java - 一个HashMap，两个线程——在这个例子中如何保证线程安全？

Question

我有一个关于线程安全和 HashMap 的问题。更具体地说，我想知道一个线程是否有可能在写入 HashMap 时尝试读取它。这是一个粗略的示例:

我有一个名为“TestClass”的类:

public class TestClass implements Runnable {

    // New thread
    TestThread testThread = new TestThread();

    @Override
    public void run() {

        // Starts the thread.
        testThread.start();

        // A copy of testHashMap is retrieved from the other thread.
        // This class often reads from the HashMap.
        // It's the only class that reads from the HashMap.
        while (true) {
            HashMap<String, Long> testHashMap = testThread.get();

        }
    }
}

我还有另一个名为 TestThread 的类:

public class TestThread extends Thread {

    private HashMap<String, Long> testHashMap = new HashMap<>();

    @Override
    public void run() {

        // This thread performs a series of calculations once a second.
        // After the calculations are done, they're saved to testHashMap with put().
        // This is the only thread that writes to testHashMap.

    }

    // This method returns a copy of testHashMap. This method is used by the Test class.
    public HashMap<String, Long> get() {
        return testHashMap;
    }

}

当 TestThread 写入 testHashMap 时，get() 方法是否可能会尝试复制 testHashMap？如果是这样，我如何确保本例中的线程安全？我必须创建一个synchronizedMap而不是HashMap吗？

提前致谢。

Answer 1

Is it possible that the get() method will attempt to copy testHashMap while it's being written to by TestThread?

没有。 get()方法只返回 map 。这里没有复制。

但是，是的，您必须以某种方式控制对 map 的访问，因为 HashMap不是线程安全的。您可以通过同步 HashMap ( Map<A, B> map = Collections.synchronizedMap(new HashMap<A, B>()); ) 或使用 ConcurrentHashMap 来实现此目的.