构造方法

  • jdk 1.7 ConcurrentHashMap 如果指定初始容量为32 那么就是32 但是jdk1.8 是 64
  • 可以看到实现了懒惰初始化，在构造方法中仅仅计算了 table 的大小，以后在第一次使用时才会真正创建
    1.   public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {
    2.       if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
    3.           throw new IllegalArgumentException();
    4.       if (initialCapacity < concurrencyLevel) // Use at least as many bins 初始容量< 并发度
    5.           initialCapacity = concurrencyLevel; // as estimated threads
    6.       long size = (long)(1.0 + (long)initialCapacity / loadFactor);
    7.       // tableSizeFor 仍然是保证计算的大小是 2^n, 即 16,32,64 ... 
    8.       int cap = (size >= (long)MAXIMUM_CAPACITY) ?
    9.               MAXIMUM_CAPACITY : tableSizeFor((int)size);
    10.       this.sizeCtl = cap; 
    11.   }