[ThreadLocal Hash] ThreadLocalMap中Entry的hashcode生成方式 #Java

ThreadLocalHash.java

/**                                                               
 * ThreadLocals rely on per-thread linear-probe hash maps attached
 * to each thread (Thread.threadLocals and                        
 * inheritableThreadLocals).  The ThreadLocal objects act as keys,
 * searched via threadLocalHashCode.  This is a custom hash code  
 * (useful only within ThreadLocalMaps) that eliminates collisions
 * in the common case where consecutively constructed ThreadLocals
 * are used by the same threads, while remaining well-behaved in  
 * less common cases.                                             
 */                                                               
private final int threadLocalHashCode = nextHashCode();             
                                                                    
/**                                                                 
 * The next hash code to be given out. Updated atomically. Starts at
 * zero.                                                            
 */                                                                 
private static AtomicInteger nextHashCode =                         
    new AtomicInteger();                                            
                                                                    
/**                                                                 
 * The difference between successively generated hash codes - turns 
 * implicit sequential thread-local IDs into near-optimally spread  
 * multiplicative hash values for power-of-two-sized tables. 
 * 使用0x61c88647为递增间隔，能使Entry在长度为2的n次方的长度上分布均匀
 */
private static final int HASH_INCREMENT = 0x61c88647;               
                                                                    
/**                                                                 
 * Returns the next hash code.                                      
 */                                                                 
private static int nextHashCode() {                                 
    return nextHashCode.getAndAdd(HASH_INCREMENT);                  
}