Posted 2020-03-29Updated 2021-11-12Backend16 minutes read (About 2468 words)0 visits

Servlets and JSPs

Servlet

Understanding the servlet

Tomcat可以简单的理解为一个web容器，我们编写的servelet程序必须要在web容器中才能运行，这很好理解，因为单单一个servlet java程序并不能够接收浏览器的信息，我们需要通过web容器接收到浏览器的命令，然后web容器再调用servlet处理并返回结果。
Tomcat从浏览器接收到请求后生成request和response两个对象，将这两个对象传入servlet，servelet处理后再返回Tomcat。
Servlet可以使用xml或者annotation来处理路径。

package org.shiyu.liu;

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

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

/**
 * Servlet implementation class SimpleServlet
 */
@WebServlet(description = "A simple servlet", urlPatterns = { "/SimpleServletPath" })//使用annotation来处理路径
public class SimpleServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {//从Tomcat中接收到的request和response对象
		// TODO Auto-generated method stub
		System.out.println("Hello From Get Method");
		response.setContentType("text/html");
		PrintWriter writer = response.getWriter();
		writer.println("<h3>Hello from html</h3>");
	}

}

Servlet XML configuration

我们可以不使用annonation配置servlet的url，而是直接在web.xml中进行配置
: welcome-file-list的工作原理是，按照welcome-file的.list一个一个去检查是否web目录下面存在这个文件，如果存在，继续下面的工作，先去webcontent(这里是Eclipse的工程目录根目录)下是否真的存在index.html这个文件，如果不存在去找是否存在index.jsp这个文件，以此类推。
: 关联和
: 关联和

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" id="WebApp_ID" version="4.0">
  <display-name>SimpleServletProject</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
  </welcome-file-list>
  <servlet>
  	<servlet-name>xmlServlet</servlet-name>
  	<servlet-class>org.shiyu.liu.XmlServlet</servlet-class>
  </servlet>
  <servlet-mapping>
	<servlet-name>xmlServlet</servlet-name>
	<url-pattern>/xmlServletPath</url-pattern>
  </servlet-mapping>
</web-app>

xml比annotation的优势在于，当我们改变路径的时候，如果是在xml修改，只需要重启Tomcat即可，但是如果修改java代码，那么需要重新编译。

The POST Method and Passing Parameters

package org.shiyu.liu;

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

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

/**
 * Servlet implementation class XmlServlet
 */
public class XmlServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
    
	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		response.setContentType("text/html");
		PrintWriter out = response.getWriter();
		String userName = request.getParameter("userName");//得到userName这个参数的值
		out.println("Hello " + userName);
	}
}

Request, Session, context

response, request和servlet本身都是objects，由Tomcat创建(我们只写了servlet的class，它的instantiate是由Tomcat实现的)。
request和response objects每一次有浏览器发起请求都会由Tomcat实例化一次(因为HTTP协议是stateless的)
servlet object只有一个，不同的请求有不同的servlet thread，但是都是在这一个实例下面的

由于request object在每次请求事都会被重置，有的时候我们需要记住已有的信息(比如用户的登录信息)，这个时候就要使用session。
Tomcat将session存在内存中。
https://www.zhihu.com/question/19786827
https://www.runoob.com/servlet/servlet-session-tracking.html

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
	// TODO Auto-generated method stub
	response.setContentType("text/html");
	PrintWriter out = response.getWriter();
	String userName = request.getParameter("userName");
	HttpSession session = request.getSession();
	if(userName!=null && !userName.equals("")) {
		session.setAttribute("savedUserName", userName);
	}
	out.println("Hello from GET moethd " + userName);
	out.println("Saved session user name: "+ (String)session.getAttribute("savedUserName"));
	
}

session object对于不同的用户(浏览器)有不同的值，如果我们需要一个对于所有用户都存有同样值的对象，那么可以使用context object。https://blog.csdn.net/gavin_john/article/details/51399425

Understanding init, service and ServletConfig(how a servlet is initialized, the methods that get called and the objects used on initialization and execution)

There are methods which run before the doGet and doPost. 代码都可以在HttpServlet.java中查看
init(servletConfig):第一次call servlet的时候，进行servlet初始化
service():service通过判断HTTP code来调用doGet或者其他方法

protected void service(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException {

