Sun Microsystems 2 manual Global and Static Data

For example, certain native functions (such as file storage) must be maintained on a per-application basis. In a single-tasking system, only one application is running, and so all file access is on behalf of that one application. In a multitasking system, several applications are running, and so the file access code can no longer assume that just one application exists. Instead, it must be aware of the possibility of multiple applications, so that one application doesn't accidentally operate on another application’s files. Code that is aware of this possibility is multitask safe.

While operations from different tasks can occur in an arbitrary order, no possibility exists of actual concurrency among native methods. One native thread exists in the Java Wireless Client software. When it is running a native method, no possibility exists of a context switch that causes another native method to be run at the same time. Each native method runs to completion and returns before the next native method can begin. For this reason, every native method is a critical section, and it is usually not necessary to perform any OS-level locking (such as with Pthreads mutexes) in native methods. It is only necessary to use OS-level locking if other native threads are active in the system while the Java Wireless Client software is running.

Thus, while native code that runs within Java Wireless Client software must be multitask safe, it need not be multithread safe.

Global and Static Data

In a single-tasking system, it is common for native code to use global or static variables. This works because only one application is running at a time. Global data is implicitly associated with the currently running application. In a multi-tasking system, the multiple applications likely conflict over global data. Therefore, to make your code multitask safe, you might need to rearrange your native data to be allocated on a per-task basis instead of globally.

To do this, you must allocate native data dynamically instead of statically. In addition, you need to associate each piece of native data with a particular task.

To manage native data on a per-task basis, store a pointer to the native data into an int field in a Java object. Associating the native resource with a single Java object implicitly provides multitasking safety because each Java object resides in exactly one task. It is also a good object-oriented approach.

Create a class to represent the native resource. Give the class a private field to hold the pointer to the resource, for example:

private int nativePtr;

Chapter 2 Multitasking Safety 7