创建自定义线程池

1. 创建一个单例模式的自定义线程池类

public class CustomThreadPool
{
    //#region configurable items - for demo let's have these as constants
    private const int MAX = 8; // maximum no of threads in pool
    private const int MIN = 3; // minimum no of threads in pool
    private const int MIN_WAIT = 10; // milliseconds
    private const int MAX_WAIT = 15000; // milliseconds - threshold for simple task
    private const int CLEANUP_INTERVAL = 60000; // millisecond - to free waiting threads in pool
    private const int SCHEDULING_INTERVAL = 10; // millisecond - look for task in queue in loop
    //#endregion

    //#region singleton instance of threadpool
    private static readonly CustomThreadPool _instance = new CustomThreadPool();

    private CustomThreadPool() {
        InitializeThreadPool();
    }

    public static CustomThreadPool Instance
    {
        get
        {
            return _instance;
        }
    }
    //#endregion

    private void InitializeThreadPool() { 
    //TODO: write initialization code here 
    }
}

2. 定义一些基本类型来和线程池进行通讯

public delegate void UserTask();
public class ClientHandle
{
    public Guid ID;
    public bool IsSimpleTask = false;
}
public class TaskStatus
{
    public bool Success = true;
    public Exception InnerException = null;
}

看到上面的代码了吗？UserTask是一个代理，代表了线程池中线程将要执行的任务。

3. 接下来我们给我们的自定义线程池类增加一些公共接口的方法

public ClientHandle QueueUserTask(UserTask task, Action<TaskStatus> callback)
{
    throw new Exception("not implemented yet.");
}

public static void CancelUserTask(ClientHandle handle)
{ 
    //TODO: write implementation code here
}

 4. 下面是线程池需要使用的一些内部类和类型

//#region nested private types
enum TaskState // to represent current state of a usertask
{
    notstarted,
    processing,
    completed,
    aborted
}
class TaskHandle // Item in waiting queue
{
    public ClientHandle Token; // generate this everytime an usertask is queued and return to the caller as a reference. 
    public UserTask task; // the item to be queued - supplied by the caller
    public Action<TaskStatus> callback; // optional - in case user want's a notification of completion
}

class TaskItem // running items in the pool - TaskHandle gets a thread to execute it 
{
    public TaskHandle taskHandle;
    public Thread handler;
    public TaskState taskState = TaskState.notstarted; 
    public DateTime startTime = DateTime.MaxValue;
}
//#endregion  

5. 下面我们需要做的是线程池的初始化工作，并且初始化任务队列

//private instance members
private Queue<TaskHandle> ReadyQueue = null;
private List<TaskItem> Pool = null;
private Thread taskScheduler = null;

private void InitializeThreadPool()
{
    ReadyQueue = new Queue<TaskHandle>();
    Pool = new List<TaskItem>();
    taskScheduler = new Thread(() =>
        {
            //TODO: write scheduling logic here
        });
    taskScheduler.Start();
}  

  上面需要特别注意的是，taskScheduler这个线程类对象。
  这是始终贯穿线程池生命周期的一个额外的线程，也可以说是主线程。

它的任务是监视用户任务队列，并且尽快地把它们带去执行，另外它还负责强制最大和最小

线程数的限制，做一些清理工作。

6. 接着就是实现线程初始化了，我们使用的是就近完成算法

private void InitializeThreadPool()
{
    ReadyQueue = new Queue<TaskHandle>();
    Pool = new List<TaskItem>();

    InitPoolWithMinCapacity(); // initialize Pool with Minimum capacity - that much thread must be kept ready

    DateTime LastCleanup = DateTime.Now; // monitor this time for next cleaning activity

    taskScheduler = new Thread(() =>
        {
            do
            {
                while (ReadyQueue.Count > 0 && ReadyQueue.Peek().task == null)
                    ReadyQueue.Dequeue();
                    // remove cancelled item/s - cancelled item will have it's task set to null
                        
                int itemCount = ReadyQueue.Count;
                for (int i = 0; i < itemCount; i++)
                {
                    TaskHandle readyItem = ReadyQueue.Peek(); // the Top item of queue
                    bool Added = false;

