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Serialization in java is the process of converting the state of an object into a byte stream. A byte stream is a Java I/O (Input/Output) stream, essentially a flow of data that a programmer can read from or write to. Byte Streams read or write one byte of data at a time.

However, we must convert these byte streams back to their respective Objects to use them again. This reverse process of converting an object into byte-stream is called Deserialization.

Serialization in Java can be implemented using the java.io.Serializable interface. The writeObject() method of the ObjectOutputStream class is used for serializing an Object. To make a Java object serializable, we implement the java.io.Serializable interface.

While Serialization, the writeObject() method is used. Meanwhile, in Deserialization, the readObject() method is used. Also, for an object to be made serializable, it is mandatory for the object’s class to implement the Serializable interface.

The Serializable interface tells the JVM (Java Virtual Machine) that the objects of this class are ready for serialization and/or deserialization. The code that we write is called source code. We know that machines do not understand the human language. To bridge the gap, we need to translate the source code. This is where the JVM comes in. It converts source code to something called bytecode, which is then translated to the machine language.

Advantages of Serialization

• Serialization in java enables object state persistence and facilitates object transmission over a network.
• Implemented via the java.io.Serializable interface, it marks classes to grant serialization capability.
• Alongside saving and transferring object states, serialization supports marker interfaces like Cloneable and Remote, providing essential functionalities without data members or methods.

Important Points

• Any associated objects must also implement the Serializable interface to be serialized properly.
• To exclude a non-static data member from serialization, mark it as transient.
  During deserialization, the object's constructor isn't invoked.
• When a parent class implements the Serializable interface, its child class inherits this behaviour. However, if only the child class implements Serializable, it doesn't automatically apply to the parent.
• Serialization in java only preserves non-static data members; static and transient data members are excluded from the process.

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Examples of Serialization and Deserialization in Java

Let’s look at one example to show how serialization in Java works programmatically. Here, we are creating a Student class and serializing the object of the Student class.

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;

public class Student implements java.io.Serializable {

  public String stu_Name;
  public String stu_Addr;
  public int stu_Id;

  public static void main(String[] args) {
    Student std = new Student();
    std.stu_Name = "George";
    std.stu_Addr = "ABC,XYZ";
    std.stu_Id = 1;
    try {
      FileOutputStream fileOut = new FileOutputStream("storeObject.txt");
      //Serializing object
      ObjectOutputStream out = new ObjectOutputStream(fileOut);
      out.writeObject(std);
        
      // Close the output stream.
      out.close();
        
      // Close the file.
      fileOut.close();
      System.out.printf("Object serialized");
    } catch (IOException i) {
      i.printStackTrace();
    }
  }
}

Explanation:

• The code snippet shows serialization of an Object of type Student. A text file called storeObject is created with the help of the FileOutputStream class.
• FileOutputStream is an output stream that is used to write data into a file. Next, the ObjectOutputStream class is used to write the object as an OutputStream.
• writeObject(Object ob) is used to serialize objects into byte-streams.
• So the object is converted to a byte stream. Hence, serialization is complete.

Next, let’s look at the code for deserializing the same object. We will use the readObject() method of the ObjectInputStream class to deserialise the object.

import java.io.*;

public class Student implements java.io.Serializable {

  public String stu_Name;
  public String stu_Addr;
  public int stu_Id;

  public static void main(String[] args) {
    // Create a Student object.
    Student std = new Student();
    std.stu_Name = "George";
    std.stu_Addr = "ABC,XYZ";
    std.stu_Id = 1;

    // To hold the deserialized byte-stream
    Student deserializedStudent;
    try {
      // Serializing the student object - std
      FileOutputStream fileOut = new FileOutputStream("storeObject.txt");
      ObjectOutputStream out = new ObjectOutputStream(fileOut);
      out.writeObject(std);
      out.close();
      fileOut.close();
      // Serialization complete
      System.out.printf("Object serialized");

      // Deserialization process
      FileInputStream fileIn = new FileInputStream("storeObject.txt");
      ObjectInputStream in = new ObjectInputStream(fileIn);
      
