1.JSP标签简介
JSP标签也称之为Jsp Action(JSP动作)元素，它用于在Jsp页面中提供业务逻辑功能，避免在JSP页面中直接编写java代码，造成jsp页面难以维护。
2.JSP常用标签
- <jsp:include>标签
- <jsp:forward>标签
- <jsp:param>标签
2.1 <jsp:include>标签
<jsp:include>标签用于把另外一个资源的输出内容插入进当前jsp页面的输出内容之中，这种在jsp页面执行时的引入方式称之为动态引入。即jsp:include指令为动态包含。如果被包含的页面是JSP，则先处理之后再将结果包含，而如果包含的是非*.jsp文件，则只是把文件内容静态包含进来，功能与@include类似。
语法：<jsp:include page="relativeURL | <%=expression%>" flush="true|false" </jsp:include>
page属性用于指定被引入资源的相对路径，它可以通过执行一个表达式来获得；flush属性则指定在插入其他资源的输出内容时，是否将当前的jsp页面已经输出的内容刷新到客户端。
如下所示的includeTest3.jsp代码：
<%@ 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代码，将@include指令改为<jsp:include page="includeTest3.jsp"></jsp:include>，运行结果同上，但是生成的新的includeTest1_jsp类的源码为：
/*
* 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('
');
out.write('
');
String path1 = request.getContextPath();
String basePath1 = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path1+"/";
out.write("
");
out.write("
");
out.write("<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
");
out.write("<html>
");
out.write(" <head>
");
out.write(" <base href="");
out.print(basePath1);
out.write("">
");
out.write("
");
out.write(" <title>My JSP 'includeTest.jsp' starting page</title>
");
out.write("
");
out.write(" <meta http-equiv="pragma" content="no-cache">
");
out.write(" <meta http-equiv="cache-control" content="no-cache">
");
out.write(" <meta http-equiv="expires" content="0">
");
out.write(" <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
");
out.write(" <meta http-equiv="description" content="This is my page">
");
out.write(" <!--
");
out.write(" <link rel="stylesheet" type="text/css" href="styles.css">
");
out.write(" -->
");
out.write("
");
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>
");
out.write("
");
out.write(" <body data-genuitec-lp-enabled="false" data-genuitec-file-id="wc1-8" data-genuitec-path="/MyWebProject/WebRoot/jspTest/includeTest1.jsp">
");
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>
");
out.write(" ");
org.apache.jasper.runtime.JspRuntimeLibrary.include(request, response, "includeTest3.jsp", out, false);
out.write("
");
out.write(" </body>
");
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);
}
}
}
从源码可以看出，includeTest1.jsp使用org.apache.jasper.runtime.JspRuntimeLibrary.include(request, response, "includeTest3.jsp", out, false);将includeTest3.jsp的动态内容包含进来，includeTest3.jsp的静态内容并没有包含进来。动态引入并不像@include一样生成一个Servlet源文件，而是生成两个Servlet源文件，然后通过org.apache.jasper.runtime.JspRuntimeLibrary.include(request, response, "includeTest3.jsp", out, false);方式将另一个Servlet的目标页面包含进来。
很显然，jsp：include标签是在当前jsp页面的执行期间插入被引入资源的输出内容，被动态引入的资源必须是一个能独立被WEB容器调用和执行的资源。动态包含就是指先将各个页面分别处理，处理之后再将处理之后的结果包含进来。而include指令的静态引入只能引入遵循jsp格式的文件，被引入文件与当前jsp文件共同被翻译成一个Servlet的源文件。静态包含就是将全部内容包含进来之后再进行处理，属于先包含后处理。
2.2 <jsp:forward>标签
<jsp:forward>标签用于把请求转发给另外一个资源。
语法：<jsp:forward page="relativeURL | <%=expression%>" <jsp:forward/>
page属性用于指定请求转发到的资源的相对路径，它也可以通过执行一个表达式来获得。
使用<jsp:forward>标签跳转页面，例如在includeTest1.jsp中，添加如下代码，
<jsp:forward page="/jspTest/includeTest2.jsp"></jsp:forward>
运行后输出的includeTest2.jsp输出的内容：
它的效果和下面的代码效果是一样的，
<%
request.getRequestDispatcher("/jspTest/includeTest2.jsp").forward(request,response);
%>
但是用jsp：forward可以使用jsp：param子标签向includeTest2.jsp传入一些参数，同样jsp：include也可以使用jsp：param子标签。
例如：
<jsp:forward page="/jspTest/includeTest2.jsp">
<jsp:param value="koala" name="username"/>
</jsp:forward>
这样在includeTest2.jsp中就可以通过request.getParameter("username")获取到传入的参数，
2.3 <jsp:param>标签
当使用<jsp:include>和<jsp:forward>标签引入或将请求转发给其它资源时，可以使用<jsp:param>标签向这个资源传递参数。
语法1：
<jsp:include page="relativeURL | <%=expression%>">
<jsp:param name="parameterName" value="parameterValue|<%= expression %>" />
</jsp:include>
语法2：
<jsp:forward page="relativeURL | <%=expression%>">
<jsp:param name="parameterName" value="parameterValue|<%= expression %>" />
</jsp:forward>
<jsp:param>标签的name属性用于指定参数名，value属性用于指定参数值。在<jsp:include>和<jsp:forward>标签中能使用多个<jsp:param>标签来传递多个参数。
3.关于中文乱码
3.1 在jsp页面上输入中文，请求后页面不出现乱码：保证contentType="text/html;charset="UTF-8"，pageEncoding="UTF-8" 中charset和pageEncoding的编码已知，且都支持中文，通常建议取值为UTF-8。还需保证浏览器的显示的字符编码和请求的jsp页面的编码一致。
3.2 获取中文参数值：默认参数在传输过程中使用的编码为ISO-8859-1
对于post请求：只要在获取请求信息之前，调用request.setCharacterEncoding("UTF-8")即可。
对于get请求：前面的方式对于get请求无效，可以通过修改Tomcat的server.xml文件的方式，为Connector节点添加useBodyEncodingForURI="true"属性即可。
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