What does Serializable mean?

Question

Serialization is persisting an object from memory to a sequence of bits, for instance for saving onto the disk. Deserialization is the opposite - reading data from the disk to hydrate/create an object.

In the context of your question, it is an interface that if implemented in a class, this class can automatically be serialized and deserialized by different serializers.

Though most of the users have already given the answer, but I would like to add an example for those who need it in order to explain the idea:

Let's say you have a class person like the following:

public class Person implements java.io.Serializable {
    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    public String firstName;
    public String lastName;
    public int age;
    public String address;

    public void play() {
        System.out.println(String.format(
                "If I win, send me the trophy to this address: %s", address));
    }
    @Override
    public String toString() {
        return String.format(".....Person......\nFirst Name = %s\nLast Name = %s", firstName, lastName);
    }
}

and then you create an object like this:

Person william = new Person();
        william.firstName = "William";
        william.lastName = "Kinaan";
        william.age = 26;
        william.address = "Lisbon, Portugal";

You can serialise that object to many streams. I will do that to two streams:

Serialization to standard output:

public static void serializeToStandardOutput(Person person)
            throws IOException {
        OutputStream outStream = System.out;
        ObjectOutputStream stdObjectOut = new ObjectOutputStream(outStream);
        stdObjectOut.writeObject(person);
        stdObjectOut.close();
        outStream.close();
    }

Serialization to a file:

public static void serializeToFile(Person person) throws IOException {
        OutputStream outStream = new FileOutputStream("person.ser");
        ObjectOutputStream fileObjectOut = new ObjectOutputStream(outStream);
        fileObjectOut.writeObject(person);
        fileObjectOut.close();
        outStream.close();
    }

Then:

Deserialize from file:

public static void deserializeFromFile() throws IOException,
            ClassNotFoundException {
        InputStream inStream = new FileInputStream("person.ser");
        ObjectInputStream fileObjectIn = new ObjectInputStream(inStream);
        Person person = (Person) fileObjectIn.readObject();
        System.out.println(person);
        fileObjectIn.close();
        inStream.close();
    }

It means that instances of the class can be turned into a byte-stream (for example, to be saved to a file) and then converted back into classes again. This reloading could happen in a different instance of the program, or even on a different machine. Serialisation (in any language) involves all sorts of issues, though, especially when you've got references to other objects inside the serialisable one.

Here is a detailed explanation of the Serialization: (my own blog)

Serialization:

Serialization is the process of serializing the state of an object is represented and stored in the form of a sequence of bytes. This can be stored in a file. The process to read the state of the object from the file and restoring it is called deserialization.

What is the need of Serialization?

In modern day architecture, there is always a need to store object state and then retrieve it. For example in Hibernate, to store a object we should make the class Serializable. What it does, is that once the object state is saved in the form of bytes it can be transferred to another system which can then read from the state and retrieve the class. The object state can come from a database or a different jvm or from a separate component. With the help of Serialization we can retrieve the Object state.

Code Example and explanation:

First let's have a look at the Item Class:

public class Item implements Serializable{

    /**
    *  This is the Serializable class
    */
    private static final long serialVersionUID = 475918891428093041L;
    private Long itemId;
    private String itemName;
    private transient Double itemCostPrice;
    public Item(Long itemId, String itemName, Double itemCostPrice) {
        super();
        this.itemId = itemId;
        this.itemName = itemName;
        this.itemCostPrice = itemCostPrice;
      }

      public Long getItemId() {
          return itemId;
      }

     @Override
      public String toString() {
          return "Item [itemId=" + itemId + ", itemName=" + itemName + ", itemCostPrice=" + itemCostPrice + "]";
       }


       public void setItemId(Long itemId) {
           this.itemId = itemId;
       }

       public String getItemName() {
           return itemName;
       }
       public void setItemName(String itemName) {
            this.itemName = itemName;
        }

       public Double getItemCostPrice() {
            return itemCostPrice;
        }

        public void setItemCostPrice(Double itemCostPrice) {
             this.itemCostPrice = itemCostPrice;
        }
}

In the above code it can be seen that Item class implements Serializable.

This is the interface that enables a class to be serializable.

Now we can see a variable called serialVersionUID is initialized to Long variable. This number is calculated by the compiler based on the state of the class and the class attributes. This is the number that will help the jvm identify the state of an object when it reads the state of the object from file.

For that we can have a look at the official Oracle Documentation:

The serialization runtime associates with each serializable class a version number, called a serialVersionUID, which is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization. If the receiver has loaded a class for the object that has a different serialVersionUID than that of the corresponding sender's class, then deserialization will result in an InvalidClassException. A serializable class can declare its own serialVersionUID explicitly by declaring a field named "serialVersionUID" that must be static, final, and of type long: ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L; If a serializable class does not explicitly declare a serialVersionUID, then the serialization runtime will calculate a default serialVersionUID value for that class based on various aspects of the class, as described in the Java(TM) Object Serialization Specification. However, it is strongly recommended that all serializable classes explicitly declare serialVersionUID values, since the default serialVersionUID computation is highly sensitive to class details that may vary depending on compiler implementations, and can thus result in unexpected InvalidClassExceptions during deserialization. Therefore, to guarantee a consistent serialVersionUID value across different java compiler implementations, a serializable class must declare an explicit serialVersionUID value. It is also strongly advised that explicit serialVersionUID declarations use the private modifier where possible, since such declarations apply only to the immediately declaring class--serialVersionUID fields are not useful as inherited members.

If you have noticed there is another keyword we have used which is transient.

If a field is not serializable, it must be marked transient. Here we marked the itemCostPrice as transient and don't want it to be written in a file

Now let's have a look on how to write the state of an object in the file and then read it from there.

public class SerializationExample {

    public static void main(String[] args){
        serialize();
       deserialize();
    } 

    public static void serialize(){

         Item item = new Item(1L,"Pen", 12.55);
         System.out.println("Before Serialization" + item);

         FileOutputStream fileOut;
         try {
             fileOut = new FileOutputStream("/tmp/item.ser");
             ObjectOutputStream out = new ObjectOutputStream(fileOut);
             out.writeObject(item);
             out.close();
             fileOut.close();
             System.out.println("Serialized data is saved in /tmp/item.ser");
           } catch (FileNotFoundException e) {

                  e.printStackTrace();
           } catch (IOException e) {

                  e.printStackTrace();
           }
      }

    public static void deserialize(){
        Item item;

        try {
                FileInputStream fileIn = new FileInputStream("/tmp/item.ser");
                ObjectInputStream in = new ObjectInputStream(fileIn);
                item = (Item) in.readObject();
                System.out.println("Serialized data is read from /tmp/item.ser");
                System.out.println("After Deserialization" + item);
        } catch (FileNotFoundException e) {
                e.printStackTrace();
        } catch (IOException e) {
               e.printStackTrace();
        } catch (ClassNotFoundException e) {
               e.printStackTrace();
        }
     }
}

In the above we can see an example of serialization and deserialization of an object.

For that we used two classes. For serializing the object we have used ObjectOutputStream. We have used the method writeObject to write the object in the file.

For Deserializing we have used ObjectInputStream which reads from the object from the file. It uses readObject to read the object data from the file.

The output of the above code would be like:

Before SerializationItem [itemId=1, itemName=Pen, itemCostPrice=12.55]
Serialized data is saved in /tmp/item.ser
After DeserializationItem [itemId=1, itemName=Pen, itemCostPrice=null]

Notice that itemCostPrice from deserialized object is null as it was not written.