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More randomness. Less time. Still an instance of java.util.Random.

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BetterRandom is a library that helps you get the best performance and the best pseudorandomness from your pseudorandom-number generators (PRNGs) and their seed sources. With it, you can:

  • Wrap a SplittableRandom, with its improved speed and randomness-test performance, as a java.util.Random so that methods like Collections.shuffle(List<>,Random) can use it.
    • Have it automatically split when accessed from a new thread, so that it's even faster than a ThreadLocalRandom but still safe to pass between threads.
  • Wrap any Supplier<Random> to create a Random that's thread-local, but can be accessed just like any other Random.
  • Use AesCounterRandom, a PRNG that's as secure as AES but much faster than SecureRandom.
  • Use any of 5 other great PRNG algorithms.
  • Use the above PRNGs anywhere you can use java.util.Random, because they're subclasses of java.util.Random.
  • Be confident that your PRNGs will serialize and deserialize correctly.
  • Make the most of a custom PRNG algorithm by extending the abstract class BaseRandom.
  • Keep track of how much entropy your PRNG has left (bits of output versus bits of seed).
  • Automatically reseed your PRNGs whenever it's possible and beneficial, without blocking the threads that use them while a seed is generated. The RandomSeederThread class does this and can use any of the following seed sources:
    • /dev/random
    • A SecureRandom
    • Automatically choose the best of the above three with DefaultSeedGenerator.
    • Or roll your own by implementing just one method from the SeedGenerator interface.

But java.util.Random is already fast enough for me!

Yeah, but it's not random enough for you. Monte Carlo simulations have been known to give misleading results because of low-quality PRNGs, and the implementation in java.util.Random is low-quality for two reasons:

  • It's a linear congruential generator, a notoriously bad algorithm.
  • It only has 48 bits of internal state, so some long and double values are impossible to generate. This means "uniform distributions" aren't uniform.

Many standard tests of randomness amount to Monte Carlo simulations. And since widespread pseudorandom number generators (PRNGs) pass most but not all such tests in standard suites such as BigCrush and Dieharder, this suggests that any Monte Carlo simulation may turn out to be a test of randomness, and to give misleading or untrustworthy results because of an unfortunate choice of PRNG. There are two ways to minimize this risk, both of which BetterRandom can help with:

  • Have several different PRNG algorithms available, all with the same interfaces, so that you can swap one out and compare results.
  • Reseed PRNGs as often as possible, ideally with a seed source that continues to receive entropy in parallel with your simulation.

Don't take chances on randomness -- get it right with BetterRandom.

Professional support

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GitHub Sponsors can get mentions or ad space in this section of this page, both on GitHub and GitHub Pages. You can even get a mention in the Javadocs. Get publicity from a library that was downloaded 13,854 times from Maven Central in October 2019. Visit for details.

Get it from MavenCentral

  • Get the latest version from Maven Central.
  • Get dependency codes for Maven, Gradle, SBT, Ivy, Grape, Leiningen and Buildr from (more full-featured than Maven Central, but not always up-to-date).

At both links, choose BetterRandom if using JDK 8+ and/or Android API 24+ at both compile-time and runtime. Otherwise, choose BetterRandom-Java7.

Quick start: picking a PRNG

When you need an instance of java.util.Random, the recommended BetterRandom solution depends on how you're using it.

It's for cryptography or gambling

Only AesCounterRandom has cryptographically secure output. Continuous reseeding is recommended if you don't need reproducible output.

  • If you need reproducible output, use:
    byte[] seed;
    // Use an input seed if provided, or else:
    if (seed == null) {
      seed = DEFAULT_SEED_GENERATOR.generateSeed(AesCounterRandom.MAX_SEED_LENGTH_BYTES);
      // Then output the seed
    random = new AesCounterRandom(seed);
  • If you need multi-thread concurrency (incompatible with reproducibility), use:
    new ReseedingThreadLocalRandomWrapper(AesCounterRandom::new, new RandomSeederThread(DEFAULT_SEED_GENERATOR))
  • Otherwise, use:
    random = new AesCounterRandom();
    new RandomSeederThread(DEFAULT_SEED_GENERATOR).add(random);

It's for a scientific simulation, competitive multiplayer game, or professional creative work

For these purposes, the SplittableRandomAdapter family is fastest and passes the Dieharder pseudo-randomness tests. Continuous reseeding is recommended if you don't need reproducible output.

