Random Misc

http://java-performance.info/java-util-random-java-util-concurrent-threadlocalrandom-multithreaded-environments/

java.util.Random

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

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 anythingelse out of its 64 byte L1 cache line.

• Do not share an instance of java.util.Random between several threads in any circumstances, wrap it in ThreadLocal instead.
• From Java 7 prefer java.util.concurrent.ThreadLocalRandom to java.util.Random in allcircumstances - it is backwards compatible with existing code, but uses cheaper operations internally.

• Java7在所有情形下都更推荐使用java.util.concurrent.ThreadLocalRandom——它向下兼容已有的代码且运营成本更低。

http://www.importnew.com/12460.html
How to reuse a Random seed in java so that I nextInt() the same values again?

Fix the seed, that's it.

  Random randomGenerator = new Random(5);
  System.out.println(randomGenerator.nextInt(Integer.MAX_VALUE));
  randomGenerator = new Random(5);
  System.out.println(randomGenerator.nextInt(Integer.MAX_VALUE));

Result :

1568779487

1568779487

http://stackoverflow.com/questions/12458383/java-random-numbers-using-a-seed

If you're giving the same seed, that's normal, that's an important feature allowing tests.

Check this to understand pseudo random generation and seeds :http://en.wikipedia.org/wiki/Pseudorandom_number_generator

A pseudorandom number generator (PRNG), also known as a deterministic random bit generator DRBG, is an algorithm for generating a sequence of numbers that approximates the properties of random numbers. The sequence is not truly random in that it is completely determined by a relatively small set of initial values, called the PRNG's state, which includes a truly random seed.