Java synchronized Keyword
Example
Use the synchronized modifier to prevent race conditions between threads:
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.
Related Pages
Read more about modifiers in our Java Modifiers Tutorial.
Read more about threads in our Java Threads Tutorial.
