Where to put application level beans in Spring MVC? Into root-context.xml or into servlet-context.xml?
Beans declared in root-context.xml (services) are visible for beans in servlet-context.xml (controllers), but not the other way around. Therefore there can be a dependency from controllers to services, but the reverse dependency is forbidden by Spring.
That being said put MVC-independent code (services, DAOs, etc.) in main context and put web-only stuff in servlet context.
Related
What is the difference between DispatcherServlet and ContextLoaderListener in Spring framework? Do we need to configure both of them in web.xml when we are using spring framework?
AFAIK each DispatcherServlet will have a WebApplicationContext. By default the DispatcherServlet looks for a spring configuration file named [appname]-servlet.xml under WEB-INF folder.
Do we need to configure DispatcherServlet?
Yes, every spring application should configure DispatcherServlet as it is the one through which all the requests are routed. It decides the appropriate method of the controller class to handle the request. Once controller returns the model along with the logical view, DispatcherServlet takes the help of ViewResolver to resolve the view (generally JSPs) and will pass the model data to the view, which is finally rendered on the browser.
Do we need to configure ContextLoaderListener?
No, this is not mandatory. Spring applications can live with out ContextLoaderListener.
Why do we need ContextLoaderListener?
Usually when we build multi-tier applications we don't want to clutter all the beans in one config file [appname]-servlet.xml. For example if you configure spring security you wanted to include all those beans in security-context.xml, in the same way all the beans belonging to service layer are configured in applicationContext.xml and some would like to configure beans belonging to DAO layer in dao-context.xml. So when you configure all these beans in different context files, you need to let know spring that these files exist as spring only knows about [appname]-servlet.xml. ContextLoaderListener will help spring recognize all the other context files.
Hope this helps!
The root WebApplicationContext is a Spring Application Context shared across the application.
A DispatcherServlet instance actually has its own
WebApplicationContext.
One can have multiple DispatcherServlet instances in an application, and each will have its own WebApplicationContext.
The root WebApplicationContext is shared across
the application, so if you have a root WebApplicationContext and
multiple DispatcherServlets, the DispatcherServlets will share the
root WebApplicationContext.
However, for a simple Spring MVC application, one can even have a situation where there is no need to have a root WebApplicationContext. A DispatcherServlet would still have its own WebApplicationContext, but it doesn’t actually need to have a parent root WebApplicationContext.
So, which beans should go in the root Web Application Context and which beans should go in the DispatcherServlet’s Web Application Context?
Well, general beans such as services and DAOs make their way in root Web Application Context, and more web-specific beans such as controllers are included in DispatcherServlet’s Web Application Context.
When DispatcherServlet starts up, it creates a Spring application context and starts
loading it with beans declared in the configuration files or classes that it’s given.
But in Spring web applications, there’s often another application context. This
other application context is created by ContextLoaderListener
Whereas DispatcherServlet is expected to load beans containing web components
such as controllers, view resolvers, and handler mappings, ContextLoaderListener is
expected to load the other beans in your application. These beans are typically the
middle-tier and data-tier components that drive the back end of the application.
Good luck!
I start to learn spring recently.
My goal is to use spring MVC to do restful api
I know spring MVC is web framework in spring
I know that in spring,there is beans.xml
And in spring MVC , there is servletname-servlet.xml
I want to know where is difference??
Is it means if I use spring MVC,I don't need to use beans.xml??
Please give me some way or give me example project link with spring and spring MVC together
The servletname-servlet.xml defines the beans for one servlet's app context. There can be number of servlets in a webapp and for every servlet we have servletname-servlet.xml (e.g. spring1-servlet.xml for servlet1, spring2-servlet.xml for servlet2).
Beans defined in servletname-servlet.xml can reference beans in beans.xml, but not vice versa.
All Spring MVC controllers must go in the servletname-servlet.xml context.
Beans.xml contain beans that are shared between all servlets in a webapp.Usually the beans.xml context is not necessary if you have only one servlet in your webapp.
You could define all your beans in servletname-servlet.xml but it's not a good practice.
