Chapter 11
The java.io Package

Externalizable

Name

Externalizable

Synopsis

Interface Name:

java.io.Externalizable

Super-interface:

java.io.Serializable

Immediate Sub-interfaces:

None

Implemented By:

None

Availability:

New as of JDK 1.1

The Externalizable interface is an extension of the Serializable interface. Whereas a Serializable object is automatically saved and loaded (in most cases), an Externalizable object has sole responsibility for saving and loading its state via the writeExternal() and readExternal() methods. If a class implements the Externalizable interface, it must handle any versioning issues that occur.

The methods of Externalizable are public, which can pose a security risk. If security is a concern, Externalizable objects should not write or read sensitive information, or the Serializable interface should be used instead.

Interface Declaration

public abstract interface java.io.Externalizable
                          extends java.io.Serializable {
  // Methods
  public abstract void readExternal(ObjectInput in);
  public abstract void writeExternal(ObjectOutput out);
}

Methods

readExternal

 public abstract void readExternal(ObjectInput in) throws IOException, ClassNotFoundException

Parameters

in: The object input stream to use.

Throws

ClassNotFoundException

If the class of the object being deserialized cannot be found.

IOException

If any kind of I/O error occurs.

Description

This method reads an object from the given stream. This method has full responsibility for restoring the object's state. The implementation of readExternal() should read data in the format that is written out by writeExternal(). In general, an implementation should call methods of DataInput to read primitive types and methods of ObjectInput to read objects, strings, and arrays.

writeExternal

 public abstract void writeExternal(ObjectOutput out) throws IOException

Parameters

out: The object output stream to use.

Throws

IOException: If any kind of I/O error occurs.

Description

This method writes an object to the given stream. This method has full responsibility for saving the object's state. The implementation of writeExternal() should write data in the format that is read by readExternal(). In general, an implementation should call methods of DataOutput to write primitive types and methods of ObjectOutput to write objects, strings, and arrays.