Object Serialization Stream Protocol

Topics:

• Overview
• Stream Elements
• Stream Protocol Versions
• Grammar for the Stream Format
• Example

6.1 Overview

The stream format satisfies the following design goals:

• Is compact and is structured for efficient reading.

• Allows skipping through the stream using only the knowledge of the structure and format of the stream. Does not require invoking any per class code.
• Requires only stream access to the data.

6.2 Stream Elements

A basic structure is needed to represent objects in a stream. Each attribute of the object needs to be represented: its classes, its fields, and data written and later read by class-specific methods. The representation of objects in the stream can be described with a grammar. There are special representations for null objects, new objects, classes, arrays, strings, and back references to any object already in the stream. Each object written to the stream is assigned a handle that is used to refer back to the object. Handles are assigned sequentially starting from 0x7E0000. The handles restart at 0x7E0000 when the stream is reset.

A class object is represented by the following:

• Its ObjectStreamClass object.

An ObjectStreamClass object for a Class that is not a dynamic proxy class is represented by the following:

• The Stream Unique Identifier (SUID) of compatible classes.

• A set of flags indicating various properties of the class, such as whether the class defines a writeObject method, and whether the class is serializable, externalizable, or an enum type
• The number of serializable fields
• The array of fields of the class that are serialized by the default mechanismFor arrays and object fields, the type of the field is included as a string which must be in "field descriptor" format (e.g., "Ljava/lang/Object;") as specified in section 4.3.2 of The Java^TM Virtual Machine Specification, Second Edition.
• Optional block-data records or objects written by the annotateClass method
• The ObjectStreamClass of its supertype (null if the superclass is not serializable)

An ObjectStreamClass object for a dynamic proxy class is represented by the following:

• The number of interfaces that the dynamic proxy class implements

• The names of all of the interfaces implemented by the dynamic proxy class, listed in the order that they are returned by invoking the getInterfaces method on the Class object.
• Optional block-data records or objects written by the annotateProxyClass method.
• The ObjectStreamClass of its supertype, java.lang.reflect.Proxy.

The representation of String objects consists of length information followed by the contents of the string encoded in modified UTF-8. The modified UTF-8 encoding is the same as used in the Java^TM Virtual Machine and in the java.io.DataInput and DataOutput interfaces; it differs from standard UTF-8 in the representation of supplementary characters and of the null character. The form of the length information depends on the length of the string in modified UTF-8 encoding. If the modified UTF-8 encoding of the given String is less than 65536 bytes in length, the length is written as 2 bytes representing an unsigned 16-bit integer. Starting with the Java^TM 2 platform, Standard Edition, v1.3, if the length of the string in modified UTF-8 encoding is 65536 bytes or more, the length is written in 8 bytes representing a signed 64-bit integer. The typecode preceding the String in the serialization stream indicates which format was used to write the String.

Arrays are represented by the following:

• Their ObjectStreamClass object.

• The number of elements.
• The sequence of values. The type of the values is implicit in the type of the array. for example the values of a byte array are of type byte.

Enum constants are represented by the following:

• The ObjectStreamClass object of the constant's base enum type.

• The constant's name string.

New objects in the stream are represented by the following:

• The most derived class of the object.