I have a session scoped bean which is injected into a singleton bean using a proxy.
From the singleton bean I need to call a certain init-method of the session scoped bean, which needs the singleton bean as parameter.
Furthermore I can not change the source of the session scoped bean.
To elaborate the case:
The singleton bean is a Service and the session scoped bean is a Vaadin4Spring eventbus, for which I need to call the method eventBus.subscribe(this).
There seems to be no way to check, if I am already subscribed and I must not call subscribe twice on a certain eventBus instance.
Is there a way to tell Spring to call eventBus.subscribe(this) when the proxy links to an eventBus instance for the first time?
Can't you just go the other way round, inject the service into your eventbus, and call this.subscribe(serviceBean) in a #PostConstruct annotated method in the eventbus? Then it would only be called once when the eventbus is constructed.
You could add #Lazy to the Singleton. You do not need the Proxy.
Related
I have been trying to understand spring beans. As per my understanding, by default all beans are singleton and a singleton bean with lazy-init property set to true is created when it is first requested and singleton bean with lazy-init property set to false is created when the application context is created.
So, in an application, when a user request comes in ( where each request is a separate thread), do all these threads share the same singleton beans when requested within the program/class?
Yes, if the bean is created with default scope, the bean is shared across threads. However, another scope can be used to achieve the behaviour you mentioned.
See: https://docs.spring.io/spring-framework/docs/3.0.0.M3/reference/html/ch04s04.html?
Yes, by default (scope == 'singleton'), all threads will share the same singleton bean. There are two other bean scopes, session and request, that may be what you're looking for. The request scope creates a bean instance for a single HTTP request while session scope maintains a unique bean for each HTTP Session.
For a list and description of all of the Spring bean scopes, check out: Spring bean scopes
The best way to understand this is to understand how #Autowired annotation works.
Or in other words to understand "Spring Dependency Injection".
You said, "
by default all beans are singleton
We use #Autowired when injecting one layer into another between two layers.
If exemplify: Suppose, I want to inject UserService UserController. UserService is a Singleton bean. Creates an instance from UserService and Stores its cache. When we want to inject this service with #Autowired annotation. This annotation brings the UserService stored in the cache.
In this way, Even if we do this inject operation in many classes.#Autowired inject an instance of UserService with singleton bean instead of dealing with one UserService at each time. It saves us a big burden.
This is the fundamental working logic of Spring Singleton Bean.
Also, Spring Ioc Container manages this whole process.
I was in the impression that whenever a class implements an interface then JDK based proxies are created.
I have a bean which implements an interface with prototype scope which is injected in a bean with singleton scope. I tried using proxyMode = ScopedProxyMode.INTERFACES JDK proxy was created which is right, but when I tired proxyMode = ScopedProxyMode.TARGET_CLASS then CGLIB based proxy is created even though my bean implements an interface.
what happens when
proxyMode = ScopedProxyMode.TARGET_CLASS
and
proxyMode = ScopedProxyMode.INTERFACES
what is the difference between them? when to use which one?
One more question I have is what is the correct way to inject prototype bean into singleton bean?
In general, no proxy is needed in a scenario where a prototype scoped bean is injected into a singleton scoped bean.
The reference documentation states:
You do not need to use the <aop:scoped-proxy/> in conjunction with beans that are scoped as singletons or prototypes.
What happens without a proxy?
When the Spring context is being created and the singleton bean instantiated, a new instance of the prototype bean is created and injected into the singleton. Every invocation of our singleton now uses the same instance of the prototype.
Why would you need the proxy?
Maybe you want to alter the default behavior and create a new prototype bean instance for every invocation of the singleton bean. With the proxy, the singleton bean will keep the same proxy instance for all invocations. With every invocation, the proxy will get a new prototype bean instance from the Spring context.
The proxy modes
The proxy modes are self-descriptive. See ScopeProxyMode Javadoc:
ScopeProxyMode.INTERFACES - Create a JDK dynamic proxy implementing all interfaces exposed by the class of the target object.
ScopeProxyMode.TARGET_CLASS - Create a class-based proxy (uses CGLIB).
Does the singleton bean depend on an interface implemented by the prototype bean? Use ScopeProxyMode.INTERFACES. Otherwise, use ScopeProxyMode.TARGET_CLASS.
A method injection would be an alternate approach to get the same behavior as with the proxy.
As I know per default are controllers in Spring MVC singletons. HttpServletRequest passed offen to the controller handler method. And its ok, while HttpServletRequest is request-scoped, but I see often HttpServletRequest gets #Autowired into the controller field, like this:
#Controller("CMSProductComponentController")
#RequestMapping(CMSProductComponentController.CONTROLLER_PATH)
public class CMSProductComponentController {
#Autowired
private HttpServletRequest request;
}
Could be this a problem? And more general question: What happens if inject a reqeust-scoped component into a singleton?
No, for HttpServletRequest it will not be a problem and it shouldn't for other request scoped beans. Basically, Spring will generate a proxy HttpServletRequest that wraps some kind of ObjectFactory (RequestObjectFactory for HttpServletRequest) (YMMV) that knows how to retrieve the actual instance. When you use any of the methods of this proxy, they will delegate to that instance.
