Serializing java.util.Date

Question

Does anyone know how a java.util.Date gets serialized? I mean explain to me exactly what each byte is? I tried writing out a long then a date and I can see matches but the

Answer 1

The details of the format of Java object serialisation are specified in Java Object Serialization Specification. Other than magic and version numbers , details of the Date class and the fact the object is a Date is written to the stream.

The API doc for Date serialised form is:

The value returned by getTime() is emitted (long). This represents the offset from January 1, 1970, 00:00:00 GMT in milliseconds.

Note that it actually breaks the spec by not calling defaultWriteObject or putFields.

Answer 2

/**
 * Save the state of this object to a stream (i.e., serialize it).
 *
 * @serialData The value returned by <code>getTime()</code>
 *         is emitted (long).  This represents the offset from
 *             January 1, 1970, 00:00:00 GMT in milliseconds.
 */
private void writeObject(ObjectOutputStream s)
     throws IOException
{
    s.writeLong(getTimeImpl());
}

therefore, it's the long value representing the offset from Jan 1 1970 00:00:00 GMT in milliseconds.

EDIT: however this is preceeded and succeeded by some headers:

0x73 - being the code for an ordinary object (TC_OBJECT)    
0x72 - being the code for a class description (TC_CLASSDESC)    
"java.util.Date" - the name of the class    
7523967970034938905L - the serialVersionUID    
0|0x02|0x01 - flags including SC_SERIALIZABLE & SC_WRITE_METHOD    
0 - number of fields    
0x78 - TC_ENDBLOCKDATA    
null - there is no superclass descriptor    
the time (long milliseconds since epoch)    
0x78 - TC_ENDBLOCKDATA