singleton

问题 In an existing project, I am to inherit a Controller class (MVC) declared as Singleton so as to define my own treatment. How to appropriately derive this Singleton class? First, I expand on context and need for this inheritance. The application that I am added to the existing software wants to use a MVC module that performs almost same task as the one I am willing to perform. It is using the same methods up to signature and slight modifications. Rewriting my own MVC module would definitively

问题 I have a MyTask class which implements Runnable and there can be many such objects instantiated at any given moment. There are certain properties that I would like to autowire into MyTask class. But I think that if I mark MyTask with @Component then it will become a spring-managed singleton correct? That's not what I want, I need many independent instances of this class to be run by a TaskExecutor. So my question(s): a) Am I fundamentally wrong in my understanding of @Component annotation?

问题 I was drawn inspiration from this stackoverflow question How can one create a Java class instance that is guaranteed to be available only once for the entire JVM process? Every Application that runs on that JVM should then be able to use that singleton instance. 回答1: You can as a matter of fact implement such a singleton. The problem that was described to you in the comments is the possibility of a class being loaded by multiple ClassLoader s. Each of these ClassLoader s can then define a

问题 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

问题 I want Stats.singleton.twitter_count += 1 and I thought I could do class Stats: singleton_object = None @property @staticmethod def singleton(): if Stats.singleton_object: return Stats.singleton_object Stats.singleton_object = Stats() return Stats.singleton() But it throws an exception: >>> Stats.singleton.a = "b" Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'property' object has only read-only attributes (assign to .a) 回答1: Singletons are pointless in

问题 I read a lot on stackoverflow regarding the creation of singleton classes using enum. I must have missed something because i can't reach the INSTANCE anywhere. this is my code: public class UserActivity { private DataSource _dataSource; private JdbcTemplate _jdbcTemplate; static enum Singleton { INSTANCE; private static final UserActivity singleton = new UserActivity(); public UserActivity getSingleton() { return singleton; } } public UserActivity() { this._dataSource = MysqlDb.getInstance()

问题 I have an interface class Interface with pure virtual methods. class Interface { public: virtual void DoSomething() = 0; } Also, there is a class Implementation that implements the interface. class Implementation : public Interface { void DoSomething(); } In my program I need to have a Singleton (single instance) of Implementation/Interface pair. There are many pairs of Implementation/Interface classes in the program, but very few Implementation classes for one Interface class. QUESTIONS:

问题 How can I throw an exception from an enum constructor? eg: public enum RLoader { INSTANCE; private RLoader() throws IOException { .... } } produces the error Unhandled exception type IOException 回答1: Because instances are created in a static initializer, throw an ExceptionInInitializerError instead. 回答2: I have a case where I want to use enums as keys in some settings classes. The database will store a string value, allowing us to change the enum constants without having to udpate the

问题 I am developing ASP.NET application and I have problem with the EF 4.0 model. The EF model detects the newly added and deleted data, but not the modified data from the database. Here is an example of the problem what I have. A- Database: Script to generate the "Employees" database table CREATE TABLE [dbo].[Employees] ( [id] [int] IDENTITY(1, 1) NOT NULL, [name] [nvarchar](50) NULL, CONSTRAINT [PK_Employees] PRIMARY KEY CLUSTERED ( [id] ASC ) WITH ( PAD_INDEX = OFF, STATISTICS_NORECOMPUTE =

问题 Effective Java #77 states that we have to use readResolve to preserve the Singleton guarentee during serialization. They have used the example. public class Elvis implements Serializable{ public static final Elvis INSTANCE = new Elvis(); private Elvis() { ... } public void leaveTheBuilding() { ... } and they suggest using If the Elvis class is made to implement Serializable, the following readResolve method suffices to guarantee the singleton property: // readResolve for instance control -