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1. Overview

Generating random values is a very common task. This is why Java provides the java.util.Random class.

However, this class doesn’t perform well in a multi-threaded environment.

In a simplified way, the reason for poor performance of Random in a multi-threaded environment is due to contention – given that multiple threads share the same Random instance.

To address that limitation, Java introduced the java.util.concurrent.ThreadLocalRandom class in JDK 7 – for generating random numbers in a multi-threaded environment.

Let’s see how ThreadLocalRandom performs and how to use it in real-world applications.

2. ThreadLocalRandom over Random

ThreadLocalRandom is a combination of ThreadLocal and Random classes, which is isolated to the current thread. Thus, it achieves better performance in a multithreaded environment by simply avoiding any concurrent access to the Random objects.

The random number obtained by one thread is not affected by the other thread, whereas java.util.Random provides random numbers globally.

Also, unlike Random, ThreadLocalRandom doesn’t support setting the seed explicitly. Instead, it overrides the setSeed(long seed) method inherited from Random to always throw an UnsupportedOperationException if called.

Let’s now take a look at some of the ways to generate random int, long and double values.

3. Generating Random Values Using ThreadLocalRandom

As per the Oracle documentation, we just need to call ThreadLocalRandom.current() method, and it will return the instance of ThreadLocalRandom for the current thread. We can then generate random values by invoking available instance methods of the class.

Let’s generate a random int value without any bounds:

int unboundedRandomValue = ThreadLocalRandom.current().nextInt());

Next, let’s see how we can generate a random bounded int value, meaning a value between a given lower and upper limit.

Here’s an example of generating a random int value between 0 and 100:

int boundedRandomValue = ThreadLocalRandom.current().nextInt(0, 100);

Please note, 0 is the inclusive lower limit and 100 is the exclusive upper limit.

We can generate random values for long and double by invoking nextLong() and nextDouble() methods in a similar way as shown in the examples above.

Java 8 also adds the nextGaussian() method to generate the next normally-distributed value with a 0.0 mean and 1.0 standard deviation from the generator’s sequence.

As with the Random class, we can also use the doubles(), ints() and longs() methods to generate streams of random values.

4. Comparing ThreadLocalRandom and Random Using JMH

Let’s see how we can generate random values in a multi-threaded environment, by using the two classes, then compare their performance using JMH.

First, let’s create an example where all the threads are sharing a single instance of Random. Here, we’re submitting the task of generating a random value using the Random instance to an ExecutorService:

ExecutorService executor = Executors.newWorkStealingPool();
List<Callable<Integer>> callables = new ArrayList<>();
Random random = new Random();
for (int i = 0; i < 1000; i++) {
    callables.add(() -> {
         return random.nextInt();
    });
}
executor.invokeAll(callables);

Let’s check the performance of the code above using JMH benchmarking:

# Run complete. Total time: 00:00:36
Benchmark                                            Mode Cnt Score    Error    Units
ThreadLocalRandomBenchMarker.randomValuesUsingRandom avgt 20  771.613 ± 222.220 us/op

Similarly, let’s now use ThreadLocalRandom instead of the Random instance, which uses one instance of ThreadLocalRandom for each thread in the pool:

ExecutorService executor = Executors.newWorkStealingPool();
List<Callable<Integer>> callables = new ArrayList<>();
for (int i = 0; i < 1000; i++) {
    callables.add(() -> {
        return ThreadLocalRandom.current().nextInt();
    });
}
executor.invokeAll(callables);

Here’s the result of using ThreadLocalRandom:

# Run complete. Total time: 00:00:36
Benchmark                                                       Mode Cnt Score    Error   Units
ThreadLocalRandomBenchMarker.randomValuesUsingThreadLocalRandom avgt 20  624.911 ± 113.268 us/op

Finally, by comparing the JMH results above for both Random and ThreadLocalRandom, we can clearly see that the average time taken to generate 1000 random values using Random is 772 microseconds, whereas using ThreadLocalRandom it’s around 625 microseconds.

Thus, we can conclude that ThreadLocalRandom is more efficient in a highly concurrent environment.

To learn more about JMH, check out our previous article here.

5. Conclusion

This article illustrated the difference between java.util.Random and java.util.concurrent.ThreadLocalRandom.

We also saw the advantage of ThreadLocalRandom over Random in a multithreaded environment, as well as performance and how we can generate random values using the class.

ThreadLocalRandom is a simple addition to the JDK, but it can create a notable impact in highly concurrent applications.

And, as always, the implementation of all of these examples can be found in the GitHub project.

I just announced the new Spring 5 modules in REST With Spring:

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