Externalization

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• In serialization every things take care by JVM and programmer have does not any control.
• In serialization is always possible to save total object to the file and it is not possible to save part of the object, which may create performance problem. 
• To overcome this problem we should go for externalization.
• The main advantage of externalization over serialization is every thing take care by programmer and JVM does not have any control.
• Based on our requirement we can save either total object or part of the object, which improve performance of the system.
• To provide externalizability for any java object, compulsory the corresponding class implements Externalizable interface.
• Externalizable interface define 2 methods.

 

 

• Externalizable is the child interface of Serializable interface 

 

 

1. public void writeExternal(ObjectOutputStream oos) throws IOException

• This method will be executed automatically at the time of serialization.  
• Within this method we have to write code to save required variables to the file.

 

2. public void readExternal(ObjectInputStream ois) throws IOException 

• This method will be executed automatically at the time of de-serialization.  
• Within this method we have to write code to read required variable from the file and assign to current object.
• But strictly speaking at the time of de-serialization JVM will create a separate new object by executing public no-argument constructor.
• On that object JVM will call readExternal().
• Hence every Externalizable implemented class should compulsory contain public no-argument constructor, otherwise we will get runtime exception: InvalidClassException
  

 Ex.

• If the class implements Serializable interface then total object will be saved to the file. In this case

• If the class implements Externalizable interface then only required variable will be saved to the file. In this case

Note: 

In serialization transient keyword will play role but in Externalization transient keyword would not play any role. Of course transient keyword not required in Externalization.

 

Important: Default constructor must have public modifier.

 

Difference between Serialization and Externalization

Serialization

• In default Serialization every this take care by JVM, and programmers does not have any control.  
• In this case it is always possible to save total object to the file.
• Not possible to save part of the object.
• Relatively performance is low.
• Best choice for save total object to the file.
• Marker interface so have no method.
• Implemented class not required to contain public no argument constructor. 
• transient keyword play a role.

Externalization

• In Customized serialization every this take care by programmers and JVM does not have any control.
• Based on our requirement we can save either total object or part of the object.
• Possible to save part of the object.
• Performance is high.
• Best choice for save part of the object to file.
• Not marker interface, have 2 methods writeExternal() and readExternal().
• Implemented calss should compulsory contain public no argument constructor, otherwise get runtime exception: InvalidClassException.
• No use of transient keyword.
  

 

Related Topic 

1. Serialization in Java
2. Object graph in java serialization
3. Customized Serialization
4. Serialization in inheritance
5. Externalization
6. SerialVersionUID