终于研究明白了， concurrenthashmap 的 get 然后 put 的并发问题

我参考的是这篇文章： https://www.cnkirito.moe/java-ConcurrentHashMap-CAS/ 网上论述这个的文章实在是不多。

我的测试代码如下：

public class test2 { private final static Map<String, Table> map new ConcurrentHashMap<>(); private static final String KEY = "key";

public static void increase1(String key) { Table oldTable = map.get(key); int value = oldTable.getI(); oldTable.setI(value + 1); map.put(key, oldTable); } public static void increase2(String key) { Table oldTable; Table newTable = new Table(0); while(true) { oldTable = map.get(KEY); newTable.setI(oldTable.getI() + 1); if(map.replace(KEY, oldTable, newTable)) break; } } public static int getTableValue(String key) { return map.get(key).getI(); } public static void main(String[] args) { map.put(KEY, new Table(0)); ExecutorService executor = Executors.newFixedThreadPool(10); int callTime = 1000; CountDownLatch countDownLatch = new CountDownLatch(callTime); for(int i=0; i<callTime; i++) { executor.execute(new Runnable() { @Override public void run() { increase2(KEY); countDownLatch.countDown(); } }); } try { countDownLatch.await(); } catch (InterruptedException e) { e.printStackTrace(); } executor.shutdown(); System.out.println("调用次数：" + getTableValue(KEY)); } 

}

class Table { private int i;

public Table(int i) { this.i = i; } public int getI() { return i; } public void setI(int i) { this.i = i; } 

}

测试结果：使用 increase1 的话，调用次数是不停变动的，存在并发错误。而用 increase2 的话，恒定都是 1000.