Java is unstable and / or synchronized

Question

I have a static method that should generate a unique identifier based on the current timestamp, as shown in the codes below. To make sure that the newly generated identifier does not match the previously generated identifier (due to a very fast computer, so the millisecond does not change), I set a loop to compare the newly generated identifier with the previously generated identifier. If they match, it will generate another identifier.

public class Util {

    protected static String uniqueID;

    public static String generateUniqueID() {
        SimpleDateFormat timstampFormat = new SimpleDateFormat("yyyyMMddHHmmssSSS");
        do {
            String timestamp = timstampFormat.format(new Date());
            if (!timestamp.equals(uniqueID)) {
                uniqueID = timestamp;
                return uniqueID;
            }
        } while (true);
    }

}

I want the codes above to work when a method is called by multiple threads.

If I just put the volatile keyword in the uniqueID variable, would that be good enough? Should I have a synchronized block?

, volatile?

.

:

, volatile?

public class Util {

    private static volatile String uniqueID;

    public static synchronized String generateUniqueID() {
        uniqueID = UUID.randomUUID().toString();
        return uniqueID;
    }

}

Answer 1

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Answer 2

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private static final AtomicLong time = new AtomicLong(0);
public static long uniqueTimedId() {
    while(true) {
         long now = System.currentTimeMillis();
         long value = time.get();
         long next = now > value ? now : value + 1;
         if (time.compareAndSwap(value, next))
            return next;
    }
}

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private static final AtomicLong time = new AtomicLong(0);
public static long uniqueTimedId() {
    while(true) {
         long now = System.currentTimeMillis() * 1000;
         long value = time.get();
         long next = now > value ? now : value + 1;
         if (time.compareAndSwap(value, next))
            return next;
    }
}

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private static String uniqueID;

public static synchronized String generateUniqueID() {
    uniqueID = UUID.randomUUID().toString();
    // without synchronized, one thread could read what another thread wrote
    return uniqueID;
}

, , .

// nothing is shared, so no thread safety issues.
public static String generateUniqueID() {
    return UUID.randomUUID().toString();
}

Answer 3

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Answer 4

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public static String generateUniqueID() {
    return UUID.randomUUID().toString();
}

public static UUID randomUUID()

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