一、JSP中的JAVA代碼塊：JSP小腳本
JSP的Java代碼塊也稱為JSP小腳本，是以<% %>括起來的部分，其中可以編寫Java代碼。編寫在Java代碼塊中的代碼，會被JSP翻譯引擎將其原樣的植入到Servlet的_jspService()方法中作為方法中的Java語句出現。所以，其中存放的必須是以分號結尾的Java語句。
需要注意的是，JSP代碼塊可以放置在JSP頁面的任意位置(頭部、主體、<html>標簽外)，可以放置任意數量。但它們都會被按照順序放置到Servlet的_jspService()方法中。也就是說，其執行順序是與其在JSP頁面中的出現順序是一致的。
二、由於JSP小腳本是被翻譯到Servlet的_jspService()方法中，所以JSP小腳本中是不能出現如下內容的：
2.1 聲明的變量是不能添加權限訪問控制符的。(Illegal modifier for parameter a; only final is permitted)
2.2 不能定義方法。
2.3 不能定義靜態語句塊。
三、示例
3.1 JSP源碼
<% System.out.println("我是Java代碼塊！我可以放在位置一！"); %> <%@ 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"> <% System.out.println("我是Java代碼塊！我可以放在位置二！"); %> <html> <head> <% int a=2; %> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>first_jsp</title> <% System.out.println("我是Java代碼塊！我可以放在位置三！"); %> </head> <% int b=5; %> <body> <!-- HTML注釋 --> <%--JSP注釋 --%> My First jsp! <% System.out.println("我是Java代碼塊！我可以放在位置四！"); %> </body> <% int num=a*b; System.out.println("num="+num); %> </html> <% System.out.println("我是Java代碼塊！我可以放在位置五！"); %>
3.2將上面的JSP頁面 經Tomcat遵循JSP規范將其翻譯為.java源文件內容如下：
/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/7.0.85 * Generated at: 2019-01-22 22:15:21 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 { 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 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 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 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; System.out.println("我是Java代碼塊！我可以放在位置一！"); out.write("\r\n"); 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"); System.out.println("我是Java代碼塊！我可以放在位置二！"); out.write("\r\n"); out.write("<html>\r\n"); out.write("<head>\r\n"); int a=2; out.write("\r\n"); out.write("<meta http-equiv=\"Content-Type\" content=\"text/html; charset=UTF-8\">\r\n"); out.write("<title>first_jsp</title>\r\n"); System.out.println("我是Java代碼塊！我可以放在位置三！"); out.write("\r\n"); out.write("</head>\r\n"); int b=5; out.write("\r\n"); out.write("<body>\r\n"); out.write("<!-- HTML注釋 -->\r\n"); out.write("\r\n"); out.write("My First jsp!\r\n"); System.out.println("我是Java代碼塊！我可以放在位置四！"); out.write("\r\n"); out.write("</body>\r\n"); int num=a*b; System.out.println("num="+num); out.write("\r\n"); out.write("</html>\r\n"); System.out.println("我是Java代碼塊！我可以放在位置五！"); } 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); } } }
3.3 JSP源碼和其遵循JSP規范被Tomcat翻譯成.java源文件的圖解
測試結果截圖：
jsp小腳本中是可以使用Java注釋的，原樣編譯。