Race Conditions
A race condition occurs when two
threads attempt to use the same resource at the same time. The
following class demonstrates a simple race condition. Two threads
simultaneously try to increment a counter. If each thread can
complete the increment( )
method in its entirety
without the other thread executing, then all is fine, and the counter
monotonically increases. Otherwise, the thread context switcher has
the opportunity to interrupt one thread in the middle of executing
the increment( )
method, and let the other thread
run through this method. Note that the thread can actually be
interrupted anywhere, not necessarily in the middle of the
increment( )
method, but I’ve greatly
increased the likelihood of an interruption in the
increment( )
method by including a print statement
there:
package tuning.threads; public class ThreadRace implements Runnable { //global counter static int num=0; public static void increment( ) { int n = num; //This next line gives the context switcher an ideal //place to switch context. System.out.print(num+" "); //And when it switches back, n will still be the old //value from the old thread. num = n + 1; } public static void main(String args[]) { ThreadRace d1 = new ThreadRace( ); ThreadRace d2 = new ThreadRace( ); Thread d1Thread = new Thread(d1); Thread d2Thread = new Thread(d2); d1Thread.start( ); d2Thread.start( ); } public void run( ) { for (int i = 200; i >= 0 ; i--) { increment( ); } } }
The output from executing this class on a ...
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