Check the condition for a loop
Checking the condition for waiting in a loop is crucial to prevent spurious wake-ups and ensure correctness in multithreaded programs.
Table of Contents
1. Spurious Wake-ups
Sometimes, a waiting thread can be awakened even though no other thread has called notify() or notifyAll(). These are known as spurious wake-ups and can occur due to various reasons, including JVM implementation details or system interrupts. Checking the condition in a helps guard against such spurious wake-ups by ensuring that the thread only proceeds when the condition it was waiting for is actually satisfied.
2. Concurrency Issues
In a multithreaded environment, the state of shared variables can change unexpectedly. Without a loop to recheck the condition after waking up, a waiting thread might mistakenly proceed assuming that the condition is met, leading to incorrect behavior or race conditions.
Let’s demonstrate the importance of checking the condition in a with an example:
java
public class ConditionCheckExample {
private boolean condition = false;
public synchronized void waitForCondition() throws InterruptedException {
// Without loop
if (!condition) {
wait(); // wait until condition becomes true
}
System.out.println("Condition is now true, proceeding...");
}
public synchronized void setConditionTrue() {
condition = true;
notifyAll(); // notify all threads waiting on this object
}
public static void main(String[] args) {
ConditionCheckExample example = new ConditionCheckExample();
Thread thread1 = new Thread(() -> {
try {
example.waitForCondition();
} catch (InterruptedException e) {
e.printStackTrace();
}
});
Thread thread2 = new Thread(() -> {
example.setConditionTrue();
});
thread1.start();
thread2.start();
}
}
To fix this, let's modify the waitForCondition() method to include a loop for condition checking:
java
public synchronized void waitForCondition() throws InterruptedException {
while (!condition) {
wait(); // wait until condition becomes true
}
System.out.println("Condition is now true, proceeding...");
}
Example condition for a loop
In this example, if the condition is false initially, and the waiting thread is awakened due to a spurious wake-up, it will proceed to execute even though the condition is not satisfied. This can lead to incorrect behavior in the program.
With the loop, the waiting thread will recheck the condition after waking up, ensuring that it only proceeds when the condition is actually true, thus avoiding potential issues caused by spurious wake-ups.