【Head First Servlets and JSP】笔记15：建立一个JSP页面来显示被访问了多少次

1、这是一个非常简单的程序，它看起来是这个样子的：

实际功能就是，每访问该页面一次count数加1，在服务器重启前（JVM重启前），这个次数将持续累加。

2、因为这个程序过于简单，所以我希望可以通过HTML内置Java来完成。

这样做的好处就是：可以把写servlet的事情交给web容器来做。

3、代码如下：

package Sample;

public class Counter {
    private static int count;
    public static int getCount() {
        ++count;
        return  count;
    }
}

BasicCounter.jsp

<%@ page import="Sample.Counter" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Basic Counter</title>
</head>
<body>
    <p>The page count is: </p>
    <%
        out.println(Counter.getCount());
    %>
</body>
</html>

web.xml使用默认的即可

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">
</web-app>

4、启动服务器后，输入BasicCounter.jsp的绝对路径即可访问。

5、println是servlet才用的语句！在JSP中还可以改进代码，如下

<%@ page import="Sample.Counter" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Basic Counter</title>
</head>
<body>
    <p>The page count is: </p>
    <%=Counter.getCount()%>
</body>
</html>

这是一种等价写法，事实上web容器在把JSP转换成.java时，还是会把它改写为out.println...

还可以这么玩 ↓

<%@ page import="Sample.HelloWorld" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>Simple jsp page</title>
  </head>
  <body>
    <h3 class="message"><%=HelloWorld.getMessage()%></h3>
    <%=Math.random()%>
  </body>
</html>

每次访问这个JSP都会获得一个新的随机数。

6、目前为止，我们已经看到了三种不同形态的JSP元素：

<%=Counter.getCount()%>

<%@ page import="Sample.Counter" %>

 <%
        out.println(Counter.getCount());
    %>

7、实际上，我们甚至不需要创建额外的cOUNTER类，我们尝试一下这段代码

<%@ page import="Sample.Counter" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Basic Counter</title>
</head>
<body>
    <%
        int count = 0;
    %>
    <p>The page count is: </p>
    <%= ++count %>
</body>
</html>

这段代码是错的。结果很容易猜到：count的结果永远是1，因为每调用一次页面count都会被重新初始化一次。

要想知道容器到底对你的JSP做了什么，

唯一的办法就是去产看源码。

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.77
 * Generated at: 2017-06-14 08:18:57 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.*;
import Sample.Counter;

public final class BasicCounter_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;

      out.write("\r\n");
      out.write("\r\n");
      out.write("<html>\r\n");
      out.write("<head>\r\n");
      out.write("    <title>Basic Counter</title>\r\n");
      out.write("</head>\r\n");
      out.write("<body>\r\n");

    int count = 0;

      out.write("\r\n");
      out.write("<p>The page count is: </p>\r\n");
      out.print( ++count );
      out.write("\r\n");
      out.write("</body>\r\n");
      out.write("</html>\r\n");
    } 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);
    }
  }
}

8、在实际应用中，统计自身被调用多少次并不是 servlet 的职责，可以通过过滤器或者 AOP 切面实现。

posted @ 2017-06-14 15:33 xkfx 阅读(566) 评论(0) 收藏举报

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【Head First Servlets and JSP】笔记15：建立一个JSP页面来显示被访问了多少次

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