◎ Scripting Elements 描述語言元素 ◎

JSP的Scripting Elements 包含三部份: 

1. Scriptlets (程式碼區段):以<% 與 %>包括的元素

 

<html><body><center>
This is Test App<br>
<%
    String str1;
    int int1;
%>

<%
    str1 = "This is a aaa!";
    int1 = 10;
%>
Output str1 is: <%=str1%> <br>
Output int1 is: <%=int1%> <br>
</center></body></html>
package org.apache.jsp;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;

public final class TestApp_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {

  private static java.util.List _jspx_dependants;

  public Object getDependants() {
    return _jspx_dependants;
  }

  public void _jspService(HttpServletRequest request, HttpServletResponse response)
        throws java.io.IOException, ServletException {

    JspFactory _jspxFactory = null;
    PageContext pageContext = null;
    HttpSession session = null;
    ServletContext application = null;
    ServletConfig config = null;
    JspWriter out = null;
    Object page = this;
    JspWriter _jspx_out = null;
    PageContext _jspx_page_context = null;


    try {
      _jspxFactory = JspFactory.getDefaultFactory();
      response.setContentType("text/html");
      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("<html><body><center> \r\n");
      out.write("This is Test App<br>\r\n");

    String str1;
    int int1;


      out.write("\r\n");
      out.write(" \r\n");

    str1 = "This is a aaa!";
    int1 = 10;

      out.write("\r\n");
      out.write("Output str1 is: ");
      out.print(str1);
      out.write(" <br>\r\n");
      out.write("Output int1 is: ");
      out.print(int1);
      out.write(" <br>\r\n");
      out.write("</center></body></html>");
    } catch (Throwable t) {
      if (!(t instanceof SkipPageException)){
        out = _jspx_out;
        if (out != null && out.getBufferSize() != 0)
          out.clearBuffer();
        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
      }
    } finally {
      if (_jspxFactory != null) _jspxFactory.releasePageContext(_jspx_page_context);
    }
  }
}

 

2. Declarations (宣告):以<%! 與 %>包括的元素

在JSP中<%%>間 宣告的變數,將轉換為Servlet的方法中一個區域變數,而<%!%>間宣告的變數,它在 Servlet中會對應至一個 field 成員,此屬性的有效範圍是整個Servlet類別產生的實例,因此,屬性是一個實例變數。 由於Servlet被載入後,將會一直存在於JVM中,直到容器規定的存在期限過期,或是關閉伺服器後才會清除Servlet產生的實例,故使用<%!與% >宣告變數時,必須小心資料共用與執行緒安全的問題。

<%!與%>之間也可以宣告method,而它將在轉換為Servlet之後, 成為Servlet類別的一個method member。

<html><body><center>
This is Test App<br>
<%!
    String str1;
    int int1;
%>

<%
    str1 = "This is a aaa!";
    int1 = 10;
%>
Output str1 is: <%=str1%> <br>
Output int1 is: <%=int1%> <br>
</center></body></html>
package org.apache.jsp;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;

public final class TestApp_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {


    String str1;
    int int1;


  private static java.util.List _jspx_dependants;

  public Object getDependants() {
    return _jspx_dependants;
  }

  public void _jspService(HttpServletRequest request, HttpServletResponse response)
        throws java.io.IOException, ServletException {

    JspFactory _jspxFactory = null;
    PageContext pageContext = null;
    HttpSession session = null;
    ServletContext application = null;
    ServletConfig config = null;
    JspWriter out = null;
    Object page = this;
    JspWriter _jspx_out = null;
    PageContext _jspx_page_context = null;


    try {
      _jspxFactory = JspFactory.getDefaultFactory();
      response.setContentType("text/html");
      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("<html><body><center> \r\n");
      out.write("This is Test App<br>\r\n");
      out.write("\r\n");
      out.write(" \r\n");

    str1 = "This is a aaa!";
    int1 = 10;

      out.write("\r\n");
      out.write("Output str1 is: ");
      out.print(str1);
      out.write(" <br>\r\n");
      out.write("Output int1 is: ");
      out.print(int1);
      out.write(" <br>\r\n");
      out.write("</center></body></html>");
    } catch (Throwable t) {
      if (!(t instanceof SkipPageException)){
        out = _jspx_out;
        if (out != null && out.getBufferSize() != 0)
          out.clearBuffer();
        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
      }
    } finally {
      if (_jspxFactory != null) _jspxFactory.releasePageContext(_jspx_page_context);
    }
  }

 

利用 <%! …… %> 敘述宣告函數時,該函數將成為 servlet 的方法,定義語法如下:

<html><body><center>
This is Test App<br>
<%!
  
回傳資料型別  函數名稱(引數串列){
         ……  //程式碼
     return 回傳值;
 }

%>

<%
    str1 = "This is a aaa!";
    int1 = 10;
%>
Output str1 is: <%=str1%> <br>
Output int1 is: <%=int1%> <br>
</center></body></html>

 

3. Expressions (運算式):以<%= 與 %>包括的元素,注意這個表示法中最後不需要加上分號

<%= expression %>是一個運算式表示,它可以視作 <% out.println(expression); %> 的一個簡單表示。
例如:
<% out.println(new java.util.Date()); %>
<%= new java.util.Date() %>

<%=new out.println ("大家好")%>
<% out.println ("大家好");%>


 

 

若在web.xml中加上<script-invalid>標籤,表示設定所有的JSP網頁都不可以使用Scriptlet

...
<web-app ..>
    ....
    <jsp-config>
        <jsp-property-group>
            <url-pattern>*.jsp</url-pattern>
            <script-invalid>true</script-invalid>
        </jsp-property-group>
    </jsp-config>
....
</web-app>

 

 

 

參考:良葛格學習筆記 JSP/Servlet

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