singleton

问题 Let's say in scala I have an ADT as follows: sealed trait Animal object Animal { case class Lion(name: String) extends Animal case class Elephant(name:String) extends Animal case object Tiger extends Animal } Here, is it preferable to declare Tiger as a case object or should it be declared as an empty case class i.e case class Tiger() ? Does one have any advantage over other? 回答1: If there is the only Tiger it should be an object. If there can be several equal Tiger s it should be a class.

问题 Effective Java 2nd describes the Enum Implementation as the best practice to implement a Singleton in Java. But the advantage of that implementation against the Static Holder Implementation is that the enum can prevent the reflection attack. So, there comes the question: Why do we need to prevent the reflection attack of singleton? The other implementations of Java Singleton are just resolving the issues of multiple threads and lazy initialization . These problems will and often appear at the

问题 I have a simple singleton class in Java that looks like the following one: public class Singleton{ private static Singleton instance; private Singleton(){} public static Singleton getInstance(){ if (instance == null){ instance = new Singleton(); } return instance; } public void doSomething(){ ... } } My code also contains two classes, from now on called A and B, that both have the following structure: public class Foo{ ... public void bar(){ ... Singleton.getInstance().doSomething(); ... } ..

问题 Abstract member method is illegal in a singleton object scala> object Foo { | def g: Int | } def g: Int ^ On line 2: error: only traits and abstract classes can have declared but undefined members as is abstract value member scala> object Foo { | val x: Int | } val x: Int ^ On line 2: error: only traits and abstract classes can have declared but undefined members however abstract type member is legal in a singleton object scala> object Foo { | type A | } object Foo so clearly the sense in

问题 Abstract member method is illegal in a singleton object scala> object Foo { | def g: Int | } def g: Int ^ On line 2: error: only traits and abstract classes can have declared but undefined members as is abstract value member scala> object Foo { | val x: Int | } val x: Int ^ On line 2: error: only traits and abstract classes can have declared but undefined members however abstract type member is legal in a singleton object scala> object Foo { | type A | } object Foo so clearly the sense in

问题 I am playing around with Spring Boot and I am trying to construct an instance of ServiceImpl to be resolved when a Service is required. Currently I am annotating the implementation as @Component but this does not give me the chance to construct the instance as I want. The ServiceImpl should be constructed with a String containing a path to a file on disk. I would like to do this in the main method of the @SpringBootApplication class of the application. Maybe it's just me coming from a long

问题 I am playing around with Spring Boot and I am trying to construct an instance of ServiceImpl to be resolved when a Service is required. Currently I am annotating the implementation as @Component but this does not give me the chance to construct the instance as I want. The ServiceImpl should be constructed with a String containing a path to a file on disk. I would like to do this in the main method of the @SpringBootApplication class of the application. Maybe it's just me coming from a long

问题 I am playing around with Spring Boot and I am trying to construct an instance of ServiceImpl to be resolved when a Service is required. Currently I am annotating the implementation as @Component but this does not give me the chance to construct the instance as I want. The ServiceImpl should be constructed with a String containing a path to a file on disk. I would like to do this in the main method of the @SpringBootApplication class of the application. Maybe it's just me coming from a long

问题 I recently came across the Nifty Counter Idiom. My understanding is that this is used to implement globals in the standard library like cout, cerr, etc. Since the experts have chosen it, I assume that it's a very strong technique. I'm trying to understand what the advantage is over using something more like a Meyer Singleton. For example, one could just have, in a header file: inline Stream& getStream() { static Stream s; return s; } static Stream& stream = getStream(); The advantage is you

问题 I'm trying to implement class inherited singleton as described here (Method 2). Going over the question and the extensive chosen answer I tried to implement the following: class Singleton(object): _instance = None def __new__(cls, *args, **kwargs): if not isinstance(cls._instance, cls): cls._instance = object.__new__(cls, *args, **kwargs) cls._instance._initialized = False return cls._instance class A(Singleton): def __init__(self): print "Init is called" class B(Singleton): def __init__(self