复习了一遍 Singleton 的 5 种写法

public class SingletonDemo { public static class Singleton1{ private static final Singleton1 instance = new Singleton1(); private Singleton1(){} public static Singleton1 getInstance(){ return instance; } } public static class Singleton2{ private static Singleton2 instance; private Singleton2(){}; public static synchronized Singleton2 getInstance(){ if(instance == null){ instance = new Singleton2(); } return instance; } } public static class Singleton3{ private static volatile Singleton3 instance; private Singleton3(){} public static Singleton3 getInstance(){ if(instance == null){ synchronized (Singleton3.class){ if(instance == null){ instance = new Singleton3(); } } } return instance; } } public static class Singleton4{ private static final Singleton4 instance; static{ instance = new Singleton4(); } private Singleton4(){} public static Singleton4 getInstance(){ return instance; } } public static class Singleton5{ private static class SingletonHolder{ private static final Singleton5 instance = new Singleton5(); } private Singleton5(){} public static Singleton5 getInstance(){ return SingletonHolder.instance; } } }