Jsp和Servlet关系

为什么会出现Jsp？

其实对于服务器来说它只认识Servlet，我们完全可以在Servlet用resp.getWriter().write("");画出网页的界面，但是仅仅一个很简单的界面就要重复的书写resp.getWriter().write("")，并且这还没有加上js、css以及jquery。

但是我们在Servlet写的是java代码可以有逻辑的判断，但是在html、css中无法加入逻辑导致页面是静态的。为了解决这一问题出现了jsp，在jsp中可以书写Java代码、js代码、html代码等。

我们都知道浏览器发送请求到服务器，服务器通过解析其中Servlet的别名找到对应的Servlet并执行其中的service方法做出处理和响应，那么对于服务器来说它只认识Servlet，那么我们编辑的Jsp到底是如何被服务器识别的？

实际上服务器还是将.jsp当作Servlet的别名在你局部配置下的web.xml找有没有对应的Servlet，如果没有找到对应的Servlet就去Tomcat的公共配置下的web.xml（Tomcat安装目录conf下的web.xml）找对应的Servlet。

会找到如下内容：

org.apache.jasper.servlet.JspServlet将对应的jsp文件转为Servlet。

那么转的Servlet在什么地方呢？打开Tomcat下的work文件夹一直点到：找你正在运行的项目的jsp

测试的jsp：

<%@ page language="java" contentType="text/html; charset=utf-8"
    pageEncoding="utf-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
666
</body>
</html>

被编译好的Java文件：

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/8.5.46
 * Generated at: 2019-10-16 09:05:10 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 index_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent,
                 org.apache.jasper.runtime.JspSourceImports {

  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 {

    final java.lang.String _jspx_method = request.getMethod();
    if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
      response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSPs only permit GET POST or HEAD");
      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=ISO-8859-1");
      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("\r\n");
      out.write("<!DOCTYPE html PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\" \"http://www.w3.org/TR/html4/loose.dtd\">\r\n");
      out.write("<html>\r\n");
      out.write("<head>\r\n");
      out.write("<meta http-equiv=\"Content-Type\" content=\"text/html; charset=ISO-8859-1\">\r\n");
      out.write("<title>Insert title here</title>\r\n");
      out.write("</head>\r\n");
      out.write("<body>\r\n");
      out.write("666\r\n");
      out.write("</body>\r\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);
    }
  }
}

但是我们发现这个类并没有继承HttpServlet而是继承了org.apache.jasper.runtime.HttpJspBase并且里面也没有service方法而是_jspservice方法。那么Tomcat就会到它的父类中找service方法并进行调用。

实际上它的父类org.apache.jasper.runtime.HttpJspBase继承了HttpServlet。

父类中Service方法就是调用了子类的_jspservice方法（所以说实际上Tomcat执行的就是_jspservice方法）

总结：实际上jsp的本质还是servlet，只不过是为了方面花界面所单独拉出来的一部分而已，到最终的实现还是会回到Servlet当中。

展望：jsp已经是一门比较落后的技术来，因为html、css、js是前端写的，而后端要在其中加入逻辑判断，那么首先你一定得认识前端语言的代码，否则你如何嵌套？这样很不利于前后端的分离。之所以学习是因为很多框架和项目中依然会用到它，但是最终还是会被时代所淘汰。

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