一、JSP中的声明语句块
在JSP页面中使用<%！%>括起来的部分，称为声明语句块。声明语句块中的内容，将被JSP引擎翻译到Servlet的类体中，是不会被包含到某个方法中的。
这样的话，在JSP的声明语句块中就可以声明实例变量、实例方法、静态方法、静态代码块等内容。并且，这些内容均可被JSP的的Java代码块中的代码访问。因为可以访问类属性的。不过，还是不建议在JSP的声明语句块中声明实例变量。因为JSP是运行在单例多线程环境下的，实例变量将会引起线程安全问题。
需要注意的是，在JSP的声明语句块中，是不能编写某些普通的Java语句的(比如System.out.println("");)。因为这些代码最终会被放置在Java类体当中的，而不属于某个方法，所以类体中不能出现诸如：System.out.println("");的java语句的。
二、示例
2.1 jsp源码
<%@ 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声明)</title></head><body><!-- 我是HTML注释，我注释的内容是会被发送到客户端的 ,可以右键源码的形式查看我所注释的内容。--><%--我是JSP注释，我注释的内容不会被发送到客户端的,客户端是不可以通过右键源码的形式查看我所注释内容的。--%>jsp声明！<%!//声明一个静态私有的类变量aprivate static int a=11;/**声明一个方法*/private static int getNum(int a,int b){ ???return ?a+b;}%></body><% ???/*结果*/ ???System.out.println("getNum(3,5)的返回值为："+getNum(3,5));%></html>2.2 jsp引擎翻译后的.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 {//声明一个静态私有的类变量aprivate static int a=11;/**声明一个方法*/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声明)</title>\r\n"); ?????out.write("</head>\r\n"); ?????out.write("<body>\r\n"); ?????out.write("<!-- 我是HTML注释，我注释的内容是会被发送到客户端的 ,可以右键源码的形式查看我所注释的内容。-->\r\n"); ?????out.write("\r\n"); ?????out.write("jsp声明！\r\n"); ?????out.write("\r\n"); ?????out.write("</body>\r\n"); ???/*结果*/ ???System.out.println("getNum(3,5)的返回值为："+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 对比图
三、JSP中的表达式块
在JSP页面中使用<%= %>括起来的部分，称为表达式块。其可在JSP页面中输出变量、常量及它们组成的各种表达式的值。注意是表达式而不是语句，是不能有分号的。该表达式将被JSP引擎翻译到_jspService()方法的out.write()中输出。
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声明)</title></head><body><!-- 我是HTML注释，我注释的内容是会被发送到客户端的 ,可以右键源码的形式查看我所注释的内容。--><%--我是JSP注释，我注释的内容不会被发送到客户端的,客户端是不可以通过右键源码的形式查看我所注释内容的。--%>jsp声明！<%!//声明一个静态私有的类变量aprivate static int a=11;/**声明一个方法*/private static int getNum(int a,int b){ ???return ?a+b;}%>
<br/>a+2的值为：<%=a+2 %></body><% ???/*结果*/ ???System.out.println("getNum(3,5)的返回值为："+getNum(3,5));%></html>结果截图如下：
JSP中的声明语句块和表达式块
原文地址：https://www.cnblogs.com/aeon/p/10306999.html