Serialization

Serialization
in Java

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Serialization

Defined:
- The process of creating a "frozen" representation of a "live" (i.e., in-memory) object
Uses:
- Copy objects
- Provide persistence
- Transport objects over a communications channel

Overview of Serialization in Java

The Serializable Interface:
- Implemented by classes to indicate that their Objects can be serialized
- Has no methods (serves only to identify the semantics of being serializable)
Serialization:
- Handled by the writeObject() method in the ObjectOutputStream class
Deserialization:
- Handled by the readObject() method in the ObjectInputStream class

The Serialization Process

An ObjectOutputStream maintains a mapping from instances and classes to handles.
When writeObject( ) is passed an instance that has not yet been written to the stream the instance is assigned a reference handle, the handle is written to the stream, and the instance data is written to the stream.
When writeObject() is called with an argument that has already been written to the stream, the handle is written to the stream, and no further operations are necessary.
When an instance is being serialized and it contains another serializable instance, the serialization of the first instance is suspended and the second instance is serialized. After the second instance is fully serialized, the serialization of the first instance resumes.

Versioning

A Potential Problem:
- The class changes between when it is serialized and when it is deserialized
Avoiding this Problem:
- Provide a version number within the class and objects
A "Tricky" Implementation:
- Use a static attribute within the class

Serialization

An Example

Date d; FileOutputStream out; ObjectOutputStream serializer; String s; out = new FileOutputStream("objects.dat"); serializer = new ObjectOutputStream(out); s = new String("Today"); d = new Date(); serializer.writeObject(s); serializer.writeObject(d); serializer.flush(); out.close();

Deserialization

An Example

Date d; FileInputStream in; ObjectInputStream deserializer; String s; in = new FileInputStream("objects.dat"); deserializer = new ObjectInputStream(in); s = (String)deserializer.readObject(); d = (Date)deserializer.readObject(); in.close();

A Complete Example

A Simple Class javaexamples/serialization/Course.java

A Complete Example (cont.)

A Reasonably Complicated Collection javaexamples/serialization/CourseDatabase.java

A Complete Example (cont.)

Serialization javaexamples/serialization/SerializationDriver.java

A Complete Example (cont.)

Deserialization javaexamples/serialization/DeserializationDriver.java

Advanced Serialization in Java

Transients:
- Values that can be recalculated (and, hence, don't need to be serialized) can be declared transient
For Special Handling You Should Implement:
- private void readObject(ObjectInputStream stream)
- private void writeObject(ObjectOutputStream stream)

Advanced Serialization in Java (cont.)

Preventing Serialization/Deserialization:
- Implement writeObject() and readObject() methods that always throw the NotSerializableException
- The exception will be caught by the ObjectOutputStream which will abort the serialization process.
Keeping Complete Control:
- Implement the Externalizable interface
Specifying an Alternative Object for Serialization:
- Implement the writeReplace() and readReplace() methods

Formatting the Serialized Object

One Approach:
- Use a binary representation
Another Approach:
- Use XML

There's Always More to Learn