JSP中的聲明語句塊和表達式塊
阿新 • • 發佈:2019-01-23
建議 import lse cto exp contex ssis express private
一、JSP中的聲明語句塊
在JSP頁面中使用<%!%>括起來的部分,稱為聲明語句塊。聲明語句塊中的內容,將被JSP引擎翻譯到Servlet的類體中,是不會被包含到某個方法中的。
這樣的話,在JSP的聲明語句塊中就可以聲明實例變量、實例方法、靜態方法、靜態代碼塊等內容。並且,這些內容均可被JSP的的Java代碼塊中的代碼訪問。因為可以訪問類屬性的。不過,還是不建議在JSP的聲明語句塊中聲明實例變量。因為JSP是運行在單例多線程環境下的,實例變量將會引起線程安全問題。
需要註意的是,在JSP的聲明語句塊中,是不能編寫某些普通的Java語句的(比如System.out.println("");)。因為這些代碼最終會被放置在Java類體當中的,而不屬於某個方法,所以類體中不能出現諸如:System.out.println("");的java語句的。
二、示例
2.1 jsp源碼
<%@ 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"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>JSP_Declaration(JSP聲明)</title> </head> <body> <!-- 我是HTML註釋,我註釋的內容是會被發送到客戶端的 ,可以右鍵源碼的形式查看我所註釋的內容。--> <%--我是JSP註釋,我註釋的內容不會被發送到客戶端的,客戶端是不可以通過右鍵源碼的形式查看我所註釋內容的。--%> jsp聲明! <%! //聲明一個靜態私有的類變量a private static int a=11; /** 聲明一個方法 */ private staticint getNum(int a,int b){ return a+b; } %> </body> <% /*結果*/ System.out.println("getNum(3,5)的返回值為:"+getNum(3,5)); %> </html>
2.2 jsp引擎翻譯後的.java源文件如下:
/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/7.0.85 * Generated at: 2019-01-22 23:04:10 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 Declaration_jsp extends org.apache.jasper.runtime.HttpJspBase implements org.apache.jasper.runtime.JspSourceDependent { //聲明一個靜態私有的類變量a private static int a=11; /** 聲明一個方法 */ private static int getNum(int a,int b){ return a+b; } 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; 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"); out.write("<html>\r\n"); out.write("<head>\r\n"); out.write("<meta http-equiv=\"Content-Type\" content=\"text/html; charset=UTF-8\">\r\n"); out.write("<title>JSP_Declaration(JSP聲明)</title>\r\n"); out.write("</head>\r\n"); out.write("<body>\r\n"); out.write("<!-- 我是HTML註釋,我註釋的內容是會被發送到客戶端的 ,可以右鍵源碼的形式查看我所註釋的內容。-->\r\n"); out.write("\r\n"); out.write("jsp聲明!\r\n"); out.write("\r\n"); out.write("</body>\r\n"); /*結果*/ System.out.println("getNum(3,5)的返回值為:"+getNum(3,5)); out.write("\r\n"); out.write("</html>"); } 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); } } }
2.3 對比圖
三、JSP中的表達式塊
在JSP頁面中使用<%= %>括起來的部分,稱為表達式塊。其可在JSP頁面中輸出變量、常量及它們組成的各種表達式的值。註意是表達式而不是語句,是不能有分號的。該表達式將被JSP引擎翻譯到_jspService()方法的out.write()中輸出。
3.1示例
<%@ 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"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>JSP_Declaration(JSP聲明)</title> </head> <body> <!-- 我是HTML註釋,我註釋的內容是會被發送到客戶端的 ,可以右鍵源碼的形式查看我所註釋的內容。--> <%--我是JSP註釋,我註釋的內容不會被發送到客戶端的,客戶端是不可以通過右鍵源碼的形式查看我所註釋內容的。--%> jsp聲明! <%! //聲明一個靜態私有的類變量a private static int a=11; /** 聲明一個方法 */ private static int getNum(int a,int b){ return a+b; } %>
<br/> a+2的值為:<%=a+2 %> </body> <% /*結果*/ System.out.println("getNum(3,5)的返回值為:"+getNum(3,5)); %> </html>
結果截圖如下:
JSP中的聲明語句塊和表達式塊