1.懒汉-线程不安全

1 public class Singleton {   2     private static Singleton instance;   3     private Singleton (){}   4    5     public static Singleton getInstance() {   6     if (instance == null) {   7         instance = new Singleton();   8     }   9     return instance;  10     }  11 }

2.懒汉-线程安全（效率很低，99%情况下不需要同步。）

1 public class Singleton {   2     private static Singleton instance;   3     private Singleton (){}   4     public static synchronized Singleton getInstance() {   5     if (instance == null) {   6         instance = new Singleton();   7     }   8     return instance;   9     }  10 }

3.饿汉

1 public class Singleton {  2     private static Singleton instance = new Singleton();  3     private Singleton (){}  4     public static Singleton getInstance() {  5     return instance;  6     }  7 }

4.饿汉2

1 public class Singleton {   2     private Singleton instance = null;   3     static {   4     instance = new Singleton();   5     }   6     private Singleton (){}   7     public static Singleton getInstance() {   8     return this.instance;   9     }  10 }

5.静态内部类

1 public class Singleton {  2     private static class SingletonHolder {  3     private static final Singleton INSTANCE = new Singleton();  4     }  5     private Singleton (){}  6     public static final Singleton getInstance() {  7     return SingletonHolder.INSTANCE;  8     }  9 }

6.枚举

1 public enum Singleton {  2     INSTANCE;  3     public void whateverMethod() {  4     }  5 }

7.双重校验锁

1 public class Singleton {   2     private volatile static Singleton singleton;   3     private Singleton (){}   4     public static Singleton getSingleton() {   5     if (singleton == null) {   6         synchronized (Singleton.class) {   7         if (singleton == null) {   8             singleton = new Singleton();   9         }  10         }  11     }  12     return singleton;  13     }  14 }