2.1 简介
竞争条件：多个线程同时使用共享对象。需要同步这些线程使得共享对象的操作能够以正确的顺序执行
线程同步问题：多线程的执行并没有正确的同步，当一个线程执行递增和递减操作时，其他线程需要依次等待
线程同步解决方案：
无须共享对象：大部分时候可以通过重新设计来移除共享对象，去掉复杂的同步构造，避免多线程使用单一对象
必须共享对象：只使用原子操作，一个操作只占用一个量子的时间，无须实现其他线程等待当前操作完成
内核模式：将等待的线程置于阻塞状态，消耗少量的CPU资源，但会引入至少一次上下文切换，适用于线程等待较长时间
用户模式：只是简单的等待，线程等待会浪费CPU时间但是可以节省上下文切换消耗的CPU时间，适用于线程等待较短时间
混合模式：先尝试用户模式，如果等待时间较长，则会切换到内核模式
2.2 执行基本的原子操作
using System;
using System.Threading; namespace MulityThreadNote
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Incorrect counter");
//没有限定，会遇到竞争条件，得出的结果大部分不是正确的
var c = new Counter();
var t1 = new Thread(() => TestCounter(c));
var t2 = new Thread(() => TestCounter(c));
var t3 = new Thread(() => TestCounter(c));
t1.Start();
t2.Start();
t3.Start();
t1.Join();
t2.Join();
t3.Join();
Console.WriteLine($"Total count:{c.Count}");
Console.WriteLine("--------------------------");
Console.WriteLine("Correct counter");
//使用Interlocked类提供的原子操作方法，无需锁定任何对象可得出正确结果
var c1 = new CounterNoLock();
t1 = new Thread(() => TestCounter(c1));
t2 = new Thread(() => TestCounter(c1));
t3 = new Thread(() => TestCounter(c1));
t1.Start();
t2.Start();
t3.Start();
t1.Join();
t2.Join();
t3.Join();
Console.WriteLine($"Total count:{c1.Count}");
Console.ReadLine();
}
static void TestCounter(CounterBase c)
{
for (int i = ; i < ; i++)
{
c.Increment();
c.Decrement();
}
}
class Counter : CounterBase
{
private int _count;
public int Count { get { return _count; } } public override void Decrement()
{
_count--;
} public override void Increment()
{
_count++;
}
} class CounterNoLock : CounterBase
{
private int _count;
public int Count { get { return _count; } } public override void Decrement()
{
//Interlocked提供了Increment()、Decrement()和Add等基本数学操作的原子方法
Interlocked.Decrement(ref _count);
} public override void Increment()
{
Interlocked.Increment(ref _count);
}
} abstract class CounterBase
{
public abstract void Increment();
public abstract void Decrement();
}
}
}
注释：Interlocked提供了Increment()、Decrement()和Add等基本数学操作的原子方法，不用锁也可以得出正确结果
2.3 使用Mutex类
using System;
using System.Threading; namespace MulityThreadNote
{
class Program
{
static void Main(string[] args)
{
const string MutexName = "CSharpThreadingCookbook";
//Mutex是一种原始的同步方式，只对一个线程授予对共享资源的独占访问
//定义一个指定名称的互斥量，设置initialOwner标志为false
using (var m = new Mutex(false, MutexName))
{
//如果互斥量已经被创建，获取互斥量，否则就执行else语句
if (!m.WaitOne(TimeSpan.FromSeconds(), false))
{
Console.WriteLine("Second instance is running!");
}
else
{
Console.WriteLine("Running!");
Console.ReadLine();
m.ReleaseMutex();
}
}
//如果再运行同样的程序，则会在5秒内尝试获取互斥量，如果第一个程序按下了任何键，第二个程序开始执行。
//如果等待5秒钟，第二个程序将无法获取该互斥量
}
}
}
注释：互斥量是全局操作对象，必须正确关闭，最好用using
2.4 使用SemaphoreSlim类
using System;
using System.Threading; namespace MulityThreadNote
{
class Program
{
static void Main(string[] args)
{
//启动6个线程，启动的顺序不一样
for (int i = ; i <= ; i++)
{
string threadName = "Thread " + i;
int secondsToWait = + * i;
var t = new Thread(() => AccessDatabase(threadName, secondsToWait));
t.Start();
} Console.ReadLine();
}
//SemaphoreSlim的构造函数参数为允许的并发线程数目
static SemaphoreSlim semaphore = new SemaphoreSlim(); static void AccessDatabase(string name, int seconds)
{
Console.WriteLine($"{name} waits to access a database");
semaphore.Wait();
Console.WriteLine($"{name} was granted an access to a database");
Thread.Sleep(TimeSpan.FromSeconds(seconds));
Console.WriteLine($"{name} is Completed");
//调用Release方法说明线程已经完成，可以开启一个新的线程了
semaphore.Release();
}
}
}
注释：这里使用了混合模式，允许我们在等待时间很短的情况下无需上下文切换。SemaphoreSlim并不使用Windows内核信号量，而且也不支持进程间同步
