java - 单例对象——在静态 block 中或在 getInstance() 中；应该使用哪个

Question

下面是实现单例的两种方式。各有什么优缺点？

静态初始化:

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

惰性初始化是:

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

Answer 1

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;
       }
   }
   

   • 延迟加载
   • 表现不佳

2. 双重检查锁定和 volatile

   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;
       }
   }
   

   • 延迟加载
   • 高性能
   • JDK 应该是 1,5++

3. On Demand Holder 成语

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

   • 延迟加载
   • 高性能
   • 不能用于非静态类字段