在文章开始之前首先要思考的问题是为什么要建立对象池。这和.NET垃圾回收机制有关,正如下面引用所说,内存不是无限的,垃圾回收器最终要回收对象,释放内存。尽管.NET为垃圾回收已经进行了大量优化,例如将托管堆划分为 3 Generations(代)并设定新建的对象回收的最快,新建的短生命周期对象将进入 Gen 0(新建对象大于或等于 85,000 字节将被看作大对象,直接进入 Gen 2),而 Gen 0 通常情况下分配比较小的内存,因此Gen 0 将回收的非常快。而高频率进行垃圾回收导致 CPU 使用率过高,当 Gen 2 包含大量对象时,回收垃圾也将产生性能问题。

构造对象池

.Net Core 在(Base Class Library)基础类型中添加了 ArrayPool,但 ArrayPool 只适用于数组。针对自定义对象,参考MSDN有一个实现,但没有初始化池大小,且从池里取对象的方式比较粗糙,完整的对象池应该包含:

  • 池大小
  • 初始化委托
  • 实例存取方式(FIFO、LIFO 等自定义方式,根据个人需求实现获取实例方式)
  • 获取实例策略

1. 定义对象存取接口,以实现多种存取策略,例如 FIFO、LIFO

/// <summary>
/// 对象存取方式
/// </summary>
public interface IAccessMode<T>
{
    /// <summary>
    /// 租用对象
    /// </summary>
    /// <returns></returns>
    /// <exception cref="InvalidOperationException"></exception>
    T Rent();

    /// <summary>
    /// 返回实例
    /// </summary>
    /// <param name="item"></param>
    void Return(T item);
}

2. 实现存取策略

FIFO

FIFO通过Queue实现,参考

public sealed class FIFOAccessMode<T> : Queue<T>, IAccessMode<T>
{
    private readonly int _capacity;
    private readonly Func<T> _func;
    private int _count;

    public FIFOAccessMode(int capacity, Func<T> func) : base(capacity)
    {
        _capacity = capacity;
        _func = func;
        InitialQueue();
    }

    public T Rent()
    {
        Interlocked.Increment(ref _count);
        return _capacity < _count ? _func.Invoke() : Dequeue();
    }

    public void Return(T item)
    {
        if (_count > _capacity)
        {
            var disposable = (IDisposable)item;
            disposable.Dispose();
        }
        else
        {
            Enqueue(item);
        }
        Interlocked.Decrement(ref _count);
    }

    private void InitialQueue()
    {
        for (var i = 0; i < _capacity; i++)
        {
            Enqueue(_func.Invoke());
        }
    }
}
LIFO

在LIFO中借助Stack特性实现进栈出栈,因此该策略继承自Stack,参考

public sealed class LIFOAccessModel<T> : Stack<T>, IAccessMode<T>
{
    private readonly int _capacity;
    private readonly Func<T> _func;
    private int _count;

    public LIFOAccessModel(int capacity, Func<T> func) : base(capacity)
    {
        _capacity = capacity;
        _func = func;
        InitialStack();
    }

    public T Rent()
    {
        Interlocked.Increment(ref _count);
        return _capacity < _count ? _func.Invoke() : Pop();
    }

    public void Return(T item)
    {
        if (_count > _capacity)
        {
            var disposable = (IDisposable)item;
            disposable.Dispose();
        }
        else
        {
            Push(item);
        }
        Interlocked.Decrement(ref _count);
    }

    private void InitialStack()
    {
        for (var i = 0; i < _capacity; i++)
        {
            Push(_func.Invoke());
        }
    }
}

注意:以上两个实现都遵循池容量不变原则,但租用的实例可以超过对象池大小,返还时还将检测该实例直接释放还是进入池中。而如何控制池大小和并发将在下面说明。

3.Pool实现

public class Pool<T> : IDisposable where T : IDisposable
{
    private int _capacity;
    private IAccessMode<T> _accessMode;
    private readonly object _locker = new object();
    private readonly Semaphore _semaphore;

    public Pool(AccessModel accessModel, int capacity, Func<T> func)
    {
        _capacity = capacity;
        _semaphore = new Semaphore(capacity, capacity);
        InitialAccessMode(accessModel, capacity, func);
    }

    private void InitialAccessMode(AccessModel accessModel, int capacity, Func<T> func)
    {
        switch (accessModel)
        {
            case AccessModel.FIFO:
                _accessMode = new FIFOAccessMode<T>(capacity, func);
                break;
            case AccessModel.LIFO:
                _accessMode = new LIFOAccessModel<T>(capacity, func);
                break;
            default:
                throw new NotImplementedException();
        }
    }

    public T Rent()
    {
        _semaphore.WaitOne();
        return _accessMode.Rent();
    }

    public void Return(T item)
    {
        _accessMode.Return(item);
        _semaphore.Release();
    }

    public void Dispose()
    {
        if (!typeof(IDisposable).IsAssignableFrom(typeof(T))) return;

        lock (_locker)
        {
            while (_capacity > 0)
            {
                var disposable = (IDisposable)_accessMode.Rent();
                _capacity--;
                disposable.Dispose();
            }

            _semaphore.Dispose();
        }
    }
}

在Pool中如何控制程序池并发,这里我们引入了 Semaphore 以控制并发,这里将严格控制程序池大小,避免内存溢出。

4.使用

Student 类用作测试

public class Student : IDisposable
{
    public string Name { get; set; }

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    private bool _disposed;

    protected virtual void Dispose(bool disposing)
    {
        if (_disposed)
            return;

        if (disposing)
        {
            Name = null;
             //Free any other managed objects here.
        }

        _disposed = true;
    }
}
public void TestPool()
{
    Func<Student> func = NewStudent;
    var pool = new Pool<Student>(AccessModel.FIFO, 2, func);
    for (var i = 0; i < 3; i++)
    {
        Student temp = pool.Rent();
        //todo:Some operations
        pool.Return(temp);
    }

    Student temp1 = pool.Rent();

    pool.Return(temp1);

    pool.Dispose();
}

public Student NewStudent()
{
    return new Student();
}

总结:至此,一个完整的对象池建立完毕。

02-15 17:04