使用java.util.concurrent语义的Java并发支持

Question

我正在尝试以线程安全的方式支持以下数据结构的修改(deactivate()函数调用)-

 private static Map<String, Set<Integer>> dbPartitionStatus = new HashMap<String, Set<DBPartitionId>>();

 public void deactivate(DBPartitionId partition) throws Exception {
    synchronized (dbPartitionStatus) {
        Set<DBPartitionId> partitions = dbPartitionStatus.get(serviceName);
        if (partitions == null) {
            partitions = new HashSet<DBPartitionId>();
        }
        partitions.add(partition);
        dbPartitionStatus.put(serviceName, partitions);
    }
}

如果我将同步替换为ConcurrentHashMap & ConcurrentSkipListSet duo，将会出现一些竞争情况。

我想知道这里是否有更干净的方法来实现同步(使用java.util.concurrent)。

Answer 1

在以下实现中应为无争用条件：

private final static ConcurrentMap <String, Set <DBPartitionId>> dbPartitionStatus = 
    new ConcurrentHashMap <String, Set <DBPartitionId>> ();

public void deactivate (DBPartitionId partition) {
    Set <DBPartitionId> partitions = dbPartitionStatus.get (serviceName);
    if (partitions == null)
    {
        partitions = new ConcurrentSkipListSet <DBPartitionId> ();
        Set <DBPartitionId> p = 
            dbPartitionStatus.putIfAbsent (serviceName, partitions);
        if (p != null) partitions = p;
    }
    partitions.add (partition);
}

Answer 2

我个人看不出这种方法有什么问题：

private static ConcurrentHashMap<String, ConcurrentSkipListSet<DBPartitionId>> dbPartitionStatus = new ConcurrentHashMap<>();

public bool deactivate(DBPartitionId partition) throws Exception {
  ConcurrentSkipListSet<DBPartitionId> partitions = dbPartitionStatus.get(serviceName);
  if (partitions == null) {
    // Create a new set
    partitions = new ConcurrentSkipListSet<DBPartitionId>();
    // Attempt to add, if we add, ev will be null.
    ConcurrentSkipListSet<DBPartitionId> ev  = dbPartitionStatus.put(serviceName, partitions);
    // If non-null, someone else has added it, so now use it.
    if (ev != null)
      partitions = ev;
  }
  // will return true if added succesfully...
  return partitions.add(partition);
}

map中还有putIfAbsent()方法，它可以在“原子”操作中对map进行get/put操作，但是在这种情况下，它有额外的开销，每次都必须构造一个空集来传递。