Tag Archives: Random

java.util.Random and java.util.concurrent.ThreadLocalRandom in multithreaded environments

by Mikhail Vorontsov

Generating pseudorandom data

There are 2 types of random generators in Java: pseudorandom and secure. Pseudorandom generators are transforming a seed into a new portion of pseudorandom data based on some formula. Secure random generators are using some machine specific sources of actually random events (file/socket access, for example) to generate its data.

Secure random generators:

  • should be used only when cryptographically strong random data is required
  • are slow
  • could be waiting for external events (“stuck”) if you have requested a lot of random data (Linux /dev/random is an example of such generator)

Pseudorandom generators, on the other hand, depend only on the initial “seed” value, so you can generate the same sequence of pseudorandom events if you will provide the same seed to the algorithm. In general case, such generators are fast because they are CPU bound only (do not rely on any IO). We will review the evolution of pseudorandom generators in Java and the reasons behind these changes.


java.util.Random is available from Java 1.0. It is a thread safe class, so you may share (in theory) instances of this class between several threads and do not expect to get the same random data in 2 threads at the same time. Such thread safety is achieved via using an AtomicLong for the generator seed.

Random uses AtomicLong CAS (compare-and-set) operations for updating its seed. Despite being a light non-blocking primitive used in a lot of non-blocking algorithms, CAS behaves really poorly under the high contention. Wait for test results to see how poorly it behaves.


java.util.concurrent.ThreadLocalRandom was added in Java 7 as an attempt to overcome all performance issues associated with java.util.Random. This new class extends java.util.Random, so you may pass it to all logic expecting the old java.util.Random.

Here are main ThreadLocalRandom implementation details:

  • Internally it does not use an AtomicLong seed from Random. Instead it uses an ordinary long.
  • You can not create an instance of ThreadLocalRandom yourself, because its constructor is not public. Instead you should use its static factory ThreadLocalRandom.current(). This factory method queries internal ThreadLocal<ThreadLocalRandom>
  • It is CPU-cache aware, so it uses 8 long dummy fields for padding, thus pushing anything else out of its 64 byte L1 cache line.

All of these changes are important, which will be revealed by the following tests.

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