都说 HashMap 是线程不安全的，到底体现在哪儿？
在jdk1.8中对HashMap做了很多优化，这里先分析在jdk1.7中的问题，相信
大家都知道在jdk1.7多线程环境下HashMap容易出现死循环，这里我们先用
代码来模拟出现死循环的情况：
public class HashMapTest {
public static void main(String[] args) {
HashMapThread thread0 = new HashMapThread();
HashMapThread thread1 = new HashMapThread();
HashMapThread thread2 = new HashMapThread();
HashMapThread thread3 = new HashMapThread();
HashMapThread thread4 = new HashMapThread();
thread0.start();
thread1.start();
thread2.start();
thread3.start();
thread4.start();
}
}
class HashMapThread extends Thread {
private static AtomicInteger ai = new AtomicInteger();
private static Map<Integer, Integer> map = new HashMap<>();
@Override
public void run() {
while (ai.get() < 1000000) {
map.put(ai.get(), ai.get());
ai.incrementAndGet();
}
}
}
