Java Object Serialization

Java Object Serialization, often referred to by third-party sources as simply Java serialization, is the native serialization format for the Java programming language, analogous to the Pickle format for Python. It appears to have been introduced in 1997, with version 1.1.

Versions
There are three different things in or attributes of a Java Object Serialization stream that are referred to by "version": The "protocol version" can be either 1 or 2. The second version was made standard in the Java platform in version 1.2 in December 1998. Java versions at and above 1.1.7 can read both versions, but will default to writing and reading 2. There is nothing in the stream to indicate the "protocol version"; users must decide on it (fill this in with heuristics).
 * "protocol version", which corresponds to the usual meaning of "version";
 * an integer field in the header called "STREAM_VERSION" or just "version", which has been 5 since at least JDK 1.1.4, released between 1997 and 1998 , meaning that it has had the value of 5 for almost all of the format's existence;
 * and the "version" not of the Object Serialization format, but of whatever format is built on top of it.

Specifications

 * Protocol version 1
 * Protocol version 2