What's more, this is done lazily, so it won't fail at initialization. It will however fail if you try to use the bean when there is no request available (or if you haven't registered the RequestScope).
The following is in response to the comments and to clarify in general.
Regarding the proxy-mode attribute of #Scope or the XML equivalent, the default is ScopedProxyMode.NO. However, as the javadoc states
This proxy-mode is not typically useful when used with a non-singleton
scoped instance, which should favor the use of the INTERFACES or
TARGET_CLASS proxy-modes instead if it is to be used as a dependency.
With request scoped beans, this proxy-mode value will not work. You'll need to use INTERFACES OR TARGET_CLASS depending on the configuration you want.
With scope set to request (use the constant WebApplicationContext.SCOPE_REQUEST), Spring will use RequestScope which
Relies on a thread-bound RequestAttributes instance, which can be
exported through RequestContextListener, RequestContextFilter or
DispatcherServlet.
Let's take this simple example
#Component
#Scope(proxyMode = ScopedProxyMode.INTERFACES, value = WebApplicationContext.SCOPE_REQUEST)
public class RequestScopedBean {
public void method() {}
}
...
#Autowired
private RequestScopedBean bean;
Spring will generate two bean definitions: one for your injected bean, a singleton, and one for the request scoped bean to be generated on each request.
From those bean definitions, Spring will initialize the singleton as a proxy with the types of your target class. In this example, that is RequestScopedBean. The proxy will contain the state it needs to produce or return the actual bean when it is needed, ie. when a method is called on the proxy. For example, when
bean.method();
is called.
This state is basically a reference to the underlying BeanFactory and the name of the request-scoped bean definition. It will use these two to generate a new bean and then call method() on that instance.
The documentation states
The Spring IoC container manages not only the instantiation of your
objects (beans), but also the wiring up of collaborators (or
dependencies). If you want to inject (for example) an HTTP request
scoped bean into another bean, you must inject an AOP proxy in place
of the scoped bean. That is, you need to inject a proxy object that
exposes the same public interface as the scoped object but that can
also retrieve the real, target object from the relevant scope (for
example, an HTTP request) and delegate method calls onto the real
object.
All eagerly loaded request scoped beans, if implemented correctly, will be proxies. Similarly, request scoped beans that aren't eagerly loaded will either be proxies themselves or be loaded through a proxy. This will fail if there is no HttpSerlvetRequest bound to the current thread. Basically, a proxy is necessary somewhere in the bean dependency chain for request scoped beans.
What happens if inject a reqeust-scoped component into a singleton?
Try it and you'll get a BeanCreationException¹ during application context initialization. The error message clearly explains why this doesn't happen with HttpServletRequest:
Scope 'request' is not active for the current thread; consider defining a scoped proxy for this bean if you intend to refer to it from a singleton;
So obviously HttpServletRequest is a scoped proxy. If you want to use beans of smaller scopes in singletons they have to be proxies. The documentation elaborates about smaller scoped dependencies in Scoped beans as dependencies.
[1]: unless you didn't change the default behaviour for proxyMode, which is NO or try to inject it with #Lazy. The latter might result into a valid application context but might lead to request scoped beans acting like singletons (e.g. if a request scoped bean is injected into a singleton).
I have created stateless session bean in Java. Now I want to invoke a method of another stateless session bean. Some things are missing in my code. Usual way of invoking method does not fit here. Being invoked method at another stateless session bean retrieves data from the Internet.
Likewise, how to invoke a method from #Stateless bean of a simple Java class. I build a REST web service with Java and somehow I can't invoke methods being at simple Java class from #Stateless beans.
Cheers
Just inject it with #EJB
#Stateless
public class StatelessBean1 {
#EJB
private StatelessBean2 bean;
}
There's nothing special about invoking methods on a stateless session bean. You use the exact same syntax as with every other kind of bean.
As Bozho indicated, the only thing special about EJBs is that you can't construct an instance using the new operator. You need to inject an instance or alternatively do a JNDI lookup. After that, the normal Java rules apply.
It really shouldn't need to be explained but to be sure, calling a method on a stateless session bean called 'bean':
bean.someMethod(someArgument);
I need to initialize every new http session with some values. How do I do that?
I tried to create a session-scoped component and initializing session in #PostConstruct, but session-scoped beans are not eagerly created until I request access them.
If you are absolutely certain that your want eager initialization, you can do the following:
define an interceptor for all beans
defina a <lookup-method> for that interceptor:
<lookup-method name="getCurrentSessionBean"
bean="yourSessionBeanToInitialize"/>
define the interceptor abstract, with an abstract method getCurrentSessionBean()
create a flag initialized on the bean
on each interception, call the lookup method and it will return an instance of the bean from the current session. If it is not initialized (the flag), initialize it
you can also use #PostConstruct and spare the initizlied flag
Another option is to:
define a HttpSessionListener in web.xml (or with annotations if using servlet 3.0)
use WebApplicationContextUtils.getRequiredWebApplicationContext(servletContext) to obtain the context
call getBean(..) to get an instance of the session-scoped bean
it will be initialized with #PostConstruct at that point
The first option is "more spring", the second is easier and faster to implement.