如果从多个线程中使用，则HashMap＃put / remove是否会遇到任何ConcurrentModificationException

Question

In Test.java

public static Map<String, Integer> testMap=new HashMap<String, Integer>();

In TestServlet.java

public void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws IOException {
  Test.testMap.put("cool", 1);
}

In TestServlet2.java

public void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws IOException {
  Test.testMap.remove("cool");
}

In test.html , there is a function to call TestServlet every 3 secs

mTimer = setTimeout('getTest();',3000); //call TestServlet in 3 seconds

In test2.html , there is a function to call TestServlet2 every 2 secs

mTimer = setTimeout('getTest2();',2000); //call TestServlet in 2 seconds

Now, I run test.html & test2.html at the same time & no problem occur.

Of course, if I replace public static Map<String, Integer> testMap=new HashMap<String, Integer>(); with public static List<String> testList=new ArrayList<String>(); , then I got ConcurrentModificationException

My question is that? Can HashMap suffer any ConcurrentModificationException or be Thread-safe?

I have not tried to modify the value of each key of the Map so I don't know what is going on.

Answer 1

HashMap s are not thread safe and should be used wisely. If you want to use key value pairs type of data structure ConcurrentHashMap is recommended in multithreaded environment.

Answer 2

Yes - you can see from the source code that the iterator throws such an exception:

808         final Entry<K,V> nextEntry() {
809             if (modCount != expectedModCount)
810                 throw new ConcurrentModificationException();

And modCount is incremented in the put method and other places.

Note that this is not necessarily restricted to the issue of thread-safety. A same-thread sequence of operations could cause the exception to be thrown if a put operation was done while between two iterator operations.