Spring MVC difference between context component-scan and annotation-driven tags

Question

Some days ago I began to study this Spring Hello World Tutorial  http   viralpatel net blogs spring-3-mvc-create-hello-world-application-spring-3-mvc   In this tutorial Spring DispatcherServlet is configured using the spring-servlet xml file  this one     lt  xml version  1 0  encoding  UTF-8   gt    lt beans xmlns  http   www springframework org schema beans      xmlns xsi  http   www w3 org 2001 XMLSchema-instance      xmlns p  http   www springframework org schema p      xmlns context  http   www springframework org schema context   xsi schemaLocation       http   www springframework org schema beans      http   www springframework org schema beans spring-beans xsd     http   www springframework org schema context      http   www springframework org schema context spring-context xsd  gt    lt context component-scan base-package  net viralpatel spring3 controller    gt    lt bean id  viewResolver      class  org springframework web servlet view UrlBasedViewResolver  gt       lt property name  viewClass          value  org springframework web servlet view JstlView    gt       lt property name  prefix  value   WEB-INF jsp     gt       lt property name  suffix  value   jsp    gt   lt  bean gt      In this file I am using the context component-scan tag to say that Spring have to scan my file searching the annotation  so for example  when the controller class finds that a method is annotated by  RequestMapping   hello   annotation knows that this method handles the HTTP Request toward the URL ending with   hello   This is simple     Now my doubt is related to the Spring MVC template project that I could automatically build in STS Eclipse    When I create a new Spring MVC project in STS I have that my DispatcherServlet is configured by a file named servlet-context xml that contains some configuration similar to the previous example file   In this file  I still have  the component scan tag    lt context component-scan base-package  com mycompany maventestwebapp    gt    but I have also another tag  that look like have similar task   this one    lt annotation-driven   gt    What is the difference between these two tags  An other  strange  thing is that the previous example  that don t use the annotation-driven tag  is very similar to the project create by STS using the Spring MVC Template project but if I delete the annotation-driven tag from its configuration file the project don t run and give me the following error  HTTP Status 404 -  And in the stacktrace I have   WARN   org springframework web servlet PageNotFound - No mapping found for HTTP request with URI   maventestwebapp   in DispatcherServlet with name  appServlet   But why  The previous example works well without annotation-driven tag  and this controller class is very similar  In fact  there is only one method that handles HTTP request toward     path  This is the code of my controller class   package com mycompany maventestwebapp   import java text DateFormat  import java util Date  import java util Locale   import org slf4j Logger  import org slf4j LoggerFactory  import org springframework stereotype Controller  import org springframework ui Model  import org springframework web bind annotation RequestMapping  import org springframework web bind annotation RequestMethod          Handles requests for the application home page      Controller public class HomeController    private static final Logger logger   LoggerFactory getLogger HomeController class           Simply selects the home view to render by returning its name       RequestMapping value        method   RequestMethod GET  public String home Locale locale  Model model        logger info  Welcome home  The client locale is       locale        Date date   new Date        DateFormat dateFormat   DateFormat getDateTimeInstance DateFormat LONG  DateFormat LONG  locale        String formattedDate   dateFormat format date        model addAttribute  serverTime   formattedDate         return  home       Can someone help me to understand this thing   Thank you very much

User · Answer

lt context component-scan base-package      gt     tells Spring to scan those packages for Annotations    lt mvc annotation-driven gt     registers a RequestMappingHanderMapping  a RequestMappingHandlerAdapter  and an ExceptionHandlerExceptionResolver to support the annotated controller methods like  RequestMapping   ExceptionHandler  etc  that come with MVC   This also enables a ConversionService that supports Annotation driven formatting of outputs as well as Annotation driven validation for inputs  It also enables support for  ResponseBody which you can use to return JSON data   You can accomplish the same things using Java-based Configuration using  ComponentScan basePackages                 and  EnableWebMvc in a  Configuration class   Check out the 3 1 documentation to learn more   http   static springsource org spring docs 3 1 x spring-framework-reference html mvc html mvc-config

User · Answer

lt mvc annotation-driven   gt  means that you can define spring beans dependencies without actually having to specify a bunch of  elements in XML or implement an interface or extend a base class  For example  Repository to tell spring that a class is a Dao without having to extend JpaDaoSupport or some other subclass of DaoSupport  Similarly  Controller tells spring that the class specified contains methods that will handle Http requests without you having to implement the Controller interface or extend a subclass that implements the controller    When spring starts up it reads its XML configuration file and looks for  lt bean elements within it if it sees something like  lt bean class  com example Foo    gt  and  Foo was marked up with  Controller it knows that the class is a controller and treats it as such  By default  Spring assumes that all the classes it should manage are explicitly defined in the beans XML file   Component scanning with  lt context component-scan base-package  com mycompany maventestwebapp    gt  is telling spring that it should search the classpath for all the classes under com mycompany maventestweapp and look at each class to see if it has a  Controller  or  Repository  or  Service  or  Component and if it does then Spring will register the class with the bean factory as if you had typed  lt bean class         gt  in the XML configuration files    In a typical spring MVC app you will find that there are two spring configuration files  a file that configures the application context usually started with the Spring context listener     lt listener gt       lt listener-class gt org springframework web context ContextLoaderListener lt  listener-class gt   lt  listener gt    And a Spring MVC configuration file usually started with the Spring dispatcher servlet  For example    lt servlet gt           lt servlet-name gt main lt  servlet-name gt           lt servlet-class gt org springframework web servlet DispatcherServlet lt  servlet-class gt           lt load-on-startup gt 1 lt  load-on-startup gt       lt  servlet gt       lt servlet-mapping gt           lt servlet-name gt main lt  servlet-name gt           lt url-pattern gt   lt  url-pattern gt       lt  servlet-mapping gt    Spring has support for hierarchical bean factories  so in the case of the Spring MVC  the dispatcher servlet context is a child of the main application context  If the servlet context was asked for a bean called  abc  it will look in the servlet context first  if it does not find it there it will look in the parent context  which is the application context    Common beans such as data sources  JPA configuration  business services are defined in the application context while MVC specific configuration goes not the configuration file associated with the servlet    Hope this helps

User · Answer

Annotation-driven indicates to Spring that it should scan for annotated beans  and to not just rely on XML bean configuration   Component-scan indicates where to look for those beans   Here s some doc  http   static springsource org spring docs current spring-framework-reference html mvc html mvc-config-enable

[java] Spring MVC: difference between <context:component-scan> and <annotation-driven /> tags?

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