Example
Use the synchronized modifier to prevent race conditions between threads:
java
public class Main implements Runnable {
public static int a, b;
public static void main(String[] args) {
a = 100;
b = 100;
// Check the total amount shared between a and b before the transfers
System.out.println("Total before: " + (a + b));
// Run threads which will transfer amounts between a and b
Thread thread1 = new Thread(new Main());
Thread thread2 = new Thread(new Main());
thread1.start();
thread2.start();
// Wait for the threads to finish running
try {
thread1.join();
thread2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
// Check the total amount shared between a and b after the transfers
// It should be the same amount as before
System.out.println("Total after: " + (a + b));
}
public void run() {
for (int i = 0; i < 10000000; i++) {
transfer();
}
}
public static synchronized void transfer() {
// Choose a random amount to transfer
int amount = (int) (5.0 * Math.random());
// Transfer between a and b
if (a > b) {
a -= amount;
b += amount;
} else {
a += amount;
b -= amount;
}
}
}
Definition and Usage
The synchronized keyword is a modifier that locks a method so that only one thread can use it at a time. This prevents problems that arise from race conditions between threads.
In the example above, removing the synchronized keyword from the transfer() method may cause the values of a and b to be modified by another thread in between operations. This would result in the total amount between the two variables to change.