Is Singleton an anti-pattern? If yes, then Dependency Injection in Spring also Anti-pattern (because Spring promotes Singleton bean which is default)?
There's an important difference between hard-coding a singleton and using Spring to create a singleton bean: the latter is just configuration.
As such Spring doesn't prevent testability: it's a single use-case where a bean is used as a singleton. You can easily use the same spring by manually instantiating it in a test, or replacing it with a mocked implementation of the same interface.
If you code a class as a singleton then you can't easily replace it, without rewriting it. Testing is just a single example where you might want to replace it. If you realize that you'll need two different instances of that bean, then you're stuck with a hard-coded singleton as well.
The Singleton pattern is no anti-pattern, it is a patter to restrict the number of instances
for an object.
But abuse the singleton to provide global instances is a anti-pattern.
For the spring part of your question see Joachim Sauer answer.
Related
Can someone explain the difference between Spring “prototype” bean scope and using “new” operator? Also what is the advantage of declaring the bean with “prototype” scope over “new” operator?
Prototype beans do mean that a new instance of the bean is created every time it is requested (in which case you might think you may as well be instantiating it yourself when needed using new).
But the key thing is that they still satisfy the dependency injection design pattern (https://en.wikipedia.org/wiki/Dependency_injection) which among many other things makes unit testing with mocked dependencies significantly easier.
I have a doubt regarding dependency injection,Suppose my controller ,service,dao all are singleton so usually when we create the controller we inject the service as a instance variable of that class, but according to the singleton pattern if our controller is stateless then only we would not face any concurrency issue but here We are declaring the service dependency so it should not be stateless so we have to take care of synchronization?
Please clear this doubt as I am beginner so I hope its natural to have this doubt in mind,I don't know if I am thinking totally wrong.Please help.
All beans in Spring are Singleton by default. This includes any #Controller, #Service, #Repository and others, as well as any xml defined bean.
You could read this and this
From Java basic variable tutorial:
Local Variables Similar to how an object stores its state in fields, a
method will often store its temporary state in local variables. The
syntax for declaring a local variable is similar to declaring a field
(for example, int count = 0;). There is no special keyword designating
a variable as local; that determination comes entirely from the
location in which the variable is declared — which is between the
opening and closing braces of a method. As such, local variables are
only visible to the methods in which they are declared; they are not
accessible from the rest of the class.
If your service and controller are stateless, it's ok to inject one to another.
You should not declare any variable which keeps a state in these classes. final variables are ok.
If all operations are defined in methods and they don't use any variables of the classes, dependency injection that you're doing is totally safe.
That's why you need to use #Autowired when you declare a dependent service. Effectively handing the initialization process to the Spring framework instead of instantiating it yourself. Since Spring only has stateless beans, you're injecting one stateless singleton to another stateless singleton, so there's no need to manage thread manually.
I am new to spring mvc web development. I have one query.
Suppose we are having different service classes. So do we have one instance of those classes per request OR only single instance of that class gets create. Actually i want to use instance variables , so with each request new instance will get created or it will be like singleton type of behavior. Hopefully i am able to explain my question.
you can have either, the default is a singleton - one instance. But this can be changed using bean scope.
obligatory link to offical docs correct chapter
(personally never needed to use anything other singleton)
If you have not defined any scope explicitly, it will be singleton by default, singleton means there will be one object per spring container, for your context, one object for all your request threads. In case of singleton scope, be cautious while using member variables, because thread safety comes into picture.
If you are modifying state of a member variable inside your singleton scoped bean, you need to write thread safe code because multiple threads are accessing your member variable and a race condition may occur.
Moreover, you can define other scopes too using #Scope at the class level(i.e above #Component) or at method level above #Bean annotations.
Generally, we keep using the default scope (i.e singleton scope), this way spring container also does not waste time in creating the new object of the bean asked for, though it would be a little overhead creating an object on each request thread.
If you want a new object on every bean injection, you can have prototype scope for that bean.
Spring has two two types of DI: setter DI and construction DI.
Constructor-based DI fixes the order in which the dependencies need to be injected. Setter based DI does not offer this.
Setter-based DI helps us to inject the dependency only when it is required, as opposed to requiring it at construction time.
I do not see any other significant differences, as both types of Spring DI provide the same features - both setter and constructor DI inject the dependency when the code starts up. Granted, constructor DI will do it through the constructor while setter DI will do it through a setter right after constructing the object, but it does not make any difference for the developer in terms of performance, etc. Both also offer means to specify the order of dependency injection as well.
I'm looking for a scenario where one provides a distinct advantage over the other or where one type is completely unusable.
When it comes to Spring specific pros and cons:
Constructor injection (from the definition) does not allow you to create circular dependencies between beans. This limitation is actually an advantage of constructor injection - Spring can resolve circular dependencies when setter injection is used without you even noticing.
