Data types

Opaque objects will be presented as Java objects. This introduces the option of simplifying the user's task in managing these objects. MPI destructors can be absorbed into Java object destructors, which are called automatically by the Java garbage collector. We adopt this strategy as the general rule. Explicit calls to MPI destructor functions are typically omitted from the Java user interface. An exception is made for the Comm classes. MPI_COMM_FREE is a collective operation, so the user must ensure that calls are made at consistent times by all processors involved--the call can't be left to the vagaries of the garbage collector.

• Advice to implementors. For JNI-based ``wrapper'' implementations based on the C binding of MPI, one should ensure that all MPI functions, including the MPI_..._free object destructors, are called before MPI_Finalize is called. Because invocation of Java destructors is under the control of the garbage collector there is no guarantee that all Java-level finalize methods will be called before the explicit call to the Java-level MPI.finish. A possible solution is to maintain a global count of the number of outstanding MPI_..._free calls. This is initialized to 1 by MPI.init and incremented by relevant constructors. All relevant Java finalize methods and MPI.finish should decrement this counter, and be prepared to call the physical MPI_Finalize function when and only when the count falls to zero.(End of advice to implementors.)