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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 flag indicating if the class defined a writeObject method
  • 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 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 depends on the length of the UTF encoded string. If the UTF encoding of the given String is less than 65536 bytes in length, the String is written in the standard Java UTF-8 format. Starting with the Java 2 SDK, Standard Edition, v1.3, strings for which the UTF encoding length is greater than or equal to 65536 bytes are written in a variant "long" UTF format. The "long" UTF format is identical to the standard Java UTF-8 format, except that it uses 8 bytes to write the length of the UTF string, instead of 2 bytes. 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.
New objects in the stream are represented by the following:

  • The most derived class of the object.
  • Data for each serializable class of the object, with the highest superclass first.For each class the stream contains the following:

All primitive data written by classes is buffered and wrapped in block-data records, regardless if the data is written to the stream within a writeObject method or written directly to the stream from outside a writeObject method. This data can only be read by the corresponding readObject methods or be read directly from the stream. Objects written by the writeObject method terminate any previous block-data record and are written either as regular objects or null or back references, as appropriate. The block-data records allow error recovery to discard any optional data. When called from within a class, the stream can discard any data or objects until the endBlockData.


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