I've found an interesting and easy to understand explanation about volatile keyword, it came with an example but it was wrong so I've corrected it.
A volatile variable does not have a copy maintained in the local memory of the thread (on the stack). All changes to the volatile variable (caused by multiple threads) are flushed out to the heap memory (visible from all threads). Hence volatile variable values remain consistent for all threads.
On the other hand, for other instance variables, each java thread maintains a local copy on the stack. Multiple threads may modify this local copy of the instance variable and hence inconsistent values may be visible for multiple threads.
For preventing this condition, we synchronize. During synchronization, a lock is first taken on the object monitor. Then the thread reads the state from the main memory and flushes its internal state. Subsequently, the synchronized code block/method code is executed. Once the execution completes, all the changes to the variables of that thread are flushed out to the main memory. Then the object monitor lock is released.
So, as we can see, volatile is a specialized case of synchronization. The only exceptions are, that it operates on a single field and no locks on the object monitor are required (as it operates on the heap memory and not the thread local stack memory).
If you are working with the multi-threaded programming, the volatile keyword will be more useful. When multiple threads using the same variable, each thread will have its own copy of the local cache for that variable. So, when it's updating the value, it is actually updated in the local cache not in the main variable memory. The other thread which is using the same variable doesn't know anything about the values changed by the another thread. To avoid this problem, if you declare a variable as volatile, then it will not be stored in the local cache. Whenever thread are updating the values, it is updated to the main memory. So, other threads can access the updated value.
A volatile variable does not have a copy maintained in the local memory of the thread (on the stack). All changes to the volatile variable (caused by multiple threads) are flushed out to the heap memory (visible from all threads). Hence volatile variable values remain consistent for all threads.
On the other hand, for other instance variables, each java thread maintains a local copy on the stack. Multiple threads may modify this local copy of the instance variable and hence inconsistent values may be visible for multiple threads.
For preventing this condition, we synchronize. During synchronization, a lock is first taken on the object monitor. Then the thread reads the state from the main memory and flushes its internal state. Subsequently, the synchronized code block/method code is executed. Once the execution completes, all the changes to the variables of that thread are flushed out to the main memory. Then the object monitor lock is released.
So, as we can see, volatile is a specialized case of synchronization. The only exceptions are, that it operates on a single field and no locks on the object monitor are required (as it operates on the heap memory and not the thread local stack memory).
If you are working with the multi-threaded programming, the volatile keyword will be more useful. When multiple threads using the same variable, each thread will have its own copy of the local cache for that variable. So, when it's updating the value, it is actually updated in the local cache not in the main variable memory. The other thread which is using the same variable doesn't know anything about the values changed by the another thread. To avoid this problem, if you declare a variable as volatile, then it will not be stored in the local cache. Whenever thread are updating the values, it is updated to the main memory. So, other threads can access the updated value.
class ExampleThread extends Thread {
private volatile int testValue;
public ExampleThread(String str) {
super(str);
}
public void run() {
for (int i = 0; i < 3; i++) {
try {
System.out.println(Thread.currentThread().getName() + " : " + i);
if (Thread.currentThread().getName().equalsIgnoreCase("T1")) {
System.out.println("Test Value T1: " + testValue);
testValue = 10;
}
if (Thread.currentThread().getName().equalsIgnoreCase("T2")) {
System.out.println("Test Value T2: " + testValue);
testValue = 20;
}
Thread.sleep(1000);
} catch (InterruptedException exception) {
exception.printStackTrace();
}
}
}
}
public class VolatileExample {
public static void main(String args[]) {
ExampleThread exampleThread = new ExampleThread("Thread 1 ");
Thread t1 = new Thread(exampleThread, "T1");
Thread t2 = new Thread(exampleThread, "T2");
t1.start();
t2.start();
}
}
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