ConcurrentHashMap 可以一边遍历一边修改吗

是的，ConcurrentHashMap可以一边遍历一边修改。这是因为ConcurrentHashMap内部使用了一种叫做分段锁（Segment）的机制，将整个Map分成了多个小的Segment，每个Segment内部都有一个锁。在进行修改操作时，只需要锁住当前需要修改的Segment，而不是整个Map，这样就可以实现并发修改而不会出现线程安全问题。而在进行遍历操作时，只需要获取每个Segment的锁，就可以同时遍历整个Map，而不会出现ConcurrentModificationException等异常。

以下是一个演示代码：

from concurrent.futures import ThreadPoolExecutor
import time
import random
import string
from collections.abc import MutableMapping

class ConcurrentHashMap(MutableMapping):
    def __init__(self, *args, **kwargs):
        self._lock = ThreadPoolExecutor(max_workers=1)
        self._segments = [{} for _ in range(16)]
        self.update(dict(*args, **kwargs))

    def __getitem__(self, key):
        segment = self._get_segment(key)
        with self._lock:
            return segment[key]

    def __setitem__(self, key, value):
        segment = self._get_segment(key)
        with self._lock:
            segment[key] = value

    def __delitem__(self, key):
        segment = self._get_segment(key)
        with self._lock:
            del segment[key]

    def __iter__(self):
        with self._lock:
            for segment in self._segments:
                yield from segment.keys()

    def __len__(self):
        with self._lock:
            return sum(len(segment) for segment in self._segments)

    def _get_segment(self, key):
        return self._segments[hash(key) % len(self._segments)]

# 测试代码
def test_concurrent_modify():
    d = ConcurrentHashMap()
    for i in range(100):
        d[i] = ''.join(random.choices(string.ascii_uppercase + string.digits, k=10))

    def modify():
        for i in range(100):
            d[i] = ''.join(random.choices(string.ascii_uppercase + string.digits, k=10))

    def iterate():
        for k, v in d.items():
            pass

    with ThreadPoolExecutor(max_workers=2) as executor:
        executor.submit(modify)
        executor.submit(iterate)

    print('ConcurrentHashMap can modify and iterate concurrently.')

test_concurrent_modify()