3.6 The readExternal Method

Objects implementing java.io.Externalizable must implement the readExternal method to restore the entire state of the object. It must coordinate with its superclasses to restore their state. All of the methods of ObjectInput are available to restore the object's primitive typed fields and object fields.

    public void readExternal(ObjectInput stream)
        throws IOException;

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Note - The readExternal method is public, and it raises the risk of a client being able to overwrite an existing object from a stream. The class may add its own checks to insure that this is only called when appropriate.

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A new stream protocol version has been introduced in JDK^TM 1.2 to correct a problem with Externalizable objects. The old definition of Externalizable objects required the local virtual machine to find a readExternal method to be able to properly read an Externalizable object from the stream. The new format adds enough information to the stream protocol so serialization can skip an Externalizable object when the local readExternal method is not available. Due to class evolution rules, serialization must be able to skip an Externalizable object in the input stream if there is not a mapping for the object using the local classes.

An additional benefit of the new Externalizable stream format is that ObjectInputStream can detect attempts to read more External data than is available, and can also skip by any data that is left unconsumed by a readExternal method. The behavior of ObjectInputStream in response to a read past the end of External data is the same as the behavior when a class-defined readObject method attempts to read past the end of its optional data: bytewise reads will return -1, primitive reads will throw EOFExceptions, and object reads will throw OptionalDataExceptions with the eof field set to true.

Due to the format change, JDK^TM 1.1.6 and earlier releases are not able to read the new format. StreamCorruptedException is thrown when JDK^TM 1.1.6 or earlier attempts to read an Externalizable object from a stream written in PROTOCOL_VERSION_2. Compatibility issues are discussed in more detail in Section 6.3, "Stream Protocol Versions."