2.5 使用AutoResetEvent类
using System;
using System.Threading; namespace MulityThreadNote
{
class Program
{
static void Main(string[] args)
{
Thread t = new Thread(() => Process());
t.Start();
Console.WriteLine("Waiting for another thread to complete work");
//开启一个线程后workEvent等待，直到收到Set信号
workEvent.WaitOne();
Console.WriteLine("First operation is complete");
Console.WriteLine("Performing an operation on a main thread");
Thread.Sleep(TimeSpan.FromSeconds());
mainEvent.Set();
Console.WriteLine("Now running the second operation on a second thread");
workEvent.WaitOne();
Console.WriteLine("Second operation is complete");
Console.ReadLine();
}
//初始状态为unsignaled，子线程向主线程发信号
private static AutoResetEvent workEvent = new AutoResetEvent(false);
//初始状态为unsignaled，主线程向子线程发信号
private static AutoResetEvent mainEvent = new AutoResetEvent(false); static void Process(int seconds)
{
Console.WriteLine("Starting a long running work...");
Thread.Sleep(TimeSpan.FromSeconds(seconds));
Console.WriteLine("Work is done!");
workEvent.Set();//将事件设为终止状态允许一个或多个线程继续
Console.WriteLine("Waiting for a main thread to complete its work"); mainEvent.WaitOne();//阻止当前线程，直到mainEvent收到信号
Console.WriteLine("Starting second operation...");
Thread.Sleep(TimeSpan.FromSeconds(seconds));
Console.WriteLine("Work is done");
workEvent.Set();
}
}
}
注释：AutoResetEvent采用的是内核模式，所以等待时间不能太长
2.6 使用ManualResetEventSlim类
using System;
using System.Threading; namespace MulityThreadNote
{
class Program
{
static void Main(string[] args)
{
Thread t1 = new Thread(() => TravelThroughGates("Thread 1", ));
Thread t2 = new Thread(() => TravelThroughGates("Thread 2", ));
Thread t3 = new Thread(() => TravelThroughGates("Thread 3", ));
t1.Start();
t2.Start();
t3.Start();
Thread.Sleep(TimeSpan.FromSeconds());
Console.WriteLine("The gates are now open");
mainEvent.Set();//将时间设置为有信号，从而让一个或多个等待该事件的线程继续
Thread.Sleep(TimeSpan.FromSeconds());
mainEvent.Reset();//将事件设置为非终止，从而导致线程受阻
Console.WriteLine("The gates have been closed!");
Thread.Sleep(TimeSpan.FromSeconds());
Console.WriteLine("The gates are now open for the second time");
mainEvent.Set();
Thread.Sleep(TimeSpan.FromSeconds());
Console.WriteLine("The gates have been closed!");
mainEvent.Reset();
Console.ReadLine();
}
static ManualResetEventSlim mainEvent = new ManualResetEventSlim(false); static void TravelThroughGates(string threadName, int seconds)
{
Console.WriteLine($"{threadName} falls to sleep");
Thread.Sleep(TimeSpan.FromSeconds(seconds));
Console.WriteLine($"{threadName} waits for the gates to open!");
mainEvent.Wait();//阻止当前线程
Console.WriteLine($"{threadName} enter the gates!");
}
}
}
注释：ManualResetEventSlim工作方式像人群通过的大门，一直保持大门敞开直到调用reset，set相当于打开大门，reset相当于关闭大门
2.7 使用CountDownEvent类
using System;
using System.Threading; namespace MulityThreadNote
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Starting two operations");
Thread t1 = new Thread(() => PerformOperation("Operation 1 is completed", ));
Thread t2 = new Thread(() => PerformOperation("Operation 2 is completed", ));
t1.Start();
t2.Start();
//开启了两个线程，调用Wait方法阻止当前线程，知道所有线程都完成
countdown.Wait();
Console.WriteLine("Both operations have been completed");
countdown.Dispose();
Console.ReadLine();
}
//计数器初始化CountdownEvent实例，计数器表示：当计数器个数完成操作发出信号
static CountdownEvent countdown = new CountdownEvent(); static void PerformOperation(string message, int seconds)
{
Thread.Sleep(TimeSpan.FromSeconds(seconds));
Console.WriteLine(message);
