前言

了解Servlet的生命週期是很重要的，因為知道他的流程我們才能在不同階段做我們想要做的事。

0.創建module

(1)創建module day06

請參考Day05創建module

(2)day06範例建立

@WebServlet("/")
public class DemoLifeCycleServlet implements Servlet {
    @Override
    public void init(ServletConfig config) throws ServletException {
        System.out.println("servlet init");
    }

    @Override
    public ServletConfig getServletConfig() {
        return null;
    }

    @Override
    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        System.out.println("service");
    }

    @Override
    public String getServletInfo() {
        return "";
    }

    @Override
    public void destroy() {
        System.out.println("servlet destroy");
    }
}

一、Servlets 生命週期

Servlet Life Cycle，也就是從Servlet實例化開始到銷毀的過程，可分成以下階段

1. 實例化 -> constructor (tomcat啟動或第一次請求)
2. 初始化 -> init (實例化後)
3. 接收處理請求 -> service (每次請求)
4. 銷毀 -> destory (關閉tomcat)

根據上面的範例我們啟動server後可以訪問locahost:8080/多次後再關閉server，會看到以下log

[2024-08-19 10:59:18,064] Artifact day06:war exploded: Artifact is deployed successfully
[2024-08-19 10:59:18,065] Artifact day06:war exploded: Deploy took 291 milliseconds
DemoLifeCycleServlet constructor
servlet init
service
service
service
servlet destroy

Servlet在tomcat是singleton(單例)

Servlet的成員變數會被多執行續共享，故不適合在方法中修改成員變數，會有線程安全問題

二、loadonstartup

default是-1，tomcat啟動時不實例化servlet
正整數，tomcat啟動時實例化servlet，按照數字小到到大，序號衝突tomcat會自動調整

@WebServlet(value = "/",loadOnStartup = 10)
public class DemoLifeCycleServlet implements Servlet {
  //略
}

可以看到與上面log不同的是這裡啟動tomcat就把servlet帶起來了

DemoLifeCycleServlet constructor
servlet init
[2024-08-19 11:29:52,441] Artifact day06:war exploded: Artifact is deployed successfully
[2024-08-19 11:29:52,441] Artifact day06:war exploded: Deploy took 287 milliseconds

xml設定方式

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="https://jakarta.ee/xml/ns/jakartaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="https://jakarta.ee/xml/ns/jakartaee https://jakarta.ee/xml/ns/jakartaee/web-app_6_0.xsd"
         version="6.0">
  <display-name>Archetype Created Web Application</display-name>

  <servlet>
    <servlet-name>DemoLifeCycle</servlet-name>
    <servlet-class>com.swj.DemoLifeCycleServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>DemoLifeCycle</servlet-name>
    <url-pattern>/</url-pattern>
  </servlet-mapping>
</web-app>

三、default servlet

從tomcat目錄下conf/web.xml可以找到default servlet的設定，靜態資源會透過default servlet來完成response，要注意的是未來使用SpringMVC時因為他有DispatchServlet，若未再特別設置讓DefaultServlet生效，將會造成靜態資源無法得到response的問題。

<servlet-mapping>
        <servlet-name>default</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>
<servlet>
    <servlet-name>default</servlet-name>
    <servlet-class>org.apache.catalina.servlets.DefaultServlet</servlet-class>
    <init-param>
        <param-name>debug</param-name>
        <param-value>0</param-value>
    </init-param>
    <init-param>
        <param-name>listings</param-name>
        <param-value>false</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>

四、生命週期小結

reference：A Servlet's Life cycle - NTU SG

Reference

• Dependency Scope - Maven doc
• MIME type - mdn web docs
• A Servlet's Life cycle - NTU SG