Java Concurrent API's Lock not working as intended [duplicate]

Question

I am trying to synchronize 2 threads using Java Concurrent's lock API. The code basically increments a counter to a certain number using 2 threads but the result i am getting is less than the said number even after using locks. Code-

import java.util.concurrent.*; import java.util.concurrent.locks.ReentrantLock; class DemoClass { public int count =0; public ReentrantLock lock = new ReentrantLock(); public void increment() { lock.lock(); try { count++; } finally { lock.unlock(); } } } public class Main { public static void main(String[] args) { ExecutorService executor = Executors.newFixedThreadPool(2); DemoClass demoClass = new DemoClass(); int i=0; for(;i<=10000;i++) { executor.submit(demoClass::increment); } executor.shutdown(); System.out.println( demoClass.count); // 10000 } }

Not sure what is it i am doing wrong here.

"// 10000" Note that the correct result is 10001, because you used <=10000. — Andy Turner
– Andy Turner, Commented May 20, 2021 at 11:54

Andy Turner · Accepted Answer · 2021-05-20 17:35:35Z

You're not waiting for all the threads to complete. executor.shutdown() just stops executor accepting new work.

You need to use executor.awaitTermination(...) after executor.shutdown();.

Ideone demo

Also, you should surround the read of demoClass.count with a lock/unlock pair, in order to establish the happens-before relationship between the writes to the variable in the thread and the read.

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