单例模式作为常用的设计模式，在项目中经常使用。手写单例模式，考察面试者的java基础，关键字的理解。面试了很多面试者，能完整写出来的，线程安全的，不到三分之一。还是比较考研基础的。
常用写法：
饿汉模式：
public class Singleton { private final static Singleton INSTANCE = new Singleton(); private Singleton(){} public static Singleton getInstance(){ return INSTANCE; } }懒汉模式：
public class Singleton { private static Singleton singleton; private Singleton() {} public static Singleton getInstance() { if (singleton == null) { singleton = new Singleton(); } return singleton; } }双重校验：
public class Singleton { private static volatile Singleton singleton; private Singleton() {} public static Singleton getInstance() { if (singleton == null) { synchronized (Singleton.class) { if (singleton == null) { singleton = new Singleton(); } } } return singleton; } }静态内部类：
public class Singleton { private Singleton() {} private static class SingletonInstance { private static final Singleton INSTANCE = new Singleton(); } public static Singleton getInstance() { return SingletonInstance.INSTANCE; } }