This question already has answers here:
Singleton design pattern vs Singleton beans in Spring container
(11 answers)
Closed 6 years ago.
Singleton means Single object for a class.In spring we can create multiple beans for same class (bean is nothing but an object) using singleton scope.For example see the below code
<bean id="abc" class="com.test.Abc"/>
<bean id="def" class="com.test.Abc"/>
In this scenario spring will create 2 singletons (I have refereed some SO links).How we can call it as singleton , because more than one object is created for the same class.
As per the definition
The Singleton's purpose is to control object creation, limiting the number of objects to only one. Since there is only one Singleton instance, any instance fields of a Singleton will occur only once per class.
Each bean is a singletone, no matter how many times u create the same bean ( as long as the scope is set to default or singleton) you will only have on object .
It makes since for 2 beans with the same type to create different object as you may set different constructor params or properties
Related
This question already has answers here:
Singleton design pattern vs Singleton beans in Spring container
(11 answers)
Closed 5 years ago.
I know Spring can create singleton beans. Does it mean that we needn't use 'Singleton Pattern' to create a java bean by myself? And what's the difference between the spring singleton and singleton pattern?
Spring singleton is unique ApplicationContext (per Spring container) instance. Meaning if you create a new ApplicationContext then you would get a new instance of the bean even if it's singleton.
However original Java singleton means one instance per Classloader. Meaning that singleton instance remains same for a particular classloader. In most cases that's fine, however suppose if you need a true singleton, single instance per JVM then there's some extra work to do. Look at this example https://stackoverflow.com/a/47445573/5343269
Answer to your question is if you are instantiating a single spring container in your application then spring singleton bean can be treated as a singleton, however that's true only for Spring components. Meaning this instance can not be accessed by any class that's not a Spring bean.
To be safe, don't rely on the spring singleton and create your own singleton class.
I am new to Spring concept, so I have little confusion about #Component which by default is a singleton which creates an instance whenever a class is loaded and the same instance is reused; same happens with new operator. If a class is declared as singleton we can change the properties of the class using setters and getters same with new operator also.
When you call new , you are creating the instance during runtime manually.
Assume , you have a controller being called 'N' number of times which im turn calls a Service.
Java
In plain java, you will be creating a New Object by calling new. Which means , you are creating 'n'number of objects
Spring
In Spring, when you just deploy the application in the server or load the spring XML/Configuration class, the Spring creates an instances of all the classes which are annotated , and stores in the spring container. Now,even if your controller is called 'n'times, spring will use the same object again and again
Because you wont call new instead use another annotation called Autowired
#Component annotation says that the object is managed by Spring: i.e. the framework creates it. Usually you should use the dependenci injection mechanism using Spring: I mean you don't need to create the component instance by yourself but inject the provided one into your code.
Read about the IoC paradigm here: https://en.wikipedia.org/wiki/Inversion_of_control
Also you can actually change the scope of the Spring managed bean to different then singleton (see tutorial here) but if you already implenemted some singleton class on your own you can't override it's behavior
BTW you shouldn't use the new operator for singletons (actually you can't because of private constructor), use method like getInstance() instead
This question already has answers here:
Singleton design pattern vs Singleton beans in Spring container
(11 answers)
Closed 3 years ago.
Is it common that in Java programming using Spring bean to control singleton instance instead of making a class with the singleton pattern?
It depends. If you need an utility class, which has no dependencies and that just provide a bunch of commons methods, I think it is quick and straightforward to build your own singleton.
But if you want to build a business class which needs to be injected with other dependencies, and probably, you want to define interfaces to avoid coupling between classes, I think Spring (or other DI frameworks) are better (easier) than build your own singleton.
These are the differences :
JVM handles the lifecycle of Singleton pattern classś object where Spring context handles life cycle of Spring singleton bean.
Singleton pattern classś object will be single for whole JVM where if you have multiple SpringContext in same JVM you can have multiple objects of it.
It depends on how loosely you interpret the Singleton pattern.
