Servlet - Java Web Core Component_java webcore-优快云博客

本文链接：https://blog.youkuaiyun.com/afandaafandaafanda/article/details/46764021

本文探讨了Servlet作为JavaWeb中的关键组件的作用，并对比了直接实现Servlet接口与继承HttpServlet类的方法。此外，还介绍了Struts2框架如何通过Action简化Servlet的doGet和doPost方法。

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在Java Web的世界里，Tomcat 等Web Server被称作servlet容器。这一称谓的由来就是因为，Tomcat 实际上运行JSP 页面和Servlet。servlet实际上包含了诸多的业务逻辑，后来经过分层思想的演化，业务逻辑会被独立成dao,service等层，但在servlet中还是会调用service层的逻辑。这时候servlet实际上是页面和逻辑层的桥梁，差不多是MVC中的控制器（C）的意思。

正是因为这种原因，structs2这个很好的MVC框架才诞生出来，这时候通过Action，ActionSupport，等接口屏蔽了servlet的doGet,doPost等方法。为什么需要屏蔽这些方法呢？我们分别看看doGet和doPost里面做了些什么。

public void doGet(ServletRequest rep, ServletResponse req)
			throws ServletException, IOException {

	System.out.println("doGet");
	req.setContentType("text/html");
	PrintWriter out = req.getWriter();
	out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
	out.println("<HTML>");
	out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");
	out.println("  <BODY>");
	out.print("    This is ");
	out.print(this.getClass());
	out.println(", using the GET method");
	out.println("  </BODY>");
	out.println("</HTML>");
	out.flush();
	out.close();		
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
	System.out.println("doPost");
	response.setContentType("text/html");
	PrintWriter out = response.getWriter();
	out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
	out.println("<HTML>");
	out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");
	out.println("  <BODY>");
	out.print("    This is ");
	out.print(this.getClass());
	out.println(", using the POST method");
	out.println("  </BODY>");
	out.println("</HTML>");
	out.flush();
	out.close();
}

是的,您没有看错，确实在拼html标签。动态的部分来自业务逻辑代码返回的，而静态部分需要在这里拼接字符串，效率很低，而且很容易出错。

下面我们来看看实现servlet的两种方式：实现Servlet接口和继承httpServlet

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


public class MyServlet2 implements Servlet{

	public MyServlet2(){
		System.out.println("MyServlet2");
	}
	
	public void destroy() {
		// TODO Auto-generated method stub
		System.out.println("destroy");
	}

	public ServletConfig getServletConfig() {
		// TODO Auto-generated method stub
		System.out.println("ServletConfig");
		return null;
	}

	public String getServletInfo() {
		System.out.println("getServletInfo");
		// TODO Auto-generated method stub
		return null;
	}

	public void init(ServletConfig arg0) throws ServletException {
		// TODO Auto-generated method stub
		System.out.println("init");
	}

	public void service(ServletRequest rep, ServletResponse req)
			throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(rep,req);
		System.out.println("service");
	}
	
	public void doGet(ServletRequest rep, ServletResponse req)
			throws ServletException, IOException {

		System.out.println("doGet");
		req.setContentType("text/html");
		PrintWriter out = req.getWriter();
		out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
		out.println("<HTML>");
		out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");
		out.println("  <BODY>");
		out.print("    This is ");
		out.print(this.getClass());
		out.println(", using the GET method");
		out.println("  </BODY>");
		out.println("</HTML>");
		out.flush();
		out.close();
	}
	
	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		System.out.println("doPost");
		response.setContentType("text/html");
		PrintWriter out = response.getWriter();
		out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
		out.println("<HTML>");
		out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");
		out.println("  <BODY>");
		out.print("    This is ");
		out.print(this.getClass());
		out.println(", using the POST method");
		out.println("  </BODY>");
		out.println("</HTML>");
		out.flush();
		out.close();
	}

}

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


public class MyServlet extends HttpServlet {

	/**
	 * Constructor of the object.
	 */
	public MyServlet() {
		super();
		System.out.println("constructor.");
	}

	/**
	 * Destruction of the servlet. <br>
	 */
	public void destroy() {
		System.out.println("destroy");
		super.destroy(); // Just puts "destroy" string in log
		// Put your code here
	}

	/**
	 * The doGet method of the servlet. <br>
	 *
	 * This method is called when a form has its tag value method equals to get.
	 * 
	 * @param request the request send by the client to the server
	 * @param response the response send by the server to the client
	 * @throws ServletException if an error occurred
	 * @throws IOException if an error occurred
	 */
	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		System.out.println("doGet");
		response.setContentType("text/html");
		PrintWriter out = response.getWriter();
		out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
		out.println("<HTML>");
		out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");
		out.println("  <BODY>");
		out.print("    This is ");
		out.print(this.getClass());
		out.println(", using the GET method");
		out.println("  </BODY>");
		out.println("</HTML>");
		out.flush();
		out.close();
	}

	/**
	 * The doPost method of the servlet. <br>
	 *
	 * This method is called when a form has its tag value method equals to post.
	 * 
	 * @param request the request send by the client to the server
	 * @param response the response send by the server to the client
	 * @throws ServletException if an error occurred
	 * @throws IOException if an error occurred
	 */
	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		System.out.println("doPost");
		response.setContentType("text/html");
		PrintWriter out = response.getWriter();
		out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
		out.println("<HTML>");
		out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");
		out.println("  <BODY>");
		out.print("    This is ");
		out.print(this.getClass());
		out.println(", using the POST method");
		out.println("  </BODY>");
		out.println("</HTML>");
		out.flush();
		out.close();
	}

	/**
	 * Initialization of the servlet. <br>
	 *
	 * @throws ServletException if an error occurs
	 */
	public void init() throws ServletException {
		// Put your code here
		System.out.println("init");
	}

}

这两个servlet算是最简单的两个servlet了，因为只包含doGet和doPost方法，实际开发过程中，肯定比这个复杂，但是大量地拼接字符串不是好的做法。

我们实际使用structs的Action,实现Action接口或者继承ActionSupport

package com.wicresoft.action;

import com.opensymphony.xwork2.Action;

public class IndexAction2 implements Action {

	public String execute() throws Exception {
		System.out.println("IndexAction2 executed.");
		// TODO Auto-generated method stub
		return SUCCESS;
	}
}

package com.wicresoft.action;

import com.opensymphony.xwork2.ActionSupport;

public class IndexAction3 extends ActionSupport{

	@Override
	public String execute() throws Exception {
		System.out.println("IndexAction3 executed.");
		// TODO Auto-generated method stub
		//return super.execute();
		return "success";
	}
	
	public String UserAdd(){
		return SUCCESS;
	}
	
	public String Add(){
		return SUCCESS;
	}
	
}

值得注意的是，这两种方式和实现servlet的两种方式遥相呼应。

Servlet生命周期分为三个阶段：
1，初始化阶段调用init()方法
2，响应客户请求阶段　　调用service()方法
3，终止阶段　　调用destroy()方法

Servlet被装载后，Servlet容器创建一个Servlet实例并且调用Servlet的init()方法进行初始化。在Servlet的整个生命周期内，init()方法只被调用一次。

在servlet的实例被创建后，如果有请求过来，servlet容器会开新的线程来处理请求，而不再创建新的servlet的实例，也是因此,init()方法只会被调用一次！