Java单例模式的线程安全,饿汉和懒汉模式详解

//如果是多线程 需要加锁
class MultiThreadSingleton
{
public:
    ~MultiThreadSingleton()
    {
        cout << "~MultiThreadSingleton()" << endl;
    }   
    static MultiThreadSingleton* getInstance()
    {
        if(instance == nullptr)
        {
            //初次创建时加锁
            pthread_mutex_lock(&mutex);
            instance = new MultiThreadSingleton();
            pthread_mutex_unlock(&mutex);
        }
        return instance;
    }
    void SingletonOP()
    {
        cout << "SingletonOP!" << endl;
    }
private:
    MultiThreadSingleton()
    {
        pthread_mutex_init(&mutex,NULL);
        cout << "MultiThreadSingleton()" << endl;
    }
    static pthread_mutex_t mutex;
    static MultiThreadSingleton* instance;
};
//懒汉模式  即需要的时候才去实例化对象
MultiThreadSingleton*  MultiThreadSingleton::instance = nullptr;
pthread_mutex_t MultiThreadSingleton::mutex;
//------------
MultiThreadSingleton* sig3 = MultiThreadSingleton::getInstance();
sig3->SingletonOP();
delete sig3;

//局部变量懒汉模式
static MultiThreadSingleton* getInstance()
{
     //局部变量
     static MultiThreadSingleton localInstance;
     return &localInstance;
}
//-----------------------------
//懒汉局部变量
 MultiThreadSingleton* sig4 = MultiThreadSingleton::getInstance();
 MultiThreadSingleton* sig5 = MultiThreadSingleton::getInstance();
 sig4->SingletonOP();
 if(sig4 == sig5) cout << "Test!" << endl;

class MultiThreadSingleton
{
public:
    ~MultiThreadSingleton()
    {
        cout << "~MultiThreadSingleton()" << endl;
    }   
    static MultiThreadSingleton* getInstance()
    {
        // if(instance == nullptr)
        // {
        //     return new MultiThreadSingleton();
        // }
        return instance;
    }
    void SingletonOP()
    {
        cout << "SingletonOP!" << endl;
    }
private:
    MultiThreadSingleton()
    {
        cout << "MultiThreadSingleton()" << endl;
    }
    static MultiThreadSingleton* instance;

};
//饿汉模式  即在类加载时时就创建对象，适合访问量较大的时候，用空间换时间
MultiThreadSingleton*  MultiThreadSingleton::instance = new MultiThreadSingleton();