I have read the concepts of Singleton design pattern and understood that for making a class singleton we have to do the following steps :
1)Private constructor to restrict instantiation of the class from other classes.
2)Private static variable of the same class that is the only instance of the class.
3)Public static method that returns the instance of the class, this is the global access point for outer world to get the instance of the singleton class.
So my class looks like this :
public class Singleton {
private static Singleton singleton =new Singleton();;
/* A private Constructor prevents any other
* class from instantiating.
*/
private Singleton(){
System.out.println("Creating new now");
}
/* Static 'instance' method */
public static Singleton getInstance( ) {
return singleton;
}
/* Other methods protected by singleton-ness */
public void demoMethod( ) {
System.out.println("demoMethod for singleton");
}
}
But here how we can ensure that only one instance of Singleton is created ? Suppose I have 2 classes Singletondemo1 and Singletondemo2.
In Singletondemo1 , I am calling the getInstance() and craete an object. Same way I can do that in Singletondemo2 also.
So how we will ensure only object is created and also it is thread safe.
In many of the Singleton examples, I have come across constructor having comment as "Exists only to defeat instantiation", can you please give me details and explain more about it.
It's common to create a private constructor when implementing the Singleton pattern so that the default constructor cannot be used to instantiate multiple Singleton objects.
See the example from Wikipedia's Singleton pattern article.
public class SingletonDemo {
private static SingletonDemo instance = null;
private SingletonDemo() { }
public static synchronized SingletonDemo getInstance() {
if (instance == null) {
instance = new SingletonDemo ();
}
return instance;
}
}
By making a private constructor, you insure that the compiler can't make a default constructor with the same signature, which forces any client code to call the getInstance() method.
if I have the interface interfaceA
public interface IInterfaceA
{
void MethodA();
void MethodB();
}
and I have the classA
class ClassA:IInterfaceA
{
public void MethodA()
{
}
public void MethodB()
{
}
}
it's ok that I use ninject's bind,but when it comes that I have a method that called MethodC,I think the method should only exists in classA(just for classA) and should not be defined in InterfaceA,so how to use ninject'bind when just calling like this:
var a = _kernel.get<IInterfaceA>()
should I convert the result into ClassA ? (is that a bad habbit?) or there are another solution
Usually this is needed when you want interface separation but need both interfaces to be implemented by the same object since it holds data relevant to both interfaces. If that is not the case you would be able to separate interfaces and implementation completely - and then you should do so.
For simplicitys sake i'm going to asume Singleton Scope, but you could also use any other scope.
Create two interfaces instead:
public interface IInterfaceA {
{
void MethodA();
}
public interface IInterfaceC {
void MethodC();
}
public class SomeClass : IInterfaceA, IInterfaceC {
....
}
IBindingRoot.Bind<IInterfaceA, IInterfaceB>().To<SomeClass>()
.InSingletonScope();
var instanceOfA = IResolutionRoot.Get<IInterfaceA>();
var instanceOfB = IResolutionRoot.Get<IInterfaceB>();
instanceOfA.Should().Be(instanceOfB);
Does this answer your question?
I'm using a SQLconnector singleton class with a lock object. My question now is that if multiple users use this class the same time, will this give issues?
FYI... Singleton code
public sealed class SqlConnector
{
private static volatile SqlConnector instance;
private static object syncRoot = new Object();
public static SqlConnector Instance
{
get
{
if (instance == null)
{
lock (syncRoot)
{
if (instance == null)
{
instance = new SqlConnector();
}
}
}
return instance;
}
}
public Execute(...)
{
//what ever
}
}
It is not a very good practice to use a SqlConnector as a Singleton.
The connection Pooling will handle it for you.
From http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlconnection.aspx :
To deploy high-performance applications, you must use connection
pooling. When you use the .NET Framework Data Provider for SQL Server,
you do not have to enable connection pooling because the provider
manages this automatically, although you can modify some settings. For
more information, see SQL Server Connection Pooling (ADO.NET).
SQL Server Connection Pooling (ADO.NET)
Class SqlConnector should be thread-safe
We have a concrete singleton service which implements Ninject.IInitializable and 2 interfaces. Problem is that services Initialize-methdod is called 2 times, when only one is desired. We are using .NET 3.5 and Ninject 2.0.0.0.
Is there a pattern in Ninject prevent this from happening. Neither of the interfaces implement Ninject.IInitializable. the service class is:
public class ConcreteService : IService1, IService2, Ninject.IInitializable
{
public void Initialize()
{
// This is called twice!
}
}
And module looks like this:
public class ServiceModule : NinjectModule
{
public override void Load()
{
this.Singleton<Iservice1, Iservice2, ConcreteService>();
}
}
where Singleton is an extension method defined like this:
public static void Singleton<K, T>(this NinjectModule module) where T : K
{
module.Bind<K>().To<T>().InSingletonScope();
}
public static void Singleton<K, L, T>(this NinjectModule module)
where T : K, L
{
Singleton<K, T>(module);
module.Bind<L>().ToMethod(n => n.Kernel.Get<T>());
}
Of course we could add bool initialized-member to ConcreteService and initialize only when it is false, but it seems quite a bit of a hack. And it would require repeating the same logic in every service that implements two or more interfaces.
