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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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package jsr166y; |
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import java.util.*; |
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import java.util.Random; |
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/** |
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* A random number generator with the same properties as class {@link |
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* Random} but isolated to the current Thread. Like the global |
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* generator used by the {@link java.lang.Math} class, a |
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* ThreadLocalRandom is initialized with an internally generated seed |
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* that may not otherwise be modified. When applicable, use of |
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* ThreadLocalRandom rather than shared Random objects in concurrent |
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* programs will typically encounter much less overhead and |
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* contention. ThreadLocalRandoms are particularly appropriate when |
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* multiple tasks (for example, each a {@link ForkJoinTask}), use |
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* random numbers in parallel in thread pools. |
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* A random number generator isolated to the current thread. Like the |
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* global {@link java.util.Random} generator used by the {@link |
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* java.lang.Math} class, a {@code ThreadLocalRandom} is initialized |
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* with an internally generated seed that may not otherwise be |
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* modified. When applicable, use of {@code ThreadLocalRandom} rather |
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* than shared {@code Random} objects in concurrent programs will |
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* typically encounter much less overhead and contention. Use of |
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* {@code ThreadLocalRandom} is particularly appropriate when multiple |
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* tasks (for example, each a {@link ForkJoinTask}) use random numbers |
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* in parallel in thread pools. |
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* |
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* <p>Usages of this class should typically be of the form: |
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* {@code ThreadLocalRandom.current().nextX(...)} (where |
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* {@code X} is {@code Int}, {@code Long}, etc). |
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* When all usages are of this form, it is never possible to |
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* accidently share ThreadLocalRandoms across multiple threads. |
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* accidently share a {@code ThreadLocalRandom} across multiple threads. |
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* |
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* <p>This class also provides additional commonly used bounded random |
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* generation methods. |
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*/ |
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public class ThreadLocalRandom extends Random { |
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// same constants as Random, but must be redeclared because private |
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private final static long multiplier = 0x5DEECE66DL; |
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private final static long addend = 0xBL; |
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private final static long mask = (1L << 48) - 1; |
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private static final long multiplier = 0x5DEECE66DL; |
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private static final long addend = 0xBL; |
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private static final long mask = (1L << 48) - 1; |
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/** |
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* The random seed. We can't use super.seed. |
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private long rnd; |
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/** |
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* Initialization flag to permit the first and only allowed call |
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* to setSeed (inside Random constructor) to succeed. We can't |
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* allow others since it would cause setting seed in one part of a |
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* program to unintentionally impact other usages by the thread. |
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* Initialization flag to permit calls to setSeed to succeed only |
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* while executing the Random constructor. We can't allow others |
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* since it would cause setting seed in one part of a program to |
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* unintentionally impact other usages by the thread. |
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*/ |
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boolean initialized; |
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/** |
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* Constructor called only by localRandom.initialValue. |
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* We rely on the fact that the superclass no-arg constructor |
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* invokes setSeed exactly once to initialize. |
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*/ |
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ThreadLocalRandom() { |
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super(); |
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initialized = true; |
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} |
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/** |
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* Returns the current Thread's ThreadLocalRandom. |
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* Returns the current thread's {@code ThreadLocalRandom}. |
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* |
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* @return the current Thread's ThreadLocalRandom |
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* @return the current thread's {@code ThreadLocalRandom} |
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*/ |
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public static ThreadLocalRandom current() { |
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return localRandom.get(); |
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} |
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/** |
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* Throws UnsupportedOperationException. Setting seeds in this |
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* generator is unsupported. |
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* Throws {@code UnsupportedOperationException}. Setting seeds in |
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* this generator is not supported. |
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* |
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* @throws UnsupportedOperationException always |
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*/ |
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public void setSeed(long seed) { |
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if (initialized) |
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throw new UnsupportedOperationException(); |
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initialized = true; |
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rnd = (seed ^ multiplier) & mask; |
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} |
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