线程安全延迟初始化方案 #10

Posted 5 years ago · 1 mins reading

Java

Initialization On Demand Holder idiom

java
public class InstanceFactory { 
    private static class InstanceHolder {
        public static Instance instance = new Instance(); 
    } public static Instance getInstance() {
        return InstanceHolder.instance ;　　// 这里将导致InstanceHolder类被初始化  
    } 
}

DoubleCheckedLocking

java
public class SafeDoubleCheckedLocking {


    private volatile static Instance instance; 


    public static Instance getInstance() { 
        if (instance == null) { 
            synchronized (SafeDoubleCheckedLocking.class) { 
                if (instance == null) instance = new Instance(); // instance为volatile，现在没问题了 
            } 
        } 
        return instance;
    }
}

结语

字段延迟初始化降低了初始化类或创建实例的开销，但增加了访问被延迟初始化的字段 的开销。在大多数时候，正常的初始化要优于延迟初始化。如果确实需要对实例字段使用线程 安全的延迟初始化，请使用上面介绍的基于volatile的延迟初始化的方案；如果确实需要对静 态字段使用线程安全的延迟初始化，请使用上面介绍的基于类初始化的方案。

c++

c++中的常见单例

cpp
class Singleton {
public:
    static Singleton &GetInstance();
private:
    Singleton();
    ~Singleton();
    Singleton(const Singleton &);
    const Singleton &opertor = (const Singleton &);


}


Singleton &Singleton::GetInstance() {
    static Singleton instance;
    return instance;
}

DoubleCheckedLocking

cpp
Singleton &Singleton::GetInstance() {
    static Singleton *instance = nullptr;


    if (!instance) {
        Mutex mutex;
        ScopedLock lock(&mutex);




        if (!instance) {
            instance = new Singleton();
        }
    }


    return *instance;
}

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