Java单例模式

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

 1 public class Singleton {
 2 
 3     private static Singleton singleton;
 4     static {
 5         singleton = new Singleton();
 6     }
 7     private Singleton(){}
 8 
 9     public static Singleton getInstance(){
10         return singleton;
11     }
12 }

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

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

 1 public class Singleton {
 2     /**由于 return singleton = new Singleton(); 这行代码在JVM中会分为三步处理
 3      * 1.给singleton分配内存空间
 4      * 2.调用singleton的构造函数开初始化
 5      * 3.将singleton对象指向分配的内存空间（这步执行了就singleton就不是null了）
 6      * 
 7      * 由于CPU会对这三个步骤重排序，如果顺序是1 3 2，那么就可能出现singleton就不是空的，但并没有初始化singleton
 8      * 这样第二个线程可能拿到的就是为初始化的singleton，所以使用volatile来修饰singleton,防止重排序的问题
 9      */
10     private static volatile Singleton singleton;
11     private Singleton(){}
12 
13     public static Singleton getInstance(){
14         if (singleton == null){
15             synchronized (Singleton.class) {
16                 if (singleton == null) {
17                     return singleton = new Singleton();
18                 }
19             }
20         }
21         return singleton;
22     }
23 }

 1 public class Singleton {
 2     
 3     private static class SingletonInstance{
 4         private static final Singleton singleton = new Singleton();
 5     }
 6     private Singleton(){}
 7 
 8     public static Singleton getInstance(){
 9         return SingletonInstance.singleton;
10     }
11 }

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