JSP开始

在我们开始做web程序时，我们已经发现，在后端写前端代码是一件非常难受的事情，写着不方便，修改也不方便，有没有一个技术，可以把前后端分离开，让它帮我们去做这个前端代码翻译成后端的呢？ 这个技术就是JSP

当我们启动服务器时我们可以看到这样一个东西

• Dcatalina.base=/Users/leojackasher/Library/Caches/JetBrains/IntelliJIdea2023.2/tomcat/3e69f6af-7bb2-4fc9-aa67-46d855f11965
  当我们打开这个地址我们可以看到jsp里面会有帮我们翻译的前端代码
  
  + ```java
    /*
     * Generated by the Jasper component of Apache Tomcat
     * Version: Apache Tomcat/10.1.19
     * Generated at: 2024-03-16 06:10:37 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 jakarta.servlet.*;
    import jakarta.servlet.http.*;
    import jakarta.servlet.jsp.*;
    
    public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase
        implements org.apache.jasper.runtime.JspSourceDependent,
                     org.apache.jasper.runtime.JspSourceImports,
                     org.apache.jasper.runtime.JspSourceDirectives {
    
      private static final jakarta.servlet.jsp.JspFactory _jspxFactory =
              jakarta.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.LinkedHashSet<>(3);
        _jspx_imports_packages.add("jakarta.servlet");
        _jspx_imports_packages.add("jakarta.servlet.http");
        _jspx_imports_packages.add("jakarta.servlet.jsp");
        _jspx_imports_classes = null;
      }
    
      private volatile jakarta.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 boolean getErrorOnELNotFound() {
        return false;
      }
    
      public jakarta.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 jakarta.servlet.http.HttpServletRequest request, final jakarta.servlet.http.HttpServletResponse response)
          throws java.io.IOException, jakarta.servlet.ServletException {
    
        if (!jakarta.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 jakarta.servlet.jsp.PageContext pageContext;
        jakarta.servlet.http.HttpSession session = null;
        final jakarta.servlet.ServletContext application;
        final jakarta.servlet.ServletConfig config;
        jakarta.servlet.jsp.JspWriter out = null;
        final java.lang.Object page = this;
        jakarta.servlet.jsp.JspWriter _jspx_out = null;
        jakarta.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("    <title>JSP - Hello World</title>\n");
          out.write("</head>\n");
          out.write("<body>\n");
          out.write("<h1>");
          out.print( "Hello World!" );
          out.write("\n");
          out.write("</h1>\n");
          out.write("<br/>\n");
          out.write("<a href=\"hello-servlet\">Hello Servlet</a>\n");
          out.write("</body>\n");
          out.write("</html>");
        } catch (java.lang.Throwable t) {
          if (!(t instanceof jakarta.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);
        }
      }
    }
  
  

• index.jsp会被服务器翻译生产java文件，编译成class文件执行

• 仔细观看该文件第一个类，继承的HttpJspBase

• public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase
  我们可以打开源码看看，这个HttpJspbase到底是什么，
  
  + ```java
    public abstract class HttpJspBase extends HttpServlet implements HttpJspPage 
    //原来最终还是继承的HttpServlet，依旧是一个servlet对象 
  
  
  
  我们再回到JSP.java文件
  
  out.write("\n");
        out.write("<!DOCTYPE html>\n");
        out.write("<html>\n");
        out.write("<head>\n");
        out.write("    <title>JSP - Hello World</title>\n");
        out.write("</head>\n");
        out.write("<body>\n");
        out.write("<h1>");
        out.print( "Hello World!" );
        out.write("\n");
        out.write("</h1>\n");
        out.write("<br/>\n");
        out.write("<a href=\"hello-servlet\">Hello Servlet</a>\n");
        out.write("</body>\n");
        out.write("</html>");
  //这是核心打印的地方，所以jsp文件里面都会打印出来
  
  

JSP内置对象和Page使用

前面我们有使用过<%@page session=“false”%>,用于关闭JSP自动创建session对象，那么这样的属性还有哪些？

• <%@page errorPage=""%>
  //当jsp出现异常时，自动跳转页面
  + ```jsp
    <%@page iserrorPage="true"%>
    //将报错信息输出到命令台
  
  

• <%@page conentText="text/html; charset=utf-8"%>
  

我们已经用了几个对象了,request, response,session,exception,printWriter,

还有pageContent,config,servletContext，page