单身和异常 [英] Singleton and Exception

问题描述

这里我有一个Singleton（使用Bill Pugh的方法，在Wiki中记录为Singleton） 。

  private static class SingletonObjectFactoryHolder {
 // 1 
 private static final ObjectFactory INSTANCE = new ObjectFactory ）; 
} 
 
 private ObjectFactory（）throws异常{
 // 2 
 //创建工厂
} 
 
 
 public static ObjectFactory getInstance（）{
 // 3 
 return SingletonObjectFactoryHolder.INSTANCE; 
} 
  

如果在2处抛出异常，我想将其传播到呼叫者。
但是，我不能从第1行引发异常。

所以，如果单例对象不是，我唯一的选择是返回null对象创建正确？

谢谢

PS我意识到，如果通过不同的类加载器加载Singleton，如果加载反射，但它对我的目的是足够好的。

//更新

我很好奇，我不能重新排列我的设计，以抛出异常？

此外，我不需要任何同步（类加载器保证静态内部只有当getInstance（）被调用时，类才会加载一次）。因此，线程安全和懒惰地实例化？

  private static class SingletonObjectFactoryHolder {
 // 1 
 public static ObjectFactory getInstance（）throws Exception {
 return new ObjectFactory（）; 
} 
} 
 
 private ObjectFactory（）throws异常{
 // 2 
 //创建工厂
} 
 
 
 public static ObjectFactory getInstance（）{
 // 3 
 return SingletonObjectFactoryHolder.getInstance（）; 
} 
  

再次感谢

解决方案

使用一个静态的初始化程序，并将 Exception 作为 ExceptionInInitializerError 。单击链接以阅读Javadoc，您将看到它适合于此特定的功能需求：在静态初始化期间处理异常。实际上，一个单例实际上不过是一个静态和延迟初始化的全局对象。

  private static class SingletonObjectFactoryHolder {
 private static final ObjectFactory INSTANCE; 
 static {
 try {
 INSTANCE = new ObjectFactory（）; 
} catch（Exception e）{
 throw new ExceptionInInitializerError（e）; 
} 
} 
} 
  

不需要双重检查锁定成语，被认为是反模式，在某些情况下甚至不安全。

Whats the best way to design a singleton class that could throw an exception?

Here I have a Singleton (using Bill Pugh's method, documented in Wiki for Singleton).

    private static class SingletonObjectFactoryHolder{
    //1  
        private static final ObjectFactory INSTANCE = new ObjectFactory();
    }

    private ObjectFactory() throws Exception{
    //2
            //create the factory
    }


    public static ObjectFactory getInstance(){
    //3
        return SingletonObjectFactoryHolder.INSTANCE;
    }

If an exception is thrown at 2, I would like to propagate it to the caller. However, I can't throw an exception from line 1.

So, is my only option to return a null object if the singleton object wasn't created correctly?

Thanks

P.S I do realize that this Singleton can break if its loaded via different classloaders or if loaded reflexively, but it's good enough for my purpose.

//UPDATE

I am curious, can I not rearrange my design as below to throw exceptions?

Also, I don't need any synchronization (the classloader guarantees that the static inner class will only loaded once and only when getInstance() is called). Thus, thread-safe and lazily-instantiated?

 private static class SingletonObjectFactoryHolder{
        //1  
           public static ObjectFactory getInstance() throws Exception{
         return new ObjectFactory();
           }
 }

 private ObjectFactory() throws Exception{
        //2
        //create the factory
 }


 public static ObjectFactory getInstance(){
        //3
    return SingletonObjectFactoryHolder.getInstance();
 }

Thanks again.

解决方案

Use a static initializer and rethrow the Exception as ExceptionInInitializerError. Click the link to read the Javadoc, you'll see that it suits exactly for this particular functional requirement: handling exceptions during static initialization. A singleton is in fact nothing less or more than a statically and lazily initialized global object.

private static class SingletonObjectFactoryHolder{
    private static final ObjectFactory INSTANCE;
    static {
        try {
            INSTANCE = new ObjectFactory();
        } catch (Exception e) {
            throw new ExceptionInInitializerError(e);
        }
    }
}

No need for double checked locking idiom which is considered an anti-pattern and in some circumstances even unsafe.

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