CSC352: Java Threads and Synchronization Examples
--D. Thiebaut (talk) 21:12, 4 September 2013 (EDT)
Contents
A Badly Written (and Flawed) Multithreaded Computation of Pi
package DT;
public class UnsynchronizedThreadExample {
static int sum = 0;
class PiThreadBad extends Thread {
private int N; // the total number of samples/iterations
public PiThreadBad( int Id, int N ) {
super( "Thread-"+Id ); // give a name to the thread
this.N = N;
}
@Override
public void run() {
for ( int i=0; i<N; i++ )
sum ++;
}
}
public void process( int N ) {
PiThreadBad t1 = new PiThreadBad( 0, N );
PiThreadBad t2 = new PiThreadBad( 1, N );
//--- start two threads ---
t1.start();
t2.start();
//--- wait till they finish ---
try {
t1.join();
t2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println( "sum = " + sum );
}
public static void main(String[] args) {
int N = 100000000;
UnsynchronizedThreadExample U = new UnsynchronizedThreadExample();
U.process( N );
}
}
Output
sum = 180612836 Execution time: 19 ms
Note that the sum should really be 200000000, as both threads increment sum 100000000 times. The result is certainly incorrect.
Note also that the execution time is quite fast: 19 ms.
A Synchronized Version of the Same Program
We decide to put the statement that increments the variable sum into a function, and ask Java to synchronize around the function, i.e. make sure than only one thread at a time runs through this function. In other word, the synchronized function becomes atomic for threads.
package DT;
public class SynchronizedThreadExample {
int sum = 0;
Integer lock=0;
SynchronizedThreadExample() {
sum = 0;
}
class PiThreadBad extends Thread {
private int N; // the total number of samples/iterations
public PiThreadBad( int Id, int N ) {
super( "Thread-"+Id ); // give a name to the thread
this.N = N;
}
@Override
public void run() {
for ( int i=0; i<N; i++ )
synchronized( lock ) {
sum++;
}
}
}
public void process( int N ) {
long startTime = System.currentTimeMillis();
PiThreadBad t1 = new PiThreadBad( 0, N );
PiThreadBad t2 = new PiThreadBad( 1, N );
//--- start two threads ---
t1.start();
t2.start();
//--- wait till they finish ---
try {
t1.join();
t2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println( "sum = " + sum );
System.out.println( "Execution time: " + (System.currentTimeMillis()-startTime) + " ms" );
}
public static void main(String[] args) {
int N = 100000000;
SynchronizedThreadExample U = new SynchronizedThreadExample();
U.process( N );
}
}
Output
sum = 200000000 Execution time: 8448 ms
Note that the result is now correct. However the execution time is 400 longer!