      //Deserialization
      deserializedStudent = (Student) in.readObject();
      in.close();
      fileIn.close();

      // Printing the deserialized object.
      System.out.println("Deserialized Student...");
      System.out.println("Name: " + deserializedStudent.stu_Name);
      System.out.println("Address: " + deserializedStudent.stu_Addr);
    } catch (IOException i) {
      i.printStackTrace();
    } catch (Exception e) {
      System.out.println("Class not found");
      e.printStackTrace();
      return;
    }
  }
}

Output:

Deserialized Student...
Name: George
Address: ABC,XYZ

Explanation:

• Just like the previous code snippet, the object is first serialized.
• Here, the storeObject.txt file is accessed with the help of the FileInputStream, which is used to read data from a file.
• Next, the object is retrieved with the help of the ObjectInputStream, which converts the objects from the byte stream that was written using ObjectOutputStream.
• readObject() method is used to deserialize objects.

Hence, the process from object to byte stream is complete.

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Java Serialization with Inheritance (IS-A Relationship)

When a parent class implements the Serializable interface, the child classes do not have to do so. This is a case of Serialization with Inheritance. Inheritance is when a child class extends a parent class and inherits its properties. Hence, the names - parent and child.

Example:

import java.io.*;

class Child implements java.io.Serializable {

  public String stu_Name;

  Child(String stu_Name) {
    this.stu_Name = stu_Name;
  }
}

public class Student extends Child {

  public String stu_Addr;
  public int stu_Id;

  public Student(String stu_Name, String stu_Addr, int stu_Id) {
    super(stu_Name);
    this.stu_Addr = stu_Addr;
    this.stu_Id = stu_Id;
  }

  public static void main(String[] args) {
    Student s = new Student("George", "ABC, XYZ", 1);
    try {
      FileOutputStream fileOut = new FileOutputStream("storeObject.txt");
      ObjectOutputStream out = new ObjectOutputStream(fileOut);
      out.writeObject(s);
      out.close();
      fileOut.close();
      System.out.printf("Object serialized");
    } catch (IOException i) {
      i.printStackTrace();
    }
  }
}

Since Student extends Child, which extends the Serializable interface, there is no need for the Student class itself to extend this interface.
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Java Serialization with Aggregation (HAS-A Relationship)

If a class Student has an object of type Child and if class Student were to implement the Serializable interface and not B, Java would throw a NotSerializableException. Let’s see in the code:

Example:

import java.io.*;

class Child {

  public String stu_Name;

  Child(String stu_Name) {
    this.stu_Name = stu_Name;
  }
}

// Non-serializable class
public class Student implements java.io.Serializable {

  String stu_Addr;
  int stu_Id;
  Child child;

  public Student(String stu_Addr, int stu_Id) {
    this.stu_Addr = stu_Addr;
    this.stu_Id = stu_Id;
  }
}

The above code snippet would throw a NotSerializableException when a class Student object is serialized. Class Student is said to have a relationship with class Child. This is called Aggregation (HAS-A Relationship).
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Java Serialization with the Static Data Member

Static data members are volatile and can change during the serialization to deserialization process. Static means the data member is the only copy of the class, and any change made to it will be felt throughout the program. In other words, it belongs to the whole class.

Consider the below code where x is a static variable:

Example:

import java.io.*;

public class Main implements java.io.Serializable {

  static int x = 50;

  public static void main(String[] args) {
    Main s = new Main();
    System.out.println("Value before serialization: " + x);
    try {
      // Serialization
      FileOutputStream fileOut = new FileOutputStream("storeObject.txt");
      ObjectOutputStream out = new ObjectOutputStream(fileOut);
      out.writeObject(s);
      out.close();

      // Serialization complete
      // Static member value changed
      x = 48;
      fileOut.close();
      System.out.println("Object serialized");

      // Deserialization
      FileInputStream fileIn = new FileInputStream("storeObject.txt");
      ObjectInputStream in = new ObjectInputStream(fileIn);

      s = (Main) in.readObject();
      in.close();
      fileIn.close();

