What is Dispatcher Servlet in Spring

Question

In this image  which I got from here   HTTP request sends something to Dispatcher Servlet      My Question is what does Dispatcher Servlet do   Is it something like getting the information thrown from the web page and throwing it to the controller

User · Answer

You can say Dispatcher Servlet acts as an entry and exit point for any request. Whenever a request comes it first goes to the Dispatcher Servlet(DS) where the DS then tries to identify its handler method ( the methods you define in the controller to handle the requests ), once the handler mapper (The DS asks the handler mapper) returns the controller the dispatcher servlet knows the controller which can handle this request and can now go to this controller to further complete the processing of the request. Now the controller can respond with an appropriate response and then the DS goes to the view resolver to identify where the view is located and once the view resolver tells the DS it then grabs that view and returns it back to you as the final response. I am adding an image which I took from YouTube from the channel Java Guides.

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The job of the DispatcherServlet is to take an incoming URI and find the right combination of handlers  generally methods on Controller classes  and views  generally JSPs  that combine to form the page or resource that s supposed to be found at that location   I might have   a file  WEB-INF jsp pages Home jsp and a method on a class   RequestMapping value   pages Home html   private ModelMap buildHome         return somestuff       The Dispatcher servlet is the bit that  knows  to call that method when a browser requests the page  and to combine its results with the matching JSP file to make an html document   How it accomplishes this varies widely with configuration and Spring version   There s also no reason the end result has to be web pages   It can do the same thing to locate RMI end points  handle SOAP requests  anything that can come into a servlet

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I know this question is marked as solved already but I want to add a newer image explaining this pattern in detail source  spring in action 4      Explanation  When the request leaves the browser  1   it carries information about what the user is asking for  At the least  the request will be carrying the requested URL  But it may also carry additional data  such as the information submitted in a form by the user   The first stop in the request   s travels is at Spring   s DispatcherServlet  Like most Java- based web frameworks  Spring MVC funnels requests through a single front controller servlet  A front controller is a common web application pattern where a single servlet delegates responsibility for a request to other components of an application to per- form actual processing  In the case of Spring MVC  DispatcherServlet is the front controller  The DispatcherServlet   s job is to send the request on to a Spring MVC controller  A controller is a Spring component that processes the request  But a typical application may have several controllers  and DispatcherServlet needs some help deciding which controller to send the request to  So the DispatcherServlet consults one or more handler mappings  2  to figure out where the request   s next stop will be  The handler mapping pays particular attention to the URL carried by the request when making its decision  Once an appropriate controller has been chosen  DispatcherServlet sends the request on its merry way to the chosen controller  3   At the controller  the request drops off its payload  the information submitted by the user  and patiently waits while the controller processes that information   Actually  a well-designed controller per- forms little or no processing itself and instead delegates responsibility for the business logic to one or more service objects   The logic performed by a controller often results in some information that needs to be carried back to the user and displayed in the browser  This information is referred to as the model  But sending raw information back to the user isn   t suffi- cient   it needs to be formatted in a user-friendly format  typically HTML  For that  the information needs to be given to a view  typically a JavaServer Page  JSP   One of the last things a controller does is package up the model data and identify the name of a view that should render the output  It then sends the request  along with the model and view name  back to the DispatcherServlet  4   So that the controller doesn   t get coupled to a particular view  the view name passed back to DispatcherServlet doesn   t directly identify a specific JSP  It doesn   t even necessarily suggest that the view is a JSP  Instead  it only carries a logical name that will be used to look up the actual view that will produce the result  The DispatcherServlet consults a view resolver  5  to map the logical view name to a spe- cific view implementation  which may or may not be a JSP  Now that DispatcherServlet knows which view will render the result  the request   s job is almost over  Its final stop is at the view implementation  6   typically a JSP  where it delivers the model data  The request   s job is finally done  The view will use the model data to render output that will be carried back to the client by the  not- so-hardworking  response object  7

