How Singleton bean can be Autowired in different places spring boot

singleton class in spring boot example
when to use prototype scope in spring
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inject request scoped bean into singleton
spring boot autowired singleton
spring @autowired
spring objectfactory
proxymode = scopedproxymode target_class

I'm confused at this point, and i know all spring boot applications beans are singleton, according to my understanding if we have class annotated with @Service annotation that bean can be @Autowired in only one class (correct me if i'm wrong) here is the code that works fine, but i'm trying to understand how it works? how one bean can be @Autowired in two different classes?

How SampleService bean can be @Autowired in SampleController2 and SampleController3 at a time ?

And is this recommended approach? and in this case two threads can parallely change the data inside bean?


public class SampleController2 {

private SampleService2 sampleservice2;

public void mockCall1() {



public class SampleController3 {

private SampleService2 sampleservice2;

public void mockCall1() {



 public class SampleService2 {

public void m1() {
    System.out.println("bean is autowired");

Here is a simplified view of what Spring does on startup:

// Create bean: sampleService2
SampleService2 sampleService2 = new SampleService2();

// Create bean: sampleController2
SampleController2 sampleController2 = new SampleController2();
sampleController2.sampleservice2 = sampleService2; // because @Autowired

// Create bean: sampleController3
SampleController3 sampleController3 = new SampleController3();
sampleController3.sampleservice2 = sampleService2; // because @Autowired

As you can see, the singleton bean sampleService2 is autowired into both sampleController2 and sampleController3.

The beans are added to a repository, so you can look them up by name or type at any later point in time.

4.4 Bean scopes, How do you inject prototype beans in Singleton in spring? Beans defined in spring framework are singleton beans. There is an attribute in bean tag called ‘singleton’. If it is specified as true then bean becomes singleton and if it sets to false then the bean becomes a prototype bean, it is non-singleton class. By default this property is set to true.

By default, as you mentioned, all Spring beans are singletons, but your second assumption is wrong: the same bean can be autowired in many other beans.

In fact that's the whole point of them being singletons.

That also means two different threads could change the state of the same bean indeed. You would most of the time want to keep your beans stateless for that reason.

If you really ever need to have one different instance of a bean for each place where it is autowired, you can change the scope of that bean to prototype. See Spring bean scopes docs.

Injecting Prototype bean into a Singleton bean in Spring, We can also inject the ApplicationContext directly into a bean. To achieve this, either use the @Autowire annotation or implement the  The @Bean annotation is a way of telling the Spring container to manage the bean returned by that so-annotated method. Like in the examples above, the @Bean methods are generally used inside configuration classes, which are proxied by CGLIB — the result of the bean-defining method is registered as a Spring bean,

Can singleton bean be injected into prototype bean?, And how is that single bean auto-wired in different classes? You may It was only needed to clarify simple-yet-important points. That is why I So, multiple threads can refer to the singleton bean at the same time. Dynamically Schedule the Same Task with Multiple Cron Expression Using Spring Boot  Spring Bean Autowiring – @Autowired. In Spring framework, declaring bean dependencies in configuration files is a good practice to follow, so the Spring container is able to autowire relationships between collaborating beans.

Prototype vs Singleton Spring Beans, 3 Differences between Prototype beans and Singleton beans As you can see, programatically, a prototype bean class looks exactly If you autowire the bean in multiple places, Spring will still provide you with the same instance. In our spring boot test, we will request our test bean and we will request  Spring bean dependencies are defined in the XML files (Also Read : Introduction to Spring Boot), the same can be automatically detected by the Spring container by using the @Autowired annotation. This would eliminate using the XML configurations. Introduced in Spring 2.5, the @Autowired annotation can be applied to

Injecting Prototype Beans into a Singleton Instance in Spring , Singleton Beans with Prototype-bean Dependencies; 1.5.4. Request Fine-​tuning Annotation-based Autowiring with Qualifiers; 1.9.5. The location path or paths supplied to an ApplicationContext constructor are resource strings that let the container load It can be useful to have bean definitions span multiple XML files. I have met the same problem when run app in tomcat. Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.mail.javamail.JavaMailSender' available: expected at least 1 bean which qualifies as autowire candidate.

Spring: Serving Multiple Requests With the Singleton Bean, The location path or paths supplied to an ApplicationContext constructor are actually resource strings It can be useful to have bean definitions span multiple XML files. You can still override this default behavior so that singleton beans will The Spring container can autowire relationships between collaborating beans. One @Configuration class may directly reference bean instances registered from another using Spring's @Autowired annotation. Given that both these configuration classes are supplied to the application context at runtime, the AccountRepository bean declared in ConfigOne will be autowired (injected) into the AccountRepository field in ConfigTwo .

  • To understand this you have to understand dependency injection. Dependency injection is the concept behind the keyword Autowired. In other words, sampleService2 variable in both controllers are simple declarations which means that there is no instance object associated. Keep in mind that Spring core is responsible to create the instances separately when the need is felt.
  • i went through dependency injection, you mean it will create two instances? or a single instance ?
  • just because something is a singleton doesn't mean it can be only used in one place. you can have the same instance used in two different places
  • definitely, it will create two instances are created because those beans are not necessarily invoked at the same time.
  • @Solace, can you give me some more info? how does singleton bean can be autowired in different places? just i want to explain in few words, there is a singleton bean and it is associated with reference in controller1, again how can same bean associated with controller2 reference?
  • Thank you so much for the valuable information @Andreas, i have one more question, according to my understanding when ever spring container starts, it will create beans and save a copy (singleton), so when ever request comes operations will be performed on bean and save back the updated copy, am i right? what if at a time two threads requested the bean? how the one thread bean updates reflect to another thread ?
  • @Deadpool Most singleton beans are stateless, i.e. after setup they are immutable. A web request is processed by a controller (and a service), but the beans themselves are not updated. --- Also, "save back the updated copy" implies more than one instance, which violates the singleton aspect. All requests are processed in parallel by a single instance, but since the instance is stateless, there are no multi-threading issues.
  • Thank you @Andres, correct me if i'm wrong, spring bean are singleton and immutable,but properties inside bean can be updated by multiple threads and it is developer responsibility to deal that, so in this case what happen if one thread updating some instance property of that bean and second thread is requesting bean at that time?
  • @Deadpool You are wrong. Spring bean being immutable means that they don't update fields of the bean, because if they did, they wouldn't be immutable. Since most singleton beans are intended to process web requests in parallel, they should always be stateless, i.e. immutable, ensuring they are always thread-safe. --- What do you mean by "second thread is requesting bean"? Normally, all beans are autowired together, and hence connected at startup, so there is no "requesting" of beans at request-time, since they already have access to each other.
  • Thank you so much @Andreas, i think i'm confusing at stateless and stateful`, but i got it now thanks you
  • you mean prototype? could you please give an example or link? so where i can get more info
  • can you give me some more info? how does singleton bean can be autowired in different places? just i want to explain in few words, there is a singleton bean and it is associated with reference in controller1, again how can same bean associated with controller2 reference?