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声明的语法。
1.1、JSP声明基础语法
JSP声明，是指：在JSP文件中，使用JSP的声明语法，定义Java成员变量、定义Java方法。也就是说，JSP声明是在编译之后的JSP程序源代码的类里面，嵌入定义的Java变量、嵌入定义的Java方法。语法规则如下所示：
<%!
在这里定义变量、定义方法
%>注意：必须先定义JSP声明，后续才能使用定义的JSP声明。
案例代码如下所示：
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>JSP基础语法之JSP声明</title>
</head>
<body>
<h3>JSP基础语法之JSP声明</h3>
<%!
// 在这里定义Java变量
int i = 0;
// 在这里定义Java方法
public void demoMethod() {
System.out.println("JSP基础语法之JSP声明");
}
%>
<%
// 使用定义的变量
System.out.println("JSP基础语法之JSP声明的变量i=" + i);
// 使用定义的方法
demoMethod();
%>
</body>
</html>启动工程，浏览器访问http://localhost:8080/servlet/demo.jsp，IDEA控制台可以看到如下日志：
1.2、JSP声明的本质
JSP声明，你可以理解成：就是在JSP编译之后的Servlet程序中，在该类里面定义Java变量、定义Java方法、定义代码块。JSP声明中的代码，最终会被嵌入到JSP编译生成的Servlet类里面。查看上面JSP文件编译之后的源代码：
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/8.5.98
* Generated at: 2024-02-24 11:31:26 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 demo_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent,
org.apache.jasper.runtime.JspSourceImports {
// 在这里定义Java变量
int i = 0;
// 在这里定义Java方法
public void demoMethod() {
System.out.println("JSP基础语法之JSP声明");
}
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JSP声明</title>\r\n");
out.write("</head>\r\n");
out.write("<body>\r\n");
out.write(" <h3>JSP基础语法之JSP声明</h3>\r\n");
out.write(" ");
out.write("\r\n");
out.write("\r\n");
out.write(" ");
// 使用定义的变量
System.out.println("JSP基础语法之JSP声明的变量i=" + i);
// 使用定义的方法
demoMethod();
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);
}
}
}通过上面源代码可以看到，JSP声明中定义的代码，最终会被嵌入到类的最前面位置，如下所示：
到此，JSP声明的语法就介绍完啦。
今天就到这里，未完待续~~