javaweb之jsp標簽
1.JSP標簽簡介
JSP標簽也稱之為Jsp Action(JSP動作)元素,它用於在Jsp頁面中提供業務邏輯功能,避免在JSP頁面中直接編寫java代碼,造成jsp頁面難以維護。
2.JSP常用標簽
- <jsp:include>標簽
- <jsp:forward>標簽
- <jsp:param>標簽
2.1 <jsp:include>標簽
<jsp:include>標簽用於把另外一個資源的輸出內容插入進當前jsp頁面的輸出內容之中,這種在jsp頁面執行時的引入方式稱之為動態引入。即jsp:include指令為動態包含。如果被包含的頁面是JSP,則先處理之後再將結果包含,而如果包含的是非*.jsp文件,則只是把文件內容靜態包含進來,功能與@include類似。
語法:<jsp:include page="relativeURL | <%=expression%>" flush="true|false" </jsp:include>
page屬性用於指定被引入資源的相對路徑,它可以通過執行一個表達式來獲得;flush屬性則指定在插入其他資源的輸出內容時,是否將當前的jsp頁面已經輸出的內容刷新到客戶端。
如下所示的includeTest3.jsp代碼:
<%@ page language="java" import="java.util.*" pageEncoding="ISO-8859-1"%> <% String path3 = request.getContextPath(); String basePath3 = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path3+"/"; %> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <base href="<%=basePath3%>"> <title>My JSP ‘includeTest3.jsp‘ starting page</title> <meta http-equiv="pragma" content="no-cache"> <meta http-equiv="cache-control" content="no-cache"> <meta http-equiv="expires" content="0"> <meta http-equiv="keywords" content="keyword1,keyword2,keyword3"> <meta http-equiv="description" content="This is my page"> <!-- <link rel="stylesheet" type="text/css" href="styles.css"> --> </head> <body> <h2>"includeTest3.jsp‘s content"</h2> </body> </html>
修改上篇中includeTest1.jsp代碼,將@include指令改為<jsp:include page="includeTest3.jsp"></jsp:include>,運行結果同上,但是生成的新的includeTest1_jsp類的源碼為:
/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/8.5.9 * Generated at: 2018-10-20 13:53:54 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.jspTest; import javax.servlet.*; import javax.servlet.http.*; import javax.servlet.jsp.*; import java.util.*; public final class includeTest1_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("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=ISO-8859-1"); 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‘); out.write(‘\n‘); String path1 = request.getContextPath(); String basePath1 = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path1+"/"; out.write("\r\n"); out.write("\r\n"); out.write("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\r\n"); out.write("<html>\r\n"); out.write(" <head>\r\n"); out.write(" <base href=\""); out.print(basePath1); out.write("\">\r\n"); out.write(" \r\n"); out.write(" <title>My JSP ‘includeTest.jsp‘ starting page</title>\r\n"); out.write(" \r\n"); out.write("\t<meta http-equiv=\"pragma\" content=\"no-cache\">\r\n"); out.write("\t<meta http-equiv=\"cache-control\" content=\"no-cache\">\r\n"); out.write("\t<meta http-equiv=\"expires\" content=\"0\"> \r\n"); out.write("\t<meta http-equiv=\"keywords\" content=\"keyword1,keyword2,keyword3\">\r\n"); out.write("\t<meta http-equiv=\"description\" content=\"This is my page\">\r\n"); out.write("\t<!