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JSP基础语法

jsp表达式

<%--将程序的结果输出到客户端，可写变量或者表达式--%><%=new Date()%>

jsp脚本片段

<%--jsp脚本片段--%><%    int sum = 0;    for (int i = 0; i < 100; i++) {        sum += i;    }    out.println("
sum:" + sum + "
");%>

脚本片段的再实现

<%    for (int i = 0; i < 5; i++) {%>
这是第<%=i%>次展示
<%    }%>

jsp声明

<%!    int globalVar = 0;    static {        System.out.println("看看在哪...");    }    public void look(){        System.out.println("再看看在哪...");    }%>

看源码分析<% %> / <%! %>的区别(看注释的地方)

整体源码：index_jsp.java

/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/8.5.47 * Generated at: 2021-01-22 03:30:32 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.*;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 {   //<%! %>中声明的    /*int globalVar = 0;    static {        System.out.println("看看在哪...");    }    public void look(){        System.out.println("再看看在哪...");    }*/  private static final javax.servlet.jsp.JspFactory _jspxFactory =          javax.servlet.jsp.JspFactory.getDefaultFactory();  private static java.util.Map
   
     _jspx_dependants;  private static final java.util.Set
    
      _jspx_imports_packages;  private static final java.util.Set
     
       _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
      
        getDependants() {       return _jspx_dependants;  }  public java.util.Set
       
         getPackageImports() {       return _jspx_imports_packages;  }  public java.util.Set
        
          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("\n"); out.write("\n"); out.write("\n"); out.write("\n"); out.write("
         
Hello World!
\n"); out.write('\n'); out.print(new Date()); out.write('\n'); out.write('\n');//以下为<%= %>和<% %>中声明的 /*int sum = 0; for (int i = 0; i < 100; i++) { sum += i; } out.println("
         
sum:" + sum + "
"); out.write("\n"); out.write("\n"); out.write("
         
\n"); for (int i = 0; i < 5; i++) { out.write("\n"); out.write("
         
这是第"); out.print(i); out.write("次展示
\n"); } out.write("\n"); out.write("
         
\n"); out.write("\n"); out.write("\n"); out.write("\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); } }}
        
       
      
     
    
   

由此可知<%! %>中声明的是放在index_jsp类中，其他的声明放在_jspService方法中

<% %><%= %><%! %><%--注释--%>

注释

   <%--jsp注释--%>

如有不对的地方欢迎指出，大家共同进步！