Thanks for all the answers! I learned something from all of them! (I am having a hard time to decide which one mark correct).
We ended up creating IActivable interface and extending ninject kernel (it also removed nicely code level dependencies to ninject, allthough attributes still remain).
Ninject 3
Ninject 3.0 now supports multiple generic types in the call to bind, what you are trying to do can be easily accomplished in a single chained statement.
kernel.Bind<IService1, IService2>()
.To<ConcreteService>()
.InSingletonScope();
Ninject 2
You are setting up two different bindings K=>T and L=>T. Requesting instances of L will return transient instances of T. Requesting K will return a singleton instance of T.
In Ninject 2.0, an objects scope is per service interface bound to a scope callback.
When you have
Bind<IFoo>...InSingletonScope();
Bind<IBar>...InSingletonScope();
you are creating two different scopes.
You are saying
"Binding to IFoo will resolve to the same object that was returned
when .Get was called."
and
"Binding to IBar will resolve to the same object that was returned
when .Get was called."
you can chain the bindings together, but you will need to remove IInitializable as it will cause duplicate initialization when the instance is activated:
kernel.Bind<IBoo>()
.To<Foo>()
.InSingletonScope();
.OnActivation(instance=>instance.Initialize());
kernel.Bind<IBaz>()
.ToMethod( ctx => (IBaz) ctx.Kernel.Get<IBoo>() );
or
kernel.Bind<Foo>().ToSelf().InSingletonScope()
.OnActivation(instance=>instance.Initialize());
kernel.Bind<IBaz>().ToMethod( ctx => ctx.Kernel.Get<Foo>() );
kernel.Bind<IBoo>().ToMethod( ctx => ctx.Kernel.Get<Foo>() );
in order to get multiple interfaces to resolve to the same singleton instance. When I see situations like this, I always have to ask, is your object doing too much if you have a singleton with two responsibilities?
Update : Pretty sure using V3's multiple Bind overloads will address this; See this Q/A
Good question.
From looking at the source, the initialize bit happens after each Activate. Your Bind...ToMethod counts as one too. The strategy is pretty uniformly applied - there's no way to opt out in particular cases.
Your workaround options are to use an explicit OnActivation in your Bind which will do it conditionally (but to do that in a general way would require maintaining a Set of initialized objects (havent looked to see if there is a mechanism to stash a flag against an activated object)), or to make your Initialize idempotent through whatever means is cleanest for you.
EDIT:
internal interface IService1
{
}
internal interface IService2
{
}
public class ConcreteService : IService1, IService2, Ninject.IInitializable
{
public int CallCount { get; private set; }
public void Initialize()
{
++CallCount;
}
}
public class ServiceModule : NinjectModule
{
public override void Load()
{
this.Singleton<IService1, IService2, ConcreteService>();
}
}
Given the following helpers:
static class Helpers
{
public static void Singleton<K, T>( this NinjectModule module ) where T : K
{
module.Bind<K>().To<T>().InSingletonScope();
}
public static void Singleton<K, L, T>( this NinjectModule module )
where T : K, L
{
Singleton<T, T>( module );
module.Bind<K>().ToMethod( n => n.Kernel.Get<T>() );
module.Bind<L>().ToMethod( n => n.Kernel.Get<T>() );
}
}
#Ian Davis et al. The problem is that:
class Problem
{
[Fact]
static void x()
{
var kernel = new StandardKernel( new ServiceModule() );
var v1 = kernel.Get<IService1>();
var v2 = kernel.Get<IService2>();
var service = kernel.Get<ConcreteService>();
Console.WriteLine( service.CallCount ); // 3
Assert.AreEqual( 1, service.CallCount ); // FAILS
}
}
Because each activation (per Bind) initialises each time.
EDIT 2: Same when you use the following slightly more stripped down version:
static class Helpers
{
public static void Singleton<K, L, T>( this NinjectModule module )
where T : K, L
{
module.Bind<T>().ToSelf().InSingletonScope();
module.Bind<K>().ToMethod( n => n.Kernel.Get<T>() );
module.Bind<L>().ToMethod( n => n.Kernel.Get<T>() );
}
}
I think one of the option is, you create the object your self in the module and bind your object the each of the interfaces.
BTW, try not to use any container specific code in your production code. If you have to do that, use some helper and isolate them in the module project.
public class ServiceModule : NinjectModule
{
public override void Load()
{
ConcreteService svc = new ConcreteService();
Bind<IService1>().ToConstant(svc);
Bind<IService2>().ToConstant(svc);
....
}
}