浏览器送一个HTTP请求到java web 服务(tomcat)。这个web服务检查，如果是servelt请求，servlet 容器 将通过请求。servlet容器然后将找出请求的是哪一个servlet，并且**servlet.servlet 通过调用servlet.service() **。

      一旦servlet 被**，servlet 处理 请求，然后产生结果。这个响应然后送回给浏览器。

3、servlet 生命周期

    servlet 容器管理 servlet 生命周期，生命周期包含下面步骤：

1. Load Servlet Class
2. Create Instance of Servlet：建立servlet 实例，应该已经初始化类的数据。
3. Call the servlets init() method. //有什么用？
4. Call the servlets service() method. //应该处理浏览器请求。
5. Call the servlets destroy() method.

   Step 1, 2 and 3 are executed only once：

    You can force the container to load the servlet when the container starts up . See web.xml Servlet Configuration for more details about that.

Step 4 is executed multiple times - 每一个http对servlet请求一次。
Step 5 is executed when the servlet container unloads the servlet.
                         

                                        The Java Servlet life cycle

Load Servlet Class

Before a servlet can be invoked the servlet container must first load its class definition. This is done just like any other class is loaded.

Create Instance of Servlet：仅仅建立，未初始化数据

When the servlet class is loaded, the servlet container creates an instance of the servlet.

Typically, only a single isntance of the servlet is created, and concurrent requests to the servlet are executed on the same servlet instance. This is really up to the servlet container to decide, though. But typically, there is just one instance.

Call the Servlets init() Method：允许初始化对象。

When a servlet instance is created, its init() method is invoked. The init() method allows a servlet to initialize itself before the first request is processed.

You can specify init parameters to the servlet in the web.xml file. See web.xml Servlet Configuration for more details.

Call the Servlets service() Method

As long as the servlet is active in the servlet container, the service() method can be called. Thus, this step in the life cycle can be executed multiple times.

Call the Servlets destroy() Method

When a servlet is unloaded by the servlet container, its destroy() method is called. This step is only executed once, since a servlet is only unloaded once.

A servlet is unloaded by the container if the container shuts down, or if the container reloads the whole web application at runtime.

总结：浏览器一个http请求----------到达tomact，tomcat 一看是servlet----去配置文件找对应servlet---然后把servlet对象装入内存-