherited from superclass which is not serializable. Therefore while restoring it the non-serializable superclass constructor runs and its value is set to 8 and is not same as the value saved during serialization which is 4.