Interface Externalizable

All Superinterfaces:
Serializable
All Known Subinterfaces:
RemoteRef, ServerRef
All Known Implementing Classes:
DataFlavor
public interface Externalizable extends Serializable
A Serializable class that fully implements its own protocol for reading and writing to a serialization stream.

Only the identity of the class of an Externalizable instance is written in the serialization stream and it is the responsibility of the class to save and restore the contents of its instances. The writeExternal and readExternal methods give the class complete control over the format and contents of the stream for an object and its supertypes. These methods must explicitly coordinate with the supertype to save its state. These methods supersede customized implementations of writeObject and readObject methods.

Object Serialization uses the Serializable and Externalizable interfaces. Object persistence mechanisms can use them as well. Each object to be stored is tested for the Externalizable interface. If the object supports Externalizable, the writeExternal method is called. If the object does not support Externalizable and does implement Serializable, the object is saved using ObjectOutputStream.

When an Externalizable object is reconstructed, an instance is created using the public no-arg constructor, then the readExternal method called.

An Externalizable instance can designate a substitution object via the writeReplace and readResolve methods documented in the Serializable interface.

Record classes can implement Externalizable, but it is ignored: instances will receive the same treatment as other Serializable instances, as defined by the Java Object Serialization Specification, Section 1.13, "Serialization of Records".

Value classesPREVIEW that are not records are permitted to implement Externalizable, but the class has no mutable fields, and so is unlikely to be able to properly implement the readExternal method. Instead, the writeReplace method should be used to designate an alternative object for serialization. At deserialization time, the alternative object can implement readResolve to construct the expected value class instance.

Since:
1.1
See Also:

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    readExternal(ObjectInput in)
    The object implements the readExternal method to restore its contents by calling the methods of DataInput for primitive types and readObject for objects, strings and arrays.
    void
    writeExternal(ObjectOutput out)
    The object implements the writeExternal method to save its contents by calling the methods of DataOutput for its primitive values or calling the writeObject method of ObjectOutput for objects, strings, and arrays.

  • Method Details

    • writeExternal

      void writeExternal(ObjectOutput out) throws IOException
      The object implements the writeExternal method to save its contents by calling the methods of DataOutput for its primitive values or calling the writeObject method of ObjectOutput for objects, strings, and arrays.
      Parameters:
      out - the stream to write the object to
      Throws:
      IOException - Includes any I/O exceptions that may occur

    • readExternal

      void readExternal(ObjectInput in) throws IOException, ClassNotFoundException
      The object implements the readExternal method to restore its contents by calling the methods of DataInput for primitive types and readObject for objects, strings and arrays. The readExternal method must read the values in the same sequence and with the same types as were written by writeExternal.
      Parameters:
      in - the stream to read data from in order to restore the object
      Throws:
      IOException - if I/O errors occur
      ClassNotFoundException - If the class for an object being restored cannot be found.