Java Singleton Mode Analysis

(1)/* thread security, but efficiency is relatively low. At the beginning, we need to load a class new object. This is a hunger Singleton1 mode */public class Singleton1 {private Singleton1 () {} private static final Singleton1 instance = new Singleton1 (); public static Singleton1 getInstancei () {return instance ;}} (2) // full-Chinese Singleton mode, but when multiple threads access the instance, it is not safe to return not the same object public class Singleton {private Singleton () {} private static Singleton instance; public static Singleton getInstance () {if (instance = null) instance = new Singleton (); return instance ;}} (3) // although secure, the efficiency is very low. At one time, only one thread can access and return an object public class Singleton2 {private Singleton2 () {} private static Singleton2 instance; public static synchronized Singleton2 getInstance () {if (instance = null) instance = new Singleton2 (); return instance ;}} (4) /* thread security and high efficiency multiple threads can access */public class Singleton3 {private static Singleton3 instance; private Singleton3 () {} public static Singleton3 getIstance () {if (instance = null) {synchronized (Singleton3.class) {if (instance = null) {instance = new Singleton3 () ;}} return instance ;}}