On the other hand if you use constructor injection CGLIB is not able to create a proxy, forcing you to either use interface-based proxies or a dummy no-arg constructor. See: SPR-3150
You should be deciding based on design considerations, not tool (Spring) considerations. Unfortunately, Spring has trained us to use setter injection because when it was originally conceived, there was no such thing as an "annotation" in Java, and in XML, setter injection works and looks much better. Today, we're freed from those constraints, thus allowing it to be a design decision again. Your beans should use constructor injection for any dependencies that are required by the bean and setter injection for dependencies that are optional and have a reasonable default, more or less as OOD has been telling us from the beginning.
Constructor Injection: We are injecting the dependencies through Constructor.
Generally we can use for Mandatory dependencies.
If you use the Constructor injection there is one disadvantage called "Circular Dependency".
Circular Dependency: Assume A and B. A is dependent on B. B is dependent on A. In this constructor injection will be failed. At that time Setter injection is useful.
If Object state is not inconsistent it won't create Object.
Setter Injection: We are injecting the dependencies through Setter methods.
This is useful for Non-Mandatory dependencies.
It is possible to re injecting dependencies by using Setter Injection. It is not possible in Constructor injection.
As per the content from spring.io from Spring 5 onwards
Since you can mix constructor-based and setter-based DI, it is a good rule of thumb to use constructors for mandatory dependencies and setter methods or configuration methods for optional dependencies. Note that use of the #Required annotation on a setter method can be used to make the property a required dependency.
The Spring team generally advocates constructor injection as it enables one to implement application components as immutable objects and to ensure that required dependencies are not null. Furthermore constructor-injected components are always returned to client (calling) code in a fully initialized state. As a side note, a large number of constructor arguments is a bad code smell, implying that the class likely has too many responsibilities and should be refactored to better address proper separation of concerns.
Setter injection should primarily only be used for optional dependencies that can be assigned reasonable default values within the class. Otherwise, not-null checks must be performed everywhere the code uses the dependency. One benefit of setter injection is that setter methods make objects of that class amenable to reconfiguration or re-injection later. Management through JMX MBeans is therefore a compelling use case for setter injection.
Here is the link for above quote
But, all of the injections types are available and none of them are deprecated. At a high-level you get the same functionality across all injection types.
In short, choose the injection type that works best for your team and project.
Recommendations from the Spring team and independent blog posts will vary over time. There is no hard-fast rule.
If a particular injection style was not recommended by the Spring team, then they would mark it as deprecated or obsolete. That is not the case with any of the injection styles.
Prefer setter injection.
Think what would be without spring (as Ryan noted). Would you pass the dependencies in constructor? If there are too many dependencies this seems wrong. On the other hand the constructor may be used to enforce the valid state of the object - require all dependencies and verify if they are non-null.
Proxies are another thing (As Tomasz noted) - you will need a dummy constructor which defeats the whole idea.
There is a 3rd option btw - field injection. I tend to be using that, although it is not such a good design decision, because it saves an extra setter, but if this is used outside of spring I will have to add the setter.
My 2 cents.
Assume a classA with 10 fields, with few injected dependencies.
Now if you need entire classA with all fields then you can go for constructor injection.
But if you need only one of the injected field to use in that class you can use setter injection.
This way,
You will not create new object each time.
You do not need to worry about circular dependency issue(BeanCurrentlyInCreationException).
You will not have to create other fields for class A so you have much more flexible code
Since you can mix both, Constructor DI- and Setter-based DI, it is a good rule of thumb to use constructor arguments for mandatory dependencies and setters for optional dependencies.
Note that the use of a #Required annotation on a setter can be used to make setters required dependencies.
Probably it's not main advantage. But let me explain mechanism of injection in Spring.
The meaning of the difference these two approaches is that with the way of injection using #Inject, #Autowire and so on, Spring will inject one bean into another using reflection, and with the way of the constructor, we ourselves use the constructor in order to initialize one bean by another bean without using reflection.
Therefore, the way with constructor better other option, at least that we don't use reflection-mechanism because reflection is an expensive operation from the machine-side.
P.S. Please consider, that correct use of construction DI it's when you manually create bean through constructor with params, even though you can you create using constructor without any of them.
no, even Constructor Injection happen , injection is still working, but just limited initialize , setter injection is optional and flexible. but it may generally for the parameter class , a spring bean with other spring beans
From Spring documentation http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/beans.html#beans-child-bean-definitions:
The remaining settings are always taken from the child definition: depends on, autowire mode, dependency check, singleton, scope, lazy init.
I think there is a good reason for not inheriting these settings, but can't think of one. What are the reasons?
I think it's because it would cause more confusion. Imagine the time spent debugging why your bean is not in the default (singleton) scope. Or the case when the child bean is injected into a bean that the parent depends-on. You will get a circular dependency without being able to notice it.
Bean inheritance is only in terms of properties injected, and not in terms of bean settings.
One of the main reasons is that Spring is used to inject implementations. Usually, extended classes are the implementations and it's natural to pick up the configuration details from their definitions.
Other practical reason is that annotations are not inherited. If an interface has annotated methods this annotation is not automatically visible in the implementing/extending classes without some reflection gymnastics.