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lt  xml version  1 0  encoding  UTF-8    gt   lt  -- was   lt  xml version  1 0  encoding  UTF-8   gt  -- 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 aop  http   www springframework org schema aop         xmlns context  http   www springframework org schema context         xmlns mvc  http   www springframework org schema mvc         xmlns tx  http   www springframework org schema tx         xsi schemaLocation  http   www springframework org schema beans http   www springframework org schema beans spring-beans-4 0 xsd        http   www springframework org schema aop http   www springframework org schema aop spring-aop-4 0 xsd        http   www springframework org schema tx http   www springframework org schema tx spring-tx-4 0 xsd         http   www springframework org schema mvc http   www springframework org schema mvc spring-mvc-4 0 xsd                http   www springframework org schema context http   www springframework org schema context spring-context-4 0 xsd  gt        lt bean class  org springframework web servlet mvc support ControllerClassNameHandlerMapping   gt       lt context component-scan base-package  com demo    gt       lt context annotation-config   gt        lt mvc annotation-driven   gt         lt bean id  viewResolver            class  org springframework web servlet view InternalResourceViewResolver            p prefix   WEB-INF jsp             p suffix   jsp    gt        lt bean id  jdbcTemplate  class  org springframework jdbc core JdbcTemplate  gt           lt property name  dataSource  ref  datasource    gt       lt  bean gt               lt bean id  datasource  class  org springframework jdbc datasource DriverManagerDataSource  gt           lt property name  driverClassName  value  com mysql jdbc Driver    gt             lt property name  url  value  jdbc mysql   localhost 3306 employee    gt           lt property name  username  value  username    gt           lt property name  password  value  password    gt       lt  bean gt     lt  beans gt

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In Spring MVC  all incoming requests go through a single servlet  This servlet - DispatcherServlet - is the front controller  Front controller is a typical design pattern in the web applications development  In this case  a single servlet receives all requests and transfers them to all other components of the application   The task of the DispatcherServlet is to send request to the specific Spring MVC controller   Usually we have a lot of controllers and DispatcherServlet refers to one of the following mappers in order to determine the target controller    BeanNameUrlHandlerMapping  ControllerBeanNameHandlerMapping  ControllerClassNameHandlerMapping  DefaultAnnotationHandlerMapping  SimpleUrlHandlerMapping    If no configuration is performed  the DispatcherServlet uses BeanNameUrlHandlerMapping and DefaultAnnotationHandlerMapping by default   When the target controller is identified  the DispatcherServlet sends request to it  The controller performs some work according to the request   or delegate it to the other objects   and returns back to the DispatcherServlet with the Model and the name of the View   The name of the View is only a logical name  This logical name is then used to search for the actual View  to avoid coupling with the controller and specific View   Then DispatcherServlet refers to the ViewResolver and maps the logical name of the View to the specific implementation of the View   Some possible Implementations of the ViewResolver are    BeanNameViewResolver  ContentNegotiatingViewResolver  FreeMarkerViewResolver  InternalResourceViewResolver  JasperReportsViewResolver  ResourceBundleViewResolver  TilesViewResolver  UrlBasedViewResolver  VelocityLayoutViewResolver  VelocityViewResolver  XmlViewResolver  XsltViewResolver    When the DispatcherServlet determines the view that will display the results it will be rendered as the response   Finally  the DispatcherServlet returns the Response object back to the client

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DispatcherServlet is Spring MVC s implementation of the front controller pattern   See description in the Spring docs here   Essentially  it s a servlet that takes the incoming request  and delegates processing of that request to one of a number of handlers  the mapping of which is specific in the DispatcherServlet configuration

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Dispatcher Controller are displayed in the figure all the incoming request is in intercepted by the dispatcher servlet that works as front controller  The dispatcher servlet gets an entry to handler mapping from the XML file and forwords the request to the Controller

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We can say like DispatcherServlet taking care of everything in Spring MVC   At web container start up    DispatcherServlet will be loaded and initialized by calling init   method init   of DispatcherServlet will try to identify the Spring     Configuration Document with naming conventions like      servlet name-servlet xml  then all beans can be identified    Example   public class DispatcherServlet extends HttpServlet        ApplicationContext ctx   null       public void init ServletConfig cfg              1  try to get the spring configuration document with default naming conventions         String xml    servlet name     -servlet xml              if it was found then creates the ApplicationContext object         ctx   new XmlWebApplicationContext xml                     So  in generally DispatcherServlet capture request URI and hand over to HandlerMapping  HandlerMapping search mapping bean with method of controller  where controller returning logical name view   Then this logical name is send to DispatcherServlet by HandlerMapping  Then DispatcherServlet tell ViewResolver to give full location of view by appending prefix and suffix  then DispatcherServlet give view to the client

[java] What is Dispatcher Servlet in Spring?

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