Synchronization on immutable objects (in java)
java, multithreading, synchronization
Solution
synchronized (won)
{
if (!won)
won = true;
}
Here you have a transient race condition which you don't notice because it goes away after the first execution of the `run` method. After that the `won` variable constantly points to the same instance of `Boolean` representing `true`, which thus serves properly as a mutex lock.
This is not to say that you should ever write such code in real projects. All lock objects should be assigned to `final` variables to make sure they never change.
Problem
Code snippet - 1 ``` class RequestObject implements Runnable { private static Integer nRequests = 0; @Override public void run() { synchronized (nRequests) { nRequests++; } } } ``` Code snippet - 2 ``` public class Racer implements Runnable { public static Boolean won = false; @Override public void run() { synchronized (won) { if (!won) won = true; } } } ``` I was having a race condition with the first code snippet. I understood that this was because I was obtaining a lock on an immutable object(of type Integer). I have written a second code snippet which is again impervious to 'Boolean' being immutable. But this works(no race condition is displayed in an output run). If I have understood the solution to my previous question properly the below is is one possible way in which things can go wrong - Thread 1 gets a lock on an object(say A) pointed by `won` - Thread 2 tries to now get a lock on the object pointed to by `won` and gets in the wait queue for A - Thread 1 goes into the synchronized block, verifies that A is false and creates a new object(say B) by saying `won = true`(A thinks it won the race). - 'won' now points to B. Thread 1 releases the lock on object A(no longer pointed to by `won`) - Now, thread-2 which was in the wait queue of object A gets woken up and gets a lock on object A which is still `false`(immutably so). It now goes into the synchronized block and assumes that it has also won, which is not correct. Why is the second code snippet working fine all the time??