Sun Microsystems 2 manual Multitask Safety and Multithread Safety

The following list summarizes the multitasking safety issues to consider when you update or add native code for your port:

■Multitask safety and multithread safety

■Native global or static data

■Singletons

Multitask Safety and Multithread Safety

Many systems today are multithreaded, which requires code that runs in these systems to be thread safe or multithread safe. For example, POSIX Threads (Pthreads) enables multiple native threads to run in the same OS process. Each native thread has access to native memory, so any data structures in native memory must be protected from concurrent access. This is typically accomplished through the use of locks to provide mutual exclusion.

Similarly, multiple Java platform threads can run in the same Java virtual machine. Each Java platform thread has access to the objects in the Java virtual machine, so these objects must also be protected from concurrent access. Java code typically accomplishes this through the use of synchronized code blocks or higher-level constructs.

In the Java Wireless Client software, each task has one or more threads. Each of these threads has concurrent access to the objects in that task, and so the same multithread safety issues occur in Java Wireless Client software as in conventional Java virtual machines. However, a thread in one task has access only to objects within its task, and it has no access to objects in any other task. The multitasking nature of Java Wireless Client software thus has no impact on the multithreaded safety of applications.

All of the tasks in Java Wireless Client software run in a single OS process, and therefore they all have access to the same native memory and data structures. Threads from different tasks are scheduled arbitrarily. Therefore, native methods that read or update native data structures must be prepared to deal with operations from different tasks being interleaved in an arbitrary order. Furthermore, because different tasks are generally running different applications, these different applications are likely to place quite different demands on the underlying native system.

6 Multitasking Guide • May 2007