--\r\n"); out.write("\t<link rel=\"stylesheet\" type=\"text/css\" href=\"styles.css\">\r\n"); out.write("\t-->\r\n"); out.write("\r\n"); out.write(" <script>\"undefined\"==typeof CODE_LIVE&&(!function(e){var t={nonSecure:\"51550\",secure:\"51555\"},c={nonSecure:\"http://\",secure:\"https://\"},r={nonSecure:\"127.0.0.1\",secure:\"gapdebug.local.genuitec.com\"},n=\"https:\"===window.location.protocol?\"secure\":\"nonSecure\";script=e.createElement(\"script\"),script.type=\"text/javascript\",script.async=!0,script.src=c[n]+r[n]+\":\"+t[n]+\"/codelive-assets/bundle.js\",e.getElementsByTagName(\"head\")[0].appendChild(script)}(document),CODE_LIVE=!0);</script></head>\r\n"); out.write(" \r\n"); out.write(" <body data-genuitec-lp-enabled=\"false\" data-genuitec-file-id=\"wc1-8\" data-genuitec-path=\"/MyWebProject/WebRoot/jspTest/includeTest1.jsp\">\r\n"); out.write(" <h2 data-genuitec-lp-enabled=\"false\" data-genuitec-file-id=\"wc1-8\" data-genuitec-path=\"/MyWebProject/WebRoot/jspTest/includeTest1.jsp\">\"includeTest1.jsp‘s content\"</h2>\r\n"); out.write(" "); org.apache.jasper.runtime.JspRuntimeLibrary.include(request, response, "includeTest3.jsp", out, false); out.write("\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); } } }
從源碼可以看出,includeTest1.jsp使用org.apache.jasper.runtime.JspRuntimeLibrary.include(request, response, "includeTest3.jsp", out, false);將includeTest3.jsp的動態內容包含進來,includeTest3.jsp的靜態內容並沒有包含進來。動態引入並不像@include一樣生成一個Servlet源文件,而是生成兩個Servlet源文件,然後通過org.apache.jasper.runtime.JspRuntimeLibrary.include(request, response, "includeTest3.jsp", out, false);方式將另一個Servlet的目標頁面包含進來。
很顯然,jsp:include標簽是在當前jsp頁面的執行期間插入被引入資源的輸出內容,被動態引入的資源必須是一個能獨立被WEB容器調用和執行的資源。動態包含就是指先將各個頁面分別處理,處理之後再將處理之後的結果包含進來。而include指令的靜態引入只能引入遵循jsp格式的文件,被引入文件與當前jsp文件共同被翻譯成一個Servlet的源文件。靜態包含就是將全部內容包含進來之後再進行處理,屬於先包含後處理。
2.2 <jsp:forward>標簽
<jsp:forward>標簽用於把請求轉發給另外一個資源。
語法:<jsp:forward page="relativeURL | <%=expression%>" <jsp:forward/>
page屬性用於指定請求轉發到的資源的相對路徑,它也可以通過執行一個表達式來獲得。
使用<jsp:forward>標簽跳轉頁面,例如在includeTest1.jsp中,添加如下代碼,
<jsp:forward page="/jspTest/includeTest2.jsp"></jsp:forward>
運行後輸出的includeTest2.jsp輸出的內容:
它的效果和下面的代碼效果是一樣的,
<% request.getRequestDispatcher("/jspTest/includeTest2.jsp").forward(request,response); %>
但是用jsp:forward可以使用jsp:param子標簽向includeTest2.jsp傳入一些參數,同樣jsp:include也可以使用jsp:param子標簽。
例如:
<jsp:forward page="/jspTest/includeTest2.jsp"> <jsp:param value="koala" name="username"/> </jsp:forward>
這樣在includeTest2.jsp中就可以通過request.getParameter("username")獲取到傳入的參數,
2.3 <jsp:param>標簽
當使用<jsp:include>和<jsp:forward>標簽引入或將請求轉發給其它資源時,可以使用<jsp:param>標簽向這個資源傳遞參數。
語法1:
<jsp:include page="relativeURL | <%=expression%>"> <jsp:param name="parameterName" value="parameterValue|<%= expression %>" /> </jsp:include>
語法2:
<jsp:forward page="relativeURL | <%=expression%>"> <jsp:param name="parameterName" value="parameterValue|<%= expression %>" /> </jsp:forward>
<jsp:param>標簽的name屬性用於指定參數名,value屬性用於指定參數值。在<jsp:include>和<jsp:forward>標簽中能使用多個<jsp:param>標簽來傳遞多個參數。
javaweb之jsp標簽