JSP基础 web服务器如何访问 jsp 页面

时间 2020-07-03 标签 jsp 基础 web 服务器如何访问 jsp 页面

JSP

JSP全名为Java Server Pages，中文名叫 java服务器页面，其简单来讲就是一个servlet 。
JSP能够跨平台使用，既能运行在Linux操做系统上，也能够运行在Windows操做平台上。
在传统的HTML文件中，加入了java代码。
如何在jsp中写入java代码呢？

1.得到变量的值  <%=new Date()%>>
  2. 定义局部变量 ：写一段java代码
  严格遵照java规范
  <%
   java代码
  %>
  3.全局变量 <%!  全局变量或方法  %>
  EL表达式：${} 只能输出值 不能输出对象

<%
//局部变量
  String name = "hyf";
  int age = 18;
  int j = 0;
  Date date = new Date();
%>
<%!
  //全局变量，跨session执行
  int i = 0;
%>
<html>
  <head>
    <title>$Title$</title>
  </head>
  <body>
<h1>
  日期:<%=new Date()%>
  年龄:<%=age%>
  名字:<%=name%>
  i:<%=i++%>
  j：<%=j++%>
</h1>

咱们是如何经过jsp文件访问服务器的呢？
（1）首先，当用户请求一个jsp文件时，其实就是在执行servlet，服务器会先把 jsp 转化为servlet，因此咱们访问jsp时，其实并非访问jsp，而是在访问jsp翻译过的servlet；好比如下这段jsp代码

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

<%--
  1.得到变量的值  <%=new Date()%>>
  2. 定义局部变量 ：写一段java代码
  严格遵照java规范
  <%
   java代码
  %>
  3.全局变量 <%!  全局变量或方法  %>
  EL表达式：${} 只能输出值 不能输出对象
--%>
<%
  String name = "hyf";
  int age = 18;
  int j = 0;
  Date date = new Date();
%>
<%!
  //跨session执行
  int i = 0;
%>
<html>
  <head>
    <title>$Title$</title>
  </head>
  <body>
<h1>
  日期:<%=new Date()%>
  年龄:<%=age%>
  名字:<%=name%>
  i:<%=i++%>
  j：<%=j++%>
</h1>
  </body>
</html>

（2）当咱们经过浏览器访问 index.jsp 时，服务器会首先将index.jsp翻译成 index_jsp.class,咱们能够访问index_jsp.java文件
html

/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/9.0.20 * Generated at: 2019-07-16 04:33:44 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.*;
import java.util.Date;

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


  //跨session执行
  int i = 0;

  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 = new java.util.HashSet<>();
    _jspx_imports_classes.add("java.util.Date");
  }

  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 只容许 GET、POST 或 HEAD。Jasper 还容许 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("\n");
      out.write("\n");
      out.write("\n");
      out.write("\n");
      out.write('\n');

  String name = "hyf";
  int age = 18;
  int j = 0;
  Date date = new Date();

      out.write('\n');
      out.write("\n");
      out.write("<html>\n");
      out.write(" <head>\n");
      out.write(" <title>$Title$</title>\n");
      out.write(" </head>\n");
      out.write(" <body>\n");
      out.write("<h1>\n");
      out.write(" 日期:");
      out.print(new Date());
      out.write("\n");
      out.write(" 年龄:");
      out.print(age);
      out.write("\n");
      out.write(" 名字:");
      out.print(name);
      out.write("\n");
      out.write(" i:");
      out.print(i++);
      out.write("\n");
      out.write(" j：");
      out.print(j++);
      out.write("\n");
      out.write("\n");
      out.write("\n");
      out.write("\n");
      out.write("</h1>\n");
      out.write(" </body>\n");
      out.write("</html>\n");
    } 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);
    }
  }
}

咱们能够看到 index_jsp.java 是继承类 HttpJspBase 这个类，而 HttpJspBase 这个类是继承 Serlvet 这个类的。
在web页面输出100个Hello World

<%for (int i = 0;i < 100;i++){%>
    <h1>Hello World!</h1>
    <%}%>