Sun Microsystems 2 manual Multitasking Safety

C H A P T E R 2

Multitasking Safety

The Java Wireless Client software provides the ability to run multiple MIDlets concurrently in a single OS process. From the standpoint of the OS, there is one process and one Java virtual machine. However, from the standpoint of a Java application, it appears as if it is running in its own, independent virtual machine, isolated from other Java applications.

These apparently independent virtual machines are called tasks. Each MIDlet runs in its own task. When a new MIDlet is started, a new task is created for it. When a MIDlet exits, its task is destroyed. The Application Management Software (AMS) runs in a dedicated task, and it is running the entire time the Java Wireless Client software is active.

Although tasks cannot interact (they cannot access each other’s objects, for example), they do share native code, process resources, and external resources. This sharing leads to some implementation issues, both in native code and in Java programming language code (Java code). The Java Wireless Client software takes these issues into account. Those who want to integrate existing libraries, which might not have been written with multitasking in mind, also need to be aware of the issues so that they can add the source code correctly.

Code integrated into the Java Wireless Client software must be multitask safe. That is, it must maintain the independence of one task from another. Because tasks run in a single operating system process, the OS process can run native code for any task. To keep tasks from interfering with each other and with each other’s data, the code must be made aware of the task on whose behalf it is being called and possibly allocating resources. When the task context is established, the code is considered multitask safe.

This chapter points out issues that might arise in ensuring native code is multitask safe. It also provides techniques for making the code multitasking safe in different situations.

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