In my interpretation, Singleton implies that there is exactly one instance per given context. The context is usually the JVM (the ClassLoader, actually), as that is the easiest to implement with plain Java code, but nothing stops me from mapping the pattern to different contexts, one of them being Spring's ApplicationContext.
According to this definition, one could argue that the "request" and "session" scopes also hold singleton objects, just with a narrower scope.
See also this previous answer of mine, where I go into more detail.
Simply, when you use spring bean singleton, you will have one instance per spring container.
However, when you implement the singleton pattern by yourself (java based design pattern in our case), you will have one instance per java class loader.
I understand Object Creation and Thread safety are two different things.
Singleton is a way of constructing an object, some design pattern we follow to construct the object.
Spring Bean - Using Singleton scope, we guarantee that only 1 instance will be there, but still Spring bean is not thread safe.. why..??
Cannot be thread safe, precisely because just one instance is created, so every request(thread) will use the very same instance, so concurrency issues may happens.
If for example your scope is prototype, Spring wont use singleton and then it will create an instance per getClass invokation.
Same thing for request or session, instance will be created by request or per user session.
As a rule, use the prototype scope for all state-full beans and the singleton scope for stateless beans.
you can read in the documentation how it´s working
http://www.tutorialspoint.com/spring/spring_bean_scopes.htm
It is generally said that choose only those beans as singleton that don’t have state. I am new to Spring and reading about bean scope in Spring.
My first query was what this STATE actually mean with respect to Bean?
Secondly, Why it is recommended to use singleton scope with only stateless bean? Is there some thread safe related limitations or Is there any other reason?
Any member attributes of class is referred to as its state.
Classes with no mutable state are best suitable for becoming a Spring (singleton) beans. Mutable state refers to those member attributes to which new values can be assigned after the object has been constructed.
Beans like DAOs which have member attributes such as JpaRespository can be considered to have fairly immutable state, as no one assigns new values to JpaRespository attribute once DAO object has been initialized.
Only beans which have no state (no member attributes) or have immutable state (member variables whose values are not updated once they have been assigned a value) are the ideal candidates for becoming Spring bean. This is because most Spring beans are configured as Singleton beans, and are used by multiple threads in the container. Imagine if you had mutable state and multiple threads were trying to update state of singleton bean, you will never have predictable result.
If your bean is not Singleton, but instead it is a Prototype bean, then, that bean can have state as such beans are created and destroyed as per need.
An object (and a bean is an object) has a state if the calls to methods do not only depend on the parameter given to the method call but also to the previous calls.
In principle if you have any instance variables you have likely a state. The only exception to this is that singletons stored in instance (not static) variables (which is quite usual in spring programming) don't count to the state.
When using a multithreaded environment a state in a singleton can have side effects. E.g. one thread sets the state (e.g. a connection to open) and another thread does not expect this state. Then one thread could fail.
In which state of object you want to run your program.When you design and write your class you do not know whether client use it as singleton OR prototype. The container configures object .So,it might be singleton OR prototype .For example TransactionAwareDataSourceProxy
class of spring framework is written to run in either protoype Or singleton.The class is written in such a way that it is safe from multithrading access .As the state that is shared by all threads is made common via DI container with some setter method.So,the configuration as singleton is made at startup via container and its common value is used by all threads Or single thread.The setting of instance variable is made only once with some setter method.
For example in class TransactionAwareDataSourceProxy there is reobtainTransactionalConnections as instance variable.
public TransactionAwareDataSourceProxy()
{
reobtainTransactionalConnections = false;
}
public TransactionAwareDataSourceProxy(DataSource targetDataSource)
{
super(targetDataSource);
reobtainTransactionalConnections = false;
}
public void setReobtainTransactionalConnections(boolean reobtainTransactionalConnections)
{
this.reobtainTransactionalConnections = reobtainTransactionalConnections;
}
These are the places in code where reobtainTransactionalConnections is iniatialized .If you decided to make this class singleton via DI container you set in once either with constructor injection OR setter injection.So,if we make setter injection then setReobtainTransactionalConnections will be either true Or false and remains same throughout the object life time.I think thats the advantages of configuring object with container ,the object state control can be easily made