23、面向对象的设计模式之单例模式--基础概念及概念

public class EagerSingleton
{
    // 由于在定义变量的时候实例化单例类，在类加载时就已经创建了单个对象。
    private static EagerSingleton instance = new EagerSingleton();
    private EagerSingleton()
    {

    }
    public static EagerSingleton GetInstance()
    {
        return instance;
    }
}

//在第一次调用GetInstance()方法时实例化，在类加载并不自行实例化。这种技术叫做延迟加载（Lazy Load),就是在需要的时候在加载实例。
//为了避免多线程调用GetInstance方法。加lock
public class LazySingleton
{
    private static LazySingleton instance = null;
    private LazySingleton()
    {

    }
    public static LazySingleton GetInstance()
    {
        if(instance == null)
        {
            new LazySingleton();
        }
        return instance;
    }
}

public class SingleThreadSingleton
{
    private static SingleThreadSingleton instance = null;
    private SingleThreadSingleton()
    {

    }

    public static SingleThreadSingleton GetInstance()
    {
        if(instance == null)
        {
            instance = new SingleThreadSingleton();
        }
        return instance;
    }
}

public class MultiThreadSingleton
{
    private static MultiThreadSingleton instance;
    private static readonly object syncRoot = new object();//静态的只读的辅助对象
    private MultiThreadSingleton()
    {

    }

    public static MultiThreadSingleton GetInstance()
    {
        lock (syncRoot) // 同一时刻枷锁的这部分程序只有一个线程进入。但是有一个问题这样会影响效率
        {
            if(instance == null)
            {
                instance = new MultiThreadSingleton();
            }
        }
        return instance;
    }
}

public class MultiThreadSingleton
{
    //  volatile 保证严格意义的多线程编译器在代码编译时对指令不进行微调
    private static volatile MultiThreadSingleton instance = null;
    private static readonly object syscRoot = new object();
    private MultiThreadSingleton()
    {

    }
    public static MultiThreadSingleton GetInstance()
    {
        
        if(instance == null)// 多个线程调用GetInstance方法时，都可以通过第一重if(instance == null)，可以提升效率
        {
            lock (syscRoot) //只有一个线程时进入
            {
                if(instance == null) // 如果没有这层，第一个线程创建了实例，第二个线程还可以创建实例。
                {
                    instance = new MultiThreadSingleton();
                }
            }
        }
        return instance;
    }
}

public class StaticSingleton
{
    public static readonly StaticSingleton instance = new StaticSingleton();
    private StaticSingleton()
    {

    }
}

public class StaticSingleton
{
    public static readonly StaticSingleton instance;
    private StaticSingleton()
    {

    }

    static StaticSingleton()
    {
        instance = new StaticSingleton();
    }
}