A new kind of type is described whose objects ("qualifiers") have bracket methods which can modify the run-time behaviour of other objects ("targets"). Bracket methods can qualify either specific methods of a target or can separately qualify their reader and writer methods, thus allowing general qualifiers to be developed for standard activities such as synchronisation, monitoring and protection. Qualifiers are associated with a target when it is created, in the form of a qualifier list. Individual qualifiers can be dynamically added to and removed from the list even while the target object is active.

The paper describes how the idea of persistent objects is integrated into the Timor programming language. The strategy adopted allows types to be instantiated at two levels: as "files", i.e. objects accessible at the operating system level, and as local objects within files, which resemble objects found in conventional object oriented programs. File objects (with associated methods) can be instantiated and manipulated via capabilities, which are accessible both internally and via the operating system. Local objects are accessible via references, which are not visible at the operating system level. PERSISTENT OBJECTS AND CAPABILITIES IN TIMOR104 J OURNAL OF OBJECT TECHNOLOGY V OL. 6, NO. 4 PERSISTENT OBJECTS AND CAPABILITIES IN TIMOR 106 J OURNAL OF OBJECT TECHNOLOGY V OL. 6, NO. 4 final op String name(String name); final enq String name(); and the abstract reference spouse corresponds to the methods: final op Person* spouse(Person* spouse); final enq Person* spouse(); PERSISTENT OBJECTS AND CAPABILITIES IN TIMOR 122 J OURNAL OF OBJECT TECHNOLOGY

In earlier papers qualifying types were presented as a technique for dynamically qualifying objects in a general way. This paper considers how such types can be composed statically into the definitions of other types.

This paper extends the concept of qualifying types by describing how their implementations can include not only bracket methods which are applied when a method of a target object is invoked, but also further "call-out" bracket methods which can be applied to invocations by the target object of the methods of other objects. This additional technique can be used for example to provide enhanced synchronisation in qualifying types, as an aid to confining the activities of an object, and as a means of providing parallel activities associated with the sending and receiving of information, e.g. encryption and decryption, data compression.

The paper explains the concept of persistent processes and threads in Timor, showing how it is based on the in-process (procedure oriented) model of process execution. In their Timor form processes and threads can persist even when a user is logged out. They can also invoke remote persistent objects with the same semantics as invocation of local objects.