问题
We know that whenever a request for a servlet comes, servlet container will first check web.xml file for url and corresponding servlet classes. This is fine, But the confusion comes after that. Suppose I am using load-on-startup property. SO, the servlet should be ready before first call comes in. In that case container need servlet Config object to make servlet in work. But again load-on-start up and init-parameter for servlet is defined in web.xml file. So when exactly container uses web.xml?( when load-on-startup property is used then container can not obviously wait for first call, again container has to read web.xml file to know whether that property is used with any servlet.)
It will be better if some one can clear my confusion. Please provide some dependable link also if possible.
回答1:
web.xml is read as soon as you deploy your application on a web server. For the sake of understanding, you can assume container is nothing but your web server. Although web server has more than just a web container.
Web server reads the web.xml, and loads the context config, load on startup servelts,etc. web.xml is the file through which you tell your container/server about your application. Your web application sits inside the web server, and server intercepts all the incoming requests, decides to which application the request should be forwarded depending on the context.
回答2:
Loading and Instantiation:
The servlet container is responsible for loading and instantiating servlets. The loading and instantiation can occur when the container is started, or delayed until the container determines the servlet is needed to service a request.
When the servlet engine is started, needed servlet classes must be located by the servlet container. The servlet container loads the servlet class using normal Java class loading facilities. The loading may be from a local file system, a remote file system, or other network services. And as the servlets are declared in the web.xml, this file is loaded and read by the container during container startup.
Initialization:
After the servlet object is instantiated, the container must initialize the servlet before
it can handle requests from clients.The container initializes the servlet instance by calling the init method of the Servlet interface with a unique (per servlet declaration) object implementing the ServletConfig interface.
Ref: JSR-000315 JavaTM Servlet 3.0
来源：https://stackoverflow.com/questions/18996025/when-container-call-deployment-descriptor-web-xml