Deadlock freedom through object ownership

Active objects are an attractive method of introducing con-currency into Java-like languages by decoupling method ex-ecution from invocation. In this paper, we show how owner-ship is used in the Java [14] subset language CoJava [17] to prevent deadlock associated with active object method calls. This approach builds on existing type-based approaches that eliminates data races and data-based deadlock in concurrent systems. The novel addition is the use of ownership to or-ganize active objects, thus preventing deadlock from arising when objects are allowed to block awaiting responses from others. Typechecking is used to prevent threads from sharing mu-table data, thus CoJava is free of data races and data-based deadlock. Behavioural deadlock is prevented by the use of promise objects which prevent clients from blocking indefi-nitely while awaiting responses. Ownership imposes a hier-archy on active objects; this allows owners to safely block while waiting for responses from owned objects. The paper also discusses the implications of this approach to specifica-tion with JML, formal reasoning about programs, and the consequences to runtime assertion checking. Categories and Subject Descriptor