Singleton 对象 - 在静态块中或在 getInstance() 中；应该使用哪个

下面是两种实现单例的方法。各自的优点和缺点是什么？

静态初始化：

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

延迟初始化是：

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

1. 同步访问器

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

   • 惰性加载
   • 低性能

2. 双重检查锁定和易失性

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

   • 惰性加载
   • 高性能
   • JDK应该是1,5++

3. 按需持有者习语

   public class Singleton {
   
       public static class SingletonHolder {
           public static final Singleton HOLDER_INSTANCE = new Singleton();
       }
   
       public static Singleton getInstance() {
           return SingletonHolder.HOLDER_INSTANCE;
       }
   }
   

   • 惰性加载
   • 高性能
   • 不能用于非静态类字段