      // Value of static member not revived
      System.out.println("Value after serialization: " + x);
    } catch (IOException i) {
      i.printStackTrace();
    } catch (Exception e) {
      System.out.println("Class not found");
      e.printStackTrace();
      return;
    }
  }
}

Output:

Value before serialization: 50
Object serialized
Value after serialization: 48

Explanation:

• In the above code, the class has a static member of type int, with the value 50, when an object of this class was serialized. After this object was serialized, the program changed the value of the static member to 48.
• When this object is deserialized, the value of the static member is not restored to its original value of 50, which was the value when the object was serialized, because the static member belongs to its class and not the object.
• Hence, the process of serialization and deserialization only works on the object of a class. It doesn't affect a static member of a class, and therefore, when the object was deserialized, the value of a static member is found to be 48 and not 50.

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Java Serialization with Array or Collection

When dealing with arrays or collections, all contained objects must be serializable. If any object within them lacks serializability, the serialization process will encounter failure.

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SerialVersionUID

The serialVersionUID is a constant. So, while the whole object to byte-stream to object takes place, we need to be sure that the conversion from byte-stream to object is correct.

• We use the serialVersionUID attribute to remember versions of a Serializable class and verify that a class and the serialized object are compatible.
• Please note that this serialVersionUID is optional. If the programmer does not define this constant, Java does it for us.

private static final long serialVersionUID = 1234567L;

The serialization at runtime associates with each serializable class a serialVersionUID, which is used during deserialization to verify that the object that was converted to byte-stream is the one that is being retrieved.

What if we do not want to save, i.e. serialize, the state of some data member of the class that gets serialized?

In that case, we use a reserved keyword with the data member(s) that we do not want to serialize, like below:

transient String stu_Addr;

Take a look at the example below:

import java.io.*;

public class Student implements java.io.Serializable {

  public String stu_Name;
  transient String stu_Addr;
  public int stu_Id;

  public static void main(String[] args) {
    Student s = new Student();
    s.stu_Name = "George";
    s.stu_Addr = "ABC,XYZ";
    s.stu_Id = 1;
    try {
      FileOutputStream fileOut = new FileOutputStream("storeObject.txt");
      ObjectOutputStream out = new ObjectOutputStream(fileOut);
      out.writeObject(s);
      out.close();
      fileOut.close();

      // Serialization complete
      System.out.println("Object serialized");

      FileInputStream fileIn = new FileInputStream("storeObject.txt");
      ObjectInputStream in = new ObjectInputStream(fileIn);

      // Deserialization in process.
      s = (Student) in.readObject();

      // Close input stream and the file.
      in.close();
      fileIn.close();
    } catch (IOException i) {
      i.printStackTrace();
      return;
    } catch (Exception e) {
      System.out.println("Class not found");
      e.printStackTrace();
      return;
    }
    // Print the values of the deserialized object.
    System.out.println("Deserialized Student...");
    System.out.println("Name: " + s.stu_Name);
    System.out.println("Address: " + s.stu_Addr);
  }
}

Output:

Object serialized
Deserialized Student...
Name: George
Address: null

Explanation:

The last print statement will return null as stu_Addr is a transient variable, and as such, it will not get serialized and will lose its value.

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Java Transient vs Final

Regarding serialization, final variables are serialized directly by their values. Therefore, marking a final variable as transient serves no purpose, as the compiler assigns the value to the final variable.

In Java, marking a data member as transient means it won't be serialized when the object is written to a file or transferred over a network.

class Student implements Serializable{  
 transient int id; 
 public Student(int id) {  
  this.id = id;  
 }  
}  

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Conclusion

• Serialization in java involves converting an object's state into a byte stream, while deserialization reverses this process by converting a byte stream back into an object.
• In Java, serialization is achieved using the writeObject(Object obj) method, while deserialization is done using the readObject() method.
• To ensure compatibility between serialized objects and loaded classes, we use the serialVersionUID attribute to track versions of a Serializable class.
• If a parent class implements java.io.Serializable, its child objects are automatically serializable without implementing java.io.Serializable themselves.
• If a class contains an object of another non-serializable class as a data member, the class as a whole cannot be serialized.
• Static members of a class remain unaffected by serialization in java.

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