jsp中java代码块_jsp小脚本

一、JSP中的JAVA代码块：JSP小脚本　　

　　JSP的Java代码块也称为JSP小脚本，是以<% %>括起来的部分，其中可以编写Java代码。编写在Java代码块中的代码，会被JSP翻译引擎将其原样的植入到Servlet的_jspService()方法中作为方法中的Java语句出现。所以，其中存放的必须是以分号结尾的Java语句。
　　需要注意的是，JSP代码块可以放置在JSP页面的任意位置(头部、主体、<html>标签外)，可以放置任意数量。但它们都会被按照顺序放置到Servlet的_jspService()方法中。也就是说，其执行顺序是与其在JSP页面中的出现顺序是一致的。
二、由于JSP小脚本是被翻译到Servlet的_jspService()方法中，所以JSP小脚本中是不能出现如下内容的：
　　2.1 声明的变量是不能添加权限访问控制符的。(Illegal modifier for parameter a; only final is permitted)
　　2.2 不能定义方法。
　　2.3 不能定义静态语句块。

三、示例

　　3.1 JSP源码

<% System.out.println("我是Java代码块！我可以放在位置一！"); %>
<%@ 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">
<% System.out.println("我是Java代码块！我可以放在位置二！"); %>
<html>
<head>
<% int a=2; %>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>first_jsp</title>
<% System.out.println("我是Java代码块！我可以放在位置三！"); %>
</head>
<% int b=5; %>
<body>
<!-- HTML注释 -->
<%--JSP注释 --%>
My First jsp!
<% System.out.println("我是Java代码块！我可以放在位置四！"); %>
</body>
<%
int num=a*b;
System.out.println("num="+num); 
%>
</html>
<% System.out.println("我是Java代码块！我可以放在位置五！"); %>

　　3.2将上面的JSP页面经Tomcat遵循JSP规范将其翻译为.java源文件内容如下：

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.85
 * Generated at: 2019-01-22 22:15:21 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 index_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {

  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;

 System.out.println("我是Java代码块！我可以放在位置一！"); 
      out.write("
");
      out.write("
");
      out.write("<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
");
 System.out.println("我是Java代码块！我可以放在位置二！"); 
      out.write("
");
      out.write("<html>
");
      out.write("<head>
");
 int a=2; 
      out.write("
");
      out.write("<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
");
      out.write("<title>first_jsp</title>
");
 System.out.println("我是Java代码块！我可以放在位置三！"); 
      out.write("
");
      out.write("</head>
");
 int b=5; 
      out.write("
");
      out.write("<body>
");
      out.write("<!-- HTML注释 -->
");
      out.write("
");
      out.write("My First jsp!
");
 System.out.println("我是Java代码块！我可以放在位置四！"); 
      out.write("
");
      out.write("</body>
");

int num=a*b;
System.out.println("num="+num); 

      out.write("
");
      out.write("</html>
");
 System.out.println("我是Java代码块！我可以放在位置五！"); 
    } 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);
    }
  }
}

　　3.3 JSP源码和其遵循JSP规范被Tomcat翻译成.java源文件的图解

　　测试结果截图：

　　jsp小脚本中是可以使用Java注释的，原样编译。

jsp中java代码块_jsp小脚本

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