if (!map.containsKey(key))
return map.put(key, value);
else
return map.get(key);
Thread A calls containsKey and finds out that the key is not present, but is immediately suspended.
Thread B calls containsKey and finds out that the key is not present, and has the time to insert its value v2.
Thread A resumes and inserts v1, "peacefully" overwriting (since put is threadsafe) the value inserted by thread B.
在多线程的环境中，必须使用 ConcurrentHashMap.putIfAbsent(K key, V value) 方法
java
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