Singleton对象 - 在静态块或getInstance()中; 哪个应该用

Question

以下是实现单例的两种方法.各有哪些优缺点？

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;
    }
}

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;
       }
   }
   

   • 懒加载
   • 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;
       }
   }
   

   • 懒加载
   • 高性能
   • JDK应该是1.5 ++

3. 按需持有人的习语

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

   • 懒加载
   • 高性能
   • 不能用于非静态类字段

Answer 2

第一个是急切的初始化.急切初始化甚至在使用它之前就会创建实例,这不是最佳实践.

第二个是懒惰初始化.惰性实现在单线程环境中运行良好,但是当涉及多线程系统时,如果多个线程同时位于if循环内,则会导致问题.它将破坏单例模式,两个线程将获得单例类的不同实例.

请访问:http://www.journaldev.com/1377/java-singleton-design-pattern-best-practices-with-examples#static-block-initialization了解更多信息