                    foreach (TaskItem ti in Pool)
                    {
                        if (ti.taskState == TaskState.completed)
                        {
                            // if in the Pool task state is completed then a different
                            // task can be handed over to that thread
                            ti.taskHandle = readyItem;
                            ti.taskState = TaskState.notstarted;
                            Added = true;
                            ReadyQueue.Dequeue();
                            break;
                        }
                    }
                    if (!Added && Pool.Count < MAX)
                    {
                    // if all threads in pool are busy and the count is still less than the
                    // Max limit set then create a new thread and add that to pool
                        TaskItem ti = new TaskItem() { taskState = TaskState.notstarted };
                        ti.taskHandle = readyItem;
                        // add a new TaskItem in the pool
                        AddTaskToPool(ti);
                        Added = true;
                        ReadyQueue.Dequeue();
                    }
                    if (!Added) break; // It's already crowded so try after sometime
                }
                if ((DateTime.Now - LastCleanup) > TimeSpan.FromMilliseconds(CLEANUP_INTERVAL))
                // It's long time - so try to cleanup Pool once.
                {
                    CleanupPool();
                    LastCleanup = DateTime.Now;
                }
                else
                {
                    // either of these two can work - the combination is also fine for our demo. 
                    Thread.Yield();
                    Thread.Sleep(SCHEDULING_INTERVAL); // Dont run madly in a loop - wait for sometime for things to change.
                    // the wait should be minimal - close to zero
                }
            } while (true);
        });
    taskScheduler.Priority = ThreadPriority.AboveNormal;
    taskScheduler.Start();
}

private void InitPoolWithMinCapacity()
{
    for (int i = 0; i <= MIN; i++)
    {
        TaskItem ti = new TaskItem() { taskState = TaskState.notstarted };
        ti.taskHandle = new TaskHandle() { task = () => { } };
        ti.taskHandle.callback = (taskStatus) => { };
        ti.taskHandle.Token = new ClientHandle() { ID = Guid.NewGuid() };
        AddTaskToPool(ti);
    }
}

private void AddTaskToPool(TaskItem taskItem)
{
    taskItem.handler = new Thread(() =>
    {
        do
        {
            bool Enter = false;

            // if aborted then allow it to exit the loop so that it can complete and free-up thread resource.
            // this state means it has been removed from Pool already.
            if (taskItem.taskState == TaskState.aborted) break; 

            if (taskItem.taskState == TaskState.notstarted)
            {
                taskItem.taskState = TaskState.processing;
                taskItem.startTime = DateTime.Now;
                Enter = true;
            }
            if (Enter)
            {
                TaskStatus taskStatus = new TaskStatus();
                try
                {
                    taskItem.taskHandle.task.Invoke(); // execute the UserTask
                    taskStatus.Success = true;
                }
                catch (Exception ex)
                {
                    taskStatus.Success = false;
                    taskStatus.InnerException = ex;
                }
                if (taskItem.taskHandle.callback != null && taskItem.taskState != TaskState.aborted)
                {
                    try
                    {
                        taskItem.taskState = TaskState.completed;
                        taskItem.startTime = DateTime.MaxValue;

                        taskItem.taskHandle.callback(taskStatus); // notify callback with task-status
                    }
                    catch
                    {

                    }
                }
            }
            // give other thread a chance to execute as it's current execution completed already
            Thread.Yield(); Thread.Sleep(MIN_WAIT); //TODO: need to see if Sleep is required here
        } while (true); // it's a continuous loop until task gets abort request
    });
    taskItem.handler.Start();
    Pool.Add(taskItem);
}

private void CleanupPool()
{
    throw new NotImplementedException();
}

7. 用户任务队列实现

public ClientHandle QueueUserTask(UserTask task, Action<taskstatus> callback)
{
    TaskHandle th = new TaskHandle() 
        { 
            task = task, 
            Token = new ClientHandle() 
                { 
                    ID = Guid.NewGuid() 
                }, 
            callback = callback 
        };
    ReadyQueue.Enqueue(th);
    return th.Token;
}

8. 最后便是测试代码

CustomThreadPool MyPool;

private void Form1_Load(object sender, EventArgs e)
{
    MyPool = CustomThreadPool.Instance;
}

void showMessage(string message)
{
    MessageBox.Show(message);
}
int x = 0;

private void btnStart_Click(object sender, EventArgs e)
{
    x++;
    int arg = x;
    MyPool.QueueUserTask(() => 
        { 
            showMessage(arg.ToString()); 
        }, 
        (ts) => 
        { 
            showMessage(ts.Success.ToString()); 
        });
}