学而时习之，不亦悦乎！
netty源码
public final class DefaultEventExecutorChooserFactory implements EventExecutorChooserFactory {
public static final DefaultEventExecutorChooserFactory INSTANCE = new DefaultEventExecutorChooserFactory();
private DefaultEventExecutorChooserFactory() { }
@SuppressWarnings("unchecked")
@Override
public EventExecutorChooser newChooser(EventExecutor[] executors) {
if (isPowerOfTwo(executors.length)) {
return new PowerOfTwoEventExecutorChooser(executors);
} else {
return new GenericEventExecutorChooser(executors);
}
}
//！！！判断一个数是否是2的倍数！！！
private static boolean isPowerOfTwo(int val) {
return (val & -val) == val;
}
private static final class PowerOfTwoEventExecutorChooser implements EventExecutorChooser {
private final AtomicInteger idx = new AtomicInteger();
private final EventExecutor[] executors;
PowerOfTwoEventExecutorChooser(EventExecutor[] executors) {
this.executors = executors;
}
@Override
public EventExecutor next() {
return executors[idx.getAndIncrement() & executors.length - 1];
}
}
private static final class GenericEventExecutorChooser implements EventExecutorChooser {
private final AtomicInteger idx = new AtomicInteger();
private final EventExecutor[] executors;
GenericEventExecutorChooser(EventExecutor[] executors) {
this.executors = executors;
}
@Override
public EventExecutor next() {
return executors[Math.abs(idx.getAndIncrement() % executors.length)];
}
}
}
HashMap
int一共4字节32位，2的倍数的特征就是二进制只有一个1.随意给定一个数，把它变成最近的2的倍数，其实只需要关注最高位的1，如果此时没有低位1，它就是2的倍数；如果有低位的1，那它的高一位1就是它最接近的2的倍数。例如00000100 00000000 00000000 00000000这个数，它就是2的倍数；例如00000111 00000000 11111111 11001100这个数，低位有1，那最接近的就是00001000 00000000 00000000 00000000.这里采用的算法是，拿到一个数，先减一，确保本身就是2的倍数不会变大一倍，然后把最高位的1向下传递，比如00000111 00000000 11111111 11001100变成00000111 11111111 11111111 11111111，这个时候加1，就变成了00001000 00000000 00000000 00000000。如果是00000100 00000000 00000000 00000000本身就是2的倍数，减1变成了00000011 11111111 11111111 11111111 11111111，把高位1传递后还是自己，加1变回了00000100 00000000 00000000 00000000。
最主要的原理就是只关心最高位的1.
/**
* Returns a power of two size for the given target capacity.
*/
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
原文：https://www.cnblogs.com/zby9527/p/12213032.html