单例模式

单例模式

1. 静态常量饿汉式

   class Singleton {
       private static Singleton instance = new Singleton;
       private Singleton() {}
       public static Singleton getInstance() {
           return instance;
       }
   }
   

2. 静态代码块饿汉式

   class Singleton {
       private static Singleton instance;
       static {
           instance = new Singleton();
       }
       private Singleton() {}
       public static Singleton getInstance() {
           return instance;
       }
   }
   

3. 线程不安全懒汉式

   class Singleton {
       private static Singleton singleton;
       private Singleton() {}
       public static Singleton getInstance() {
           if (singleton == null) {
               singleton = new Singleton();
           }
       }
       return singleton;
   }
   

4. 线程安全懒汉式

   class Singleton {
       private static Singleton singleton;
       private Singleton() {}
       public static synchronized Singleton getInstance() {
           if (singleton == null) {
               singleton = new Singleton();
           }
       }
       return singleton;
   }
   

5. 同步代码块懒汉式

   class Singleton {
       private static Singleton singleton;
       private Singleton() {}
       public static Singleton getInstance() {
           if (singleton == null) {
               synchronized (Singleton.class) {
                   singleton = new Singleton();
               }
           }
       }
       return singleton;
   }
   

6. DoubleCheck

   class Singleton {
       private static volatile Singleton singleton;
       private Singleton(){}
       public static Singleton getInstance() {
           if (singleton == null) {
               synchronized (Singleton.class) {
                   if (singleton == null) {
                       singleton = new Singleton();
                   }
               }
           }
       }
       return singleton;
   }
   

7. 静态内部类 (类加载时候不会加载静态内部类)

   class Singleton {
       private Singleton(){}
       private static class SingletonInstance {
           private static final Singleton INSTANCE = new Singleton();
       }
       public static Singleton getInstance() {
           return SingleInstance.INSTANCE;
       }
   }
   

8. 枚举方式

   enum Singleton {
       INSTANCE;
       public void method(){
           // 方法
       }
   }