Sun Microsystems 2 manual Multitask Safety

This API is now multithread safe. However, it is not multitask safe. The reason is that the thread safety properties are achieved using mechanisms that belong to the Microwave class. These include static variables (initialized, owner) of the Microwave class. Thread synchronization and wait and notify operations use the monitor lock owned by the class. All of these mechanisms are visible to the threads of a single task, and thus they provide safety among the multiple threads of a single task.

However, in a multitasking environment, the class static variables and monitor lock are replicated in each task. Thus, if a thread in task A is performing an operation on the Microwave class, the initialized variable is true and the owner variable contains a reference to the thread performing the operation. However, in task B, the initialized variable still has the value false and the owner variable is null. If a thread in task B attempts an operation, it takes the lock (even though a thread in task A “owns” the lock in task A), it calls init() for the first time (from its point of view). This results in a second call to the native mw_init() function, which is an error. The thread in task B then calls some of the setup functions and then calls cook(), possibly before the cook operation initiated from task A completes, which is another error.

Multitask Safety

Use the microwave library and the Java programming language interface to illustrate how a library must be changed to be multitask safe.

The first and simplest case is class data (static variables) that is semantically global but is replicated into each task. Sometimes the singleton pattern is used in this fashion. Inspect each singleton Java class to determine whether it needs to be a singleton only from the point of view of Java threads that have access to it, or whether it needs to be a system-wide singleton. In the former case, ordinary singleton classes are sufficient. Even though they are replicated within each task, they can be per-task singletons because each Java thread sees only one instance of the singleton class. In the latter case, for global singletons, the singleton characteristic must apply to the entire system. The typical way to accomplish this is to migrate the relevant data from Java code into native code.

In our example, the global singleton data is a static boolean variable initialized that indicates whether the microwave library has been initialized. This can simply be migrated to an int variable in native code. Once this variable is in native code, it is visible to all tasks using native methods. You can write native methods to set and get this value and take action as appropriate. This technique is illustrated in CODE EXAMPLE 2-6.

Chapter 2 Multitasking Safety 13