java - 为什么我在这个例子中得到了 java.util.ConcurrentModificationException？

Question

在下面的代码示例中，我不明白为什么 foo 方法会抛出 ConcurrentModificationException。请帮忙!

private void foo() {
        synchronized (map) {
            if (map != null && !map.isEmpty()) {
                Set<String> it = map.keySet();
                ArrayList<String> delArray = new ArrayList<>();
                for (String key : it) {
                    MapItem mapItem = map.get(key);
                    if (mapItem != null) {
                        long wakeTime = System.currentTimeMillis() - mapItem.getTimestamp();
                        if (wakeTime > MY_THRESHOLD) {
                            if (mapItem.getLock() != null) {
                                mapItem.getLock().release();
                            }
                            delArray.add(key);
                        }
                    }
                }

                if (!delArray.isEmpty()) {
                    for (String key : delArray) {
                        map.remove(key);
                    }
                }
            }
        }
    }

我在“for (String key : it) {”行出现异常

private static class MapItem {
        private PowerManager.WakeLock lock;
        private long timestamp;

        public MapItem(PowerManager.WakeLock lock, long timestamp) {
            this.lock = lock;
            this.timestamp = timestamp;
        }

        public PowerManager.WakeLock getLock() {
            return lock;
        }

        public long getTimestamp() {
            return timestamp;
        }
    }

Answer 1

您试图在遍历 map 时更改它，这是不允许的。 ConcurrentModificationException :

If a single thread issues a sequence of method invocations that violates the contract of an object, the object may throw this exception. For example, if a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will throw this exception