Are primitive datatypes thread-safe in Java

问题

Are the primitive data types like int & short thread-safe in Java? I have executed the following code and couldn\'t see expected result 500 some times.

public class SampleThree extends Thread
{
    static long wakeUpTime = System.currentTimeMillis() + (1000*20);
    static int inT;
    public static void main(String args[])
    {
        System.out.println(\"initial:\" + inT);
        for(int i=0; i<500; i++)
            new SampleThree().start();
        try {
            Thread.sleep(wakeUpTime - System.currentTimeMillis() + (1000*30));
            System.out.println(\"o/p:\" + inT);
        }
        catch(Exception e){
            e.printStackTrace();
        }
    }

    public void run()
    {
        try {
            long s = wakeUpTime - System.currentTimeMillis();
            System.out.println(\"will sleep ms: \" + s);
            Thread.sleep(s);
            inT++; // System.out.println(inT);
        }
        catch(Exception e) {
            e.printStackTrace();
        }
    }
}

Here concurrently 500 thread will update the int variable inT. Main thread after waiting for concurrent update to be completed, prints inT value.

Find similar example here

回答1:

There are three ways in which they're not safe:

long and double aren't even guaranteed to be updated atomically (you could see half of a write from a different thread)
The memory model doesn't guarantee that you'll see the latest updates from one thread in another thread, without extra memory barriers of some kind
The act of incrementing a variable isn't atomic anyway

Use AtomicInteger etc for thread-safe operations.

回答2:

Primitive types are not thread safe. Check this tutorial.

回答3:

I would suggest using classes in java.util.concurrent.atomic. They are designed for thread-safety and in some cases the JVM can take advantage of hardware features to optimize.

来源：https://stackoverflow.com/questions/9278764/are-primitive-datatypes-thread-safe-in-java

标签

java

thread-safety

primitive