java - 同步 map 内的集合

Question

下面是示例代码来说明这种情况:

public class ConnectionRegistry {

    private ConcurrentMap<String, List<Connection>> registry = new ConcurrentHashMap<>();

    public List<Connection> find(String key) {
        List<Connection> connections = registry.get(key);
        if (null == connections) {
            return Collections.emptyList();
        }
        synchronized(connections) {
            return new ArrayList(originalCopy);
        }
    }

    public void register(String key, Connection connection) {
        List<Connection> connections = registry.get(key);
        if (null == connections) {
            List<Connection> newConnections = new ArrayList<>();
            connections = registry.putIfAbsent(key, newConnections);
            if (null == connections) {
                connections = newConnections;
            }
        }
        synchronized(connections) {
            connections.add(connection);
        }
    }

}

在上面的代码中，我有一个注册表来管理按键索引的连接。我想让它线程安全，所以我使用了 ConcurrentMap 数据结构，除了 map 之外，我想确保 map 内的 List 也是线程的安全，因此我使用 synchronize 关键字，如上面的源代码所示。

但是我的 IDE 警告我，这是局部变量同步，并且使用此类同步时很难保证正确性。

有没有其他方法或好的做法来处理这种情况？

Answer 1

Vector 实现了动态数组。它与 ArrayList 类似，但有两点不同:

• vector 已同步。
• Vector 包含许多不属于集合框架的旧方法。

来自文档:

Unlike the new collection implementations, Vector is synchronized. If a thread-safe implementation is not needed, it is recommended to use ArrayList in place of Vector.

经过一些更改，代码将是:

public class ConnectionRegistry {

    private ConcurrentMap<String, List<Connection>> registry = new ConcurrentHashMap<>();

    public List<Connection> find(String key) {
        List<Connection> connections = registry.get(key);
        if (null == connections) {
            return Collections.emptyList();
        }

        return new Vector<Connection>(originalCopy);

    }

    public void register(String key, Connection connection) {
        List<Connection> connections = registry.get(key);
        if (null == connections) {
            List<Connection> newConnections = new Vector<Connection>();
            connections = registry.putIfAbsent(key, newConnections);
            if (null == connections) {
                connections = newConnections;
            }
        }

        connections.add(connection);

 }