Menu
I'm making a program in java that races a few cars against each other. Each car is a separate thread.
When cars complete the race, each one all calls this method. I've tested the method at varying timer speeds, and it seems to work fine. But I do realize that each thread is accessing the variable carsComplete, sometimes at the exact same time (at least at the scope the date command is giving me).
So my question is: is this method thread-safe?
public static String completeRace()
{
Date accessDate = new Date();
System.out.println("Cars Complete: " + carsComplete + " Accessed at " + accessDate.toString());
switch(++carsComplete)
{
case 1: return "1st";
case 2: return "2nd";
case 3: return "3rd";
default: return carsComplete + "th";
}
}
397
Example of Non-Thread-Safe Code in Java The above example is not thread-safe because ++ (the increment operator) is not an atomic operation and can be broken down into reading, update, and write operations.
On its stack(basically thread stack), local primitives and local reference variables are stored. Hence one thread does not share its local variables with any other thread as these local variables and references are inside the thread's private stack. Hence local variables are always thread-safe.
To test if the combination of two methods, a and b, is thread-safe, call them from two different threads. Put the complete test in a while loop iterating over all thread interleavings with the help from the class AllInterleavings from vmlens. Test if the result is either an after b or b after a.
Using Atomic Variable Using an atomic variable is another way to achieve thread-safety in java. When variables are shared by multiple threads, the atomic variable ensures that threads don't crash into each other.
No, you should be using something like java.util.concurrent.atomic.AtomicInteger. Look at its getAndIncrement() method.
178
Pre-increment on int is not thread safe, use AtomicInteger which is lock-free:
AtomicInteger carsComplete = new AtomicInteger();
//...
switch(carsComplete.incrementAndGet())
BTW the code below is not thread safe as well. Can you tell why?
carsComplete.incrementAndGet();
switch(carsComplete.get())
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
