ConcurrentHashMap并不是绝对线程安全的

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentHashMapDemo1 {

    private static Map<Long, Integer> widgetCacheMap = new ConcurrentHashMap<Long, Integer>();

    /**
     * @param args
     */
    public static void main(String[] args) {
        for (int i = 0; i < 10000; i++) {
            Thread tt = new Thread(new Rund());
            tt.start();
        }
    }

    static class Rund implements Runnable {

        public void run() {
            test();
        }

        /**
         * 1W次，总有那么几次线程不安全
         */
        public void test() {
            synchronized ("") {// 解决方案
                ConcurrentHashMapDemo1 tt = new ConcurrentHashMapDemo1();
                tt.set();
                int s1 = widgetCacheMap.get(1L).intValue();
                tt.change();
                int s2 = widgetCacheMap.get(1L).intValue();
                if (s1 == s2) {
                    System.out.println(s1 + ":" + s2);
                }
            }
        }

    }

    public void set() {
        Map<Long, Integer> mm = new HashMap<Long, Integer>();
        mm.put(1L, 1);
        widgetCacheMap = mm;
    }

    public void change() {
        Map<Long, Integer> mm = new HashMap<Long, Integer>();
        mm.put(1L, 2);
        widgetCacheMap = mm;
    }

}

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class ConcurrentHashMapDemo2 {

    public static void main(String[] args) throws InterruptedException {
        for (int i = 0; i < 10; i++) {
            System.out.println(test());
        }
    }

    private static int test() throws InterruptedException {
        ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<String, Integer>();
        ExecutorService pool = Executors.newCachedThreadPool();
        for (int i = 0; i < 8; i++) {
            pool.execute(new MyTask(map));
        }
        pool.shutdown();
        pool.awaitTermination(1, TimeUnit.DAYS);

        return map.get(MyTask.KEY);
    }
}

class MyTask implements Runnable {

    public static Object lock = new Object();

    public static final String KEY = "key";

    private ConcurrentHashMap<String, Integer> map;

    public MyTask(ConcurrentHashMap<String, Integer> map) {
        this.map = map;
    }

    @Override
    public void run() {
        for (int i = 0; i < 100; i++) {
            synchronized (lock) { // 解决方案
                this.addup();
            }
        }
    }

    private void addup() {
        if (!map.containsKey(KEY)) {
            map.put(KEY, 1);
        } else {
            map.put(KEY, map.get(KEY) + 1);
        }
    }
}

总结：ConcurrentHashMap是线程安全的，那是在他们的内部操作，其外部操作还是需要自己来保证其同步的，特别是静态的ConcurrentHashMap,其有更新和查询的过程，要保证其线程安全，需要syn一个不可变的参数才能保证其原子性