单例模式
一.懒汉模式
1 /** 2 * 懒汉模式：非线程安全的 3 * 所谓懒汉即用到的时候才初始化 4 */ 5 public class SingleTon { 6 //实例成员私有化，防止被引用；设置为null，延迟加载 7 private static SingleTon singleTon = null; 8 //构造器私有化，只有通过getInstance来获取实例 9 public static SingleTon getInstance(){ 10 if(singleTon == null){ 11 singleTon = new SingleTon(); 12 } 13 return singleTon; 14 } 15 //构造器私有化，防止被实例化 16 private SingleTon(){ 17 18 } 19 20 public static void main(String[] args) { 21 SingleTon singleTon1 = SingleTon.getInstance(); 22 SingleTon singleTon2 = SingleTon.getInstance(); 23 System.out.println(singleTon1==singleTon2);//true 24 } 25 }
二.饿汉模式
1 /** 2 * 饿汉模式：线程安全 3 */ 4 public class SingleTon { 5 private static final SingleTon singleTon = new SingleTon(); 6 public static SingleTon getInstance(){ 7 return singleTon; 8 } 9 private SingleTon(){ 10 11 } 12 public static void main(String[] args) { 13 SingleTon singleTon1 = SingleTon.getInstance(); 14 SingleTon singleTon2 = SingleTon.getInstance(); 15 System.out.println(singleTon1==singleTon2);//true 16 } 17 }
三.双重锁
1 /** 2 * 双重锁：懒汉模式的同步优化，线程安全 3 */ 4 public class SingleTon { 5 private static SingleTon singleTon = null; 6 public static SingleTon getInstance(){ 7 if(singleTon == null){ 8 synchronized (SingleTon.class) { 9 if(singleTon==null){ 10 singleTon = new SingleTon(); 11 } 12 } 13 } 14 return singleTon; 15 } 16 private SingleTon(){ 17 18 } 19 public static void main(String[] args) { 20 SingleTon singleTon1 = SingleTon.getInstance(); 21 SingleTon singleTon2 = SingleTon.getInstance(); 22 System.out.println(singleTon1==singleTon2);//true 23 } 24 }
四.静态内部类
1 /** 2 * 静态内部类：线程安全 3 */ 4 public class SingleTon { 5 private static class SingleTonHolder{ 6 private static SingleTon singleTon = new SingleTon(); 7 } 8 public static SingleTon getInstance(){ 9 return SingleTonHolder.singleTon; 10 } 11 private SingleTon(){ 12 } 13 public static void main(String[] args) { 14 SingleTon singleTon1 = SingleTon.getInstance(); 15 SingleTon singleTon2 = SingleTon.getInstance(); 16 System.out.println(singleTon1==singleTon2);//true 17 } 18 }
总结：单线程模式下用第一种就行，多线程下最好用第二种或第四种。因为网上很多人说第二种也没有做到线程安全。
