Java单例模式（懒汉式、饿汉式、线程安全的懒汉式）

1.饿汉式

class A{
    private A(){}
    private static A a = new A();
    public static A  getA(){
        return a;
    }
}

2.懒汉式

class B{
    private B() { }
    private static B b;
    private static B getB(){
        if(b==null){
            b = new B();
        }
        return b;
    }
}

3.线程安全的懒汉式

class C{
    private C(){}
    private static volatile C c;
    private static synchronized C getC(){
        if(c==null){
            synchronized (C.class){
                c = new C();
            }
        }
        return c;
    }
}