OPJ Tutorial: Problems with the transient modifier

"Variables may be marked transient to indicate that they are not part of the persistent state of an object. If an instance of the class Point:

  class Point {
    int x, y;
    transient float rho, theta;
  }

The OPJ specification interprets the transient modifier in the definition of persistence reachable. An object is persistence reachable if it is reachable by the standard JLS rules, unless it is reachable solely through chains of reference chains that pass through any variables marked with the transient modifier.

Unfortunately, since JDK 1.1, the transient modifier has been given a specification that is specific to Java Object Serialization, and is incompatible with the natural interpretation for orthogonal persistence. The specification is also incompatible with the ODMG Java binding. The essence of the incompatibility is an idiomatic use of transient to indicate a field that will be made persistent (serialized) but will have its representation transformed by user-code as part of the serialization process. The classic example is the java.util.Hashtable class, where the table entries themselves are marked transient.

The OPJ specification contains a proposed language change that adds sub-modifiers to the transient modifier, to distinguish object serialization and object persistence. Until this is accepted and implemented, the specification chooses to ignore the transient modifer on non-static variables and defines an API mechanism for indicating that a field is transient for object persistence purposes, namely OPTransient.mark("fieldName"). For example, here is a fragment of a class that manages an open file, in which the fd variable represents external state that is associated with the process supporting the OPJ Virtual Machine, and therefore should not be made persistent.

public class FileStream {     private transient FileDescriptor fd;     static { OPTransient.mark("fd"); } }

The method OPTransient.mark must be called in a static initializer of the class defining the field passed as argument.

Note that, since static variables are ignored by object serialization, OPJ does interpret  "static transient". It is harmless but redundant to also call OPTransient.mark on a static transient field.

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