JSP&Servlet学习笔记----第6章

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JSP与Servlet是一体两面的关系。

JSP最终还是被编译为Servlet。

<%@page contentType="text/html;charset=UTF-8" language="java" %><html>  <head>    <title>测试</title>  </head><body>测试</body></html>

上面的jsp文件最终被转译为Servlet：

/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/9.0.0.M15 * Generated at: 2017-03-02 01:55:32 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 one_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=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("\r\n");      out.write("<html>\r\n");      out.write("  <head>\r\n");      out.write("    <title>测试</title>\r\n");      out.write("  </head>\r\n");      out.write("<body>\r\n");      out.write("测试\r\n");      out.write("</body>\r\n");      out.write("</html>\r\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);    }  }}

第一次请求JSP时，容器会进行转译、编译和加载的操作，所以第一次请求jsp，响应会很慢。
如果想做JSP初始化或者收尾操作，则可以定义jspInit()或jspDestroy()方法。

指示元素：

<%@ 指示类型 [属性="值"]* %>
指示类型有：page、include、taglib

在<%!与%>之间声明的代码都将转译为Servlet中的类成员与方法。

在<%与%>之间声明的代码（不能声明方法）转译为_jspService()方法中的代码。

在此记录个bug：

<%@page contentType="text/html;charset=UTF-8" language="java" %><%@page import="java.util.*"%><%@page errorPage="errPage.jsp"%><html>  <head>    <title>测试</title>  </head><%!int x = 0;int z = Integer.parseInt("123qwe");%><body></body></html>

并没有跳转到errPage.jsp界面?

对应的Servlet

/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/9.0.0.M15 * Generated at: 2017-03-02 07:50:01 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.*;public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase    implements org.apache.jasper.runtime.JspSourceDependent,                 org.apache.jasper.runtime.JspSourceImports {int x = 0;int z = Integer.parseInt("123qwe");  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("java.util");    _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=UTF-8");      pageContext = _jspxFactory.getPageContext(this, request, response,      "errPage.jsp", 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("<html>\n");      out.write("  <head>\n");      out.write("    <title>测试</title>\n");      out.write("  </head>\n");      out.write(" \n");      out.write(" \n");      out.write("<!--\n");      out.write("HTML注释\n");      out.write("-->\n");      out.write("\n");      out.write("\n");      out.write("  \n");      out.write("\n");      out.write("  \n");      out.write("\n");      out.write("  \n");      out.write("\n");      out.write("  \n");      out.write("<body>\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);    }  }}

z变量初始化就导致异常产生了，大概根本就没有调用_jspService方法？

错误处理：

1、转译过程中

例如：<%@page buffer="16"%>

buffer属性必须指定单位，如16kb

2、编译过程中（unable to comple）

例如：Some some；没有部署Some类到服务器。

此时转译可以完成（生成*.java文件，不会生成*.class文件）。

3、运行时错误（找不到资源，程序逻辑错误等）（an exception occurred processing JSP page）

例如：NullPointerException异常

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