Servlet的生命周期

Servlet接口

• Servlet接口中有5个方法，其中三个生命周期方法、两个普通方法

1. 初始化：执行init()方法，Servlet生命周期中，仅执行一次init()方法，它是在服务器装入Servlet时执行的，即第一次访问这个Servlet才执行
2. 服务：执行service()方法，service()方法是Servlet的核心。每当一个客户请求HttpServlet对象，该对象的service()方法就要被调用
3. 销毁：执行destroy()方法，destroy()方法仅执行一次，即在服务器停止且卸载Servlet时执行该方法

public interface Servlet {
    void init(ServletConfig var1) throws ServletException;

    ServletConfig getServletConfig();

    void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;

    String getServletInfo();

    void destroy();
}

示?

• 继承关系：UserServlet->HttpServlet->GenericServlet->implements Servlet
• HttpServlet里面实现了service()方法，里面会根据不同的http method调用不同的方法，所以我们自定义servlet只需要重写对应的doXXX方法即可
• 以下示例：第一次访问userServlet中doGet()方法时，init()方法被调用；接下来再次调用userServlet中的方法时不会再调用init()方法；将服务停止会发现destroy()方法被调用

  package net.fl.web;
  
  import javax.servlet.ServletException;
  import javax.servlet.http.HttpServlet;
  import javax.servlet.http.HttpServletRequest;
  import javax.servlet.http.HttpServletResponse;
  import java.io.IOException;
  import java.io.PrintWriter;
  
  /**
   * @author fl
   * @description: TODO
   * @date 2022/5/13 2:36 下午
   */
  public class UserServlet extends HttpServlet {
      @Override
      protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          System.out.println("doGet");
          //设置响应内容
          resp.setContentType("text/html;charset=utf-8");
          PrintWriter printWriter = resp.getWriter();
          printWriter.write("<div>doGet可以了</div>");
      }
  
      @Override
      protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          System.out.println("doPost");
          //设置响应内容
          resp.setContentType("text/html;charset=utf-8");
          PrintWriter printWriter = resp.getWriter();
          printWriter.write("<div>doPost可以了</div>");
      }
  
      @Override
      public void init() throws ServletException {
          System.out.println("servlet init");
      }
  
      @Override
      public void destroy() {
          System.out.println("servlet destroy");
      }
  }
  
  

  
• HttpServlet 源码中service()方法

  protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          String method = req.getMethod();
          long lastModified;
          if (method.equals("GET")) {
              lastModified = this.getLastModified(req);
              if (lastModified == -1L) {
                  this.doGet(req, resp);
              } else {
                  long ifModifiedSince = req.getDateHeader("If-Modified-Since");
                  if (ifModifiedSince < lastModified / 1000L * 1000L) {
                      this.maybeSetLastModified(resp, lastModified);
                      this.doGet(req, resp);
                  } else {
                      resp.setStatus(304);
                  }
              }
          } else if (method.equals("HEAD")) {
              lastModified = this.getLastModified(req);
              this.maybeSetLastModified(resp, lastModified);
              this.doHead(req, resp);
          } else if (method.equals("POST")) {
              this.doPost(req, resp);
          } else if (method.equals("PUT")) {
              this.doPut(req, resp);
          } else if (method.equals("DELETE")) {
              this.doDelete(req, resp);
          } else if (method.equals("OPTIONS")) {
              this.doOptions(req, resp);
          } else if (method.equals("TRACE")) {
              this.doTrace(req, resp);
          } else {
              String errMsg = lStrings.getString("http.method_not_implemented");
              Object[] errArgs = new Object[]{method};
              errMsg = MessageFormat.format(errMsg, errArgs);
              resp.sendError(501, errMsg);
          }
  
      }
  
  
      public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
          HttpServletRequest request;
          HttpServletResponse response;
          try {
              request = (HttpServletRequest)req;
              response = (HttpServletResponse)res;
          } catch (ClassCastException var6) {
              throw new ServletException("non-HTTP request or response");
          }
  
          this.service(request, response);
      }