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POThread safe global variable in Java
 

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  1. POThread safe global variable in Java
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    <p>I am trying to understand the thread safety mechanism in java and I need some help. I have a class:</p> <pre><code>public class ThreadSafe { private Executor executor = new ScheduledThreadPoolExecutor(5); private long value = 0; public void method() { synchronized (this) { System.out.println(Thread.currentThread()); this.value++; } } private synchronized long getValue() { return this.value; } public static void main(String... args) { ThreadSafe threadSafe = new ThreadSafe(); for (int i = 0; i &lt; 10; i++) { threadSafe.executor.execute(new MyThread()); } } private static class MyThread extends Thread { private ThreadSafe threadSafe = new ThreadSafe(); private AtomicBoolean shutdownInitialized = new AtomicBoolean(false); @Override public void run() { while (!shutdownInitialized.get()) { threadSafe.method(); System.out.println(threadSafe.getValue()); } } } } </code></pre> <p>Here I am trying to make the <code>value</code> thread safe, to be accessed only by one thread at a time. When I am running this program I see that there is more than one thread operating on the <code>value</code> even if I wrap it within the <code>synchronized</code> block. Of course this loop will be infinite but its just an example, I am stopping this program manually after few seconds, so I have:</p> <pre><code>2470 Thread[pool-1-thread-3,5,main] 2470 Thread[pool-1-thread-5,5,main] 2470 Thread[pool-1-thread-2,5,main] </code></pre> <p>Different threads are accessing and changing this <code>value</code>. Can somebody explain to me why is this so? And how to make this global variable thread safe?</p>
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    1. COWasn't your intention to make long static in this example? Because in any other case there is no threadsafety issue.
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      1. USMichal Borek
    2. COMake sure you notice my answer. Your `MyThread` class should _not_ extend `Thread` and _should_ implement `Runnable`.
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