public class A { private static A a = new A(); private A(){} public static A getInstance() { return a; }}
懒汉式：私有的静态变量初始是null在公共静态方法中创建对象返回
public class B { private static B b = null; private B() {} public static B getInstance() { if (b == null) { synchronized (B.class) { if (b == null) { b = new B(); } } } return b; }}