  • If you need reproducible output, use:
    byte[] seed;
    // Get an input seed if provided, then:
    if (seed == null) {
      seed = DEFAULT_SEED_GENERATOR.generateSeed(Long.BYTES);
      // Then output the seed
    random = new SingleThreadSplittableRandomAdapter(seed); 
    See FifoFiller for an example that reads the seed from an environment variable.
  • If you need multi-thread concurrency (incompatible with reproducibility), use:
    new ReseedingSplittableRandomAdapter()
  • Otherwise, use:
    random = new SingleThreadSplittableRandomAdapter();
    new RandomSeederThread(DEFAULT_SEED_GENERATOR).add(random);  

It's for a single-player game, screensaver, playlist shuffle, etc.

  • If you need multi-thread concurrency and to be able to use a specific seed on each thread, use:
    new SplittableRandomAdapter()
  • If you need to be able to use a specific seed, but you don't need concurrency, use:
    new SingleThreadSplittableRandomAdapter()
  • Otherwise, use:
    (On OpenJDK 8 and Android API 24+, this is as good as a SplittableRandom.)

Simple tricks

Disable unless explicitly specified

// Run this before creating any PRNG instances.
DefaultSeedGenerator.set(new SeedGeneratorPreferenceList(
      new BufferedSeedGenerator(DevRandomSeedGenerator.DEV_RANDOM_SEED_GENERATOR, 128),

Specify the SecureRandom algorithm for seed generation

SecureRandom secureRandom = new SecureRandom("SHA1PRNG");
DefaultSeedGenerator.set(new SecureRandomSeedGenerator(secureRandom));

Use a different seed generator for one specific PRNG

random = new SingleThreadSplittableRandomAdapter(SecureRandomSeedGenerator.DEFAULT_INSTANCE);

Ridiculously long period

random = new Cmwc4096Random();

Full examples

Cryptographic PRNG that uses for frequent reseeding

import static io.github.pr0methean.betterrandom.seed.RandomDotOrgSeedGenerator.RANDOM_DOT_ORG_SEED_GENERATOR;

import io.github.pr0methean.betterrandom.seed.RandomSeederThread;
import io.github.pr0methean.betterrandom.seed.SeedException;
import io.github.pr0methean.betterrandom.util.BinaryUtils;

public class AesCounterRandomDemo {
  public static void main(String[] args) throws SeedException {
    AesCounterRandom random = new AesCounterRandom(RANDOM_DOT_ORG_SEED_GENERATOR);
    RandomSeederThread.add(SECURE_RANDOM_SEED_GENERATOR, random);
    byte[] randomBytes = new byte[32];
    for (int i=0; i<20; i++) {
      System.out.format("Bytes: %s\n", BinaryUtils.convertBytesToHexString(randomBytes));

ReseedingSplittableRandomAdapter for fast, high-quality, parallel duplicate-bridge dealing

package io.github.pr0methean.betterrandom.prng.adapter;

import io.github.pr0methean.betterrandom.seed.SecureRandomSeedGenerator;
import io.github.pr0methean.betterrandom.seed.SeedException;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public enum SplittableRandomAdapterDemo {

  private static final String[] VALUE_LABELS =
      {"A", "K", "Q", "J", "10", "9", "8", "7", "6", "5", "4", "3", "2"};
  private static final String[] SUIT_LABELS = {"♥️", "♣️", "♦️", "♠️"};

  public static void main(final String[] args) throws SeedException, InterruptedException {
    final String[] cards = new String[52];
    int i = 0;
    for (final String suit : SUIT_LABELS) {
      for (final String value : VALUE_LABELS) {
        cards[i] = value + suit;
    final ThreadLocal<List<String>> deckCopies =
        ThreadLocal.withInitial(() -> Arrays.asList(cards.clone()));
    final ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(4);
    final SplittableRandomAdapter random =
        new SplittableRandomAdapter(SecureRandomSeedGenerator.DEFAULT_INSTANCE);
    for (i = 0; i < 20; i++) {
      executor.submit(() -> {
        final List<String> deck = deckCopies.get();
        Collections.shuffle(deck, random);
        System.out.format("North: %s%nEast: %s%nSouth: %s%nWest: %s%n%n",
            String.join(",", deck.subList(0, 13)), String.join(",", deck.subList(13, 26)),
            String.join(",", deck.subList(26, 39)), String.join(",", deck.subList(39, 52)));
    executor.awaitTermination(1, TimeUnit.MINUTES);

Full javadocs

Javadocs for the latest snapshot, including both public and protected members (to support your subclassing), are at

Tested environments and supported versions

BetterRandom has 2 branches from which releases have been cut: one for Java 8 and newer, and one for Java 7 -- including Android API levels below 24. These produce the Maven artifacts named BetterRandom-Java7 and BetterRandom respectively.