    String method = req.getMethod();

    if (method.equals(METHOD_GET)) {
        long lastModified = getLastModified(req);
        if (lastModified == -1) {
            // servlet doesn't support if-modified-since, no reason
            // to go through further expensive logic
            doGet(req, resp);
        } else {
            long ifModifiedSince;
            try {
                ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
            } catch (IllegalArgumentException iae) {
                // Invalid date header - proceed as if none was set
                ifModifiedSince = -1;
            }
            if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                // If the servlet mod time is later, call doGet()
                // Round down to the nearest second for a proper compare
                // A ifModifiedSince of -1 will always be less
                maybeSetLastModified(resp, lastModified);
                doGet(req, resp);
            } else {
                resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
            }
        }

    } else if (method.equals(METHOD_HEAD)) {
        long lastModified = getLastModified(req);
        maybeSetLastModified(resp, lastModified);
        doHead(req, resp);

    } else if (method.equals(METHOD_POST)) {
        doPost(req, resp);

    } else if (method.equals(METHOD_PUT)) {
        doPut(req, resp);

    } else if (method.equals(METHOD_DELETE)) {
        doDelete(req, resp);

    } else if (method.equals(METHOD_OPTIONS)) {
        doOptions(req,resp);

    } else if (method.equals(METHOD_TRACE)) {
        doTrace(req,resp);

    } else {
        //
        // Note that this means NO servlet supports whatever
        // method was requested, anywhere on this server.
        //

        String errMsg = lStrings.getString("http.method_not_implemented");
        Object[] errArgs = new Object[1];
        errArgs[0] = method;
        errMsg = MessageFormat.format(errMsg, errArgs);

        resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
    }
}

所以比如连接数据库的操作就可以在init()中完成。

Servlet一个最大的缺点就是在java code中写html，JSP可以解决这个问题，JSP其实就是一种动态思想，web界面主要由html显示，其中需要改变的地方可以使用java变量。

JSP

JSP baiscs

Servlet一个最大的缺点就是在java code中写html，JSP可以解决这个问题，JSP其实就是一种动态思想，web界面主要由html显示，其中需要改变的地方可以使用java变量。
JSP和EJS里用于表示变量的方法很像(我感觉ejs应该是模仿JSP的吧)，JSP使用<% JAVA语句 %>来写java代码， <%= JAVA变量%>来显示到html中，<%! JAVA函数 %>来定义一个全局函数。

<%! 
public int add(int a, int b){
	return a+b;
}
%>
<%
int t = add(234, 123);
%>
t's value is: <%=t %>

所以其实JSP/EJS/jinja2本质上其实就是simple templating language that lets you generate HTML markup with plain JAVA/JavaScript/python。

understanding JSP

JSP在运行的时候，其实会被Tomcat转化成java class文件，也就是一个servlet，所有<% %>里面的语句都会运行在servlet的doGet method中。所以我们不能在<% %>中间写另一个函数。而<%! %>里的内容只会被放在class中。
而对于JSP中其他的html tag，其实就相当于java调用了out.println()函数。
这也是JSP可以像下面这样写的原因(和ejs类似)

<% 
for(int p = 0;p<=5;p++){
%>
<br> 
The value of p is: <%=p %>
<%	
}
%>

JSP被翻译的java class可以在Tomcat的work文件夹下面找到

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/9.0.33
 * Generated at: 2020-03-30 18:00:34 UTC
 * Note: The last modified time of this file was set to
 *       the last modified time of the source file after
 *       generation to assist with modification tracking.
 */
package org.apache.jsp;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;

public final class test_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent,
                 org.apache.jasper.runtime.JspSourceImports {

 
	public int add(int a, int b){
		return a+b;
	}
	
  private static final javax.servlet.jsp.JspFactory _jspxFactory =
          javax.servlet.jsp.JspFactory.getDefaultFactory();

  private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants;

  private static final java.util.Set<java.lang.String> _jspx_imports_packages;

  private static final java.util.Set<java.lang.String> _jspx_imports_classes;

  static {
    _jspx_imports_packages = new java.util.HashSet<>();
    _jspx_imports_packages.add("javax.servlet");
    _jspx_imports_packages.add("javax.servlet.http");
    _jspx_imports_packages.add("javax.servlet.jsp");
    _jspx_imports_classes = null;
  }

  private volatile javax.el.ExpressionFactory _el_expressionfactory;
  private volatile org.apache.tomcat.InstanceManager _jsp_instancemanager;

  public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
    return _jspx_dependants;
  }

  public java.util.Set<java.lang.String> getPackageImports() {
    return _jspx_imports_packages;
  }

  public java.util.Set<java.lang.String> getClassImports() {
    return _jspx_imports_classes;
  }

  public javax.el.ExpressionFactory _jsp_getExpressionFactory() {
    if (_el_expressionfactory == null) {
      synchronized (this) {
        if (_el_expressionfactory == null) {
          _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
        }
      }
    }
    return _el_expressionfactory;
  }

