前面几个小节，已经把JSP程序的创建以及运行原理都介绍完了，这一小节开始介绍具体的JSP语法。JSP本质上是Servlet，Servlet是采用Java语言编写的，所以JSP语法也是要满足Java语法。
JSP是经过JSP引擎渲染成Servlet程序，那么渲染过程中，就需要知道渲染的规则，这些规则也就是我们需要学习的JSP基础语法，这一小节学习JSP程序代码片段的语法。
1.1、基础语法
什么是代码片段呢？？？这里的代码片段，其实就是指：在JSP文件中使用JSP的语法，编写Java代码片段。JSP代码片段的使用语法如下：
<%
在这里面编写Java代码
%>注意：JSP代码片段，会被嵌入到编译之后的_jspService()方法里面，所以使用JSP代码片段，就相当于是在_jspService()方法体里面编写Java代码。
案例代码如下：
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>JSP基础语法之代码段</title>
</head>
<body>
<h3>JSP基础语法之代码段</h3>
<%
// JSP基础语法之代码段
out.println("使用JSP代码片段输出的内容");
%>
</body>
</html>
运行结果如下图所示：
1.2、代码片段的本质
JSP代码片段的本质是什么呢？？？我们来看看上面JSP文件，编译之后对应的Java源代码：
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/8.5.98
* Generated at: 2024-02-24 10:32:43 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 com.gitcode.servlet;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
public final class fragment_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent,
org.apache.jasper.runtime.JspSourceImports {
private static final JspFactory _jspxFactory =
JspFactory.getDefaultFactory();
private static java.util.Map<String, Long> _jspx_dependants;
private static final java.util.Set<String> _jspx_imports_packages;
private static final java.util.Set<String> _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 = null;
}
private volatile javax.el.ExpressionFactory _el_expressionfactory;
private volatile org.apache.tomcat.InstanceManager _jsp_instancemanager;
public java.util.Map<String, Long> getDependants() {
return _jspx_dependants;
}
public java.util.Set<String> getPackageImports() {
return _jspx_imports_packages;
}
public java.util.Set<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 HttpServletRequest request, final HttpServletResponse response)
throws java.io.IOException, ServletException {
final String _jspx_method = request.getMethod();
if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !DispatcherType.ERROR.equals(request.getDispatcherType())) {
response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSP 只允许 GET、POST 或 HEAD。Jasper 还允许 OPTIONS");
return;
}
final PageContext pageContext;
HttpSession session = null;
final ServletContext application;
final ServletConfig config;
JspWriter out = null;
final Object page = this;
JspWriter _jspx_out = null;
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("<html>\r\n");
out.write("<head>\r\n");
out.write(" <title>JSP基础语法之代码段</title>\r\n");
out.write("</head>\r\n");
out.write("<body>\r\n");
out.write(" <h3>JSP基础语法之代码段</h3>\r\n");
out.write(" ");
// JSP基础语法之代码段
out.println("使用JSP代码片段输出的内容");
out.write("\r\n");
out.write("</body>\r\n");
out.write("</html>\r\n");
} catch (Throwable t) {
if (!(t instanceof 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);
}
}
}通过编译之后的源代码，可以看到，JSP代码片段中的内容会被嵌入到_jspService()方法里面。
到此，JSP程序代码片段就介绍完啦。
今天就到这里，未完待续~~