Java 8 (master branch)

Beginning with version 3.1.2, continuous integration takes place on Azure Pipelines. It is designed to include the following environments:

  • Ubuntu: HotSpot 8,11,12; OpenJ9 11 and 14
  • OSX: HotSpot 8,11,14
  • Windows: HotSpot 8,11,14

HotSpot is Azul Zulu. OpenJ9 is AdoptOpenJDK. See for the implementation status of this build matrix.

For versions up to 3.1.1, continuous integration took place in the following environments:

  • Linux (on Travis): OpenJDK and Oracle JDK 8 and up
  • OS X (on Travis): OpenJDK 8 and up, Xcode 7.3 and up
  • Windows Server 2008 R2 (on Appveyor): Oracle JDK 8
  • Cygwin (on Appveyor): Oracle JDK 8
  • MinGW (on Appveyor): Oracle JDK 8

CI on BSD or Android isn't likely any time soon, since no free providers of BSD CI seem to be integrated with GitHub, and there seems to be no actively-maintained Android-app wrapper for TestNG suites. However, Android API levels 24 and up (required as both source and target) should work.

Java 7 (java7 branch)

Since version 5.0.0, the Java 7 branch is no longer being maintained with backported improvements from the Java 8 branch. Exceptions will be made at the request of a Tidelift subscriber or a Gold or higher GitHub sponsor, or possibly on other forms of paying demand. However, pull requests against this branch will still be considered and, if accepted, released.

This branch was mainly intended to support Android API levels 19 through 23; support for other environments is best-efforts.

Continuous integration takes place in OpenJDK 7 on Linux. Up to and including version 3.1.1, this was done on Travis CI; beginning with 3.1.2, Azure Pipelines is used. Until the release of version 5.0.0, the Java 7 branch was re-tested nightly.

Alternative random number generators

BetterRandom provides several pseudorandom number generators that are intended as drop-in replacements for java.util.Random.

Features common to all PRNGs in betterrandom.prng

  • Reproducible: The getSeed() function retrieves a seed that can be passed into the constructor to create another PRNG giving the same output. If any random numbers have been generated since construction or a call to setSeed(byte[]), this may rewind the state to before that happened.

  • Serializable: All these PRNGs can be serialized and deserialized, copying their full internal state.

  • setSeed(byte[]): Reseed even if more than a long is needed to do so. ** Use getNewSeedLength() to get the recommended seed size.

  • entropyBits(): Find out when the PRNG has output more random data than it has been seeded with, and thus could benefit from being reseeded. Even when the PRNG is reseeded repeatedly without being used, the entropy count won't ever go above the size of the PRNG's internal state.

  • withProbability(double): Get a boolean with any probability of being true, not just 50/50, and the PRNG will still know you're only spending 1 bit of entropy. (Technically it may be less than 1 bit, but we don't implement fractional-bit counting yet.)

  • Lockless nextGaussian().

  • doubles, ints and longs will return parallel streams whenever possible, whereas their implementations in Random always return sequential streams.

  • gaussians() and gaussians(long streamSize): Get an endless or finite stream of normally-distributed doubles.

  • setSeederThread(RandomSeederThread): Reseeds the PRNG whenever its entropy is spent, but only as long as a seed generator can keep up. See below.

SplittableRandom adapters

These classes use java8.util.SplittableRandom instances to implement the methods of Random, despite that the two classes are unrelated and have slightly different method signatures. Several adapters are available:

  • SingleThreadSplittableRandomAdapter: Simple and fast, but not thread-safe.
  • SplittableRandomAdapter: Backed by a ThreadLocal<SplittableRandom>, whose instances of SplittableRandom are all split from a single master.
  • ReseedingSplittableRandomAdapter: Also backed by a ThreadLocal<SplittableRandom>, this registers each thread's SplittableRandom instance with a RandomSeederThread (see below). This is probably the best PRNG implementation that allows concurrent access from multiple threads.