  public org.apache.tomcat.InstanceManager _jsp_getInstanceManager() {
    if (_jsp_instancemanager == null) {
      synchronized (this) {
        if (_jsp_instancemanager == null) {
          _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
        }
      }
    }
    return _jsp_instancemanager;
  }

  public void _jspInit() {
  }

  public void _jspDestroy() {
  }

  public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
      throws java.io.IOException, javax.servlet.ServletException {

    if (!javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
      final java.lang.String _jspx_method = request.getMethod();
      if ("OPTIONS".equals(_jspx_method)) {
        response.setHeader("Allow","GET, HEAD, POST, OPTIONS");
        return;
      }
      if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method)) {
        response.setHeader("Allow","GET, HEAD, POST, OPTIONS");
        response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSP 只允许 GET、POST 或 HEAD。Jasper 还允许 OPTIONS");
        return;
      }
    }

    final javax.servlet.jsp.PageContext pageContext;
    javax.servlet.http.HttpSession session = null;
    final javax.servlet.ServletContext application;
    final javax.servlet.ServletConfig config;
    javax.servlet.jsp.JspWriter out = null;
    final java.lang.Object page = this;
    javax.servlet.jsp.JspWriter _jspx_out = null;
    javax.servlet.jsp.PageContext _jspx_page_context = null;


    try {
      response.setContentType("text/html; charset=UTF-8");
      pageContext = _jspxFactory.getPageContext(this, request, response,
      			null, true, 8192, true);
      _jspx_page_context = pageContext;
      application = pageContext.getServletContext();
      config = pageContext.getServletConfig();
      session = pageContext.getSession();
      out = pageContext.getOut();
      _jspx_out = out;

      out.write("\n");
      out.write("<!DOCTYPE html>\n");
      out.write("<html>\n");
      out.write("<head>\n");
      out.write("<meta charset=\"UTF-8\">\n");
      out.write("<title>Test jsp</title>\n");
      out.write("</head>\n");
      out.write("<body>\n");
      out.write("\t");
  
	int i  = 1;
	int j = 2;
	int k;
	k = i + j;
	out.println(k);
	
      out.write("\n");
      out.write("\tThe value of k is: ");
      out.print( k );
      out.write("\n");
      out.write("\t\n");
      out.write("\t");
      out.write('\n');
      out.write('	');

	int t = add(234, 123);
	
      out.write("\n");
      out.write("\tt's value is: ");
      out.print(t );
      out.write("\n");
      out.write("\t\n");
      out.write("\t");
 
	for(int p = 0;p<=5;p++){
	
      out.write("\n");
      out.write("\t<br> \n");
      out.write("\tThe value of p is: ");
      out.print(p );
      out.write('\n');
      out.write('	');
	
	}
	
      out.write("\n");
      out.write("</body>\n");
      out.write("</html>");
    } catch (java.lang.Throwable t) {
      if (!(t instanceof javax.servlet.jsp.SkipPageException)){
        out = _jspx_out;
        if (out != null && out.getBufferSize() != 0)
          try {
            if (response.isCommitted()) {
              out.flush();
            } else {
              out.clearBuffer();
            }
          } catch (java.io.IOException e) {}
        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
        else throw new ServletException(t);
      }
    } finally {
      _jspxFactory.releasePageContext(_jspx_page_context);
    }
  }
}

page directives

用于java程序的一些全局配置

1 2	<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8" import="java.util.Date"%>

Scopes in JSP and the PageContext object

JSP中可以直接使用类似servlet的request, response, session, context objects。

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
	<% String userName = request.getParameter("userName");
	application.setAttribute("applicationContextName", "defaultName");
	if(userName!=null){
		session.setAttribute("sessionName", userName);
		application.setAttribute("applicationContextName", userName);//application context object
		pageContext.setAttribute("pageContextName", userName);//page context object
	}
	
	%>
	<br>
	The user name in the request object is : <%= userName %>
	<br>
	The user name in the session object is : <%= session.getAttribute("sessionName") %>
	<br>
	The user name in the application context object is : <%= application.getAttribute("applicationContextName") %>
	<br>
	The user name in the page context object is : <%= pageContext.getAttribute("pageContextName") %>
</body>
</html>

Using jspInit and InitParams

JSP也可以配置initParameter或者重写jspInit()函数。

MVC

Servlet(Controller)
Bean(Model)
JSP(View)

我们这里的MVC数据由JSP(View层)直接返回给user，也可以是JSP返回给COntroller，Controller处理后再返回给user，这取决于MVC的设计。

jstl

jsp的缺点是要在html中夹杂很多java code，所以产生了jstl

#Java