单例模式在Java语言中，有两种构建方式

懒汉方式。指全局的单例实例在第一次被使用时构建。饿汉方式。指全局的单例实例在类装载时构建。

在Java编程语言中，单例模式(饿汉模式)应用的例子如下述代码所示：

public class Singleton { private final static Singleton INSTANCE = new Singleton(); // Private constructor suppresses private Singleton() {} // default public constructor public static Singleton getInstance() { return INSTANCE; } }

在Java编程语言中，单例模式(懒汉模式)应用的例子如下述代码所示 (此种方法只能用在JDK5及以后版本(注意 INSTANCE 被声明为 volatie)，之前的版本使用“双重检查锁”会发生非预期行为^[1])：

public class Singleton { private static volatile Singleton INSTANCE = null; // Private constructor suppresses // default public constructor private Singleton() {} //thread safe and performance promote public static Singleton getInstance() { if(INSTANCE == null){ synchronized(Singleton.class){ //when more than two threads run into the first null check same time, to avoid instanced more than one time, it needs to be checked again. if(INSTANCE == null){ INSTANCE = new Singleton(); } } } return INSTANCE; } }

在C++编程语言中，单例模式应用的例子如下述代码所示（这里仅仅提供一个示例，这个例子对多线程的情况并不是安全的）：

// ... class lock { public: lock(); lock(lock const & l); ~lock(); lock & operator =(lock const & l); void request(); void release(); // ... }; lock::lock() { // ... } // ... lock::~lock() { // ... } // ... void lock::request() { // ... } void lock::release() { // ... } // ... // assumes _DATA_TYPE_ has a default constructor template<typename _DATA_TYPE_> class singleton { public: static _DATA_TYPE_ * request(); static void release(); private: singleton(); singleton(singleton<_DATA_TYPE_> const & s); ~singleton(); singleton<_DATA_TYPE_> & operator =(singleton<_DATA_TYPE_> const & s); static _DATA_TYPE_ * pointer; static lock mutex; // ... }; template<typename _DATA_TYPE_> _DATA_TYPE_ * singleton<_DATA_TYPE_>::pointer = 0; template<typename _DATA_TYPE_> lock singleton<_DATA_TYPE_>::mutex; template<typename _DATA_TYPE_> _DATA_TYPE_ * singleton<_DATA_TYPE_>::request() { if(singleton<_DATA_TYPE_>::pointer == 0) { singleton<_DATA_TYPE_>::mutex.request(); if(singleton<_DATA_TYPE_>::pointer == 0) { singleton<_DATA_TYPE_>::pointer = new _DATA_TYPE_; } singleton<_DATA_TYPE_>::mutex.release(); } return singleton<_DATA_TYPE_>::pointer; } template<typename _DATA_TYPE_> void singleton<_DATA_TYPE_>::release() { if(singleton<_DATA_TYPE_>::pointer != 0) { singleton<_DATA_TYPE_>::mutex.request(); if(singleton<_DATA_TYPE_>::pointer != 0) { delete singleton<_DATA_TYPE_>::pointer; singleton<_DATA_TYPE_>::pointer = 0; } singleton<_DATA_TYPE_>::mutex.release(); } } template<typename _DATA_TYPE_> singleton<_DATA_TYPE_>::singleton() { // ... } // ... int main() { int * s; s = singleton<int>::request(); // ... singleton<int>::release(); return 0; }

转载于:https://www.cnblogs.com/SunDexu/archive/2012/11/07/2759418.html