The mechanism using which the state of an object gets converted into byte stream is called serialization. Deserialization, on the other hand, is the reverse process, which includes converting the byte stream into a java object in memory. Serialization and deserialization are used to persist the object into memory.
The pictorial
representation of Serialization and deserialization is as below:
The byte stream
created by Serialization is platform-independent which means that the object
serialized on one platform can be deserialized on another platform.
To make an object
serializable the class should implement Serializable interface. Serializable
interface is present in java.io.Serializable package.
This interface is a marker interface. Marker interface means that the interface doesn't contain anything. It only marks the implementing classes to have some special capabilities. In this case, it marks the class to perform Serialization and deserialization of the objects. Cloneable and Remote are the examples of marker interface.
Advantages of serialization
·
Serialization
is used so that we can save the state of the object.
·
It
can also be used so that we can transfer the object across the network.
Important points
· No serialization of Static data members.
·
No serialization of transient data members.
·
During
deserialization constructor of the object will never be called.
·
All
the associate object must implement the Serializable interface.
·
If
the parent class is implementing the Serializable interface than its child classes
will be serialized automatically.
· If the child class is implementing the Serializable interface then the parent the class must implement the Serializable interface.
ObjectInputStream Class
This class is used
to serialize an object. There are many write methods in this class. The method
which is used frequently is :
|
public final void
writeObject(Object obj) throws IOException |
The above method sends the object to the output stream by serializing it.
ObjectOutputStream
class
This class is used to deserialize an object. Similar to
ObjectInputStream, ObjectOutputStream also have method which is used to
read/deserialized.
|
public final Object
readObject() throws IOException, ClassNotFoundException |
SerialVersionUID
During
deserialization, to verify that the sender and recipient of the serialized object
have loaded classes for that object which is compatible concerning
serialization, during serialization at runtime, associates a version number with
each serializable class. This serial version is known as serialVersionUID.
While receiving if
the receiver has loaded the class for the object which has different
serialVersionUID then during deserialization we will get InvalidClassException.
We can also explicitly declare the serialVersionUID.
This variable should be static, final and of type long. If the class doesn't declare
the UID by itself then based upon different aspects of the class, serialization
the runtime will calculate a default UID as described in Java Object Serialization
Specifications.
It is strongly recommended that every class
should have its UID declared explicitly.
Example:
Output:
|
Object is serialized Data before deserialization Name = Ram Age = 30 A = 2 B =10 Object is deserialized Data after
deserialization Name = Ram Age = 30 A = 0 B =2000 |
Description of output
As we have seen that during deserialization
the value of a and b is changed this is because of the following reasons:
·
“a” is declared as transient.
The transient variables will not be considered in serialization. The default values
will be assigned for example for Objects it will be null and for int, it will
be 0.
· “b” is defined as static. Static variables will not be serialized. The default value in the class will be considered.
Further reading:
1. Core java interview questions.
2. Java 8 interview questions.
3. Internal working of Hashmap in java.
6. Compile time polymorphism in java.
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