Serialization in Java
1. Serialization
2. Serializable Interface
3. Example of Serialization
4. Deserialization
5. Example of Deserialization
6. Serialization with Inheritance
7. Externalizable interface
8. Serialization and static datamember
Serialization
in java is a
mechanism of writing the state of an object
into a byte stream.
It is mainly used in
Hibernate, RMI, JPA, EJB, JMS technologies.
The reverse operation
of serialization is calleddeserialization.
The String class and
all the wrapper classes implements java.io.Serializable interface by default.
Advantage of Java Serialization
It is mainly used to
travel object's state on the network (known as marshaling).
java.io.Serializable interface
Serializable is a
marker interface (has no body). It is just used to "mark" java
classes which support a certain capability.
It must be implemented
by the class whose object you want to persist. Let's see the example given
below:
1. import java.io.Serializable;
2. public class Student implements Serializable{
3. int id;
4. String name;
5. public Student(int id, String name) {
6. this.id = id;
7. this.name = name;
8. }
9. }
ObjectOutputStream
class
The ObjectOutputStream
class is used to write primitive data types and Java objects to an
OutputStream. Only objects that support the java.io.Serializable interface
can be written to streams.
Constructor
Important Methods
Example of Java Serialization
In this example, we
are going to serialize the object of Student class. The writeObject() method
of ObjectOutputStream class provides the functionality to serialize the
object. We are saving the state of the object in the file named f.txt.
1. import java.io.*;
2. class Persist{
3. public static void main(String args[])throws Exception{
4. Student s1 =new Student(211,"ravi");
5.
6. FileOutputStream fout=new FileOutputStream("f.txt");
7. ObjectOutputStream out=new ObjectOutputStream(fout);
8.
9. out.writeObject(s1);
10. out.flush();
11. System.out.println("success");
12. }
13. }
success
Deserialization in java
Deserialization is the
process of reconstructing the object from the serialized state.It is the
reverse operation of serialization.
ObjectInputStream
class
An ObjectInputStream
deserializes objects and primitive data written using an ObjectOutputStream.
Constructor
Important Methods
Example
of Java Deserialization
1. import java.io.*;
2. class Depersist{
3. public static void main(String args[])throws Exception{
4.
5. ObjectInputStream in=new ObjectInputStream(new FileInputStream("f.txt"));
6. Student s=(Student)in.readObject();
7. System.out.println(s.id+" "+s.name);
8.
9. in.close();
10. }
11. }
211 ravi
Java Serialization with Inheritance (IS-A Relationship)
If a class implements
serializable then all its sub classes will also be serializable. Let's see
the example given below:
1. import java.io.Serializable;
2. class Person implements Serializable{
3. int id;
4. String name;
5. Person(int id, String name) {
6. this.id = id;
7. this.name = name;
8. }
9. }
1. class Student extends Person{
2. String course;
3. int fee;
4. public Student(int id, String name, String course, int fee) {
5. super(id,name);
6. this.course=course;
7. this.fee=fee;
8. }
9. }
Now you can serialize
the Student class object that extends the Person class which is
Serializable.Parent class properties are inherited to subclasses so if parent
class is Serializable, subclass would also be.
Java Serialization with Aggregation (HAS-A Relationship)
If a class has a
reference of another class, all the references must be Serializable otherwise
serialization process will not be performed. In such case, NotSerializableException is thrown at runtime.
1. class Address{
2. String addressLine,city,state;
3. public Address(String addressLine, String city, String state) {
4. this.addressLine=addressLine;
5. this.city=city;
6. this.state=state;
7. }
8. }
1. import java.io.Serializable;
2. public class Student implements Serializable{
3. int id;
4. String name;
5. Address address;//HAS-A
6. public Student(int id, String name) {
7. this.id = id;
8. this.name = name;
9. }
10. }
Since Address is not
Serializable, you can not serialize the instance of Student class.
Note:
All the objects within an object must be Serializable.
Java Serialization with static data member
If there is any static
data member in a class, it will not be serialized because static is the part
of class not object.
1. class Employee implements Serializable{
2. int id;
3. String name;
4. static String company="SSS IT Pvt Ltd";//it won't be serialized
5. public Student(int id, String name) {
6. this.id = id;
7. this.name = name;
8. }
9. }
Java Serialization with array or collection
Rule: In case of array
or collection, all the objects of array or collection must be serializable.
If any object is not serialiizable, serialization will be failed.
Externalizable in java
The Externalizable
interface provides the facility of writing the state of an object into a byte
stream in compress format. It is not a marker interface.
The Externalizable
interface provides two methods:
·
public void
writeExternal(ObjectOutput out) throws IOException
·
public void
readExternal(ObjectInput in) throws IOException
|
Java Transient Keyword
If you don't want to serialize
any data member of a class, you can mark it as transient.
Visit next page for more
details.
Java Transient Keyword
Java transient keyword is used in serialization. If you
define any data member as transient, it will not be serialized.
Let's take an example, I have
declared a class as Student, it has three data members id, name and age. If you
serialize the object, all the values will be serialized but I don't want to
serialize one value, e.g. age then we can declare the age data member as
transient.
Example of
Java Transient Keyword
In this example, we have
created the two classes Student and PersistExample. The age data member of the
Student class is declared as transient, its value will not be serialized.
If you deserialize the object,
you will get the default value for transient variable.
Let's create a class with
transient variable.
1.
import java.io.Serializable;
2.
public class Student implements Serializable{
3.
int id;
4.
String name;
5.
transient int age;//Now it will not be serialized
6.
public Student(int id, String name,int age) {
7.
this.id = id;
8.
this.name = name;
9.
this.age=age;
10. }
11. }
Now write the code to serialize
the object.
1.
import java.io.*;
2.
class PersistExample{
3.
public static void main(String args[])throws Exception{
4.
Student s1 =new Student(211,"ravi",22);//creating object
5.
//writing object into file
6.
FileOutputStream f=new FileOutputStream("f.txt");
7.
ObjectOutputStream out=new ObjectOutputStream(f);
8.
out.writeObject(s1);
9.
out.flush();
10.
11. out.close();
12. f.close();
13. System.out.println("success");
14. }
15. }
Output:
success
Now write the code for
deserialization.
1.
import java.io.*;
2.
class DePersist{
3.
public static void main(String args[])throws Exception{
4.
ObjectInputStream in=new ObjectInputStream(new FileInputStream("f.txt"));
5.
Student s=(Student)in.readObject();
6.
System.out.println(s.id+" "+s.name+" "+s.age);
7.
in.close();
8.
}
9.
}
211 ravi 0
As you can see, printing age of
the student returns 0 because value of age was not serialized.
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