/**
     * Maps the specified key to the specified value in this table.
     * Neither the key nor the value can be null.
     *
     * <p>The value can be retrieved by calling the {@code get} method
     * with a key that is equal to the original key.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with {@code key}, or
     *         {@code null} if there was no mapping for {@code key}
     * @throws NullPointerException if the specified key or value is null
     */
    public V put(K key, V value) {
        return putVal(key, value, false);
    }
    /** Implementation for put and putIfAbsent */
    final V putVal(K key, V value, boolean onlyIfAbsent) {
        if (key == null || value == null) throw new NullPointerException();
        int hash = spread(key.hashCode());
        int binCount = 0;
        for (Node<K,V>[] tab = table;;) {
            Node<K,V> f; int n, i, fh;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
                if (casTabAt(tab, i, null,
                             new Node<K,V>(hash, key, value, null)))
                    break;                   // no lock when adding to empty bin
            }
            else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            else {
                V oldVal = null;
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {
                            binCount = 1;
                            for (Node<K,V> e = f;; ++binCount) {
                                K ek;
                                if (e.hash == hash &&
                                    ((ek = e.key) == key ||
                                     (ek != null && key.equals(ek)))) {
                                    oldVal = e.val;
                                    if (!onlyIfAbsent)
                                        e.val = value;
                                    break;
                                }
                                Node<K,V> pred = e;
                                if ((e = e.next) == null) {
                                    pred.next = new Node<K,V>(hash, key,
                                                              value, null);
                                    break;
                                }
                            }
                        }
                        else if (f instanceof TreeBin) {
                            Node<K,V> p;
                            binCount = 2;
                            if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
                                                           value)) != null) {
                                oldVal = p.val;
                                if (!onlyIfAbsent)
                                    p.val = value;
                            }
                        }
                    }
                }
                if (binCount != 0) {
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
        addCount(1L, binCount);
        return null;
    }

import com.alibaba.fastjson.JSON;
import org.springframework.util.CollectionUtils;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ConcurrentHashMap;
/**
 * @author 小工匠
 * @version 1.0
 * @description: TODO
 * @date 2021/11/21 10:46
 * @mark: show me the code , change the world
 */
public class ConcurrentHashMapTest2 {
    // 1 创建Map , key为注册中心地址，value为客户端列表
    private static ConcurrentHashMap<String, List<String>> registMap = new ConcurrentHashMap<>();
    private static final String REGIST_SERVER_A = "注册中心A";
    private static final String REGIST_SERVER_B = "注册中心B";
    public static void main(String[] args) {
        // 2  注册 REGIST_SERVER_A
        Thread threadOne =new Thread(()->{
            List<String> list = new ArrayList<>();
            list.add("客户端一");
            list.add("客户端二");
            // 若果原集合不为空，则追加新的集合
            List<String> oldList = registMap.putIfAbsent(REGIST_SERVER_A, list);
            if (null != oldList){
                oldList.addAll(list);
            }
            System.out.println( "注册信息:" + JSON.toJSONString(registMap));
        });
        // 3 注册 REGIST_SERVER_A
        Thread threadTwo =new Thread(()->{
            List<String> list = new ArrayList<>();
            list.add("客户端三");
            list.add("客户端四");
            List<String> oldList = registMap.putIfAbsent(REGIST_SERVER_A, list);
            // 若果原集合不为空，则追加新的集合
            if (!CollectionUtils.isEmpty(oldList)){
                oldList.addAll(list);
            }
            System.out.println( "注册信息:" + JSON.toJSONString(registMap));
        });
        // 4 注册 REGIST_SERVER_B
        Thread threadThree =new Thread(()->{
            List<String> list = new ArrayList<>();
            list.add("客户端五");
            list.add("客户端六");
            List<String> oldList = registMap.putIfAbsent(REGIST_SERVER_B, list);
            if (!CollectionUtils.isEmpty(oldList)){
                oldList.addAll(list);
            }
            System.out.println("注册信息:" + JSON.toJSONString(registMap));
        });
        // 5 启动注册
        threadOne.start();
        threadTwo.start();
        threadThree.start();
    }
}