7种单例模式

七种单例模式：

懒汉式（线程不安全）

//不推荐
public class Singleton{
      private static Singleton instance;
      private Singleton();

      public static Singleton getInstance(){
            if(instance == null){
                  instance = new Singleton();
            }
            return this.instance;
      }
}

懒汉式（线程安全）

//不推荐
public class Singleton(){
      private static Singleton instance;      
      private Singleton();

      public static synchronized Singleton getInstance(){
            if(instance == null){
                  instance = new Singleton();
            }
            return this.instance;
      }
}

饿汉式

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

      public static Singleton getInstance(){
            return this.instance;
      }
}

饿汉式（变种）

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

静态内部类

//较推荐
//采用这种方式的好处是，当Singleton类被装载时，内部类SingletonHolder不一定被初始化，因为没有显式的调用getInstance()方法。
public class Singleton(){
      private static class SingletonHolder(){
            private static final Singleton INSTANCE = new Singleton();
      }

      private Singleton(){}

      public static final Singleton getInstance(){
            return SingletonHolder.INSTANCE;
      }
}

枚举

//目前最好的单例实现方式，因为枚举底层已经实现，所以代码非常简练，而且可以避免反序列化重新创建对象和多线程同步的问题
public enum Singleton{
      INSTANCE;
      public void watheverMethed(){}
}

双重校验锁

//比较推荐的方式
public class Singleton{
      private volatile static Singleton instance;      //被volatile修饰的变量可以保证其可见性（若值改变，其它线程立即可见，避免脏读）
      private Singleton(){}
      
      public static Singleton getInstance(){
            if(instance == null){
                  synchronized(Singleton.class){
                        if(instance == null){
                              instance = new Singleton();
                        }
                  }
            }
            return this.instance;
      }
}