Table of algorithms

Class Seed size (bytes) Period (bits) Speed Speed with RandomSeederThread Effect of setSeed(long) getSeed() rewinds? Algorithm author
AesCounterRandom 16-48* 2135 Slow Slow Combines with existing seed No NIST
Cmwc4096Random 16384 2131104 Medium Very slow Not supported Yes George Marsaglia
MersenneTwisterRandom 16 219937 Medium Medium Not supported Yes Makoto Matsumoto
XorShiftRandom 20 ~2160 Medium Medium Not supported Yes George Marsaglia
SplittableRandomAdapter 8** 264 Fast Fast Replaces existing seed (calling thread only) Yes Guy Steele and Doug Lea
Pcg64Random 8 262 Fast Fast Replaces existing seed Yes M. E. O'Neill

Period assumes exactly 32 or 64 bits are consumed at a time, using nextInt(), nextLong(), ints() or longs().

*Seed sizes above 32 for AesCounterRandom require jurisdiction policy files that allow 192- and 256-bit AES seeds.

**Can be reseeded independently on each thread, affecting only that thread.

Use specialized subclass ReseedingSplittableRandomAdapter.


Retrieving the internal state of an AesCounterRandom instance from its output is considered equivalent to breaking the AES cipher. Thus, this class should be able to replace SecureRandom in many applications, such as generating session keys or erasing files on a magnetic disk or tape.

AesCounterRandom only generates a permutation of the space of 128-bit integers, so if it is used to generate about 264 128-bit strings without reseeding, its statistical properties will begin to differ from those of /dev/random in that it won't have generated the same string twice. This could be prevented by using a hash function rather than a reversible cipher, but the hash functions in standard JVMs are less cryptographically secure than AES and won't run as fast on hardware featuring AES-NI.



A SeedGenerator produces seeds for PRNGs. All the provided implementations are singletons, because the seed sources cannot be parallelized. They include:

  • DevRandomSeedGenerator.DEV_RANDOM_SEED_GENERATOR: Works only on Unix-like systems; reads seeds from /dev/random.
  • RandomDotOrgSeedGenerator.RANDOM_DOT_ORG_SEED_GENERATOR: Connects to to retrieve random numbers over HTTPS. collects randomness from atmospheric noise using 9 radios, located at undisclosed addresses in Dublin and Copenhagen and tuned to undisclosed AM/FM frequencies. (The secrecy is intended to help prevent tampering with the output using a well-placed radio transmitter, and the use of AM/FM helps ensure that any such tampering would cause illegal interference with broadcasts and quickly attract regulatory attention.) Uses the legacy API by default, but can be configured to use the JSON-RPC API.
    • RandomDotOrgSeedGenerator.RATE_LIMITED_ON_FAIL: Avoids spamming or your router, by instantly reporting failure for 10 seconds after every I/O or HTTP error.
  • SecureRandomSeedGenerator.SECURE_RANDOM_SEED_GENERATOR: Uses On Oracle and OpenJDK, this in turn uses; when /dev/random isn't available, that in turn uses the timing of newly-launched threads as a source of randomness, relying on the unpredictable interactions between different configurations of hardware and software and their workloads.
  • DefaultSeedGenerator.DEFAULT_SEED_GENERATOR: Uses the best of the above three that is currently available.


This is a daemon thread that loops over all the ByteArrayReseedableRandom instances registered with it and reseeds them. Those that implement EntropyCountingRandom are skipped when they still
have entropy left from a previous seeding. Example usage:

// Obtain the seeder thread for this seed generator; launch it if it's not already running.
RandomSeederThread seederThread = RandomSeederThread.getInstance(DEFAULT_SEED_GENERATOR);

// Thread is now running, but is asleep if no PRNGs have already been added to it.

// Begin including myRandom in the loop, and wake up the thread.

// Manually trigger reseeding ASAP (myRandom must be added first)
if (myRandom instanceof EntropyCountingRandom) {

// Adding the same PRNG a second time has no effect


This class is similar to SimpleRandomSeeder, but can handle any instance of java.util.Random.


  • betterrandom/ contains BetterRandom's source code.
  • benchmark/ defines a set of JMH benchmarks for BetterRandom as a separate Maven project.
  • FifoFiller/ is a simple utility used for Dieharder randomness tests, and is also a separate Maven project.
  • etc/ contains scripts, templates used in azure.yml, and miscellaneous files used for building and testing.
  • docs/ is a Git submodule used for publishing updates to this project's Javadoc on


The following classes are forked from Uncommons Maths:

  • All of betterrandom.prng except BaseRandom and betterrandom.prng.adapter
  • All of betterrandom.seed except RandomSeederThread
  • BinaryUtils
  • Test classes corresponding to the above.


FOSSA Status


Better random-number generation for Java. Javadocs:




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