Servlet执行流程和生命周期

慕课网学习而来：https://www.imooc.com/video/5550

流   程

客户端请求（就是在浏览器输入一个url)  

如 <a href="top/chgl16/servlet/MyServlet1">跳转</a>      

# url =  "top/chgl16/servlet/MyServlet1"，top前加不加'/'都一样，毕竟前面还有主机名，比如localhost:8080/ , 但是<url-pattern>属性值的前面必须加'/'，不然等着报错

服务器端会在web.xml文件(就是servlet类的注册文件）中找到相应的url

<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd" >

<web-app>
  <display-name>Archetype Created Web Application</display-name>

  <servlet>
    <servlet-name>myServlet1</servlet-name>
    <servlet-class>top.chgl16.servlet.MyServlet1</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>myServlet1</servlet-name>
    <url-pattern>/servlet/MyServlet1</url-pattern>
  </servlet-mapping>
</web-app>

也就是从<servlet-maping>入手，找到客户端请求的url(去掉主机名那部分）,找到匹配的<url-pattern>属性，读取其<servlet-name>,然后通过<servlet-name>去<servlet>标签里面匹对，获取配对的<servlet-class>就是对应的后台servlet类了。

注：可以看到<servlet>和<servlet-mapping>优先检索<servlet-mapping>,两者中的子标签<servlet-name>可以随意写，但是要一致相同。<url-pattern>就是在浏览器运行这个servlet类的url，也可以随意任性写。但是还是规范好。

 

生命周期

1. 实例化，构造函数

2. 调用init()函数初始化

3. 客户端请求后调用service(req, resp)函数，根据请求方式去调用doGet(req, resp), doPsost(req, resp)

4. 调用doGet(req, resp)或者doPost(req, resp)方法

5. 服务器关闭后，调用销毁方法 destroy()

写了个类测试学习

package top.chgl16.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

/**
 * @Lin 2018-5-31
 * 测试Servlet的生命周期
 */
public class MyServlet1 extends HttpServlet {
    public MyServlet1() {
        System.out.println("调用了Servlet的构造函数");
    }

    @Override
    public void init() {
        System.out.println("调用了init函数");
    }

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("调用service方法");
        doPost(req, resp);        // 这里指定了doPost,源码的service方法是根据客户端请求类型决定调用哪个的
    }
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        super.doGet(req, resp);
        System.out.println("调用了doGet方法");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("调用了doPost方法");
    }

    @Override
    public void destroy() {
        System.out.println("调用了destroy方法");
    }

}

注意：

1. 重载的service方法，指定了doPost,源码的service方法是根据客户端请求类型决定调用哪个的

2. 除了构造方法，每个方法的返回值都是空void

3. init, service, doGet, doPost的方法修饰符是protected，即子类和同包可用， destroy方法必须声明为public，此为系统调用

 

运行这个MyServlet1 类的实例，可以通过启动服务器，在浏览器输入如上web.xml文件中为其配置的url    

localhost:8080/servlet/MyServlet1             # 此虚拟路径没有设置到项目名而已

或者通过junit写单元测试代码，第一种方法运行结果如下

Connected to server
[2018-05-31 06:55:24,301] Artifact servletDemo1:war exploded: Artifact is being deployed, please wait...
[2018-05-31 06:55:24,808] Artifact servletDemo1:war exploded: Artifact is deployed successfully
[2018-05-31 06:55:24,809] Artifact servletDemo1:war exploded: Deploy took 507 milliseconds
调用了Servlet的构造函数
调用了init函数
调用service方法
调用了doPost方法
31-May-2018 18:55:33.938 信息 [ContainerBackgroundProcessor[StandardEngine[Catalina]]] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory [/home/lin/tomcat/webapps/manager]
31-May-2018 18:55:33.968 信息 [ContainerBackgroundProcessor[StandardEngine[Catalina]]] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/home/lin/tomcat/webapps/manager] has finished in [29] ms
/home/lin/tomcat/bin/catalina.sh stop
Using CATALINA_BASE:   /home/lin/.IntelliJIdea2018.1/system/tomcat/index_jsp_(2)_servletDemo1
Using CATALINA_HOME:   /home/lin/tomcat
Using CATALINA_TMPDIR: /home/lin/tomcat/temp
Using JRE_HOME:        /usr/jdk1.8
Using CLASSPATH:       /home/lin/tomcat/bin/bootstrap.jar:/home/lin/tomcat/bin/tomcat-juli.jar
31-May-2018 18:55:41.887 信息 [main] org.apache.catalina.core.StandardServer.await A valid shutdown command was received via the shutdown port. Stopping the Server instance.
31-May-2018 18:55:41.888 信息 [main] org.apache.coyote.AbstractProtocol.pause Pausing ProtocolHandler ["http-nio-8080"]
31-May-2018 18:55:41.891 信息 [main] org.apache.coyote.AbstractProtocol.pause Pausing ProtocolHandler ["ajp-nio-8009"]
31-May-2018 18:55:41.892 信息 [main] org.apache.catalina.core.StandardService.stopInternal Stopping service [Catalina]
调用了destroy方法
31-May-2018 18:55:41.916 信息 [main] org.apache.coyote.AbstractProtocol.stop Stopping ProtocolHandler ["http-nio-8080"]
31-May-2018 18:55:41.918 信息 [main] org.apache.coyote.AbstractProtocol.stop Stopping ProtocolHandler ["ajp-nio-8009"]
31-May-2018 18:55:41.936 信息 [main] org.apache.coyote.AbstractProtocol.destroy Destroying ProtocolHandler ["http-nio-8080"]
31-May-2018 18:55:41.937 信息 [main] org.apache.coyote.AbstractProtocol.destroy Destroying ProtocolHandler ["ajp-nio-8009"]
Disconnected from server