java keyword volatile not behaving as expected
## Why is the Java Volatile Keyword Not Behaving as Expected?
In Java, the `volatile` keyword ensures that changes to a variable are immediately visible to all threads. However, there are certain scenarios where the `volatile` keyword may not be effective.
**1. Infinite Loops without Incrementing Variables:**
In the given code, both loops are infinite because the `j` variable is not incremented. This prevents the threads from exiting the loops and completing the task.
**2. Incorrect Use of `wait/notify`:**
The `wait` method is designed to be used with condition variables. In this case, a condition needs to be checked before calling `wait`. Additionally, the `wait` method should be called within a loop, as spurious wakeups are possible.
**3. Logical Error in Ordering:**
To ensure that before and after values are never equal, the code should call `wait` before adding a value and `notify` after reading the before value.
**4. Spurious Wakeups:**
Spurious wakeups occur when a thread is awakened from `wait` without any notification. This can cause unexpected behavior and make it difficult to reason about the code's execution.
**Revised Code:**
The following code addresses the issues mentioned above and demonstrates how to use the `volatile` keyword correctly:
```java
public class Main3 {
private static volatile int i = 0;
private static volatile boolean write = false;
private static volatile boolean read = false;
private static final Object o = new Object();
private static void add() {
i++;
}
public static void main(String[] args) throws InterruptedException {
// The first thread performs the write operation
Thread th1 = new Thread(() -> {
for (int j = 0; j < 10000; j++) {
synchronized (o) {
while (!write) {
try {
o.wait();
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
add();
write = false;
read = true;
o.notify();
}
}
});
// The second thread performs the read operation
Thread th2 = new Thread(() -> {
for (int j = 0; j < 10000; j++) {
int before;
int after;
synchronized (o) {
before = i;
write = true;
o.notify();
while (!read) {
try {
o.wait();
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
read = false;
after = i;
}
if (before == after) {
System.out.println("before: " + before + ", after: " + after);
}
}
});
th1.start();
th2.start();
th1.join();
th2.join();
System.out.println(i);
}
}
```