單例模式有兩種：餓漢模式和懶漢模式，懶漢模式的特點是延遲加載實例

//餓漢模式
class Singleton1{
private static final Singleton1 instance = new Singleton1();
private Singleton1(){}
public static Singleton1 getSingleton()
{
return instance;
}
}

//懶漢模式
class Singleton2{
private static Singleton2 instance;
private Singleton2(){}
public static Singleton2 getSingleton()
{
if(instance == null)
instance = new Singleton2();
return instance;
}
}

懶漢模式在多線程的情況下，會存在安全問題，對象會被實例化多次，可以用同步方法或者同步方法快的方式解決

//解決懶漢模式多線程的安全問題
class Singleton3{
private static Singleton3 instance;
private Singleton3(){}
public static synchronized Singleton3 getSingleton()
{
if(instance == null)
instance = new Singleton3();
return instance;
}
}

但是這種方式由於增加了判斷鎖的操作，會使得執行效率變慢

//解決懶漢模式多線程的安全問題的優化方案
class Singleton4{
private static Singleton4 instance;
private Singleton4(){}
public static Singleton4 getSingleton()
{
if(instance == null)
{
synchronized(Singleton4.class)
{
if(instance == null)
instance = new Singleton4();
}
}
return instance;
}
}

【Java學習】單例模式