SynchronizedMap ConcurrentModificationException

Question

我正在尝试理解 SynchronizedMap 并运行了以下代码。我得到以下输出，但有一个例外。根据我的理解，当线程仍在对映射执行写入或线程处于睡眠状态时，get() 方法尝试访问同步映射时会导致异常。我的理解是正确的还是我遗漏了什么？

class MapHelper1 implements Runnable {
Map<String, Integer> map;

public MapHelper1(Map<String, Integer> map) {
    this.map = map;
    new Thread(this, "MapHelper1").start();
}

public void run() {
    map.put("One", 1);
    try {
        System.out.println("MapHelper1 sleeping");
        Thread.sleep(100);
    } catch (Exception e) {
        System.out.println(e);
    }

}
}

class MapHelper2 implements Runnable { 
Map<String, Integer> map;

public MapHelper2(Map<String, Integer> map) {
    this.map = map;
    new Thread(this, "MapHelper3").start();
}

public void run() {
    map.put("two", 1);
    try {
        System.out.println("MapHelper2 sleeping");
        Thread.sleep(100);
    } catch (Exception e) {
        System.out.println(e);
    }

}
}


class MapHelper3 implements Runnable {
Map<String, Integer> map;

public MapHelper3(Map<String, Integer> map) {
    this.map = map;
    new Thread(this, "MapHelper3").start();
}

public void run() {
    map.put("three", 1);
    try {
        System.out.println("MapHelper3 sleeping");
        Thread.sleep(100);
    } catch (Exception e) {
        System.out.println(e);
    }

}
}

public class MainClass {

public static void main(String[] args) {
    Map<String, Integer> hashMap = new HashMap<>();
    Map<String, Integer> syncMap = Collections.synchronizedMap(hashMap);
    MapHelper1 mapHelper1 = new MapHelper1(syncMap);
    MapHelper2 mapHelper2 = new MapHelper2(syncMap);
    MapHelper3 mapHelper3 = new MapHelper3(syncMap);



    for (Map.Entry<String, Integer> e : syncMap.entrySet()) {
        System.out.println(e.getKey() + "=" + e.getValue());
    }

}

}

输出：

MapHelper1 sleeping
MapHelper2 sleeping
MapHelper3 sleeping


Exception in thread "main" java.util.ConcurrentModificationException
at java.base/java.util.HashMap$HashIterator.nextNode(HashMap.java:1494)
at java.base/java.util.HashMap$EntryIterator.next(HashMap.java:1527)
at java.base/java.util.HashMap$EntryIterator.next(HashMap.java:1525)
at MainClass.main(MainClass.java:137)
Command exited with non-zero status 1

编辑：当输出无异常生成时，我再次运行代码几次。为什么会出现这种行为？

Answer 1

迭代时不同步访问。使用：

synchronized(syncMap) {
  for (Map.Entry<String, Integer> e : syncMap.entrySet()) {
    System.out.println(e.getKey() + "=" + e.getValue());
  }
}

甚至在synchronizedMap()方法javadoc中

用户必须手动同步返回的 迭代其任何集合视图时的映射：

  Map m = Collections.synchronizedMap(new HashMap());
      ...
  Set s = m.keySet();  // Needn't be in synchronized block
      ...
  synchronized (m) {  // Synchronizing on m, not s!
      Iterator i = s.iterator(); // Must be in synchronized block
      while (i.hasNext())
          foo(i.next());
  }

Answer 2

我刚刚阅读了@Dariusz 的回答，我认为这是完全正确的。

要具体回答您看到随机行为的原因，完全是时机。也就是说，如果在主线程中迭代之前所有三个线程都填充了映射，那么一切都很好。但是，如果一个线程在主线程中已经在进行迭代时尝试填充，则会出现异常。

顺便说一句，我知道这是为了演示目的。但在实际代码中，最好不要在构造函数中启动线程。