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.