Usually if you create a web application in 'pure' spring (ie. without spring MVC) then you will add ContextLoaderListener as a filter to your web.xml. Then spring will look for applicationContext.xml when you will usually import beans.xml.
In servletname-servlet.xml you define servlets. Servlets can refer other beans. So it's good practice to separate front (servlets) from backend (beans.xml).
Also remember that beans declared in servletname-servlet.xml are overriding the definitions of any beans defined with the same name in the global scope.
See also better answer at: ContextLoaderListener or not?
I created a servlet filter which is using some Autowired fields. For making it work I declared it as DelegatingFilterProxy in web.xml . Before this filter, my enitre spring config was in dispatcher-servlet.xml but for some reason declaring bean for this filter was not working in dispacher-servlet. So, I declared it in applicationContext.xml. It started working then but Autwired fields inside filter were then throwing null. To tackle with it I moved
<context:component-scan base-package="com.myproj.abc" />
to applicationContext, filter started working then but url paths defined by my controller classes are no longer mapped. So I need to pull following two lines also in applicationContext
<mvc:default-servlet-handler />
<mvc:annotation-driven />
This solves the issue. But I was wondering is this the right place for all this code? Because Spring security and for static resources and view mapping all these code goes in dispatcher. In one of my other project I faced same issue and there I did like this, declared only following line in applicationContext
<context:component-scan base-package="com.myproj.abc" />
And in dispatcher-servlet I change component scan package to controller only and kept all other code there only(in dispatcher)
<context:component-scan base-package="com.myproj.abc.controller" />
Could anyone please enlighten me on this confusion.
Some terminology: ServletContext is the class. Servlet context is the Spring ApplicationContext for the DispatcherServlet. Application context, also know as root context is the ApplicationContext loaded by the ContextLoaderListener and stored in the web application's ServletContext. It is therefore available to other web application components.
The servlet context is loaded by the DispatcherServlet. The DispatcherServlet retrieves the application context from the ServletContext and uses it as the parent of the servlet context.
A servlet Filter is web application component that has no relation to the DispatcherServlet, ie. it doesn't know about the servlet context.
The javadoc for DelegatingFilterProxy states
Supports a "targetBeanName" filter init-param in web.xml, specifying
the name of the target bean in the Spring application context.
So the Filter bean has to be declared in the application context, not the servlet context.
Put all beans that have application scope in the application context.
Put all beans that have relevance to the MVC stack in the servlet context.
Your component-scan should scan the appropriate packages to support the two rules/suggestions above.
Further reading:
What is the difference between ApplicationContext and WebApplicationContext in Spring MVC?
Web-application context/ root application context and transaction manager setup
Difference between applicationContext.xml and spring-servlet.xml in Spring Framework
What type of information should be present in the dispatcher-servlet.xml and applicationContext.xml?
The applicationContext.xml is loaded by the ContextLoadeListener and should contain your shared/global beans. Things like the datasource, jms connecetionfactories, services, repositories etc.
Your dispatcher-servlet.xml is loaded by the DispatcherServlet and should contain only web related things like controllers, viewresolvers, exceptionhandlers etc.
This is a general rule of thumb of course.
In my project, we have a spring mvc application.
It has got both applicationcontext.xml as well as -servlet.xml config files.
Bean definitons are spread in both the files.
I want to know when we have -servlet.xml wats the need for applicationcontext.xml?
Please provide any explanation in this area.
applicationContext.xml will have the bean definitions of the core spring components.
project-servlet.xml will have bean definitions of indivisual servlets.
-servlet.xml can have references to applicationContext.xml not the other way round.
What you refer as applicationContext.xml is the root application context (you put beans there when you need application-wide access to them) and what you refer as the [servlet]-context.xml is a specific Spring Bean configuration xml for Spring MVC's DispatcherServlet.
servlet-context is specific to a servlet and application context is shared for whole application. So when you define a Bean in servlet-context.xml the Bean is available to the context of that specific servlet, but when you define a Bean in application-context.xml it is available in the whole application. So if you have multiple dispatcherServlet you can have separate servlet-context for each servlet. But there is only one application-context for the application