//向CountdownEvent注册信息，并减少当前计数器数值
countdown.Signal();
}
}
}
注释：如果Signal方法没有达到指定的次数，那么countdown.wait()会一直等待，所以请确保所有线程完成后都要调用Signal方法
2.8 使用Barrier类
using System;
using System.Threading; namespace MulityThreadNote
{
class Program
{
static void Main(string[] args)
{
Thread t1 = new Thread(() => PlayMusic("the gutarist", "play an amazing solo", ));
Thread t2 = new Thread(() => PlayMusic("the signer", "sing his song", ));
t1.Start();
t2.Start();
Console.ReadLine();
}
//后面的Lamda表达式是回调函数。执行完SignalAndWait后执行
static Barrier barrier = new Barrier(, b=>Console.WriteLine($"End of phase {b.CurrentPhaseNumber + 1}")); static void PlayMusic(string name, string message, int seconds)
{
for (int i = ; i < ; i++)
{
Console.WriteLine("===========================");
Thread.Sleep(TimeSpan.FromSeconds(seconds));
Console.WriteLine($"{name} starts to {message}");
Thread.Sleep(TimeSpan.FromSeconds(seconds));
Console.WriteLine($"{name} finishes to {message}");
//等所有调用线程都结束
barrier.SignalAndWait();
}
}
}
}
注释：
2.9 使用ReaderWriterlockSlim类
using System;
using System.Collections.Generic;
using System.Threading; namespace MulityThreadNote
{
class Program
{
static void Main(string[] args)
{
new Thread(Read) { IsBackground = true }.Start();
new Thread(Read) { IsBackground = true }.Start();
new Thread(Read) { IsBackground = true }.Start();
new Thread(() => Write("Thread 1")) { IsBackground = true }.Start();
new Thread(() => Write("Thread 2")) { IsBackground = true }.Start();
Thread.Sleep(TimeSpan.FromSeconds());
Console.ReadLine();
}
//实现线程安全
static ReaderWriterLockSlim _rw = new ReaderWriterLockSlim();
static Dictionary<int, int> items = new Dictionary<int, int>(); static void Read()
{
Console.WriteLine("Reading contents of a dictionary");
while (true)
{
try
{
//读锁
_rw.EnterReadLock();
foreach (var key in items.Keys)
{
Thread.Sleep(TimeSpan.FromSeconds(0.1));
}
}
finally
{
//计数为0时退出读取模式
_rw.ExitReadLock();
}
}
} static void Write(string threadName)
{
while (true)
{
try
{
int newKey = new Random().Next();
_rw.EnterUpgradeableReadLock();
if (!items.ContainsKey(newKey))
{
try
{
//写锁
_rw.EnterWriteLock();
items[newKey] = ;
Console.WriteLine($"New key {newKey} is added to a dictionary by a {threadName}");
}
finally
{
//计数为0时退出写入模式
_rw.ExitWriteLock();
}
}
Thread.Sleep(TimeSpan.FromSeconds(0.1));
}
finally
{
//计数为0时退出可升级模式
_rw.ExitUpgradeableReadLock();
}
}
}
}
}
注释：从集合读取数据时，根据当前数据决定是否获取一个写锁并修改该集合。获取写锁后集合会处于阻塞状态。
2.10 使用SpinWait类
using System;
using System.Collections.Generic;
using System.Threading; namespace MulityThreadNote
{
class Program
{
static void Main(string[] args)
{
Thread t1 = new Thread(UserModeWait);
Thread t2 = new Thread(HybridSpinWait);
Console.WriteLine("Running user mode waiting");
t1.Start();
Thread.Sleep();
_isComplete = true;
Thread.Sleep(TimeSpan.FromSeconds());
_isComplete = false;
Console.WriteLine("Running hybrid SpinWait construct waiting");
t2.Start();
Thread.Sleep();
_isComplete = true;
Console.ReadLine();
}
//volatile 一个字段可能会被多个线程同时修改，不会被编译器和处理器优化为只能被单个线程访问
static volatile bool _isComplete = false; static void UserModeWait()
{
while (!_isComplete)
{
Console.WriteLine(".");
}
Console.WriteLine();
Console.WriteLine("Waiting is complete");
} static void HybridSpinWait()
{
var w = new SpinWait();
while (!_isComplete)
{
//执行单一自旋
w.SpinOnce();
//NextSpinWillYield:获取对SpinOnce的下一次调用是否将产生处理，同时触发强制上下文切换
//显示线程是否切换为阻塞状态
Console.WriteLine(w.NextSpinWillYield);
}
Console.WriteLine("Waiting is complete");
}
}
}
注释：