Programming Java threads in the real world, Part 1

A Java programmer's guide to threading architectures

By Allen Holub, JavaWorld.com, 09/01/98

This article is the first in a series that will present real-world solutions to the problems of programming Java in a multithreaded environment. It's geared to Java programmers who understand the language-level stuff (the synchronized keyword and the various facilities of the Thread class), but want to learn how to use these language features effectively.

Platform dependence

Unfortunately, Java's promise of platform independence falls flat on its face in the threads arena. Though it's possible to write a platform-independent multithreaded Java program, you have to do it with your eyes open. This isn't really Java's fault; it's almost impossible to write a truly platform-independent threading system. (Doug Schmidt's ACE [Adaptive Communication Environment] framework is a good, though complex, attempt. See Resources for a link to his program.) So, before I can talk about hard-core Java-programming issues in subsequent installments, I have to discuss the difficulties introduced by the platforms on which the Java virtual machine (JVM) might run.

Atomic energy

The first OS-level concept that's important to understand is atomicity. An atomic operation cannot be interrupted by another thread. Java does define at least a few atomic operations. In particular, assignment to variables of any type except long or double is atomic. You don't have to worry about a thread preempting a method in the middle of the assignment. In practice, this means that you never have to synchronize a method that does nothing but return the value of (or assign a value to) a boolean or int instance variable. Similarly, a method that did a lot of computation using only local variables and arguments, and which assigned the results of that computation to an instance variable as the last thing it did, would not have to be synchronized. For example:

class some_class
{   
    int some_field;
    void f( some_class arg ) // deliberately not synchronized
    {
        // Do lots of stuff here that uses local variables
        // and method arguments, but does not access
        // any fields of the class (or call any methods
        // that access any fields of the class).
        // ...
        some_field = new_value;     // do this last.
    }
}

On the other hand, when executing x=++y or x+=y, you could be preempted after the increment but before the assignment. To get atomicity in this situation, you'll need to use the keyword synchronized.