线程安全的ArrayList和HashTable

在.net 2.0中,ArrayList和HashTable都有一个封装了同步功能的新实现.

它们可以通过ArrayList.Synchronized方法或者HashTable.Synchronized方法来获得.

下面是方法的具体实现:

 

[HostProtection(SecurityAction.LinkDemand, Synchronization=true)]
public static ArrayList Synchronized(ArrayList list)
{
    if (list == null)
    {
        throw new ArgumentNullException("list");
    }
    return new SyncArrayList(list);
}

[HostProtection(SecurityAction.LinkDemand, Synchronization=true)]
public static IList Synchronized(IList list)
{
    if (list == null)
    {
        throw new ArgumentNullException("list");
    }
    return new SyncIList(list);
}

 
[HostProtection(SecurityAction.LinkDemand, Synchronization=true)]
public static Hashtable Synchronized(Hashtable table)
{
    if (table == null)
    {
        throw new ArgumentNullException("table");
    }
    return new SyncHashtable(table);
}

 

里面的SyncHashtable之流便是同步的HashTable了,

再看看它的实现

 

[Serializable]
private class SyncHashtable : Hashtable
{
    // Fields
    protected Hashtable _table;

    // Methods
    internal SyncHashtable(Hashtable table) : base(false)
    {
        this._table = table;
    }

    internal SyncHashtable(SerializationInfo info, StreamingContext context) : base(info, context)
    {
        this._table = (Hashtable) info.GetValue("ParentTable", typeof(Hashtable));
        if (this._table == null)
        {
            throw new SerializationException(Environment.GetResourceString("Serialization_InsufficientState"));
        }
    }

    public override void Add(object key, object value)
    {
        lock (this._table.SyncRoot)
        {
            this._table.Add(key, value);
        }
    }

    public override void Clear()
    {
        lock (this._table.SyncRoot)
        {
            this._table.Clear();
        }
    }

    public override object Clone()
    {
        lock (this._table.SyncRoot)
        {
            return Hashtable.Synchronized((Hashtable) this._table.Clone());
        }
    }

    public override bool Contains(object key)
    {
        return this._table.Contains(key);
    }

    public override bool ContainsKey(object key)
    {
        return this._table.ContainsKey(key);
    }

    public override bool ContainsValue(object key)
    {
        lock (this._table.SyncRoot)
        {
            return this._table.ContainsValue(key);
        }
    }

    public override void CopyTo(Array array, int arrayIndex)
    {
        lock (this._table.SyncRoot)
        {
            this._table.CopyTo(array, arrayIndex);
        }
    }

    public override IDictionaryEnumerator GetEnumerator()
    {
        return this._table.GetEnumerator();
    }

    public override void GetObjectData(SerializationInfo info, StreamingContext context)
    {
        if (info == null)
        {
            throw new ArgumentNullException("info");
        }
        info.AddValue("ParentTable", this._table, typeof(Hashtable));
    }

    public override void OnDeserialization(object sender)
    {
    }

    public override void Remove(object key)
    {
        lock (this._table.SyncRoot)
        {
            this._table.Remove(key);
        }
    }

    internal override KeyValuePairs[] ToKeyValuePairsArray()
    {
        return this._table.ToKeyValuePairsArray();
    }

    // Properties
    public override int Count
    {
        get
        {
            return this._table.Count;
        }
    }

    public override bool IsFixedSize
    {
        get
        {
            return this._table.IsFixedSize;
        }
    }

    public override bool IsReadOnly
    {
        get
        {
            return this._table.IsReadOnly;
        }
    }

    public override bool IsSynchronized
    {
        get
        {
            return true;
        }
    }

    public override object this[object key]
    {
        get
        {
            return this._table[key];
        }
        set
        {
            lock (this._table.SyncRoot)
            {
                this._table[key] = value;
            }
        }
    }

    public override ICollection Keys
    {
        get
        {
            lock (this._table.SyncRoot)
            {
                return this._table.Keys;
            }
        }
    }

    public override object SyncRoot
    {
        get
        {
            return this._table.SyncRoot;
        }
    }

    public override ICollection Values
    {
        get
        {
            lock (this._table.SyncRoot)
            {
                return this._table.Values;
            }
        }
    }
}

这下明白了吧,就是给ArrayList和HashTable的访问方法加了个lock,

不过,这可比我们自己手写还是要靠谱的多..（注意：在进行轶代时，并不是线程安全的。如果要求绝对的线程安全，可以自行lock对象的SyncRoot）

不过有些场景可是不能lock太过的,比如读写比很高的场景(很少写,而读很多)

不过不用担心,并发环境下的缓存容器性能优化（上）：不可变的哈希表

老赵的这篇文章已经有了很好的解决办法了.