java集合synchronizedMap不工作

Question

这个问题已经有答案了:

Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop (31 个回答)

已关闭 7 年前。

当我多次运行下面的代码时，下面的代码给出 ConcurrentModificationException 。当我使用非concurrentHashMap时它工作正常。

    public static void main(String[] args) {

        Map<String, String> stringSyncedMap = Collections.synchronizedMap(new HashMap<String, String>());

        CollectionJob collectionJob1 = new CollectionJob(stringSyncedMap, "read");
        CollectionJob collectionJob2 = new CollectionJob(stringSyncedMap, "write");

        Thread thread1 = new Thread(collectionJob1, "T1");
        Thread thread2 = new Thread(collectionJob2, "T2");

        thread1.start();
        thread2.start();
    }

}

public class CollectionJob implements Runnable {

    private Map<String, String> stringCollection;
    private String operation;

    public CollectionJob(Map<String, String> stringCollection, String operation) {

        this.stringCollection = stringCollection;
        this.operation = operation;
    }

    public void run() {

        if (operation.equals("read"))
            readFromCollection();
        else
            writeToCollection();

        System.out.println("done");

    }

    public void writeToCollection() {
        for (int i = 10; i < 20; i++) {
            stringCollection.put("String " + i, "String"+i);

    }

    public void readFromCollection() {
        for (String str : stringCollection.keySet()) 
            System.out.println(str);
    }
}

我缺少什么。

Answer 1

与 ConcurrentHashMap 不同，当通过 Collections.synchronizedMap 惯用法初始化时，您需要显式同步 Map，例如使用synchronized关键字。

来自docs :

It is imperative that the user manually synchronize on the returned map when iterating over any of its collection views: [...]